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Michio Ono, Makiko Ono, Albert L. Rhoton Jr. and Margaret Barry

✓ The microsurgical anatomy of the tentorial incisura was evaluated in 25 adult cadavers using × 3 to × 40 magnification. The area surrounding the incisura is divided into the anterior, middle, and posterior incisural spaces. The anterior incisural space is located anterior to the brain stem and extends upward around the optic chiasm to the subcallosal area; the middle incisural space is located lateral to the brain stem and is intimately related to the hippocampal formation in the medial part of the temporal lobe; and the posterior incisural space is located posterior to the midbrain and corresponds to the region of the pineal gland and vein of Galen. The neural, cisternal, ventricular, and vascular relationships of each space were examined. The arterial relationships in the anterior incisural space and the venous relationships in the posterior incisural space are extremely complex, since the anterior incisural space contains all the components of the circle of Willis and the bifurcation of the internal carotid and basilar arteries, and the posterior incisural space contains the convergence of the internal cerebral and basal veins and many of their tributaries on the vein of Galen. The discussion reviews tentorial herniation and operative approaches to the incisura.

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Toshio Matsushima, Albert L. Rhoton Jr., Evandro de Oliveira and David Peace

✓ The microsurgical anatomy of the veins of the posterior fossa was defined in 25 cadavers. These veins are divided into four groups: superficial, deep, brain-stem, and bridging veins. The superficial veins are divided on the basis of which of the three cortical surfaces they drain: the tentorial surface, which faces the tentorium and is exposed in a supracerebellar approach, is drained by the superior hemispheric and vermian veins; the suboccipital surface, which is below and between the lateral and sigmoid sinuses and is exposed in a wide suboccipital craniectomy, is drained by the inferior hemispheric and inferior vermian veins; and the petrosal surface, which faces forward toward the posterior surface of the petrous bone and is retracted to expose the cerebellopontine angle, is drained by the anterior hemispheric veins. The deep veins course in the three fissures between the cerebellum and the brain stem, and on the three cerebellar peduncles. The major deep veins in the fissures between the cerebellum and brain stem are the veins of the cerebellomesencephalic, cerebellomedullary, and cerebellopontine fissures, and those on the cerebellar peduncles are the veins of the superior, middle, and inferior cerebellar peduncles. The veins of the brain stem are named on the basis of whether they drain the midbrain, pons, or medulla. The veins of the posterior fossa terminate as bridging veins, which collect into three groups: a galenic group which drains into the vein of Galen; a petrosal group which drains into the petrosal sinuses; and a tentorial group which drains into the tentorial sinuses near the torcula.

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Aneurysms of the posterior inferior cerebellar artery

A clinical and anatomical analysis

Rroger J. Hudgins, Arthur L. Day, Ronald G. Quisling, Albert L. Rhoton Jr, George W. Sypert and Francisco Garcia-Bengochea

✓ The clinical and anatomical features of 21 surgically treated saccular aneurysms of the posterior inferior cerebellar artery (PICA) are analyzed. Seventeen of these lesions originated from the PICA-vertebral junction, and four arose from distal PICA branching sites. Twelve lesions arose from the left PICA, nine were right-sided, and all were small (less than 12.5 mm). Most of these aneurysms occurred in females (16 of 21) and presented as classic subarachnoid hemorrhage. The lack of specific focal deficits prevented an accurate pre-angiographic determination of aneurysm location in most instances. Clinically significant vasospasm and aneurysm multiplicity occurred with approximately equal frequency as at other locations.

The angiographic and surgical features of these lesions are determined by the course of the vertebral artery and PICA; that is, they occur at branching sites and at curves in the parent vessel, and point in the direction in which flow would have continued if the curve at the aneurysm's origin had not been present. Aneurysms at the PICA-vertebral junction usually occur at least 1 cm above the foramen magnum level, arise distal to the PICA origin in the angle between the two vessels, and are best approached by a paramedian incision with the patient in the lateral recumbent position. Isolated clipping of the aneurysm neck is essential in this instance, as trapping may compromise vital perforating arteries of the brain stem. More distal (retromedullary) PICA aneurysms are sometimes associated with another vascular anomaly (two cases in this series), and are best handled through a bilateral suboccipital craniectomy. Clipping of the neck is the preferred treatment, but trapping is usually safe, if necessary.

