ANATOMY OF SUBARACHNOID CISTERNS OF BRAIN
- 1. Subarachnoid Cisterns Dr Piyush Ramesh Thombare Department of Neurosurgery Topiwala National Medical College, and Nair Hospital, Mumbai
- 2. Introduction • Knowledge of the neural and vascular contents of each of the basal cisterns is of particular value to the neuro- surgeon in the planning and execution of intracranial procedures. • The neurosurgeon may chart his intracranial approach like a road map in terms of the basal cisterns. • Many of the subarachnoid cisterns can be considered to be anatomically distinct compartments, but others are not, being separated from each other by a porous trabeculated wall with various sized openings. • Under normal circumstances this permits a continuous exchange of CSF from one compartment to another. • These apertures can become plugged and partially or totally obliterated after subarachnoid hemorrhage, infectious meningitis, chemical meningitis, spread of malignant cells in the subarachnoid space and spread of proteinaceous exudate hindering the normal CSF circulation
- 3. HISTORY • In 1555 Blaes (Blasius) is credited with naming of the arachnoid • Vieussens (1690) noted that the pia and arachnoid existed as two separate membranes • Ruysch (1697) showed that the arachnoid extended over the convexities of the brain. • In 1802, Bichat proposed that the arachnoid formed a serous cavity similar to the peritoneal cavity. • Magendie (1822) gave the first modern description of the subarachnoid space as containing cerebrospinal fluid • He described the basal cisterns and the extensions of the arachnoid along cranial nerves II, V VII and VII
- 4. original figure from the monograph (1875) by Key and Retzius which shows the ventricular, arachnoid, and cisternal spaces outlined by Berliner Blue. • In 1875, Key and Retzius published a monumental work in which they presented drawings of the subarachnoid space that had been injected with blue dye (Berliner-Blau) to demonstrate the extensions and divisions of the subarachnoid system • They showed the relationship of the cerebral vessels to the arachnoid and the numerous trabeculae which suspend these vessels from the walls of the cisterns.
- 5. • ln 1919 Dandy described injection of air into ' the lumbar subarachnoid space in order to outline the cerebral ventricles. • It was seen that the basal subarachnoid cisterns were also demonstrated in this manner • Locke and Naffziger (1924) undertook a corrosion cast study of the subarachnoid cisterns in dogs and humans, and demonstrated the shapes and intercommunications of the subarachnoid space. They gave general names to the subarachnoid cisterns • In 1937, Davidoff and Dyke published a textbook on the normal pneumoencephalogram in which they discussed the shape and extension of the subarachnoid cisterns in some detail. • Liliequist (1959) employed both techniques to provide a working normal anatomy of the subarachnoid space
- 6. • The cisterns are divided into two major groups • supratentorial • infratentorial
- 8. Supratentorial Cisterns • A) Anterior • 1) Carotid cistern • 2) Chiasmatic cistern • 3) Lamina terminalis cistern • 4) Olfactory cistern • 5) Sylvian cistern • B) Lateral • 1) Crural cistern • 2) Ambient cistern (anterior part) • C) Posterior • 1) Quadrigeminal cistern • 2) Velum interpositum cistern • D) Superior • 1) Corpus callosum cistern
- 9. Infratentorial Cisterns • A) Anterior • 1) Interpeduncular cistern • 2) Prepontine cistern • 3) Premedullary cistern • B) Lateral • 1) Ambient cistern (posterior part) • 2) (Superior) cerebellopontine cistern • 3) Inferior cerebellopontine or lateral cerebellomedullary cistern • C) Posterior • 1) Cisterna magna • 2) Superior cerebellar cistern • D) Superior • 1) Vermian cistern
- 12. The cisterns located in the posterior cranial fossa include paired and unpaired cisterns. • A. Unpaired cisterns • 1. Interpeduncular cistern • 2. Prepontine cistern • 3. Premedulary cistern • 4. Quadrigeminal cistern • 5. Cisterna magna • B. Paired cisterns • 1. Cerebelopontine cistern • 2. Lat Cerebelomedulary cistern
- 13. 1 Olfactory cistern 2a Callosal cistern 2b Lamina terminalis cistern 3 Chiasmatic cistern 4 Carotid cistern 5 Sylvian cistern 6 Crural cistern 7 Interpeduncular cistern . 8 Ambient cistern 9 Prepontine cistern 10 Superior cerebellar-pontine cistern 11 Inferior cerebeliar-pontine cistern (lateral cerebello- medullary) 12 Anterior spinal cistern 13 Posterior spinal cistern
- 14. The relationship between the basal cisterns and the ventral cerebral arterial system.
