Vascular responses to acute intracranial hypertension

Intracranial mechanisms for preserving brain blood flow in health and disease

The brain is an exceptionally energetically demanding organ with little metabolic reserve, and multiple systems operate to protect and preserve the brain blood supply. But how does the brain sense its own perfusion? In this review, we discuss how the brain may harness the cardiovascular system to counter threats to cerebral perfusion sensed via intracranial pressure (ICP), cerebral oxygenation and ischaemia. Since the work of Cushing over 100 years ago, the existence of brain baroreceptors capable of eliciting increases in sympathetic outflow and blood pressure has been hypothesized. In the clinic, this response has generally been thought to occur only in extremis, to perfuse the severely ischaemic brain as cerebral autoregulation fails. We review evidence that pressor responses may also occur with smaller, physiologically relevant increases in ICP. The incoming brain oxygen supply is closely monitored by the carotid chemoreceptors; however, hypoxia and other markers of ischaemia are also sensed intrinsically by astrocytes or other support cells within brain tissue itself and elicit reactive hyperaemia. Recent studies suggest that astrocytic oxygen signalling within the brainstem may directly affect sympathetic nerve activity and blood pressure. We speculate that local cerebral oxygen tension is a major determinant of the mean level of arterial pressure and discuss recent evidence that this may be the case. We conclude that intrinsic intra- and extra-cranial mechanisms sense and integrate information about hypoxia/ischaemia and ICP and play a major role in determining the long-term level of sympathetic outflow and arterial pressure, to optimize cerebral perfusion.

Pathogenesis of optic disc edema in raised intracranial pressure

Optic disc edema in raised intracranial pressure was first described in 1853. Ever since, there has been a plethora of controversial hypotheses to explain its pathogenesis. I have explored the subject comprehensively by doing basic, experimental and clinical studies. My objective was to investigate the fundamentals of the subject, to test the validity of the previous theories, and finally, based on all these studies, to find a logical explanation for the pathogenesis. My studies included the following issues pertinent to the pathogenesis of optic disc edema in raised intracranial pressure: the anatomy and blood supply of the optic nerve, the roles of the sheath of the optic nerve, of the centripetal flow of fluids along the optic nerve, of compression of the central retinal vein, and of acute intracranial hypertension and its associated effects. I found that, contrary to some previous claims, an acute rise of intracranial pressure was not quickly followed by production of optic disc edema. Then, in rhesus monkeys, I produced experimentally chronic intracranial hypertension by slowly increasing in size space-occupying lesions, in different parts of the brain. Those produced raised cerebrospinal fluid pressure (CSFP) and optic disc edema, identical to those seen in patients with elevated CSFP. Having achieved that, I investigated various aspects of optic disc edema by ophthalmoscopy, stereoscopic color fundus photography and fluorescein fundus angiography, and light microscopic, electron microscopic, horseradish peroxidase and axoplasmic transport studies, and evaluated the effect of opening the sheath of the optic nerve on the optic disc edema. This latter study showed that opening the sheath resulted in resolution of optic disc edema on the side of the sheath fenestration, in spite of high intracranial CSFP, proving that a rise of CSFP in the sheath was the essential pre-requisite for the development of optic disc edema. I also investigated optic disc edema with raised CSFP in patients, by evaluating optic disc and fundus changes by stereoscopic fundus photography and fluorescein fundus angiography. Based on the combined information from all the studies discussed above, it is clear that the pathogenesis of optic disc edema in raised intracranial pressure is a mechanical phenomenon. It is primarily due to a rise of CSFP in the optic nerve sheath, which produces axoplasmic flow stasis in the optic nerve fibers in the surface nerve fiber layer and prelaminar region of the optic nerve head. Axoplasmic flow stasis then results in swelling of the nerve fibers, and consequently of the optic disc. Swelling of the nerve fibers and of the optic disc secondarily compresses the fine, low-pressure venules in that region, resulting in venous stasis and fluid leakage; that leads to the accumulation of extracellular fluid. Contrary to the previous theories, the various vascular changes seen in optic disc edema are secondary and not primary. Thus, optic disc edema in raised CSFP is due to a combination of swollen nerve fibers and the accumulation of extracellular fluid. My studies also provided information about the pathogeneses of visual disturbances in raised intracranial pressure.

Aneurysmal subarachnoid hemorrhage models: do they need a fix?