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Hirohiko Gibo, Carla Lenkey and Albert L. Rhoton Jr.

✓ The microsurgical anatomy of the supraclinoid portion of the internal carotid artery (ICA) was studied in 50 adult cadaver cerebral hemispheres using × 3 to × 40 magnification. The ICA was divided into four parts: the C1 or cervical portion; the C2 or petrous portion; the C3 or cavernous portion; and the C4 or supraclinoid portion. The C4 portion was divided into three segments based on the origin of its major branches: the ophthalmic segment extended from the origin of the ophthalmic artery to the origin of the posterior communicating artery (PCoA); the communicating segment extended from the origin of the PCoA to the origin of the anterior choroidal artery (AChA); and the choroidal segment extended from the origin of the AChA to the bifurcation of the carotid artery. Each segment gave off a series of perforating branches with a relatively constant site of termination. The perforating branches arising from the ophthalmic segment passed to the optic nerve and chiasm, infundibulum, and the floor of the third ventricle. The perforating branches arising from the communicating segment passed to the optic tract and the floor of the third ventricle. The perforating branches arising from the choroidal segment passed upward and entered the brain through the anterior perforated substance. The anatomy of the ophthalmic, posterior communicating, anterior choroidal, and superior hypophyseal branches of the C4 portion was also examined.

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Hirohiko Gibo, Christopher C. Carver, Albert L. Rhoton Jr., Carla Lenkey and Robert J. Mitchell

✓ The microsurgical anatomy of the middle cerebral artery (MCA) was defined in 50 cerebral hemispheres. The MCA was divided into four segments: the M1 (sphenoidal) segment coursed posterior and parallel to the sphenoid ridge; the M2 (insular) segment lay on the insula; the M3 (opercular) segment coursed over the frontoparietal and temporal opercula; and the M4 (cortical) segment spread over the cortical surface. The Sylvian fissure was divided into a sphenoidal and an operculoinsular compartment. The M1 segment coursed in the sphenoidal compartment, and the M2 and M3 segments coursed in the operculoinsular compartment. The main trunk of the MCA divided in one of three ways: bifurcation (78% of hemispheres), trifurcation (12%), or division into multiple trunks (10%). The MCA's that bifurcated were divided into three groups: equal bifurcation (18%), inferior trunk dominant (32%), or superior trunk dominant (28%). The MCA territory was divided into 12 areas: orbitofrontal, prefrontal, precentral, central, anterior parietal, posterior parietal, angular, temporo-occipital, posterior temporal, middle temporal, anterior temporal, and temporopolar. The smallest cortical arteries arose at the anterior end and the largest one at the posterior end of the Sylvian fissure. The largest cortical arteries supplied the temporo-occipital and angular areas. The relationship of each of the cortical arteries to a number of external landmarks was reviewed in detail.

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Kiyotaka Fujii, Carla Lenkey and Albert L. Rhoton Jr.

✓ The microsurgical anatomy of the arteries supplying the choroid plexus in the fourth ventricle and cerebellopontine angles was examined under × 3 to × 20 magnification in brains from 25 adult cadavers. In the most common pattern, the branches of the anterior inferior cerebellar artery (AICA) supplied the portion of the choroid plexus in the cerebellopontine angle and adjacent part of the lateral recess of the fourth ventricle, and the posterior inferior cerebellar artery (PICA) supplied the choroid plexus in the roof and medial part of the lateral recess of the fourth ventricle. The superior cerebellar artery (SCA) gave rise to a choroidal branch in only one brain. The choroid plexus on each side of the midline was divided into a medial and a lateral segment. Each segment was considered two parts to facilitate the description of its blood supply. The medial segment, located in the roof of the fourth ventricle, was divided into a rostral or nodular part, and a caudal or tonsillar part. The lateral segment, located in the lateral recess of the fourth ventricle and cerebellopontine angle, was separated into a medial or peduncular part, and a lateral or floccular part. The AICA most commonly supplied all the floccular part and the lateral portion of the peduncular part, and the PICA most commonly supplied all of the tonsillar and nodular parts, and the medial portion of the peduncular part.

