Reassessment of the pathways responsible for cerebrospinal fluid absorption in the neonate

Postnatal meningeal CSF transport is primarily mediated by the arachnoid and pia maters and is not altered after intraventricular hemorrhage-posthemorrhagic hydrocephalus

BACKGROUND

CSF has long been accepted to circulate throughout the subarachnoid space, which lies between the arachnoid and pia maters of the meninges. How the CSF interacts with the cellular components of the developing postnatal meninges including the dura, arachnoid, and pia of both the meninges at the surface of the brain and the intracranial meninges, prior to its eventual efflux from the cranium and spine, is less understood. Here, we characterize small and large CSF solute distribution patterns along the intracranial and surface meninges in neonatal rodents and compare our findings to meningeal CSF solute distribution in a rodent model of intraventricular hemorrhage-posthemorrhagic hydrocephalus. We also examine CSF solute interactions with the tela choroidea and its pial invaginations into the choroid plexuses of the lateral, third, and fourth ventricles.

METHODS

1.9-nm gold nanoparticles, 15-nm gold nanoparticles, or 3 kDa Red Dextran Tetramethylrhodamine constituted in aCSF were infused into the right lateral ventricle of P7 rats to track CSF circulation. 10 min post-1.9-nm gold nanoparticle and Red Dextran Tetramethylrhodamine injection and 4 h post-15-nm gold nanoparticle injection, animals were sacrificed and brains harvested for histologic analysis to identify CSF tracer localization in the cranial and spine meninges and choroid plexus. Spinal dura and leptomeninges (arachnoid and pia) wholemounts were also evaluated.

RESULTS

There was significantly less CSF tracer distribution in the dura compared to the arachnoid and pia maters in neonatal rodents. Both small and large CSF tracers were transported intracranially to the arachnoid and pia mater of the perimesencephalic cisterns and tela choroidea, but not the falx cerebri. CSF tracers followed a similar distribution pattern in the spinal meninges. In the choroid plexus, there was large CSF tracer distribution in the apical surface of epithelial cells, and small CSF tracer along the basolateral surface. There were no significant differences in tracer intensity in the intracranial meninges of control vs. intraventricular hemorrhage-posthemorrhagic hydrocephalus (PHH) rodents, indicating preserved meningeal transport in the setting of PHH.

CONCLUSIONS

Differential CSF tracer handling by the meninges suggests that there are distinct roles for CSF handling between the arachnoid-pia and dura maters in the developing brain. Similarly, differences in apical vs. luminal choroid plexus CSF handling may provide insight into particle-size dependent CSF transport at the CSF-choroid plexus border.

Dual function of the choroid plexus: Cerebrospinal fluid production and control of brain ion homeostasis

The choroid plexus is a small monolayered epithelium located in the brain ventricles and serves to secrete the cerebrospinal fluid (CSF) that envelops the brain and fills the central ventricles. The CSF secretion is sustained with a concerted effort of a range of membrane transporters located in a polarized fashion in this tissue. Prominent amongst these are the Na+/K+-ATPase, the Na+,K+,2Cl- cotransporter (NKCC1), and several HCO3- transporters, which together support the net transepithelial transport of the major electrolytes, Na+ and Cl-, and thus drive the CSF secretion. The choroid plexus, in addition, serves an important role in keeping the CSF K+ concentration at a level compatible with normal brain function. The choroid plexus Na+/K+-ATPase represents a key factor in the barrier-mediated control of the CSF K+ homeostasis, as it increases its K+ uptake activity when faced with elevated extracellular K+ ([K+]o). In certain developmental or pathological conditions, the NKCC1 may revert its net transport direction to contribute to CSF K+ homeostasis. The choroid plexus ion transport machinery thus serves dual, yet interconnected, functions with its contribution to electrolyte and fluid secretion in combination with its control of brain K+ levels.

Perivascular spaces as a potential biomarker of Alzheimer's disease

Alzheimer's disease (AD) is a highly damaging disease that affects one's cognition and memory and presents an increasing societal and economic burden globally. Considerable research has gone into understanding AD; however, there is still a lack of effective biomarkers that aid in early diagnosis and intervention. The recent discovery of the glymphatic system and associated Perivascular Spaces (PVS) has led to the theory that enlarged PVS (ePVS) may be an indicator of AD progression and act as an early diagnostic marker. Visible on Magnetic Resonance Imaging (MRI), PVS appear to enlarge when known biomarkers of AD, amyloid-β and tau, accumulate. The central goal of ePVS and AD research is to determine when ePVS occurs in AD progression and if ePVS are causal or epiphenomena. Furthermore, if ePVS are indeed causative, interventions promoting glymphatic clearance are an attractive target for research. However, it is necessary first to ascertain where on the pathological progression of AD ePVS occurs. This review aims to examine the knowledge gap that exists in understanding the contribution of ePVS to AD. It is essential to understand whether ePVS in the brain correlate with increased regional tau distribution and global or regional Amyloid-β distribution and to determine if these spaces increase proportionally over time as individuals experience neurodegeneration. This review demonstrates that ePVS are associated with reduced glymphatic clearance and that this reduced clearance is associated with an increase in amyloid-β. However, it is not yet understood if ePVS are the outcome or driver of protein accumulation. Further, it is not yet clear if ePVS volume and number change longitudinally. Ultimately, it is vital to determine early diagnostic criteria and early interventions for AD to ease the burden it presents to the world; ePVS may be able to fulfill this role and therefore merit further research.

Cerebrospinal Fluid Homeostasis and Hydrodynamics: A Review of Facts and Theories

BACKGROUND AND PURPOSE

According to the classical hypothesis, the cerebrospinal fluid (CSF) is actively secreted inside the brain's ventricular system, predominantly by the choroid plexuses, before flowing unidirectionally in a cranio-caudal orientation toward the arachnoid granulations (AGs), where it is reabsorbed into the dural venous sinuses. This concept has been accepted as a doctrine for more than 100 years and was subjected only to minor modifications. Its inability to provide an adequate explanation to questions arising from the everyday clinical practice, in addition to the ever growing pool of experimental data contradicting it, has led to the identification of its limitations. Literature includes an increasing number of studies suggesting a more complex mechanism than that previously described. This review article summarizes the proposed mechanisms of CSF regulation, referring to the key clinical and experimental developments supporting or defying them.

METHODS

A non-systematical literature search of the major databases was performed for studies on the mechanisms of CSF homeostasis. Gray literature was additionally assessed employing a hand-search technique. No restrictions were imposed regarding the time, language, or type of publication.

CONCLUSION

CSF secretion and absorption are expected to take place throughout the entire brain's capillaries network under the regulation of hydrostatic and osmotic gradients. The unidirectional flow is defied, highlighting the possibility of its complete absence. The importance of AGs is brought into question, potentiating the significance of the lymphatic system as the primary site of reabsorption. However, the definition of hydrocephalus and its treatment strategies remain strongly associated with the classical hypothesis.

Cerebrospinal fluid production by the choroid plexus: a century of barrier research revisited

Cerebrospinal fluid (CSF) envelops the brain and fills the central ventricles. This fluid is continuously replenished by net fluid extraction from the vasculature by the secretory action of the choroid plexus epithelium residing in each of the four ventricles. We have known about these processes for more than a century, and yet the molecular mechanisms supporting this fluid secretion remain unresolved. The choroid plexus epithelium secretes its fluid in the absence of a trans-epithelial osmotic gradient, and, in addition, has an inherent ability to secrete CSF against an osmotic gradient. This paradoxical feature is shared with other 'leaky' epithelia. The assumptions underlying the classical standing gradient hypothesis await experimental support and appear to not suffice as an explanation of CSF secretion. Here, we suggest that the elusive local hyperosmotic compartment resides within the membrane transport proteins themselves. In this manner, the battery of plasma membrane transporters expressed in choroid plexus are proposed to sustain the choroidal CSF secretion independently of the prevailing bulk osmotic gradient.

