In a craniotomy to treat an anterior communicating aneurysm, the bridging veins that empty into the superior sagittal sinus near the frontal tip sometimes make the use of an interhemispheric approach difficult. These veins must be preserved as much as possible because their sacrifice causes severe wide-range disorders of the venous return. Thus, the authors present some surgical techniques for improving the preservation rate of these veins, based on investigations they have made of the microsurgical anatomy in cadavaric brains. Surgery to reach the anterior communicating artery complex can be performed by an ordinary bifronfal craniotomy, using an unilateral interhemispheric approach, when the distance between the lower edge of craniotomy and the bridging veins is more than 16 mm. In performing an unilateral interhemispheric approach, we usually dissect the right side of the falx, however, flexibility is needed when deciding on the approach side, since this depends on the vein's position. In some cases, vein dissection from the cerebral cortex enlarges the operative field. However, should angiography suggest a narrow operative field, than an additional craniotomy is required, using a basal interhemispheric approach, so that enough of an operative field is secured without sacrificing that bridging veins.

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In brain surgery, the use of the anterior interhemispheric approach depends on the number of and distance between the bridging veins of the superficial cortical vein in the frontal lobe. To determine the feasibility of using this approach, the number of bridging veins and the distances between them have been measured in 10 cases (20 hemispheres) by means of digital subtraction angiography (DSA) through an oblique projection of 30 degrees. This oblique DSA projection thus revealed the incident angle of these bridging veins into the superior sagittal sinus and the distance between them. Whenever the distance between these bridging veins is less than 3 cm, the use of the anterior interhemispheric approach becomes a difficult procedure. When these bridging veins are to be preserved, there are two surgical routes for the use of an anterior interhemispheric approach : one is a basal interhemispheric approach between the frontopolar vein and the frontal base, and the other is an anterior interhemispheric trans-callosal approach between the posterior frontal vein and the precentral vein.

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The advantages of using an interhemispheric approach to reach the third ventricular area are that the anatomy is relatively constant, the distance to the ventricle in shorter than reaching this area transcortically, the ventricular cavity can be reached without disrupting the important neural structures next to the corpus callosum and the ventricular size is irrelevant. Cadavaric brains were examined in detail to evaluate the surgically important aspects of walls through which the third ventricular area can be entered via various interhemispheric approaches. Structurally, the third ventricle has a roof, a floor, and an anterior, a posterior, and two lateral walls. Further, the lateral ventricle is divided into four components: an anterior horn, a body, an atrium, and a posterior horn and a temporal horn. Arterial and venous networks, as well as cisteral relationships, are also intimately related to the third ventricular area. The importance of each of these anatomical structures are discussed with regard to the use of various interhemispheric routes, Such as a translamina terminalis, an anterior transcallosal, a posterior transcallosal, and or an occipital transtentorial approach.

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The authors describe their use of an interhemispheric approach for the surgical treatment of an anterior communicating artery aneurysm. This surgical method is accomplished in either of two ways: by using an anterior interhemispheric (AIH) approach or a basal interhemispheric (BIH) approach. In using these approaches, space of more than 16 mm is needed from the lower margin of the craniotomy to the frontal bridging vein. Should the bridging vein be found to be positioned too low, then an additional craniotomy must be pursued to make it possible to get to the nasal part of the frontal bone. In this regard, the BIH approach is indicated so as to preserve the vein. Procedurally, the anterior portion of the corpus callosum cistern, the lamina terminalis cistern, and the chiasmatic cistern must be dissected carefully. This can be difficult because short, tight trabeculae that have developed in these cisterns are encountered. Great manipulative skill is required during this dissection process, particulal1y when dealing with certain parts of the interhemispheric fissure. In dissecting the fissure, the cardinal area requiring the greatest attention is the upper part of the fissure, where the arachnoid membrane and trabeculae must first be dissected, with the direction of this dissection proceeding downwards, the purpose being to open a V-shaped space in the fissure. Working in this manner, the planum spheniodale at the anterior cranial base will then be seen. After this, the narrow, tight space between the rectal gyri is dissected posteriorly to reach the tuberculum sellae. At this point, the approach used depends on the location of the aneurysm. Should the aneurysm be located in the anterior part of the bilateral A_2 artery, then dissection proceeds in the upper portion until this artery is confirmed. By dissecting the fissure to the proximal portion of the A_2 artery, the neck of the aneurysm and its parent arteries is reached. Conversely, should the aneurysm be located in the posterior part of the bilateral or unilateral A_2 artery, then the chiasmatic cistern is dissected from the tuberculum sellae. Thus, by dissecting the fissure from the chiasmatic cistern to the lower part of the lamina terminalis cistern, the neck of the aneurysm and its parent arteries is reached. The use of this approach is not restricted to only aneurysmal surgery, it also can be used for other organic lesions located in the anterior cranial base.