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Kiyotaka Fujii, Carla Lenkey and Albert L. Rhoton Jr.

✓ The microsurgical anatomy of the arteries supplying the choroid plexus of the lateral and third ventricles was examined in 50 formalin-fixed cerebral hemispheres using × 3 to × 20 magnification. There was marked variation in the area of choroid plexus supplied by the choroidal arteries; however, the most common pattern was for the anterior choroidal artery (AChA) to supply a portion of the choroid plexus in the inferior horn and part of the atrium; the lateral posterior choroidal artery (LPChA) to supply a portion of the choroid plexus in the atrium and posterior part of the temporal horn and body; and the medial posterior choroidal artery (MPChA) to supply the choroid plexus in the roof of the third ventricle and a portion of that in the body of the lateral ventricle. The LPChA's and MPChA's occasionally sent branches to the choroid plexus on the contralateral side. The most frequent neural branches of the three choroidal arteries were as follows: AChA branches to the optic tract, cerebral peduncle, temporal lobe, and lateral geniculate body; LPChA branches to the thalamus, geniculate bodies, fornix, and cerebral peduncles; and MPChA branches to the thalamus, pineal body, cerebral peduncle, and tegmentum of the midbrain. Each of the choroidal arteries was divided into a cisternal and plexal segment. The cisternal segments were the most common site of origin of neural branches, but they also gave rise to some plexal branches. The plexal segments occasionally gave rise to neural branches.

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Kiyotaka Fujii, Steven M. Chambers and Albert L. Rhoton Jr.

✓ The increasing use of the transsphenoidal approach to sellar tumors has created a need for more detailed information about the neurovascular relationships of the sphenoid sinus. To better define this anatomy, 25 sphenoid sinuses were examined in cadavers, with attention to the neural and vascular structures in the lateral wall of the sinus. Three structures produced prominent bulges into the lateral wall of the sinus; they were 1) the optic nerves, 2) the carotid arteries, and 3) the maxillary branches of the trigeminal nerve. Over half of these structures had a bone thickness of less than 0.5 mm separating them from the sphenoid sinus, and in a few cases, they were separated by only sinus mucosa and dura.

1) The optic canals protruded into the superolateral part of the sphenoid sinus in all except one side of one specimen. In 4% of the optic nerves, only the optic sheath and sinus mucosa separated the nerves from the sinus, and in 78%, less than a 0.5-mm thickness of bone separated them. 2) The carotid arteries produced a prominent bulge into the sphenoid sinus in all but one side of one specimen. In 8% of the carotid arteries there were areas where no bone separated the artery and the sinus. 3) The maxillary branches of trigeminal nerves bulged into the inferolateral part of the sphenoid sinus in all except one side of two specimens. One side of one specimen had no bone, and 70% had less than a 0.5-mm thickness of bone separating the nerve from the sinus. The importance of these findings in transsphenoidal surgery is reviewed.

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David G. Hardy and Albert L. Rhoton Jr.

✓ Compression and distortion of the trigeminal nerve by a tortuous and elongated superior cerebellar artery (SCA) is postulated to be a frequent cause of trigeminal neuralgia. This theory and the use of operative therapy in which the offending arterial loop is separated from the trigeminal nerve has created a need for more detailed information on the relationship of the SCA and the trigeminal nerve. In order to meet this need, 50 trigeminal nerves and the adjacent SCA were examined in 25 adult cadavers. Twenty-six of the 50 nerves examined had a point of contact with the SCA, but it was uncommon for the arterial contact to produce distortion of the nerve. In six instances, the contact was at the pontine entry zone of the trigeminal nerve, the site of arterial compression postulated to be associated with trigeminal neuralgia. Four trigeminal nerves (8%) had a point of contact with the anterior inferior cerebellar artery (AICA). The fact that large arteries are commonly in contact with the trigeminal nerve is important not only because of the controversial relationship of neurovascular contact to trigeminal neuralgia, but because of the possibility that major vessels may be encountered and injured during rhizotomy and other posterior fossa operations on the trigeminal nerve.