- 15. Carotid Cistern • Bordered • The lateral carotid membrane forms the lateral wall • The medial carotid membrane separates the carotid and the chiasmatic cisterns. • superiorly the dura over the anterior clinoid process and the orbitofrontal lobe • inferiorly by the cavernous sinus • Medially - shares a wall with the chiasmatic cistern • laterally - bounded by the mesial temporal lobe and the free margin of the tentorium. • The inferior part of the carotid cistern and superior part of the interpeduncular cistern are in apposition - sometimes creating a single (Lilie- quist's) membrane.
- 16. • The carotid cistern contains the • supraclinoid portion of the internal carotid artery, • origin of ophthalmic artery • Posterior communicating artery • anterior choroidal arteries, • small arteries to the optic nerves and pituitary stalk, • artery to the dura over the anterior clinoid process
- 17. Chiasmatic Cistern (Cisterna Chiasmatica) • This cistern encloses the subarachnoid space, around the optic nerves and chiasm. • Superiorly - tightly adherent to the superior surface of the optic nerves and chiasm and contiguous with the inferior part of the lamina terminalis cistern • Inferiorly -shares a common wall with the interpeduncular cistern • Thick arachnoid membrane joining the chiasmatic and interpeduncular cisterns being called "Liliequist's membrane“
- 18. • Anteroinferiorly it extends to the infundibulum and pituitary stalk and is bounded by the diaphragma sellae • Anteriorly - limited by the limbus sphenoidale except at the optic foramina where short extensions of the subarachnoid space follow the optic nerves into the orbit. • Laterally - shares a common wall with the carotid cistern • The chiasmatic cistern contains the optic nerves, pituitary stalk, and numerous small internal carotid branches to both structures. The ophthalmic artery enters the chiasmatic cistern within the optic canal.
- 19. • Liliequist's membrane arises from the part of the arachnoid membrane that rests against the dorsum sellae and splits into the diencephalic and mesencephalic membranes. • The diencephalic membrane is a complete membrane that separates the chiasmatic and interpeduncular cisterns. • The mesencephalic membrane, attaches along the junction of the midbrain and pons, forms an incomplete wall between the interpeduncular and prepontine cisterns with an opening through which the basilar artery ascends.
- 21. Lamina Terminalis Cistern • This cistern is defined primarily by the anterior cerebral arteries • Anteroinferiorly is the superior surface of the optic chiasm where it is contiguous with the chiasmatic cistern. • Anterosuperiorly the rostrum of the corpus callosum covers the cistern. • The posterior boundary is the lamina terminalis • Extensions laterally enclose each anterior cerebral artery
- 22. • Thickened bands of arachnoid running from the olfactory area to the optic nerve demarcate the most lateral limit of the cistern. • These form a tunnel through which the anterior cerebral artery must pass on leaving the carotid and entering the lamina terminalis cistern
- 23. • The lamina terminalis cistern contains • the anterior cerebral arteries • the most proximal A2 segments of the anterior cerebral arteries, • the anterior communicating artery complex • medial striate branches (the recurrent artery of Heubner) • frontoorbital arteries, • Anterior communicating and anterior cerebral veins also lie within the cistern
- 24. Olfactory Cistern • This cistern is formed by the arachnoid over the olfactory tract between the orbital gyri laterally and the gyrus rectus medially. • Inferiorly it is bounded rostrally by the floor of the anterior fossa including the cribriform plate of the ethmoid bone and caudally by the chiasmatic cistern. • Posteriorly it joins several other cisterns above the internal carotid artery bifurcation.
- 25. • Bilateral medial orbitofrontal arteries (arrows) arising from the A2 segments and disappearing into the olfactory sulci beneath the olfactory tracts (OL.) • The olfactory cistern contains the • olfactory bulb and tract, • parts of frontoorbital and olfactory arteries their branches, and • several frontobasal veins.