Cerebrospinal fluid outflow: a review of the historical and contemporary evidence for arachnoid villi, perineural routes, and dural lymphatics

Cerebrospinal fluid (CSF) is produced by the choroid plexuses within the ventricles of the brain and circulates through the subarachnoid space of the skull and spinal column to provide buoyancy to and maintain fluid homeostasis of the brain and spinal cord. The question of how CSF drains from the subarachnoid space has long puzzled scientists and clinicians. For many decades, it was believed that arachnoid villi or granulations, outcroppings of arachnoid tissue that project into the dural venous sinuses, served as the major outflow route. However, this concept has been increasingly challenged in recent years, as physiological and imaging evidence from several species has accumulated showing that tracers injected into the CSF can instead be found within lymphatic vessels draining from the cranium and spine. With the recent high-profile rediscovery of meningeal lymphatic vessels located in the dura mater, another debate has emerged regarding the exact anatomical pathway(s) for CSF to reach the lymphatic system, with one side favoring direct efflux to the dural lymphatic vessels within the skull and spinal column and another side advocating for pathways along exiting cranial and spinal nerves. In this review, a summary of the historical and contemporary evidence for the different outflow pathways will be presented, allowing the reader to gain further perspective on the recent advances in the field. An improved understanding of this fundamental physiological process may lead to novel therapeutic approaches for a wide range of neurological conditions, including hydrocephalus, neurodegeneration and multiple sclerosis.

Resolution of neonatal posthemorrhagic ventricular dilation coincident with patent ductus arteriosus ligation: case report

Preterm infants commonly present with a hemodynamically significant patent ductus arteriosus (hsPDA). The authors describe the case of a preterm infant with posthemorrhagic ventricular dilation, which resolved in a temporally coincident fashion to repair of hsPDA. The presence of a PDA with left-to-right shunting was confirmed at birth on echocardiogram and was unresponsive to repeated medical intervention. Initial cranial ultrasound revealed periventricular-intraventricular hemorrhage. Follow-up serial ultrasound showed resolving intraventricular hemorrhage and progressive bilateral hydrocephalus. At 5 weeks, the ductus was ligated with the goal of improving hemodynamic stability prior to CSF diversion. However, neurosurgical intervention was not required due to improvement of ventriculomegaly occurring immediately after PDA ligation. No further ventricular dilation was observed at the 6-month follow-up.Systemic venous flow disruption and abnormal patterns of cerebral blood circulation have been previously associated with hsPDA. Systemic hemodynamic change has been reported to follow hsPDA ligation, although association with ventricular normalization has not. This case suggests that the unstable hemodynamic environment due to left-to-right shunting may also impede CSF outflow and contribute to ventriculomegaly. The authors review the literature surrounding pressure transmission between a PDA and the cerebral vessels and present a mechanism by which PDA may contribute to posthemorrhagic ventricular dilation.

Complex Non-sinus-associated Pachymeningeal Lymphatic Structures: Interrelationship With Blood Microvasculature

The contribution of cranial dura mater vascular networks, as means for maintaining brain fluid movement and balance, and as the source of significant initiators and/or contributors to neurological disorders, has been overlooked. These networks consist of both blood and lymphatic vessels. The latter were discovered recently and described as sinus-associated structures thus changing the old paradigm that central nervous system lacks lymphatics. In this study, using markers specific to blood and lymphatic endothelia, we demonstrate the existence of the complex non-sinus-associated pachymeningeal lymphatic vasculature. We further show the interrelationship and possible connections between lymphatic vessels and the dural blood circulatory system. Our novel findings reveal the presence of lymphatic-like structures that exist on their own and/or in close proximity to microvessels. Of particular interest are sub-sets of vascular complexes with dual (lymphatic and blood) vessel identity representing a unique microenvironment within the cranial dura. The close association of the systemic blood circulation and meningeal lymphatics achieved in these complexes could facilitate fluid exchange between the two compartments and constitute an alternative route for CSF drainage.

Identifying the Critical Factors Governing Translaminar Pressure Differential Through a Compartmental Model

Purpose

The effective management of glaucoma is hindered by an incomplete understanding of its pathologic mechanism. While important, intraocular pressure (IOP) alone is inadequate in explaining glaucoma. Non-IOP-mediated risk factors such as cerebrospinal fluid (CSF) pressure have been reported to contribute to glaucomatous optic neuropathy. Due to the difficulty associated with experimental measurement of the salient variables, such as the retrobulbar CSF pressure, porosity of the subarachnoid space (SAS), and especially those concerned with the perioptic SAS, there remains a limited understanding of the CSF behavior contributing to the translaminar pressure gradient (TLPG), hypothesized to be a critical factor in the development of glaucoma.

Method

An integrated compartmental model describing the intracranial and orbital CSF dynamics, coupled with intraocular dynamics, is developed based on first principles of fluid mechanics. A sensitivity analysis is performed to identify anatomic characteristics that significantly affect the retrobulbar subarachnoid space (RSAS) pressure and, consequently, the TLPG.

Results

Of the 28 parameters considered, the RSAS pressure is most sensitive to CSF flow resistance in the optic nerve SAS and the potential lymphatic outflow from the optic nerve SAS into the orbital space. A parametric study demonstrates that a combination of resistance in the range of 1.600 × 1012 - 1.930 × 1012 Pa s/m3 (200.0 - 241.3 mm Hg min/mL) with 5% to 10% lymphatic CSF outflow yields RSAS pressures that are consistent with the limited number of studies in the literature.

Conclusions

The results suggest that a small percentage of lymphatic CSF outflow through the optic nerve SAS is likely. In addition, flow resistance in the orbital CSF space, hypothesized to be a function of patient-specific optic nerve SAS architecture and optic canal geometry, is a critical parameter in regulating the RSAS pressure and TLPG.

Enhanced in vitro model of the CSF dynamics

BACKGROUND

Fluid dynamics of the craniospinal system are complex and still not completely understood. In vivo flow and pressure measurements of the cerebrospinal fluid (CSF) are limited. Whereas in silico modeling can be an adequate pathway for parameter studies, in vitro modeling of the craniospinal system is essential for testing and evaluation of therapeutic measures associated with innovative implants relating to, for example, normal pressure hydrocephalus and other fluid disorders. Previously-reported in vitro models focused on the investigation of only one hypothesis of the fluid dynamics rather than developing a modular set-up to allow changes in focus of the investigation. The aim of this study is to present an enhanced and validated in vitro model of the CSF system which enables the future embedding of implants, the validation of in silico models or phase-contrast magnetic resonance imaging (PC-MRI) measurements and a variety of sensitivity analyses regarding pathological behavior, such as reduced CSF compliances, higher resistances or altered blood dynamics.