The discovery of tissue plasminogen activator to treat acute stroke is a success story of research on preventing brain injury following transient cerebral ischemia (TGI). That this discovery depended upon development of embolic animal model reiterates that proper stroke modeling is the key to develop new treatments. In contrast to TGI, despite extensive research, prevention or treatment of brain injury following aneurysmal subarachnoid hemorrhage (aSAH) has not been achieved. A lack of adequate aSAH disease model may have contributed to this failure. TGI is an important component of aSAH and shares mechanism of injury with it. We hypothesized that modifying aSAH model using experience acquired from TGI modeling may facilitate development of treatment for aSAH and its complications. This review focuses on similarities and dissimilarities between TGI and aSAH, discusses the existing TGI and aSAH animal models, and presents a modified aSAH model which effectively mimics the disease and has a potential of becoming a better resource for studying the brain injury mechanisms and developing a treatment.

Influence of acute jugular vein compression on the cerebral blood flow velocity, pial artery pulsation and width of subarachnoid space in humans

Purpose

The aim of this study was to assess the effect of acute bilateral jugular vein compression on: (1) pial artery pulsation (cc-TQ); (2) cerebral blood flow velocity (CBFV); (3) peripheral blood pressure; and (4) possible relations between mentioned parameters.

Methods

Experiments were performed on a group of 32 healthy 19–30 years old male subjects. cc-TQ and the subarachnoid width (sas-TQ) were measured using near-infrared transillumination/backscattering sounding (NIR-T/BSS), CBFV in the left anterior cerebral artery using transcranial Doppler, blood pressure was measured using Finapres, while end-tidal CO₂ was measured using medical gas analyser. Bilateral jugular vein compression was achieved with the use of a sphygmomanometer held on the neck of the participant and pumped at the pressure of 40 mmHg, and was performed in the bend-over (BOPT) and swayed to the back (initial) position.

Results

In the first group (n = 10) during BOPT, sas-TQ and pulse pressure (PP) decreased (−17.6% and −17.9%, respectively) and CBFV increased (+35.0%), while cc-TQ did not change (+1.91%). In the second group, in the initial position (n = 22) cc-TQ and CBFV increased (106.6% and 20.1%, respectively), while sas-TQ and PP decreases were not statistically significant (−15.5% and −9.0%, respectively). End-tidal CO₂ remained stable during BOPT and venous compression in both groups. Significant interdependence between changes in cc-TQ and PP after bilateral jugular vein compression in the initial position was found (r = −0.74).

Conclusions

Acute bilateral jugular venous insufficiency leads to hyperkinetic cerebral circulation characterised by augmented pial artery pulsation and CBFV and direct transmission of PP into the brain microcirculation. The Windkessel effect with impaired jugular outflow and more likely increased intracranial pressure is described. This study clarifies the potential mechanism linking jugular outflow insufficiency with arterial small vessel cerebral disease.

Distribution of acid sphingomyelinase in rodent and non-human primate brain after intracerebroventricular infusion

One treatment approach for lysosomal storage diseases (LSDs) is the systemic infusion of recombinant enzyme. Although this enzyme replacement is therapeutic for the viscera, many LSDs have central nervous system (CNS) components that are not adequately treated by systemic enzyme infusion. Direct intracerebroventricular (ICV) infusion of a high concentration of recombinant human acid sphingomyelinase (rhASM) into the CNS over a prolonged time frame (hours) has shown therapeutic efficacy in a mouse model of Niemann-Pick A (NP/A) disease. To evaluate whether such an approach would translate to a larger brain, rhASM was infused into the lateral ventricles of both rats and Rhesus macaques, and the resulting distribution of enzyme characterized qualitatively and quantitatively. In both species, ICV infusion of rhASM resulted in parenchymal distribution of enzyme at levels that were therapeutic in the NP/A mouse model. Enzyme distribution was global in nature and exhibited a relatively steep gradient from the cerebrospinal fluid compartment to the inner parenchyma. Additional optimization of an ICV delivery approach may provide a therapeutic option for LSDs with neurologic involvement.

Intracranial baroreflex yielding an early cushing response in human

The Cushing response is a pre-terminal sympatho-adrenal systemic response to very high ICP. Animal studies have demonstrated that a moderate rise of ICP yields a reversible pressure-mediated systemic response. Infusion studies are routine procedures to investigate, by infusing CSF space with saline, the cerebrospinal fluid (CSF) biophysics in patients suspected of hydrocephalus. Our study aims at assessing systemic and cerebral haemodynamic changes during moderate rise of ICP in human. Infusion studies were performed in 34 patients. This is a routine test perform in patients presenting with symptoms of NPH during their pre-shunting assessment. Arterial blood pressure (ABP) and cerebral blood flow velocity (FV) were non-invasively monitored with photoplethysmography and transcranial Doppler. The rise in ICP (8.2 +/- 5.1 mmHg to 25 +/- 8.3 mmHg) was followed by a significant rise in ABP (106.6 +/- 29.7 mmHg to 115.2 +/- 30.1 mmHg), drop in CPP (98.3 +/- 29 mmHg to 90.2 +/- 30.7 mmHg) and decrease in FV (55.6 +/- 17 cm/s to 51.1 +/- 16.3 cm/s). Increasing ICP did not alter heart rate (70.4 +/- 10.4/min to 70.3 +/- 9.1/min) but augmented the heart rate variance (0.046 +/- 0.058 to 0.067 +/- 0.075/min). In a population suspected of hydrocephalus, our study demonstrated that a moderate rise of ICP yields a reversible pressure-mediated systemic response, demonstrating an early Cushing response in human and a putative intracranial baroreflex.