- 26. Sylvian Cistern • Thickened bands of arachnoid completely enclose the origin of the middle cerebral artery • These form a tunnel through which the middle cerebral artery passes before entering the Sylvian fissure. • The most medial and inferior extent is at the origin of the middle cerebral artery from the internal carotid • The cistern narrows superiorly as the frontal and temporal lobes approach each other • The width of the cistern is usually about 0.5-1.0 cm on the surface
- 27. • In some cases the frontal and temporal lobes are closely approximated thereby covering the substance of the cistern • For this reason the sylvian cistern and its investing arachnoid can be categorized as follows: Category Cisternal Size Arachnoidal Characteristics 1 Large Transparent + fragile 2 Small Transparent + fragile 3 Large Thickened + tough 4 Small Thickened + tough
- 28. • Microsurgical dissections of the Sylvian cistern during the pterional operative approach are increasingly more difficult as the category of cistern increases according to the above classification. • Thus the exposure of a category 3 cistern is quite tedious, but easier than that of a category 4, post-meningitic cistern, which is almost impossible • The Sylvian cistern contains the middle cerebral artery and the origins of the lenticulostriate, temporopolar and anterior temporal arteries • The middle cerebral artery bifurcation • The superficial and deep Sylvian veins
- 29. Crural Cistern • Lies between the parahippocampal gyrus and the cerebral peduncle. • Extends to the carotid cistern anteriorly • lies on top of the interpeduncular cistern • the ambient cistern latero-posterior • clearly demarcated from the carotid and interpeduncular cisterns between the anterior choroidal and posterior communicating arteries.
- 30. • The importance of this cistern lies in the valuable surgical plane it establishes between the anterior choroidal and posterior communicating arteries. • The crural cistern contains • the anterior choroidal arteries • medial posterior choroidal arteries • the basal vein of Rosenthal.
- 31. Velum Interpositum Cistern • Extends from the habenular commissure to the foramen of Monro • Located beneath the splenium of the corpus callosum • above the velum interpositum, • with the roof of the third ventricle below. • Anteriorly it is beneath the fornix, converging to a point at the foramen of Monro. • It lies between the pulvinar thalami • Posteriorly there is no clear distinction from the quadrigeminal cistern.
- 32. • The cistern contains • the medial posterior choroidal artery • and the internal cerebral veins.
- 33. Corpus Callosum Cistern • It extends from the falx cerebri medially to the pia over the cingulate gyri laterally • Anteriorly it follows the falx to the crista galli and joins the lamina terminalis cistern near the rostrum of the corpus callosum. • Arachnoid forming the roof of the cistern is suspended from the inferior margin of the falx. • Inferiorly the cistern joins the quadrigeminal and velum interpositum cisterns at the end of the splenium.
- 34. • The cistern contains • the pericallosal arteries • the origins of the frontopolar and callosomarginal arteries. • Small anterior cerebral veins may be present sometimes making connections with the inferior sagittal sinus. • Also contains the posterior pericallosal veins.
- 36. Interpeduncular Cistern • situated between the cerebral peduncles and the leaves of Liliequist's membrane • roof of the cistern is formed by the inferior surface of the mesencephalon and the lower diencephalon, the posterior perforated substance, and the mammillary bodies • posterior wall - formed by the posterior perforated substance. • upper border is situated at the posterior edge of the mamillary bodies. • lower border is situated at the junction of midbrain and pons. • anteroinferior boundary is the clivus, and laterally the cistern joins the ambient cistern • superiorly is limited by the carotid and crural cisterns and the mesial temporal lobes
- 37. • bordered rostrally and caudally by Liliequist's membrane. • the diencephalic membrane separates the chiasmatic and interpeduncular cisterns. • the mesencephalic membrane separates the interpeduncular and prepontine cisterns.
- 38. • The oculomotor nerves course in the lateral wal and form the pillars to which the leaves of Liliequist's membrane attach. Also the site of attachment of other arachnoid membranes • The membranes that converge on and form a sleeve around the nerve are • the mesencephalic membrane, which separates the interpeduncular and prepontine cisterns; • the diencephalic membrane, which separates the interpeduncular and chiasmatic cisterns • the anterior pontine membrane, which separates the cerebelopontine and prepontine cisterns; • the lateral pontomesencephalic membrane, which separates the ambient and cerebelopontine cisterns; • the medial carotid membrane, which separates the chiasmatic and carotid cisterns; • the lateral carotid membrane, which forms the lateral wal of the carotid cistern.
- 39. • interpeduncular cistern contains the • the bifurcation of the basilar artery and the origins of the posterior cerebral and superior cerebellar arteries • medial posterior choroidal and thalamogeniculate arteries, and their branches • the basal veins of Rosenthal, median anterior pontomesencephalic veins, and the vein of the pontomesencephalic sulcus • and the oculomotor nerves.
