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Kenichiro Sugita and Shigeaki Kobayashi

✓ Microsurgical removal of large acoustic neurinomas, more than 3 cm in diameter, has been performed by the lateral suboccipital transmeatal approach with the patient in the lateral position in 68 cases in the past 5 years. Sixty-two patients (91%) returned to their former occupations and two died: one in the immediate postoperative period and the other 16 months later. The postoperative follow-up examination showed satisfactory facial nerve function in 70% of the patients. Hearing was preserved in six of 14 patients who had preoperatively retained their auditory function. The authors emphasize the usefulness of bipolar forceps for intraoperative stimulation of the facial nerve and for facial muscle monitoring. The value of the four-pronged hook and the multipurpose head-frame in this procedure is also discussed.

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Yuichiro Tanaka, Keizo Sakamoto, Shigeaki Kobayashi, Norio Kobayashi and Shinsuke Muraoka

✓ Five full-term neonates with a posterior fossa subdural hematoma caused by birth injury are reported. All of the patients were successfully treated with surgery. They all presented with biphasic ventricular dilatation as demonstrated by serial computerized tomography (CT) scanning. The initial dilatation due to obstructive hydrocephalus disappeared after removal of the hematoma; however, a second dilatation without obstruction of the ventricular system developed between 2 and 4 weeks after surgery. A ventriculoperitoneal shunt was required in one case because of progressive enlargement of the head size in the chronic stage. The other four patients showed transient dilatation of the ventricles without symptoms of increased intracranial pressure. Subarachnoid hemorrhage associated with posterior fossa subdural hematoma is considered to be the most likely causative factor for the delayed ventricular dilatation. The degree of dilatation roughly depended on the volume of the subarachnoid clot that was demonstrated on the initial CT scan.

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Kazuhiko Kyoshima, Shigeaki Kobayashi, Hirohiko Gibo and Takayuki Kuroyanagi

✓ Direct surgery for intra-axial lesions of the brain stem is considered a hazardous procedure, and morbidity of varying degrees cannot be avoided even with partial removal or biopsy. The main causes of morbidity relate to direct damage during removal of the lesion, selection of an entry route into the brain stem, and the direction of brain stem retraction. The authors examined the possibility of making a medullary incision and retracting the brain stem, taking into account the symptomatology and surgical anatomy, and found two safe entry zones into the brain stem through a suboccipital approach via the floor of the fourth ventricle. These safe entry zones are areas where important neural structures are less prominent. One is the “suprafacial triangle,” which is bordered medially by the medial longitudinal fascicle, caudally by the facial nerve (which runs in the brainstem parenchyma), and laterally by the cerebellar peduncle. The second is the “infrafacial triangle,” which is bordered medially by the medial longitudinal fascicle, caudally by the striae medullares, and laterally by the facial nerve. In order to minimize the retraction-related damage to important brain-stem structures, the brain stem should be retracted either laterally or rostrally in the suprafacial triangle approach and only laterally in the infrafacial triangle approach.

Three localized intra-axial brain-stem lesions were treated surgically via the safe entry zones using the suprafacial approach in two and the infrafacial approach in one. The cases are described and the approaches delineated. Both approaches are indicated for focal intra-axial lesions located unilaterally and dorsal to the medial lemniscus in the lower midbrain to the pons. Magnetic resonance imaging is useful in selecting these approaches, and intraoperative ultrasonography is helpful to confirm the exact location of a lesion before a medullary incision is made. These approaches can also be used as routes for aspiration of brain-stem hemorrhage as well as for tumor biopsy.

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Susumu Oikawa, Kazuhiko Kyoshima and Shigeaki Kobayashi

Object. The authors report on the surgical anatomy of the juxta—dural ring area of the internal carotid artery to add to the information available about this important structure.