Cushing's 'variant' response (acute hypotension) after subarachnoid hemorrhage. Association with moderate intracranial tensions and subacute cardiovascular collapse

BACKGROUND AND PURPOSE

Hypertension is considered common and appropriate with subarachnoid hemorrhage (SAH), maintaining cerebral perfusion. Hypotension, in contrast, is considered rare and detrimental. This study was designed to assess the frequency of each in both acute and subacute phases of primary SAH.

METHODS

SAH was created by arterial rupture in spontaneously breathing rats under urethane anesthesia without craniotomy (n = 32). Arterial pressure and intracranial pressure (ICP) were monitored invasively.

RESULTS

After extensive extravasation, the mean ICP rose acutely from 8 +/- 1 to 53 +/- 4 mm Hg over 2.4 +/- 0.3 minutes. Acute pressor changes occurred transiently in 71%. The most common acute response was hypotension (63%). Hypertension, in contrast, was rare (6%); the remainder was invariant (29%). Hypertension was associated with significantly lower maximum ICP values (39 +/- 4 versus 69 +/- 4 mm Hg, P < .001) with a negative correlation between hypotension and delta ICP (r = -.7, P < .01). Distinct and independent of acute responses, hypotension also occurred subacutely as a cardiovascular collapse (38%).

CONCLUSIONS

In contrast to popular belief, the most common acute response with SAH is hypotension; hypertension is rare. This, in fact, is in full agreement with Cushing: hypertension was seen only with gradual delta ICPs. In contrast, a "variant" to the classic response (hypotension) occurred with sudden delta ICPs. In the present study, hypotension stanched SAH at lower maximum ICP values, and thus with less cerebral compression. Despite this, cardiovascular collapse developed in a large proportion irrespective of acute change. Such collapse without prior hypertension (94%) implies a nonadrenergic etiology.

Effect of blood transfusion, dopamine, or normal saline on neurogenic shock secondary to acutely raised intracranial pressure

An experimental model to simulate acutely raised intracranial pressure due to a rapidly expanding intracranial space-occupying lesion was used to produce neurogenic shock. Forty-one rats in neurogenic shock (defined as a mean systemic arterial pressure (SAP) of less than 60 mm Hg) were subjected to various treatments to increase the mean SAP to a level of more than 80 mm Hg. The control group with neurogenic shock received no treatment, and the six treatment groups received infusions of: whole blood, packed cells, plasma, normal saline, dopamine, or a combination of dopamine and saline. Detrimental effects were observed after transfusion of packed cells or whole blood, which caused further deterioration of mean SAP. Although dopamine or the combination of dopamine and saline were both effective (p = 0.0001) for reversing hypotension, the combination was the most effective. If this rat paradigm correlates with human disease, these results indicate that, in the absence of hypovolemia, neurogenic shock due to acute intracranial hypertension should be treated with a combined transfusion of dopamine and normal saline, but not blood since the latter could have a detrimental effect.

Regional cerebral blood flow and CSF pressures during Cushing response induced by a supratentorial expanding mass

In order to delineate the critical blood flow pattern during the Cushing response in intracranial hypertension, regional cerebral blood flow was measured with radioactive microspheres in 12 anesthetized dogs at respiratory arrest caused either by expansion of an epidural supratentorial balloon or by cisternal infusion. Regional cerebrospinal fluid pressures were recorded and the local cerebral perfusion pressure calculated in various cerebrospinal compartments. In the 8 dogs of the balloon expansion group, the systemic arterial pressure was unmanipulated in 4, while it was kept at a constant low level (48 and 70 mm Hg) in 2 dogs and, in another 2 dogs, at a constant high level (150 and 160 mm Hg) induced by infusion of Aramine. At respiratory arrest, regional cerebral blood flow had a stereotyped pattern and was largely independent of the blood pressure level. In contrast, concomitant pressure gradients between the various cerebrospinal compartments varied markedly in the 3 animal groups, increasing with higher arterial pressure. Flow decreased by 85-100% supratentorially and by 70-100% in the upper brain stem down to the level of the upper pons, while changes in the lower brain stem were minor, on the average 25%. When intracranial pressure was raised by cisternal infusion in 4 dogs, the supratentorial blood flow pattern at respiratory arrest was approximately similar to the flow pattern in the balloon inflation group. However, blood flow decreased markedly (74-85%) also in the lower brain stem. The results constitute another argument in favour of the Cushing response in supratentorial expansion being caused by ischemia in the brain stem. The critical ischemic region seems to be located rostrally to the oblongate medulla, probably in the pons.