METHODS

The in vitro model consists of a ventricular system which is connected via the aqueduct to the cranial and spinal subarachnoid spaces. Two compliance chambers are integrated to cushion the arteriovenous blood flow generated by a cam plate unit enabling the modeling of patient specific flow dynamics. The CSF dynamics are monitored using three cranial pressure sensors and a spinal ultrasound flow meter. Measurements of the in vitro spinal flow were compared to cervical flow data recorded with PC-MRI from nine healthy young volunteers, and pressure measurements were compared to the literature values reported for intracranial pressure (ICP) to validate the newly developed in vitro model.

RESULTS

The maximum spinal CSF flow recorded in the in vitro simulation was 133.60 ml/min in the caudal direction and 68.01 ml/min in the cranial direction, whereas the PC-MRI flow data of the subjects showed 122.82 ml/min in the caudal and 77.86 ml/min in the cranial direction. In addition, the mean ICP (in vitro) was 12.68 mmHg and the pressure wave amplitude, 4.86 mmHg, which is in the physiological range.

CONCLUSIONS

The in vitro pressure values were in the physiological range. The amplitudes of the flow results were in good agreement with PC-MRI data of young and healthy volunteers. However, the maximum cranial flow in the in vitro model occurred earlier than in the PC-MRI data, which might be due to a lack of an in vitro dynamic compliance. Implementing dynamic compliances and related sensitivity analyses are major aspects of our ongoing research.

Lymphatic Vessel Network Structure and Physiology

The lymphatic system is comprised of a network of vessels interrelated with lymphoid tissue, which has the holistic function to maintain the local physiologic environment for every cell in all tissues of the body. The lymphatic system maintains extracellular fluid homeostasis favorable for optimal tissue function, removing substances that arise due to metabolism or cell death, and optimizing immunity against bacteria, viruses, parasites, and other antigens. This article provides a comprehensive review of important findings over the past century along with recent advances in the understanding of the anatomy and physiology of lymphatic vessels, including tissue/organ specificity, development, mechanisms of lymph formation and transport, lymphangiogenesis, and the roles of lymphatics in disease. © 2019 American Physiological Society. Compr Physiol 9:207-299, 2019.

Cerebrospinal fluid circulation: What do we know and how do we know it?

The central nervous system's (CNS) complicated design is a double-edged sword. On the one hand, the complexity is what gives rise to higher order thinking; but on the other hand, damage to the CNS evokes its unforgiving nature. The cerebrospinal fluid (CSF) circulation system is an intricate system embedded in and around the CNS that has been the topic of debate since it was first described in the 18^th century. It is underscored by the choroid plexus's distinct vascular network which has conventionally been seen as the most prominent structure in CSF production through a variety of active transporters and channels. Despite the ubiquity of this circulation system in vertebrates, some aspects remain understudied. Recent advances in scientific methodology and experimentation have proven to be effective tools for elucidating the mechanisms of the CSF circulation system and the pathological conditions associated with its malfunction. In this review, we capitulate the classical understanding of CSF physiology as well as a new, emerging theory on CSF production.

MR assessment of pediatric hydrocephalus: a road map

PURPOSE

This study was conducted to design a rational approach to the MR diagnosis of hydrocephalus based on a pathophysiologic reevaluation of its possible mechanisms and to apply it to the different etiological contexts.

METHOD

A review of the literature reports describing new physiologic models of production and absorption and of the hydrodynamics of the CSF was made.

RESULTS

Besides the secretion of CSF by the choroid plexuses, and its passive, pressure-dependent transdural absorption (arachnoid villi, dural clefts, cranial, and spinal nerve sheaths), water transporters, aquaporins, allow water (if not ions and organic molecules) to exchange freely between the brain parenchyma and the CSF spaces across the ependymal and the pial interfaces (including the Virchow-Robin spaces). Consequently, the CSF bulk flow is not necessarily global, and situations of balanced absorption-secretion may occur separately in different CSF compartments such as the ventricular, intracranial, or intraspinal CSF spaces. This means that rather than from a hypothetical pressure gradient from the plexuses to the dural sinuses, the dynamics of the CSF depend on the force provided in those different compartments by the arterial systolic pulsation of the pericerebral (mostly), intracerebral, and intraventricular (choroid plexuses) vascular beds.

CONCLUSION

Using MR imaging, diverse varieties of hydrocephalus may tentatively be explained by applying those concepts to the correspondingly diverse causal diseases. Hopefully, this may have an impact on the choice of the treatment strategies also.

Olfactory route for cerebrospinal fluid drainage into the cervical lymphatic system in a rabbit experimental model

The present study analyzed the anatomical association between intracranial subarachnoid space and the cervical lymphatic system. X-ray contrast medium and Microfil(®) (Microfil compounds fill and opacify microvascular and other spaces of non-surviving animals and post-mortem tissue under physiological injection pressure) were injected into the cisterna magna of the rabbit, and perineural routes of cerebrospinal fluid outflow into the lymphatic system were visualized. Under a surgical operating microscope, Microfil was found within the subarachnoid space and along the olfactory nerves. At the nasal mucosa, a lymphatic network was identified near the olfactory nerves, which crossed the nasopharyngeal region and finally emptied into the superficial and deep cervical lymph nodes. Under a light microscope, Microfil was visible around the olfactory nerves and within lymphatic vessels. These results suggested that cerebrospinal fluid drained from the subarachnoid space along the olfactory nerves to nasal lymphatic vessels, which in turn, emptied into the cervical lymph nodes. This anatomical route, therefore, allowed connection between the central nervous system and the lymphatic system.

Mechanisms of fluid movement into, through and out of the brain: evaluation of the evidence

Interstitial fluid (ISF) surrounds the parenchymal cells of the brain and spinal cord while cerebrospinal fluid (CSF) fills the larger spaces within and around the CNS. Regulation of the composition and volume of these fluids is important for effective functioning of brain cells and is achieved by barriers that prevent free exchange between CNS and blood and by mechanisms that secrete fluid of controlled composition into the brain and distribute and reabsorb it. Structures associated with this regular fluid turnover include the choroid plexuses, brain capillaries comprising the blood-brain barrier, arachnoid villi and perineural spaces penetrating the cribriform plate. ISF flow, estimated from rates of removal of markers from the brain, has been thought to reflect rates of fluid secretion across the blood-brain barrier, although this has been questioned because measurements were made under barbiturate anaesthesia possibly affecting secretion and flow and because CSF influx to the parenchyma via perivascular routes may deliver fluid independently of blood-brain barrier secretion. Fluid secretion at the blood-brain barrier is provided by specific transporters that generate solute fluxes so creating osmotic gradients that force water to follow. Any flow due to hydrostatic pressures driving water across the barrier soon ceases unless accompanied by solute transport because water movements modify solute concentrations. CSF is thought to be derived primarily from secretion by the choroid plexuses. Flow rates measured using phase contrast magnetic resonance imaging reveal CSF movements to be more rapid and variable than previously supposed, even implying that under some circumstances net flow through the cerebral aqueduct may be reversed with net flow into the third and lateral ventricles. Such reversed flow requires there to be alternative sites for both generation and removal of CSF. Fluorescent tracer analysis has shown that fluid flow can occur from CSF into parenchyma along periarterial spaces. Whether this represents net fluid flow and whether there is subsequent flow through the interstitium and net flow out of the cortex via perivenous routes, described as glymphatic circulation, remains to be established. Modern techniques have revealed complex fluid movements within the brain. This review provides a critical evaluation of the data.

Cerebrospinal fluid is drained primarily via the spinal canal and olfactory route in young and aged spontaneously hypertensive rats

BACKGROUND

Many aspects of CSF dynamics are poorly understood due to the difficulties involved in quantification and visualization. In particular, there is debate surrounding the route of CSF drainage. Our aim was to quantify CSF flow, volume, and drainage route dynamics in vivo in young and aged spontaneously hypertensive rats (SHR) using a novel contrast-enhanced computed tomography (CT) method.