Methods. Twenty sides of cadaver specimens were used in this study. The plane of the dural ring was found to incline in the posteromedial direction. Medial inclination was measured at 21.8° on average against the horizontal line in the anteroposterior view on radiographic studies. Posterior inclination was measured at 20.3° against the planum sphenoidale in the lateral projection, and the medial edge of the dural ring was located 0.4 mm above the tuberculum sellae in the same projection. The lateral edge of the dural ring was located 1.4 mm below the superior border of the anterior clinoid process. The carotid cave was situated at the medial or posteromedial aspect of the dural ring; however, two of the 20 specimens showed no cave formation. The carotid cave contained the subarachnoid space in 13 sides, the arachnoid membrane only in three sides, and the extraarachnoid space in two sides. The authors propose that the marker of the medial side of the dural ring, which is more proximal than the lateral, is the tuberculum sellae in the lateral view on radiographic studies. In the medial aspect of the dural ring the intradural space can be situated below the level of the tuberculum sellae because of the existence of the carotid cave.

Conclusions. An aneurysm arising from the medial side of the juxta—dural ring area even below the tuberculum sellae is a potential cause of subarachnoid hemorrhage.

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Kazuhiro Hongo, Shigeaki Kobayashi, Masanobu Hokama and Kenichiro Sugita

✓ A case of a 30-year-old man who showed progressive pyramidal tract signs caused by compression of the left vertebral artery is presented. Initial decompression of the vertebral artery by placing a piece of sponge between the artery and medulla had no long-term effect. The left vertebral artery distal to the origin of the posterior inferior cerebellar artery was then sectioned, decompressing the medulla oblongata. The patient's symptoms improved postoperatively. This is the first reported case of brain-stem compression by an elongated vertebral artery treated by sectioning of the artery.

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Fukuo Nakagawa, Shigeaki Kobayashi, Toshiki Takemae and Kenichiro Sugita

✓ Saccular aneurysms arising at locations other than at arterial divisions are extremely rare. The authors describe eight such aneurysms that protruded from the dorsal wall of the internal carotid artery (ICA) and were unrelated to any arterial junction. Radical surgery was performed in all eight cases. The aneurysms were saccular with a fragile wide or semifusiform neck. Intraoperative rupture occurred in three cases. From this experience, it is emphasized that these unusual protruding aneurysms of the dorsal ICA should be clipped with the clip blade parallel to the parent artery. In addition to clipping, complete wrapping with fascia or Bemsheet (cellulose fabric) is often advisable to prevent slippage of clips or postoperative rupture of residual aneurysm.

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Shigeaki Kobayashi, Kenichiro Sugita, Yuichiro Tanaka and Kazuhiko Kyoshima

✓ The authors present a new technique for surgery in the pineal region: a supracerebellar approach with the patient in the prone position. The surgeon sits on the left side of the patient, who lies prone with the head flexed and higher than the heart level (“Concorde position”). The main advantages of this arrangement over the sitting position include less fatigue on the part of the surgeon and decreased likelihood of air embolism. Fourteen patients have been operated on in this position. Pooling of blood in the operating field is rarely a problem.

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Susumu Oikawa, Masahiko Mizuno, Shinsuke Muraoka and Shigeaki Kobayashi

✓ A procedure for preventing muscle atrophy in pterional craniotomy by temporalis muscle dissection is described, along with anatomical considerations. The inferior to superior dissection of the temporalis muscle is a very simple technique and is less invasive than other approaches.

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Toru Koyama, Hiroshi Okudera and Shigeaki Kobayashi

✓ The authors' goal was to develop a computer graphics model to simulate the displacement and morphological changes that are caused by the retraction of fine intracranial structures.

The authors developed an application program to interpolate the contour of models of an artery and a retractor. The center of the displacement was determined by spatial coordinates, and the shape of the displacement of the arterial model was calculated using a cosine-based formula with representation of a brain retractor. This computer graphics model was applied to the simulation of the displacement and morphological changes that occur when retraction is performed in the optic nerve. An illustrative case is presented, in which the optic nerve was displaced by a retractor to simulate the surgery performed in a carotid cave aneurysm of the internal carotid artery.

The authors have named this methodological tool a “virtual retractor.” This new navigational system for open microneurosurgery would be useful in teaching surgical microanatomy and in presurgical operative planning.