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To the neurosurgeon, the surgical treatment of third ventricular tumors, which are located in the deep midline of the cerebrum, an area considered inaccessible, present a major technical challenge, and the authors describe various possible approaches that can be used. One surgical method is the anterior trascallosal approach, based on the location, extension, and pathological characteristics of the tumor. This approach can be considered for patients with a third ventricular tumor that has no accompanying ventriculomegaly, with the site of the craniotomy determined on the basis of the angiographical localization of the parasagittal draining vein to the superior sagittal sinus. The technical problems involved with this type of callosotomy and with the separation of the interhemispheric fissure, so as to approach the corpus callosum, are discussed. Following a lateral ventricular entry, access to the chamber is then accomplished through the diencephalic roof to expose a third ventricular tumor by using the following approaches : 1) transforaminal, 2) interforniceal, 3) transforniceal, 4) subchoroidal, 5) transchoriodal, 6) transseptal, or 7) a combination of these approaches, and the authors discuss the possible physiological damage that may occur through using each of these approaches. With particular reference to the microsurgical anatomy, the topographical landmarks encountered when the third ventricular tumor in debulked and dissected also are discussed, as well as complications resulting from brain retraction, the division of the superficial and deep veins, the procedures of the callosotomy, and surgical injuries that may occur to the fornix, diencephalon, mesencephalon, and/or the basal forebrain structures.

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The authors describe a surgical procedure, which they term the trans-lamina terminalis approach, for third ventricular tumors, said method used to treat 4 craniopharyngiomas. 4 hypothalamic astrocytomas, 2 malignant germ cell tumors, 1 giant pituitary adenoma, and 1 metastatic papillary adenocarcinoma. With regard to the 4 cases of a craniopharyngioma, they were able to perform a total removal while still preserving the pituitary stalk, one of these cases involving a recurrent tumor. To preserve the pituitary stalk, the key points are early recognition of any pituitary stalk deviation, resulting from compression by the tumor, and early identification of Dawson's arteries that irrigate the pituitary stalk and the pituitary gland. Early drilling of the planum sphenoidale and the tuberCulum sellae was found not only useful for identifying the pituitary stalk but also for the removal of a tumor in front of the chiasm. The authors used an interhemispheric approach from the narrow space between the anterior edge of the cranium and the bridging vein that flows into the superior sagittal sinus. Finally, in treating third ventricular tumors. they have found that an intentional opening of the frontal sinus, a procedure advocated by some authors and termed the anterior basal interhemispheric approach, is unnecessary for the clipping of an anterior communicating aneurysm.