METHODS

ICP was recorded in young (2-5 months) and aged (16 months) SHR. Contrast was administered into the lateral ventricles bilaterally and sequential CT imaging was used to visualize the entire intracranial CSF system and CSF drainage routes. A customized contrast decay software module was used to quantify CSF flow at multiple locations.

RESULTS

ICP was significantly higher in aged rats than in young rats (11.52 ± 2.36 mmHg, versus 7.04 ± 2.89 mmHg, p = 0.03). Contrast was observed throughout the entire intracranial CSF system and was seen to enter the spinal canal and cross the cribriform plate into the olfactory mucosa within 9.1 ± 6.1 and 22.2 ± 7.1 minutes, respectively. No contrast was observed adjacent to the sagittal sinus. There were no significant differences between young and aged rats in either contrast distribution times or CSF flow rates. Mean flow rates (combined young and aged) were 3.0 ± 1.5 μL/min at the cerebral aqueduct; 3.5 ± 1.4 μL/min at the 3rd ventricle; and 2.8 ± 0.9 μL/min at the 4th ventricle. Intracranial CSF volumes (and as percentage total brain volume) were 204 ± 97 μL (8.8 ± 4.3%) in the young and 275 ± 35 μL (10.8 ± 1.9%) in the aged animals (NS).

CONCLUSIONS

We have demonstrated a contrast-enhanced CT technique for measuring and visualising CSF dynamics in vivo. These results indicate substantial drainage of CSF via spinal and olfactory routes, but there was little evidence of drainage via sagittal sinus arachnoid granulations in either young or aged animals. The data suggests that spinal and olfactory routes are the primary routes of CSF drainage and that sagittal sinus arachnoid granulations play a minor role, even in aged rats with higher ICP.

Mechanisms of fluid movement into, through and out of the brain: evaluation of the evidence

Interstitial fluid (ISF) surrounds the parenchymal cells of the brain and spinal cord while cerebrospinal fluid (CSF) fills the larger spaces within and around the CNS. Regulation of the composition and volume of these fluids is important for effective functioning of brain cells and is achieved by barriers that prevent free exchange between CNS and blood and by mechanisms that secrete fluid of controlled composition into the brain and distribute and reabsorb it. Structures associated with this regular fluid turnover include the choroid plexuses, brain capillaries comprising the blood-brain barrier, arachnoid villi and perineural spaces penetrating the cribriform plate. ISF flow, estimated from rates of removal of markers from the brain, has been thought to reflect rates of fluid secretion across the blood-brain barrier, although this has been questioned because measurements were made under barbiturate anaesthesia possibly affecting secretion and flow and because CSF influx to the parenchyma via perivascular routes may deliver fluid independently of blood-brain barrier secretion. Fluid secretion at the blood-brain barrier is provided by specific transporters that generate solute fluxes so creating osmotic gradients that force water to follow. Any flow due to hydrostatic pressures driving water across the barrier soon ceases unless accompanied by solute transport because water movements modify solute concentrations. CSF is thought to be derived primarily from secretion by the choroid plexuses. Flow rates measured using phase contrast magnetic resonance imaging reveal CSF movements to be more rapid and variable than previously supposed, even implying that under some circumstances net flow through the cerebral aqueduct may be reversed with net flow into the third and lateral ventricles. Such reversed flow requires there to be alternative sites for both generation and removal of CSF. Fluorescent tracer analysis has shown that fluid flow can occur from CSF into parenchyma along periarterial spaces. Whether this represents net fluid flow and whether there is subsequent flow through the interstitium and net flow out of the cortex via perivenous routes, described as glymphatic circulation, remains to be established. Modern techniques have revealed complex fluid movements within the brain. This review provides a critical evaluation of the data.

Intranasal administration of oxytocin: behavioral and clinical effects, a review

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The mechanisms behind the effects of IN-applied substances need more attention.

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The mechanisms involved in the brain-distribution of IN-OT are completely unexplored.

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The possibly cascading effects of IN-OT on the intrinsic OT-system require serious investigation.

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IN-OT induces clear and specific changes in neural activation.

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IN-OT is a promising approach to treat certain clinical symptoms.

The intranasal (IN-) administration of substances is attracting attention from scientists as well as pharmaceutical companies. The effects are surprisingly fast and specific. The present review explores our current knowledge about the routes of access to the cranial cavity. ‘Direct-access-pathways’ from the nasal cavity have been described but many additional experiments are needed to answer a variety of open questions regarding anatomy and physiology.

Among the IN-applied substances oxytocin (OT) has an extensive history. Originally applied in women for its physiological effects related to lactation and parturition, over the last decade most studies focused on their behavioral ‘prosocial’ effects: from social relations and ‘trust’ to treatment of ‘autism’.

Only very recently in a microdialysis study in rats and mice, the ‘direct-nose-brain-pathways’ of IN-OT have been investigated directly, implying that we are strongly dependent on results obtained from other IN-applied substances. Especially the possibility that IN-OT activates the ‘intrinsic’ OT-system in the hypothalamus as well needs further clarification.

We conclude that IN-OT administration may be a promising approach to influence human communication but that the existing lack of information about the neural and physiological mechanisms involved is a serious problem for the proper understanding and interpretation of the observed effects.

The role of cerebrospinal fluid pressure in glaucoma and other ophthalmic diseases: A review

Glaucoma is one of the most common causes of blindness in the world. Well-known risk factors include age, race, a positive family history and elevated intraocular pressures. A newly proposed risk factor is decreased cerebrospinal fluid pressure (CSFP). This concept is based on the notion that a pressure differential exists across the lamina cribrosa, which separates the intraocular space from the subarachnoid fluid space. In this construct, an increased translaminar pressure difference will occur with a relative increase in elevated intraocular pressure or a reduction in CSFP. This net change in pressure is proposed to act on the tissues within the optic nerve head, potentially contributing to glaucomatous optic neuropathy. Similarly, patients with ocular hypertension who have elevated CSFPs, would enjoy a relatively protective effect from glaucomatous damage. This review will focus on the current literature pertaining to the role of CSFP in glaucoma. Additionally, the authors examine the relationship between glaucoma and other known CSFP-related ophthalmic disorders.

Cerebrospinal fluid pressure decreases with older age

PURPOSE

Clinical studies implicate low cerebrospinal fluid pressure (CSFP) or a high translaminar pressure difference in the pathogenesis of primary open angle glaucoma (POAG) and normal tension glaucoma (NTG). This study was performed to examine the effect of age, sex, race and body mass index (BMI) on CSFP.

METHODS

Electronic medical records from all patients who had a lumbar puncture (LP) performed at the Mayo Clinic from 1996-2009 were reviewed. Information including age, sex, race, height and weight, ocular and medical diagnoses, intraocular pressure (IOP) and LP opening pressure was obtained. Patients using medications or with medical diagnoses known to affect CSFP, and those who underwent neurosurgical procedures or where more than one LP was performed were excluded from analysis.

RESULTS

Electronic medical records of 33,922 patients with a history of having an LP during a 13-year period (1996-2009) were extracted. Of these, 12,118 patients met all entry criteria. Relative to mean CSFP at age group 20-49 (mean 11.5±2.8 mmHg), mean CSFP declined steadily after age 50, with percent reduction of 2.5% for the 50-54 age group (mean 11.2±2.7 mmHg, p<0.002) to 26.9% for the 90-95 group (mean 8.4±2.4 mmHg, p<0.001). Females had lower CSFP than males throughout all age groups. BMI was positively and independently associated with CSFP within all age groups.

