Microsurgical anatomy of the anterior commissure: correlations with diffusion tensor imaging fiber tracking and clinical relevance

A case of spontaneous basal ganglia hemorrhage with contralateral extension utilizing the canal of Gratiolet

BACKGROUND

Intracranial hemorrhage accounts for 10-20% of stroke etiologies annually. Basal ganglia is the most common site for intracranial hemorrhage accounting for 50% of all cases. Bilateral spontaneous basal ganglia hemorrhages (BGH) are rare with few reported cases.

CASE PRESENTATION

We report an unusual case of a 69-year-old female who presented with a spontaneous bilateral basal ganglia hemorrhage secondary to a right BGH with contralateral extension through the anterior commissure (AC) utilizing the Canal of Gratiolet. Clinical course and imaging findings are discussed.

CONCLUSIONS

To our knowledge, this is the first case to specifically detail the extension of spontaneous hemorrhage across the AC via the Canal of Gratiolet, and imaging findings provide a novel depiction of AC anatomy and fiber distribution in a clinical context. These findings may explain the mechanism behind this rare clinical entity.

An atlas of white matter anatomy, its variability, and reproducibility based on constrained spherical deconvolution of diffusion MRI

Virtual dissection of white matter (WM) using diffusion MRI tractography is confounded by its poor reproducibility. Despite the increased adoption of advanced reconstruction models, early region-of-interest driven protocols based on diffusion tensor imaging (DTI) remain the dominant reference for virtual dissection protocols. Here we bridge this gap by providing a comprehensive description of typical WM anatomy reconstructed using a reproducible automated subject-specific parcellation-based approach based on probabilistic constrained-spherical deconvolution (CSD) tractography. We complement this with a WM template in MNI space comprising 68 bundles, including all associated anatomical tract selection labels and associated automated workflows. Additionally, we demonstrate bundle inter- and intra-subject variability using 40 (20 test-retest) datasets from the human connectome project (HCP) and 5 sessions with varying b-values and number of b-shells from the single-subject Multiple Acquisitions for Standardization of Structural Imaging Validation and Evaluation (MASSIVE) dataset. The most reliably reconstructed bundles were the whole pyramidal tracts, primary corticospinal tracts, whole superior longitudinal fasciculi, frontal, parietal and occipital segments of the corpus callosum and middle cerebellar peduncles. More variability was found in less dense bundles, e.g., the fornix, dentato-rubro-thalamic tract (DRTT), and premotor pyramidal tract. Using the DRTT as an example, we show that this variability can be reduced by using a higher number of seeding attempts. Overall inter-session similarity was high for HCP test-retest data (median weighted-dice = 0.963, stdev = 0.201 and IQR = 0.099). Compared to the HCP-template bundles there was a high level of agreement for the HCP test-retest data (median weighted-dice = 0.747, stdev = 0.220 and IQR = 0.277) and for the MASSIVE data (median weighted-dice = 0.767, stdev = 0.255 and IQR = 0.338). In summary, this WM atlas provides an overview of the capabilities and limitations of automated subject-specific probabilistic CSD tractography for mapping white matter fasciculi in healthy adults. It will be most useful in applications requiring a reproducible parcellation-based dissection protocol, and as an educational resource for applied neuroimaging and clinical professionals.

A Fiber Dissection Study of the Anterior Commissure: Correlations with Diffusion Spectrum Imaging Tractography and Clinical Relevance in Gliomas