Spinal vasomotor reflex and Cushing response

A spinal vasomotor reflex that resembles the Cushing response (CR) can be evoked also by isolated intraspinal hypertension. The intensity of the spinal vasomotor reflex (SVR) depends primarily on the length of the isolated spinal cord, i.e., on the total vegetative elements that participate in this mechanism. The vasopressor effect produced by isolated intraspinal hypertension may be the consequence of ischaemia of the spinal vegetative structures. Cur data suggest that in the case of increased intracranial pressure, when craniospinal connections are free and there is no impaction, the CR is due not only the vasomotor centre in the brain stem, but also to spinal vegetative structures.

Fluids in the anterior part of the optic nerve in health and disease

New techniques have recently made it possible to study the flow of fluids (blood, axoplasm, and interstitial fluid) in the anterior part of the optic nerve. Blood flow has been reviewed previously; axoplasm and interstitial fluid are considered in this review. General concepts of axoplasmic transport (anterograde and retrograde) are outlined, and the role of axoplasmic transport in the pathogeneses of optic disc edema of various types, in glaucoma, and in ischemic and toxic optic neuropathies is discussed. The probable sources of interstitial fluid in the anterior part of the optic nerve are capillaries in the nerve itself, peripapillary choroid, vitreous, cerebrospinal fluid and possibly axoplasm in the local axons; the flow is defined by various barrier systems. The role of the interstitial fluid in the pathogeneses of optic edema (and associated phenomena) and in serous retinal detachment in the macular region associated with optic disc pit is discussed. Its involvement in the process of diffusion of retrobulbar medication into the optic nerve and vitreous is also considered.

Systemic vascular responses to increased intracranial pressure. 1. Effects of progressive epidural ballon expansion on intracranial pressure: and systemic circulation

This paper details the results of experimental studies, on 16 dogs with artificially-induced intracranial space-occupying lesions, of the systemic vascular responses and the intracranial pressure changes (both in the supratentorial and infratentorial compartments) induced by increasing intracranial pressure. The changes produced were divided into two phases such that phase 1 detailed the alterations observed from the start of the balloon inflation up to the initiation of the systemic pressor response. Phase 2 recorded those alterations which occurred during, and immediately after, the period of systemic hypertension (see Fitch et al., 1977). The changes observed during phase 1, and presented in this communication, were those of increasing intracranial pressures and decreasing mean arterial pressure and heart rate. These alterations were associated with decreases in supratentorial perfusion pressure and increases in transtentorial pressure gradient and arrhythmia index.

[Reticular activity and intracranial pressure. Acute and chronic intracranial hypertension (author's transl)]

Modifications of mesencephalic and bulbar reticular formation activity were studied with microelectrodes during acute and chronic intracranial hypertension. In both cases, the mesencephalic reticular activity increased progressively until a pressure level of 70 to 90 cm of CSF was reached and then fell irreversibly to less than the base value. The bulbar reticular activity followed the same pattern but more slowly. Different hypotheses are advanced to explain these modifications and their meanings.

The effect of increased intracranial pressure on pressure in the superior sagittal sinus

It is difficult to explain why rises in ICP provoke different types of response in superior sagittal sinus pressure. In most of our experimental animals there was close correlation between rises in ICP and SSSP. In the remainder, SSSP showed little increase when ICP rose. The animals with marked increase in SSSP showed a greater capacity for compensation for increased amounts of intracranial fluid.

Hydrostatically raised intracranial pressure

✓ Hydrostatic pressure with artificial cerebrospinal fluid (CSF) was applied through a needle inserted into the cisterna magna of rabbits breathing spontaneously. Blood pressure, confluens sinuum pressure and oxygen tension, respiratory rate and volume, and acid-base balance were recorded until respiratory arrest. Blood pressure and confluens sinuum pressure and respiratory volume rose; confluens sinuum oxygen and arterial carbon dioxide tension dropped. The significant similarities and differences in changes in the same parameters following local cold injury to the brain are discussed. Comparisons between different experimental models for raised intracranial pressure must take into consideration the differing reactions of the brain.

Ophthalmic arterial and venous pressures. Effects of acute intracranial hypertension

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