CONCLUSION

There is a sustained and significant reduction of CSFP with age that begins in the 6(th) decade. CSFP is consistently lower in females. BMI is positively and independently associated with CSFP in all age groups. The age where CSFP begins to decline coincides with the age where the prevalence of POAG increases. These data support the hypothesis that reduced CSFP may be a risk factor for POAG and may provide an explanation for the mechanism that underlies the age-related increase in the prevalence of POAG and NTG.

The pathophysiology of brain swelling associated with subdural hemorrhage: the role of the trigeminovascular system

INTRODUCTION

This paper reviews the evidence in support of the hypothesis that the trigeminal system mediates brain swelling associated with subdural bleeding. The trigeminovascular system has been extensively studied in migraine; it may play an important but under-recognized role in the response to head trauma. Nerve fibers originating in trigeminal ganglion cells are the primary sensors of head trauma and, through their collateral innervation of the intracranial and dural blood vessels, are capable of inciting a cascade of vascular responses and brain swelling. The extensive trigeminal representation in the brainstem initiates and augments autonomic responses. Blood and tissue injury in the dura incite neurogenic inflammatory responses capable of sensitizing dural nerves and potentiating the response to trauma.

DISCUSSION

The trigeminal system may provide the anatomo-physiological link between small-volume, thin subdural bleeds and swelling of the underlying brain. This physiology may help to explain the poorly understood phenomena of "second-impact syndrome," the infant response to subdural bleeding (the "big black brain"), as well as post-traumatic subdural effusions. Considerable age-specific differences in the density of dural innervation exist; age-specific responses of this innervation may explain differences in the brain's response to trauma in the young. An understanding of this pathophysiology is crucial to the development of intervention and treatment of these conditions. Antagonists to specific neuropeptides of the trigeminal system modify brain swelling after trauma and should be further explored as potential therapy in brain trauma and subdural bleeding.

Developmental changes in human dural innervation

INTRODUCTION

There is limited published work on the abundant innervation of the human dura mater, its role and responses to injury in humans. The dura not only provides mechanical support for the brain but may also have other functions, including control of the outflow of venous blood from the brain via the dural sinuses. The trigeminal nerve supplies sensory fibres to the dura as well as the leptomeninges, intracranial blood vessels, face, nose and mouth. Its relatively large size in embryonic life suggests an importance in development; the earliest fetal reflexes, mediated by the trigeminal, are seen by 8 weeks. Trigeminal functions vital to the fetus include the coordination of sucking and swallowing and the protective oxygen-conserving reflexes. Like other parts of the nervous system, the trigeminal undergoes pruning and remodelling throughout development.

METHODS

We have investigated changes in the innervation of the human dura with age in 27 individuals aged between 31 weeks of gestation and 60 years of postnatal life. Using immunocytochemistry with antibodies to neurofilament, we have found significant changes in the density of dural innervation with age

RESULTS

The density of innervation increased between 31 and 40 weeks of gestation, peaking at term and decreasing in the subsequent 3 months, remaining low until the sixth decade.

CONCLUSIONS

Our observations are consistent with animal studies but are, to our knowledge, the first to show age-related changes in the density of innervation in the human dura. They provide new insights into the functions of the human dura during development.

Treatment of patients with traumatic subdural effusion and concomitant hydrocephalus

Object

Traumatic subdural effusion (TSE) is a common sequela of traumatic brain injury. Surgical intervention is suggested only when TSE exerts mass effect. The authors have found that many patients with TSE exerting mass effect have concomitant hydrocephalus. Patient experiencing this occurrence were studied, and the pathogenesis of this phenomenon was discussed in the context of recent advances in the understanding of CSF circulation.

Methods

During a 2-year period, the authors' institution treated 14 patients with TSE who developed hydrocephalus, after 1 of the patients suffered subdural drainage and other 13 received subdural peritoneal shunt (SPSs). Thirteen of those who had SPSs received programmable ventriculoperitoneal shunts (VPSs) for the hydrocephalus. The clinical characteristics as well as the imaging and operative findings of these patients were reviewed.

Results

All patients with symptomatic TSE exerting mass effect received SPSs. All of these patients had a modified Frontal Horn Index of more than 0.33 at presentation, and high opening pressure on durotomy. Following a brief period (4–7 days) of clinical improvement, the condition of all patients deteriorated due to hydrocephalus. Programmable VPSs were inserted with the initial pressure set at approximately 8–10 cm H2O according to opening pressure at ventriculostomy. Shunt valve pressure was gradually decreased to 5–7 cm H2O, according to clinical and radiological follow-up.

Conclusions

Elevated modified Frontal Horn Index in patients with TSE is suggestive of concomitant hydrocephalus. The authors propose that tearing of the dura-arachnoid plane following trauma contributes to TSE and may also impede CSF circulation, causing hydrocephalus. Shunt pressure was adjusted to relative low pressure, indicating the old age of the patients and poor reexpansion of brain parenchyma after the mass effect. Subdural peritoneal shunts and VPSs are indicated in those patients with TSE exerting mass effect with concomitant hydrocephalus.

Direct venous spinal reabsorption of cerebrospinal fluid: a new concept with serial magnetic resonance cisternography in rabbits

OBJECT

For nearly 100 years it has been believed that the main reabsorption of CSF occurs in arachnoid projections into the superior sagittal sinus, but a significant number of experiments and cases conflict with this hypothesis. According to recently published studies, CSF is permanently produced and absorbed in the whole CSF system. Clusters of arachnoidal villi, which are speculated to have a role in the reabsorption of CSF, have recently been revealed in the dorsal root of the spinal nerves. Huge absorptive surface areas of microvessels have been suggested to serve a putative role in reabsorption. The authors' aim was to observe direct venous connections between the subarachnoid space and the perispinal veins.

METHODS

Eleven adult (6 months old) New Zealand white male rabbits weighing approximately 3.0 kg each were used in this experiment. After obtaining precontrast MR cisternography images, subarachnoid access was gained percutaneously via a cisternal approach by using a 20-gauge intravenous indwelling cannula. One rabbit died as a result of brainstem trauma during percutaneous cannulation before contrast administration, but contrast agent was still injected to see the possible MR imaging results of spinal CSF reabsorption after death. Magnetic resonance imaging was performed at 15, 60, 120, and 180 minutes after the administration of contrast agent. After intramuscular injections of anesthetic, 2 rabbits died 120 and 150 minutes after contrast injection, but the MR imaging study at 180 minutes after contrast injection was still performed.

RESULTS

Direct connections between the subarachnoid space and the perispinal veins were observed in all rabbits during serial MR cisternography. The enhancement power was not affected by the amount of injected contrast agent or by cervical or lumbar penetration but was increased at higher contrast concentrations or upon seizure (physical activity).

CONCLUSIONS

Extracranial reabsorption of CSF has been finally proved with direct radiological confirmation of spinal venous reabsorption of CSF using serial MR cisternography. The authors believe that this study can help to develop a more accurate model of CSF dynamics, which will allow understanding of many CSF-related diseases, as well as the development of new strategies for treatment.