- 41. Ambient Cistern • This cistern covers the lateral aspect of the mesencephalon • both supra- and infratentorial justifying its inclusion in both categories • Its medial boundary is the cerebral peduncle and the interpeduncular cistern • Its lateral boundaries are • supratentorially the mesial temporal lobe and • infratentorially the lobu-lus quadrangularis of the cerebellum. • Inferiorly it shares an arachnoid wall with the cerebellopontine cistern. • Anteriorly the cistern is related to the crural cistern.
- 42. • The ambient cistern contains • segments of the posterior cerebral artery, • numerous arteries to the midbrain from both PCA's, • the basal vein of Rosenthal. • The superior cerebellar artery and the trochlear nerve have their own arachnoid sleeves around the peduncle.
- 44. Quadrigeminal Cistern • The anterior limits -are the dorsal mesencephalon, the quadrigeminal plate, and the pineal gland. • Posteriorly arachnoid is attached to the tentorium and extends from the splenium of the corpus callosum inferiorly to the lingula of the cerebellar vermis, above the anterior medullary velum of the fourth ventricle. • The cistern is contiguous superiorly with the velum interpositum cistern and laterally with the ambient cisterns.
- 45. • anterior wall is formed by the pulvinar, superior, and inferior colliculi and the superior cerebellar peduncles. • lateral wall has an anterior and a posterior part. • anterior part is formed by the segment of the crus of the fornix that wraps around the pulvinar. • posterior part is formed by the part of the occipital cortex located below the splenium. • roof is formed by the lower surface of the splenium and the broad membranous envelope that surrounds the great vein and its tributaries. • The cistern extends downward between the cerebellum and midbrain into the cerebellomesencephalic fissure.
- 46. • The quadrigeminal cistern contains the • medial posterior choroidal arteries • the great vein of Galen and its tributaries • The trochlear nerves arise in the quadrigeminal cistern just below the inferior colliculi and course forward around the midbrain to enter the ambient cisterns. • The PCAs commonly bifurcate into their calcarine and parieto-occipital branches within the cistern. • The SCAs course through the part of the cistern that extends into the cerebelomesencephalic fissure.
- 47. • The internal cerebral veins exit the velum interpositum and the basal veins exit the ambient cisterns to reach the quadrigeminal cistern, where they join the vein of Galen. • The latter vein passes below the splenium to enter the straight sinus at the tentorial apex. • Other veins that converge on the cistern include the posterior pericallosal veins , the atrial veins, and the internal occipital veins
- 49. Prepontine Cistern • lies between the anterior surface of the pons and the clivus surrounding the basilar artery • separated from the interpeduncular cistern by the mesencephalic leaf of Liliequist's membrane. • The lateral edges of the prepontine cistern are separated from the cerebelopontine cisterns by the paired anterior pontine membranes.
- 50. • The lower boundary of the cistern is situated at the level of the pontomedulary sulcus, the site of a less wel-defined membrane called the medial pontomedulary membrane. • The inferior arachnoidal wall of the cistern is thickened as the vertebral arteries join to form the basilar artery beneath the pontomedullary sulcus • The cistern contains the • basilar artery, • the origin of the anterior inferior cerebellar artery, • the entire free course of the abducens nerve from the pons to Dorello's canal
- 54. CEREBELLOPONTINE CISTERN • The cerebelopontine cistern lies between the anterolateral surface of the pons and cerebellum • Superiorly this cistern is separated from the ambient cistern by the lateral pontomesencephalic membrane. • This membrane is attached to the brainstem at the junction of the midbrain and pons and to the outer arachnoidal membrane • Anteriorly, it intersects the oculomotor nerve. • This membrane spans the interval between the PCA and SCA.
- 55. • Inferiorly, the cerebelopontine cistern is separated from the cerebelomedulary cistern by the lateral pontomedulary membrane • This membrane stretches from the junction of the pons and medula to the outer arachnoidal membrane. • It crosses the subarachnoid space between the vestibulocochlear and glossopharyngeal nerves. • Lateraly, the cerebelopontine cistern extends to the edge of the cerebelar surface that wraps around the pons to form the cerebelopontine fissure.