The anterior commissure, which connects bilateral temporal lobes and olfactive areas, remains elusive in many aspects of its structure and functional role. To comparatively describe anatomical details of the anterior commissure using cadaveric fiber dissection (FD) and diffusion spectrum imaging (DSI) thus refining our knowledge of the tract and exploring its clinical relevance in glioma migration. Twelve normal postmortem hemispheres were treated with Klingler's method and subjected to FD with medial, inferior, and lateral approaches. The FD findings were correlated with DSI tractography results. To illustrate the clinical relevance, two patients with recurrent temporal high-grade glioma are described. Our FD and DSI tractography of the anterior commissure disclosed a new anatomical paradigm. The FD confirmed that the anterior limb (absent sometimes and variable) and the lateral/temporal extension include the rostral portion and caudal portion, respectively, of the anterior commissure fibers. The shape of the lateral/temporal extension predominantly resembles an 'H'. The DSI tractography findings corresponded to these FD results. According to the FD, the Virchow-Robin space is continuous with the subarachnoid space and very close to the anterior commissure. The two clinical cases presented severe disturbances of consciousness and behavior despite good local tumor control. Subsequent magnetic resonance images showed new lesions infiltrating the contralateral temporal lobes. FD combined with DSI provided anatomical details facilitating a better understanding of the anterior commissure. Glioma migration routes to the contralateral temporal lobe included the anterior commissure, Virchow-Robin space, and subarachnoid space and were clinically relevant.

Redundancy circuits of the commissural pathways in human and rhesus macaque brains

It has been hypothesized that the human brain has less redundancy than animals, but the structural evidence has not been identified to confirm this claim. Here, we report three redundancy circuits of the commissural pathways in primate brains, namely the orbitofrontal, temporal, and occipital redundancy circuits of the anterior commissure and corpus callosum. Each redundancy circuit has two distinctly separated routes connecting a common pair of cortical regions. We mapped their trajectories in human and rhesus macaque brains using individual and population‐averaged tractography. The dissection results confirmed the existence of these redundancy circuits connecting the orbitofrontal lobe, amygdala, and visual cortex. The volume analysis showed a significant reduction in the orbitofrontal and occipital redundancy circuits of the human brain, whereas the temporal redundancy circuit had a substantial organizational difference between the human and rhesus macaque. Our results support the hypothesis that the human brain has less redundancy in the commissural pathways than that of the rhesus macaque brain. Further studies are needed to explore its neuropathological implications.

Microsurgical anatomy of the amygdaloid body and its connections

The amygdaloid body is a limbic nuclear complex characterized by connections with the thalamus, the brainstem and the neocortex. The recent advances in functional neurosurgery regarding the treatment of refractory epilepsy and several neuropsychiatric disorders renewed the interest in the study of its functional Neuroanatomy. In this scenario, we felt that a morphological study focused on the amygdaloid body and its connections could improve the understanding of the possible  implications in functional neurosurgery. With this purpose we performed a morfological study using nine formalin-fixed human hemispheres dissected under microscopic magnification by using the fiber dissection technique originally described by Klingler. In our results the  amygdaloid body presents two divergent projection systems named dorsal and ventral amygdalofugal pathways connecting the nuclear complex with the septum and the hypothalamus. Furthermore, the amygdaloid body is connected with the hippocampus through the amygdalo-hippocampal bundle, with the anterolateral temporal cortex through the amygdalo-temporalis fascicle, the anterior commissure and the temporo-pulvinar bundle of Arnold, with the insular cortex through the lateral olfactory stria, with the ambiens gyrus, the para-hippocampal gyrus and the basal forebrain through the cingulum, and with the frontal cortex through the uncinate fascicle. Finally, the amygdaloid body is connected with the brainstem through the medial forebrain bundle. Our description of the topographic anatomy of the amygdaloid body and its connections, hopefully represents a useful tool for clinicians and scientists, both in the scope of application and speculation.

Brain structures and networks responsible for stimulation-induced memory flashbacks during forniceal deep brain stimulation for Alzheimer's disease

Introduction

Fornix deep brain stimulation (fx‐DBS) is under investigation for treatment of Alzheimer's disease (AD). We investigated the anatomic correlates of flashback phenomena that were reported previously during acute diencephalic stimulation.

Methods

Thirty‐nine patients with mild AD who took part in a prior fx‐DBS trial (NCT01608061) were studied. After localizing patients’ implanted electrodes and modeling the volume of tissue activated (VTA) by DBS during systematic stimulation testing, we performed (1) voxel‐wise VTA mapping to identify flashback‐associated zones; (2) machine learning–based prediction of flashback occurrence given VTA overlap with specific structures; (3) normative functional connectomics to define flashback‐associated brain‐wide networks.