The "Shaken Baby" syndrome: pathology and mechanisms

The "Shaken Baby" syndrome (SBS) is the subject of intense controversy; the diagnosis has in the past depended on the triad of subdural haemorrhage (SDH), retinal haemorrhage and encephalopathy. While there is no doubt that infants do suffer abusive injury at the hands of their carers and that impact can cause catastrophic intracranial damage, research has repeatedly undermined the hypothesis that shaking per se can cause this triad. The term non-accidental head injury has therefore been widely adopted. This review will focus on the pathology and mechanisms of the three physiologically associated findings which constitute the "triad" and are seen in infants suffering from a wide range of non-traumatic as well as traumatic conditions. "Sub" dural bleeding in fact originates within the deep layers of the dura. The potential sources of SDH include: the bridging veins, small vessels within the dura itself, a granulating haemorrhagic membrane and ruptured intracranial aneurysm. Most neuropathologists do not routinely examine eyes, but the significance of this second arm of the triad in the diagnosis of Shaken Baby syndrome is such that it merits consideration in the context of this review. While retinal haemorrhage can be seen clinically, dural and subarachnoid optic nerve sheath haemorrhage is usually seen exclusively by the pathologist and only rarely described by the neuroradiologist. The term encephalopathy is used loosely in the context of SBS. It may encompass anything from vomiting, irritability, feeding difficulties or floppiness to seizures, apnoea and fulminant brain swelling. The spectrum of brain pathology associated with retinal and subdural bleeding from a variety of causes is described. The most important cerebral pathology is swelling and hypoxic-ischaemic injury. Mechanical shearing injury is rare and contusions, the hallmark of adult traumatic brain damage, are vanishingly rare in infants under 1 year of age. Clefts and haemorrhages in the immediate subcortical white matter have been assumed to be due to trauma but factors specific to this age group offer other explanations. Finally, examples of the most common causes of the triad encountered in clinical diagnostic and forensic practice are briefly annotated.

Development of hydrocephalus and classical hypothesis of cerebrospinal fluid hydrodynamics: facts and illusions

According to the classical hypothesis of the cerebrospinal fluid (CSF) hydrodynamics, CSF is produced inside the brain ventricles, than it circulates like a slow river toward the cortical subarachnoid space, and finally it is absorbed into the venous sinuses. Some pathological conditions, primarily hydrocephalus, have also been interpreted based on this hypothesis. The development of hydrocephalus is explained as an imbalance between CSF formation and absorption, where more CSF is formed than is absorbed, which results in an abnormal increase in the CSF volume inside the cranial CSF spaces. It is believed that the reason for the imbalance is the obstruction of the CSF pathways between the site of CSF formation and the site of its absorption, which diminishes or prevents CSF outflow from the cranium. In spite of the general acceptance of the classical hypothesis, there are a considerable number of experimental results that do not support such a hypothesis and the generally accepted pathophysiology of hydrocephalus. A recently proposed new working hypothesis suggests that osmotic and hydrostatic forces at the central nervous system microvessels are crucial for the regulation of interstial fluid and CSF volume which constitute a functional unit. Based on that hypothesis, the generally accepted mechanisms of hydrocephalus development are not plausible. Therefore, the recent understanding of the correlation between CSF physiology and the development of hydrocephalus has been thoroughly presented, analyzed and evaluated, and new insights into hydrocephalus etiopathology have been proposed, which are in accordance with the experimental data and the new working hypothesis.

The function and structure of the cerebrospinal fluid outflow system

This review traces the development of our understanding of the anatomy and physiological properties of the two systems responsible for the drainage of cerebrospinal fluid (CSF) into the systemic circulation. The roles of the cranial and spinal arachnoid villi (AV) and the lymphatic outflow systems are evaluated as to the dominance of one over the other in various species and degree of animal maturation. The functional capabilities of the total CSF drainage system are presented, with evidence that the duality of the system is supported by the changes in fluid outflow dynamics in human and sub-human primates in hydrocephalus. The review also reconciles the relative importance and alterations of each of the outflow systems in a variety of clinical pathological conditions.

Cervical lymph nodes are found in direct relationship with the internal carotid artery: significance for the lymphatic drainage of the brain

The brain has no conventional lymphatics, but solutes injected into it drain along artery walls and reach lymph nodes in the neck. This study seeks to identify cervical lymph nodes related to the human internal carotid artery (ICA) that could act as the first regional lymph nodes for the brain. Bilateral dissections were carried out on four embalmed human heads, from the level of the carotid bifurcation in the neck, to the base of the skull. Lymph nodes from every specimen were processed for histological examination. A total of 51 deep cervical lymph nodes were identified: 12 lymph nodes (confirmed by histological examination) were observed to be in direct relationship with the ICA. These lymph nodes were found within the carotid sheath and had average diameters of 13.5 x 4.8 mm. Solutes and interstitial fluid from the brain may drain along the walls of cerebral arteries and reach these lymph nodes. They may be sites of stimulation of immune responses against antigens from the brain.

Demonstration of fluid channels in human dura and their relationship to age and intradural bleeding

PURPOSE

This paper aims to make a systematic study of human dura to establish the presence of fluid transport channels and their relationship to age.

METHODS

Samples of parasagittal dura from autopsy cases from mid-gestation to the ninth decade were examined by light microscopy.

RESULTS

We have demonstrated the presence of unlined rounded spaces, uncommon in the fetus and neonate but increasingly evident after 30 weeks of postnatal life. We have shown that intradural bleeding is inversely correlated with the presence of these channels and with age.

CONCLUSIONS

We suggest that dural maturation, involving the development of arachnoid granulations, may be related to dilatation of intradural fluid channels, allowing them to be identified histologically. The risk of reflux of blood into the dura appears to reduce with age.

The neuropathology of infant subdural haemorrhage

Subdural haemorrhage (SDH) in the infant has a different pattern from that seen in the older child and adult. It is usually a widespread, bilateral, thin film, unlike the thick, space-occupying and often unilateral clot seen in older children and adults after trauma. Whether both arise by the same mechanism is unknown, but it seems unlikely. Most SDH is said to be due to trauma but in infants there are other, atraumatic causes. Birth is also important; recent MRI studies show an incidence of almost 50% in asymptomatic neonates. Traumatic SDH is said to result from rupture of bridging veins but new insights into the anatomy of infant dura suggest a dural origin for thin film subdural bleeding in young babies. Acute SDH usually rapidly resolves, but sometimes develops into a chronic fluid collection. Healing of SDH is by formation of a granulating membrane which may confer vulnerability to rebleeding, either spontaneously or after an otherwise innocuous event. SDH has a particular significance as one of the features of the triad (together with retinal haemorrhage and encephalopathy) associated with non-accidental injury. As the possibility of non-accidental injury is often first raised by a radiologic report of subdural bleeding, it becomes critically important in the interpretation of the scan appearances to understand the unique physiology and anatomy of the infant dura.

Anatomy and development of the meninges: implications for subdural collections and CSF circulation

The dura is traditionally viewed as a supportive fibrous covering of the brain containing the dural venous sinuses but otherwise devoid of vessels and lacking any specific function. However, review of the embryology and anatomy reveals the dura to be a complex, vascularized and innervated structure, not a simple fibrous covering. The dura contains an inner vascular plexus that is larger in the infant than in the adult, and this plexus likely plays a role in CSF absorption. This role could be particularly important in the infant whose arachnoid granulations are not completely developed. Although subdural hemorrhage is frequently traumatic, there are nontraumatic conditions associated with subdural hemorrhage, and the inner dural plexus is a likely source of bleeding in these nontraumatic circumstances. This review outlines the development and age-specific vascularity of the dura and offers an alternative perspective on the role of the dura in homeostasis of the central nervous system.