- 56. • Laterally the cistern extends along the posterior petrous portion of the temporal bone entering the internal auditory meatus and extending outwards into Meckel's cave. • Trigeminal nerve has its own cisternal sleeve which forms a recess into the cerebellopontine cistern • The cerebellopontine cistern contains the anterior inferior cerebellar artery cranial nerves V, VII and VIII, and the lateral pontomesencephalic vein.
- 57. • The trigeminal nerve arises from the midpons and courses through the superolateral portion of the cistern. • The abducens nerve arises at the level of the pontomedulary sulcus and ascends just lateral to the anterior pontine membrane. • The facial and vestibulocochlear nerves arise in the inferior part of the cerebelopontine cistern just above the lateral pontomedulary membrane. • The outer arachnoid membrane extends into the internal auditory canal and surrounds the intracanalicular segment of the facial and vestibulocochlear nerves. • The flocculus projects into the cerebe lopontine cistern behind the facial and vestibulocochlear nerves.
- 58. • The SCA and AICA course through the cerebelopontine cistern • The SCA enters the cerebelopontine cistern by passing through the junction of the anterior pontine membrane and the oculomotor nerve. It courses below the trochlear nerve and the lateral pontomesencephalic membrane, and above the trigeminal nerve in its passage through this cistern. • The AICA enters the lower part of the cerebelopontine cisterns by passing through or below the anterior pontine membrane. It commonly bifurcates into its rostral and caudal trunks within this cistern. • The veins in this cistern converge and unite to form the superior petrosal veins, which empty into the superior petrosal sinus
- 59. CP angle cistern with arachnoid cyst
- 60. Lateral Cerebellomedullary Cistern (or Inferior Cerebellopontine Cistern) • Cerebellomedullary cistern lies anterior and lateral to the medulla • Its anterosuperior border is the sulcus between the medulla and the pons. • Arachnoid over cranial nerves IX, X, and the cranial portion of XI separate this cistern from the cisterna magna dorsally and from the cerebellopontine cistern superiorly. • Ventrally a less clear arachnoid sheet separates the cistern from the premedullary cistern. • Its inferior border is located at the level of the foramen magnum.
- 61. • extends from the pontomedullary sulcus superiorly to the foramen magnum inferiorly • reaches laterally along with short sleeves into the jugular and hypoglossal foramina accompanying the respective nerves. • separated from the cerebelopontine cistern by the lateral pontomedulary membrane • The cistern contains the vertebral artery, the origin of the posterior inferior cerebellar artery, the retroolivary and lateral medullary veins, • cranial nerves IX, X, XI and XII.
- 62. • The glossopharyngeal and vagus nerves and the medulary portion of the accessory nerve arise within and course through this cistern to reach the jugular foramen. • The spinal portion of the accessory nerve ascends from the posterior spinal cistern to reach the cerebellomedulary cistern. • The lateral recess of the fourth ventricle communicates with this cistern through the foramen of Luschka. • The vertebral artery enters at the lower border of this cistern and immediately leaves it to enter the premedullary cistern. • The (PICA) enters this cistern by reaching the anterior surface of the rootlets of the glossopharyngeal, vagus, and accessory nerves • From here, the artery passes dorsaly between the rootlets of these nerves and pursues a posterior course around the medula to enter the cisterna magna.
- 63. The lateral cerebello-medullary or inferior cerebello- pontine cistern containing cranial nerves IX, X, and XI (arrows). The inferior cerebello-pontine cistern has been opened revealing cranial nerves IX, X, and XI, while nerves VIII and VII remain hidden within the upper cerebello-pontine cistern (arrow).
- 64. The inferior cerebello-pontine cistern has been opened revealing cranial nerves IX, X, and XI, while nerves VIII and VII remain hidden within the upper cerebello-pontine cistern (arrow). Both upper and lower cerebellopontine cisterns have now been opened revealing nerves 7,8,9,10,11 and loop of aica( arrow)
- 65. PREMEDULLARY CISTERN • The premedulary cistern lies between the anterior surface of the medula and the arachnoid membrane covering the lower part of the clivus • Its upper border is located at the junction of the pons and medula. • It is separated from the prepontine cistern by the medial pontomedulary membrane. • Inferiorly, the premedullary cistern is continuous with the anterior spinal cistern.