Results

A distinct diencephalic region was associated with greater flashback likelihood. Fornix, bed nucleus of stria terminalis, and anterior commissure involvement predicted memory events with 72% accuracy. Flashback‐inducing stimulation exhibited greater functional connectivity to a network of memory‐evoking and autobiographical memory‐related sites.

Discussion

These results clarify the neuroanatomical substrates of stimulation‐evoked flashbacks.

Assessing the connectional anatomy of superior and lateral surgical approaches for medial temporal lobe epilepsy

The most common approaches in the treatment of epilepsy, the trans-sylvian selective amygdalohippocampectomy (SAH) and the anterior temporal lobe resection (ATLR) reach the medial temporal lobe through different surgical routes. Our aim was to delineate the white matter (WM) fiber tracts at risk in relation to trans-sylvian SAH and ATLR by defining each fascicle en route to medial temporal lobe during each approach. ATLR and trans-sylvian SAH were performedand related WM tracts en route to medial temporal region were presented in relation to the relevant approaches and surrounding neurovascular structures. The WM tracts most likely to be disrupted during trans-sylvian SAH along the roof of the temporal horn were the UF - and less commonly IFOF - at the layer of the external capsule, anterior commissure, anterior bend of optic radiations, and sublenticular internal capsule. Amygdaloid projections to the claustrum, putamen and globus pallidus, the tail of caudate and the peduncle of the lentiform nucleus were also in close proximity to the resection cavity. Fiber tracts most likely to be impaired during ATLR included the UF, ILF, IFOF, anterior commissure, optic radiations, and, less likely, the vertical ventral segment of the arcuate fascicle. Both ATLR and trans-sylvian SAH carry the risk of injury to WM pathways, which may result in unpredictable functional loss. A detailed 3-D knowledge of the related connectional anatomy will help subside neurocognitive, neuroophtalmologic, neurolinguistic complications of epilepsy surgery, providing an opportunity to tailor the surgery according to patient's unique connectional and functional anatomy.

Microsurgical anatomy of the sagittal stratum

BACKGROUND

The sagittal stratum (SS) is a critical neural crossroad traversed by several white matter tracts that connect multiple areas of the ipsilateral hemisphere. Scant information about the anatomical organization of this structure is available in literature. The goal of this study was to provide a detailed anatomical description of the SS and to discuss the functional implications of the findings when a surgical approach through this structure is planned.

METHODS

Five formalin-fixed human brains were dissected under the operating microscope by using the fiber dissection technique originally described by Ludwig and Klingler.

RESULTS

The SS is a polygonal crossroad of associational fibers situated deep on the lateral surface of the hemisphere, medial to the arcuate/superior longitudinal fascicle complex, and laterally to the tapetal fibers of the atrium. It is organized in three layers: a superficial layer formed by the middle and inferior longitudinal fascicles, a middle layer corresponding to the inferior fronto-occipital fascicle, and a deep layer formed by the optic radiation, intermingled with fibers of the anterior commissure. It originates posteroinferiorly to the inferior limiting sulcus of the insula, contiguous with the fibers of the temporal stem, and ends into the posterior temporo-occipito-parietal cortex.

CONCLUSION

The white matter fiber dissection reveals the tridimensional architecture of the SS and the relationship between its fibers. A detailed understanding of the anatomy of the SS is essential to decrease the operative risks when a surgical approach within this area is undertaken.