Impaired lymphatic cerebrospinal fluid absorption in a rat model of kaolin-induced communicating hydrocephalus

It has been assumed that the pathogenesis of hydrocephalus includes a cerebrospinal fluid (CSF) absorption deficit. Because a significant portion of CSF absorption occurs into extracranial lymphatics located in the olfactory turbinates, the purpose of this study was to determine whether CSF transport was compromised at this location in a kaolin-induced communicating (extraventricular) hydrocephalus model in rats. Under 1-3% halothane anesthesia, kaolin (n = 10) or saline (n = 9) was introduced into the basal cisterns of Sprague-Dawley rats, and the development of hydrocephalus was assessed 1 wk later using MRI. After injection of human serum albumin ((125)I-HSA) into a lateral ventricle, the tracer enrichment in the olfactory turbinates 30 min postinjection provided an estimate of CSF transport through the cribriform plate into nasal lymphatics. Lateral ventricular volumes in the kaolin group (0.073 +/- 0.014 ml) were significantly greater than those in the saline-injected animals (0.016 +/- 0.001 ml; P = 0.0014). The CSF tracer enrichment in the olfactory turbinates (expressed as percent injected/g tissue) in the kaolin rats averaged 0.99 +/- 0.39 and was significantly lower than that measured in the saline controls (5.86 +/- 0.32; P < 0.00001). The largest degree of ventriculomegaly was associated with the lowest levels of lymphatic CSF uptake with lateral ventricular expansion occurring only when almost all of the lymphatic CSF transport capacity had been compromised. We conclude that lymphatic CSF absorption is impaired in a kaolin-communicating hydrocephalus model and that the degree of this impediment may contribute to the severity of the induced disease.

Impact of ageing on lymphatic cerebrospinal fluid absorption in the rat

Several parameters associated with the cerebrospinal fluid (CSF) system show a change in the later stages of life, including elevated CSF outflow resistance. The latter implies a CSF absorption deficit. As a significant portion of CSF absorption occurs into extracranial lymphatic vessels located in the olfactory turbinates, the purpose of this study was to determine whether any age-related impediments to CSF absorption existed at this location. In previous studies, we observed rapid movement of the CSF tracer into the olfactory turbinates in young rats (peaking 30 min after injection), with the concentration of the tracer being much higher in the turbinates than in any other tissue measured. In the study reported here, (125)I-human serum albumin was injected into the lateral ventricles of 3-, 6-, 12- and 19-month-old Fisher 344 rats. The animals were sacrificed at various times after injection of the radioactive tracer, and appropriate tissue samples were extracted. At 30 min post injection, the average tracer values expressed as per cent injected/g tissue were 6.68 +/- 0.42 (n = 9, 3 months), 4.78 +/- 0.67 (n = 9, 6 months), 2.49 +/- 0.31 (n = 9, 12 months) and 2.42 +/- 0.72 (n = 9, 19 months). We conclude that lymphatic CSF transport declines significantly with age. In concert with the known drop in CSF formation, the reduction in lymphatic CSF absorption may contribute to a decrease in CSF turnover in the elderly.

High-pressure hydrocephalus: A novel analytical modeling approach

Hydrocephalus is an abnormal accumulation of cerebrospinal fluid (CSF) within ventricles and subarachnoid space (SAS) as a result of disturbances in secretion or absorption procedures. It is believed that arachnoid villi cells, which are microscopic projections of pia-arachnoid mater that extend into venous channels in sagittal sinus, are the main sites for CSF absorption, but it is tempting to speculate that a significant portion of CSF is removed from the SAS by nasal lymphatic vessels around olfactory nerve. Thus, in this paper, we propose an analytical model of CSF-lymphatic-blood circulation, in which these two output pathways for CSF absorption have been considered. Mathematical relations governing the pressures in different interacting compartments of the brain are considered. In addition, for increasing the similarity of our model to the physiological conditions, the bulk flow mechanism, which is supposed to occur during CSF absorption, has been considered in our model. We used our model to simulate hydrocephalus. The results indicate that the lymphatic disorders have more considerable effect in decreasing CSF absorption, compared to the disturbances in arachnoid villi cells. Based on our modeling, we believe that disorders in lymphatic pathway may be a cause of high-pressure hydrocephalus. Surely experimental studies are required to validate our hypothesis.

Progressive myelopathy due to meningeal thickening in shunted patients: description of a novel entity and the role of surgery

INTRODUCTION

Spinal cord compression due to meningeal thickening is a rare occurrence in shunted patients. Because of the long delay to clinical onset, this complication has not been identified as yet.

AIMS

We report on nine cases of shunt-related progressive myelopathy due to meningeal thickening (SPMMT).

MATERIALS AND METHODS

We reviewed our database of shunted children, for cases having developed progressive tetraparesis due to cervical meningeal thickening.

RESULTS

We identified nine observations of SPMMT, eight of these with hydrocephalus due to neonatal meningitis; the last case had Dandy-Walker malformation shunted at birth and suffered postoperative meningitis. The age of clinical onset of myelopathy was between 6 and 20 years (median 12.8). All patients presented with slowly progressive walking difficulties with falls and no spinal pain. Magnetic resonance imaging (MRI) showed typically a thickened dura mater with collapse of the arachnoid space, compensatory expansion of the epidural fat, and T2 hyperintensity in the spinal cord. We operated on seven patients for surgical decompression and arachnoidolysis: One died postoperatively because of shunt malfunction, and two others died later of complications of tetraplegia. Three patients were aggravated after surgery, three experienced partial improvement, but one of these subsequently deteriorated again.

CONCLUSION

SPMMT appears to be a novel and well-defined clinical and pathological entity; its pathological and radiological features are stereotyped; however, the diagnosis is delayed because of the slow pace of the disease. Although surgical decompression may be the only option, its results were poor in our experience; earlier surgery might improve this grim prognosis.

Properties of the lymphatic cerebrospinal fluid transport system in the rat: impact of elevated intracranial pressure

Previous studies suggested that a major portion of cerebrospinal fluid (CSF) is absorbed by extracranial lymphatics located in the olfactory turbinates. The objective of this study was to determine the impact of elevated intracranial pressure (ICP) on downstream cervical lymphatic pressures in the rat. Pressures were measured in the deep cervical lymph nodes using a servo-null micropressure system. A catheter was placed in a lateral ventricle and fluid was infused from a reservoir at defined ICPs. When Ringer's solution was infused, elevations of ICP from 10 to 50 cm H2O resulted on average in a reduction of diastolic cervical node pressures. In contrast, when a diluted plasma solution (80% plasma in Ringer's) was infused, downstream diastolic lymphatic pressures increased as ICP was elevated to 50 cm H2O. These data are consistent with the view that much of the CSF-derived water that convects into the lymphatics is absorbed into the ethmoidal or nodal blood vessels. This study supports the concept of fluid continuity between the subarachnoid space and extracranial lymphatics and suggests that this loss of CSF-derived water may act as a safety mechanism to reduce the volume load to the downstream lymphatic vessels.