- 66. • The rootlets forming the hypoglossal nerves arise in the posterior wall of this cistern between the medulary pyramids and the inferior olives. • The vertebral arteries enter this cistern by ascending through the foramen magnum. • They ascend obliquely through this cistern and join at the junction of the premedulary and prepontine cisterns. • The paired anterior spinal arteries arise from the vertebral arteries and join to form a single trunk that courses in the midline on the anterior surface of spinal cord.
- 68. Cisterna Magna • As the dorsal spinal subarachnoid space opens into the intracranial cavity through the foramen magnum, it widens into a large cistern, the cisterna magna. • limited anteriorly by the dorsal surface of the upper spinal cord and lower medulla, and extends to the posterior medullary velum. • Its posterior wall is formed by the arachnoid membrane that conforms to the inner surface of the occipital bone above the foramen magnum. • Superiorly in the midline it communicates with the fourth ventricle at the foramen of Magendie, forming a cephalad extension of the cistern called the vallecula • It usually ends near the lobulus pyramidalis of the vermis but may extend all the way up the tentorium.
- 69. • Superiorly, the cisterna magna projects both anterior and posterior to the cerebelar vermis. • The cisterna magna also opens behind the vermis into the posterior cerebelar incisura. • Anteriorly, it opens into the cerebe lomedulary fissure. • This cistern contains the inferior vermian branches of the posterior inferior cerebellar arteries and the median tonsillar veins. • The PICAs pass posteriorly around the medula. They enter the cisterna magna commonly divide into a • lateral trunk, which supplies the hemisphere and tonsil, • and a medial trunk, which supplies the vermis
- 71. Cisternal tap • Suboccipital access to the cisterna magna. • Usually done with patient sitting, with neck slightly flexed • A 22 gauge spinal needle is inserted exactly in the midline between the inion and the C2 spinous process, directed superiorly towards the glabella until the needle strikes the occiput or enters the cisterna magna. • If the occiput is encountered, the needle is withdrawn slightly and reinserted directed slightly inferiorly, the process is repeated until the cisterna magna is entered (a “pop” will be felt). • The distance from the skin surface to the cisterna magna is 4–6cm
- 72. SUPERIOR CEREBELLAR CISTERN • This cistern is situated between the superior part of the vermis and the arachnoid membrane that rests against the lower border of the straight sinus. • Anteriorly, it opens into the quadrigeminal cistern • Posteriorly, it communicates below the torcular with the cisterna magna. • Lateraly, it blends into the subarachnoid space over the cerebelar hemispheres. • The cistern contains the median and paramedian branches of the SCAs and the superior vermian vein.
- 74. • Opening a cisternal wall, with the resultant escape of cerebrospinal fluid, facilitates the approach to lesions in front of the brainstem and cerebellum. • Allowing cerebrospinal fluid to escape from the cisterna magna during posterior fossa operations facilitates the exposure of lesions in the cerebellopontine, cerebellomedullary, prepontine, and premedullary cisterns. • Pathological processes in the subarachnoid space may conform to cisternal boundaries. • The arachnoid septa and trabeculae separating the cisterns may prevent the spread of blood to adjacent cisterns after aneurysm rupture. • The resulting location of the blood, as seen on C T scans and MRI, often provides information pinpointing the site of a ruptured aneurysm.
- 75. • Yasargil notes that aneurysms may become invested with the arachnoidal walls of the cisterns and tension on the arachnoid membranes may be transmitted to the fundus of the aneurysm • Even when dissection is being carried out some distance away In dissecting an aneurysm, it is helpful to know which membranes may be attached to the aneurysm. • Aneurysms arising at the basilar apex and at the origin of the SCA may project into the leaves of Liliequist's membrane; • aneurysms arising at the origin of the AICA may have the anterior pontine membrane stretched around their surface • aneurysms arising at the origin of the PICA from the vertebral artery may project upward into the lateral pontomedulary membrane • aneurysms arising at the junction of the vertebral with the basilar arteries may be enmeshed in the thick trabeculae that form the medial pontomedulary membrane
- 76. • An understanding of the arachnoidal membranes is especialy important in dealing with aneurysms pointing in the direction of the oculomotor nerves. • Traction on any of the membranes converging on the oculomotor nerve may rupture these aneurysms. • The outer surface of the arachnoidal membranes that are adherent to an aneurysm may provide a plane of dissection that allows easier separation of the aneurysm from adjacent structures. • It may be necessary to leave some of the arachnoid membrane attached to the fundus and wall of the aneurysm to prevent rupture of the aneurysm before a clip is applied.
- 77. Thank you