Connection between bilateral temporal regions: Tractography using human connectome data and diffusion spectrum imaging

Temporal lobe epilepsy often propagates inter-hemispherically. Although the pathway of the propagation was verified by electrophysiology, the trajectory remains poorly defined. DTI can depict fiber trajectory but it has limited angular resolution and cannot adequately assess cortical regions. We visualized potential pathways of bitemporal epilepsy propagation using diffusion spectrum imaging (DSI) with data consisting of 8 groups of 514 directions and diffusion templates of 842 subjects from the human connectome project (HCP). We verified the results with reference to the axonal-tracing literature. Both the large population overall and individual connection properties were investigated. In both the HCP 842 atlas and DSI individual data, the bilateral temporal pole was found to connect via the anterior commissure. The splenium of the corpus callosum was divided into 3 subregions (CS1, CS2, CS3) according to the form of connections. CS1 was predominately located at the rostral third and the dorsal part of middle third of the splenium; it communicated with the bilateral parietal lobe. SC2 was predominately located at the ventral middle third of the splenium. Fibers passed through the lateral wall of the lateral ventricle and connected to regions lateral of the occipitotemporal sulci. CS3 was located at the caudal third of the splenium. Together with the hippocampal commissure, its fibers constituted the medial wall of the lateral ventricle and distributed medially to the occipitotemporal sulci. The trajectory of bilateral temporal connections was visualized in this study; the results might help in the understanding and treatment of inter-hemispherical propagation of temporal-lobe epilepsy.

The double massa intermedia

Background:

To describe the rare finding of a double massa intermedia (MI). Typically, the MI (interthalamic adhesion) is a single bridge of gray matter connecting the medial surfaces of the thalami.

Methods:

Twelve formalin- and alcohol-fixed human third ventricles were examined from superior to inferior by fiber dissection technique under ×6 to ×40 magnifications and with the endoscope.

Results:

In all hemispheres, the anterior and posterior commissure were defined. The MI, which bridges the medial surfaces of the thalami, was defined in all hemispheres. In one hemisphere, there was a second bridge between the thalami, located posteroinferior to the common MI. Endoscopic view confirmed that there was a second MI in this specimen. The MI usually traverses the third ventricle posterior to the foramen of Monro and connects the paired thalami. The MI is an important landmark during endoscopic and microscopic surgeries of the third ventricle. Although a double MI is very rare, surgeons should be aware of the possibility in their surgical planning.

Conclusion:

The surgeon should be aware of the possibility of a double MI to avoid confusion during third ventricle surgery.

Interhemispheric temporal lobe connectivity predicts language impairment in adolescents born preterm

Although language difficulties are common in children born prematurely, robust neuroanatomical correlates of these impairments remain to be established. This study investigated whether the greater prevalence of language problems in preterm (versus term-born) children might reflect injury to major intra- or interhemispheric white matter pathways connecting frontal and temporal language regions. To investigate this, we performed a comprehensive assessment of language and academic abilities in a group of adolescents born prematurely, some of whom had evidence of brain injury at birth (n = 50, mean age: 16 years, mean gestational age: 27 weeks) and compared them to a term-born control group (n = 30). Detailed structural magnetic resonance imaging and diffusion-tractography analyses of intrahemispheric and interhemispheric white matter bundles were performed. Analysis of intrahemispheric pathways included the arcuate fasciculus (dorsal language pathway) and uncinate fasciculus/extreme capsule (ventral language pathway). Analysis of interhemispheric pathways (in particular, connections between the temporal lobes) included the two major commissural bundles: the corpus callosum and anterior commissure. We found language impairment in 38% of adolescents born preterm. Language impairment was not related to abnormalities of the arcuate fasciculus (or its subsegments), but was associated with bilateral volume reductions in the ventral language pathway. However, the most significant volume reduction was detected in the posterior corpus callosum (splenium), which contains interhemispheric connections between the occipital, parietal and temporal lobes. Diffusion tractography showed that of the three groups of interhemispheric fibres within the splenium, only those connecting the temporal lobes were reduced. Crucially, we found that language impairment was only detectable if the anterior commissure (a second temporal lobe commissural pathway) was also small. Regression analyses showed that a combination of anatomical measures of temporal interhemispheric connectivity (through the splenium of the corpus callosum and anterior commissure) explained 57% of the variance in language abilities. This supports recent theories emphasizing the importance of interhemispheric connections for language, particularly in the developing brain.