Possible role of the cavernous sinus veins in cerebrospinal fluid absorption

The purpose of this investigation was to enhance our understanding of cerebrospinal fluid (CSF) absorption pathways. To achieve this, Microfil (a coloured silastic material) was infused into the subarachnoid space (cisterna magna) of sheep post mortem, and the relevant tissues examined macroscopically and microscopically. The Microfil was taken up by an extensive network of extracranial lymphatic vessels in the olfactory turbinates. In addition however, Microfil also passed consistently through the dura at the base of the brain. Microfil was noted in the spaces surrounding the venous network that comprises the cavernous sinus, in the adventitia of the internal carotid arteries and adjacent to the pituitary gland. Additionally, Microfil was observed within the endoneurial spaces of the trigeminal nerve and in lymphatic vessels emerging from the epineurium of the nerve. These results suggest several unconventional pathways by which CSF may be removed from the subarachnoid space. The movement of CSF to locations external to the cranium via these routes may lead to its absorption into veins and lymphatics outside of the skull. The physiological importance of these pathways requires further investigation.

Quantification of cerebrospinal fluid transport across the cribriform plate into lymphatics in rats

A major pathway by which cerebrospinal fluid (CSF) is removed from the cranium is transport through the cribriform plate in association with the olfactory nerves. CSF is then absorbed into lymphatics located in the submucosa of the olfactory epithelium (olfactory turbinates). In an attempt to provide a quantitative measure of this transport, 125I-human serum albumin (HSA) was injected into the lateral ventricles of adult Fisher 344 rats. The animals were killed at 10, 20, 30, 40, and 60 min after injection, and tissue samples, including blood (from heart puncture), skeletal muscle, spleen, liver, kidney, and tail were excised for radioactive assessment. The remains were frozen. To sample the olfactory turbinates, angled coronal tissue sections anterior to the cribriform plate were prepared from the frozen heads. The average concentration of 125I-HSA was higher in the middle olfactory turbinates than in any other tissue with peak concentrations achieved 30 min after injection. At this point, the recoveries of injected tracer (percent injected dose/g tissue) were 9.4% middle turbinates, 1.6% blood, 0.04% skeletal muscle, 0.2% spleen, 0.3% liver, 0.3% kidney, and 0.09% tail. The current belief that arachnoid projections are responsible for CSF drainage fails to explain some important issues related to the pathogenesis of CSF disorders. The rapid movement of the CSF tracer into the olfactory turbinates further supports a role for lymphatics in CSF absorption and provides the basis of a method to investigate the novel concept that diseases associated with the CSF system may involve impaired lymphatic CSF transport.

Development of cerebrospinal fluid absorption sites in the pig and rat: connections between the subarachnoid space and lymphatic vessels in the olfactory turbinates

The textbook view that cerebrospinal fluid (CSF) absorption occurs mainly through the arachnoid granulations and villi is being challenged by quantitative and qualitative studies that support a major role for the lymphatic circulation in CSF transport. There are many potential sites at which lymphatics may gain access to CSF but the primary pathway involves the movement of CSF through the cribriform plate foramina in association with the olfactory nerves. Lymphatics encircle the nerve trunks on the extracranial surface of the cribriform plate and absorb CSF. However, the time during development in which the CSF compartment and extracranial lymphatic vessels connect anatomically is unclear. In this report, CSF-lymphatic connections were investigated using the silastic material Microfil and a soluble Evan's blue-protein complex in two species; one in which significant CSF synthesis by the choroid plexus begins before birth (pigs) and one in which CSF secretion is markedly up regulated within the first weeks after birth (rats). We examined a total of 46 pig fetuses at embryonic (E) day E80-81, E92, E101, E110 (birth at 114 days). In rats, we investigated a total of 115 animals at E21 (birth at 21 days), postnatal (P) day P1-P9, P12, P13, P15, P22, and adults. In pigs, CSF-lymphatic connections were observed in the prenatal period as early as E92. Before this time (E80-81 fetuses) CSF-lymphatic connections did not appear to exist. In rats, these associations were not obvious until about a week after birth. These data suggest that the ability of extracranial lymphatic vessels to absorb CSF develops around the time that significant volumes of CSF are being produced by the choroid plexus and further support an important role for lymphatic vessels in CSF transport.

Subarachnoid injection of Microfil reveals connections between cerebrospinal fluid and nasal lymphatics in the non-human primate

Based on quantitative and qualitative studies in a variety of mammalian species, it would appear that a significant portion of cerebrospinal fluid (CSF) drainage is associated with transport along cranial and spinal nerves with absorption taking place into lymphatic vessels external to the central nervous system. CSF appears to convect primarily through the cribriform plate into lymphatics associated with the submucosa of the olfactory and respiratory epithelium. However, the significance of this pathway for CSF absorption in primates has never been established unequivocally. In past studies, we infused Microfil into the subarachnoid compartment of numerous species to visualize CSF transport pathways. The success of this method encouraged us to use a similar approach in the non-human primate. Yellow Microfil was injected post mortem into the cisterna magna of 6 years old Barbados green monkeys (Cercopithecus aethiops sabeus, n = 6). Macroscopic and microscopic examination revealed that Microfil was (1) distributed throughout the subarachnoid compartment, (2) located in the perineurial spaces associated with the fila olfactoria, (3) present within the olfactory submucosa, and (4) situated within an extensive network of lymphatic vessels in the nasal submucosa, nasal septum and turbinate tissues. We conclude that the Microfil distribution patterns in the monkey were very similar to those observed in many other species suggesting that significant nasal lymphatic uptake of CSF occurs in the non-human primate.

Integration of the subarachnoid space and lymphatics: is it time to embrace a new concept of cerebrospinal fluid absorption?

In most tissues and organs, the lymphatic circulation is responsible for the removal of interstitial protein and fluid but the parenchyma of the brain and spinal cord is devoid of lymphatic vessels. On the other hand, the literature is filled with qualitative and quantitative evidence supporting a lymphatic function in cerebrospinal fluid (CSF) absorption. The experimental data seems to warrant a re-examination of CSF dynamics and consideration of a new conceptual foundation on which to base our understanding of disorders of the CSF system. The objective of this paper is to review the key studies pertaining to the role of the lymphatic system in CSF absorption.

Evidence of connections between cerebrospinal fluid and nasal lymphatic vessels in humans, non-human primates and other mammalian species

Background

The parenchyma of the brain does not contain lymphatics. Consequently, it has been assumed that arachnoid projections into the cranial venous system are responsible for cerebrospinal fluid (CSF) absorption. However, recent quantitative and qualitative evidence in sheep suggest that nasal lymphatics have the major role in CSF transport. Nonetheless, the applicability of this concept to other species, especially to humans has never been clarified. The purpose of this study was to compare the CSF and nasal lymph associations in human and non-human primates with those observed in other mammalian species.

Methods

Studies were performed in sheep, pigs, rabbits, rats, mice, monkeys and humans. Immediately after sacrifice (or up to 7 hours after death in humans), yellow Microfil was injected into the CSF compartment. The heads were cut in a sagittal plane.

Results

In the seven species examined, Microfil was observed primarily in the subarachnoid space around the olfactory bulbs and cribriform plate. The contrast agent followed the olfactory nerves and entered extensive lymphatic networks in the submucosa associated with the olfactory and respiratory epithelium. This is the first direct evidence of the association between the CSF and nasal lymph compartments in humans.

Conclusions

The fact that the pattern of Microfil distribution was similar in all species tested, suggested that CSF absorption into nasal lymphatics is a characteristic feature of all mammals including humans. It is tempting to speculate that some disorders of the CSF system (hydrocephalus and idiopathic intracranial hypertension for example) may relate either directly or indirectly to a lymphatic CSF absorption deficit.