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The operative procedures and basic techniques of an occipital transtentorial approach to the posterior third ventricular tumors were described based on the operative results in 48 patients. The occipital transtentorial approach was done in a "lateral-semiprone position", a modified lateral position devised by the author : The patient's body remains lateral and the head rotated towards the floor at an approximate angle of 60 degrees. The surgeon faces the patient's back close to shoulder. This positioning allows the surgeon to sit and look down at the posterior third ventricle or pineal region with a greater comfort during operation. An additional advantage of the "lateral-semiprone position" is that the occipital lobe can be easily retracted with the aid of gravity. More importantly, the risk of an air embolism is eliminated. The occipital transtentorial approach allows a greater exposure of the supra- and infratentorial structures around the pineal region without the sacrifice of any structures or vessels. By the combination with "infrasplenial approach" which was designed by the author to expose the internal cerebral veins and the third ventricle, this occipital transtentorial approach can be applied to handle any variation of the posterior third ventricular or pineal region tumors. The minor disadvantage of this approach is the narrower space between the bilateral basal veins. The author has operated 48 cases of pineal region tumors by occipital transtentorial approach in lateral-semiprone position : 19 germ cell tumors, 15 glial tumors, 9 pineal parenchymal tumors and 5 other tumors. Of these 48 tumors, 23 were radically removed. Ten with invasive and malignant tumor were partially removed intentionally. The remaining 15 tumors were removed subtitle, because the tumors were tightly adherent to or involved in the vein of Galen or internal cerebral veins. There was no operative death in this series. Upward gaze palsy appeared postoperatively in 21 patients, although this symptom was transient in 18 patients. Three patients had homonymous hemianopsia postoperatively.

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Just before and after the surgical removal of an arteriovenous malformation (AVM), the authors intraoperatively measured the local blood flow in the cortex (LCoBF) surrounding an AVM in 24 cases (16 superficial, 8 deep AVMs), and have studied the relationship between these two time-point values and the subsequent hemodynamic complications, i. e., breakthrough swelling and/or breakthrough hemorrhaging. All LCoBFs were determined by employing a thermal diffusion probe that uses a Pertier stack. In all cases, assessment of the postoperative complications was made by CT, and in 12 of the 24 cases, the intracranial pressure (ICP) was also measured. After the removal of the AVM, an LCoBF decrease was seen in 11 cases, an LCoBF decrease in 7 cases, and no change in 6 cases. In all 11 patients with the blood flow increase, the AVM had been Superficially situated, whereas cases showing a blood flow decrease or no blood flow change had deep AVMs. As for AVM size and its relationship to the LCoBF changes after AVM removal, an LCoBF increase was observed in 5 of 12 cases with a large AVM, and in 6 of 12 cases with a small AVM. With regard to the relationship between the postoperative ICP and the LCoBF change after the AVM removal, in one case only, the ICP rose of above 25 mmHg on the third day. Postoperative CTS were used to evaluate all cases treated, and only in this case that showed an ICP above 25 mmHg was postoperative hemorrhaging seen. Attention has gradually focused on the breakthrough of the normal perfusion pressure as being the cause of hemorrhagic complications after the surgical removal of an AVM, though such complications appear to arise from more causes than had been previously thought. The results of our study would seem to indicate that the postoperative intracranial blood flow around the site of a resected AVM has no direct bearing on postoperative hemorrhaging and/or brain swelling. In cases where hemorrhaging occurs after the AVM has been removed, careful consideration should be given to other possible causes like disorders in the venous blood flow and/or hemodynamic complications in the arteries, such as an insecure hemostasis with a high pressure afterload.

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Hypertrophic cranial pachymeningitis, a rare clinical entity, is a slowly progressive lesion that can be fatal and is prone to include the dura mater at the skull base. In this report, the authors describe a case they have experienced, summarize 14 previous cases, and discuss the manifestations presented, i. e., the signs and symptoms, the pathological and radiological findings, the etiologies of these cases, and the therapies that were used. With the exception of the authors' case, a palliative effect was achieved with a steroid hormone. The author's case involved a 63-year-old man whose chief complaints were headaches and nausea. CT revealed an unusual enhancement of the tentorium and ventricular dilatation. MRI further disclosed marked hypertrophy of the suboccipital dura mater and the tentorium. Thus, hydrocephalus due to meningeal carcinomatosis was suspected. As serial cytological examinations of the cerebrospinal fluid revealed no malignancy, ventriculo-peritoneal shunting and a subsequent biopsy of the suboccipital dura mater were performed. No malignant cells were found and the histological diagnosis was hypertrophic pachmeningitis. After surgery, the patient no longer experienced headaches or nasuea. Of the 14 previous cases, 3 were associated with hydrocephalus, two of these 3 patients subsequently underwent ventriculo-peritoneal shunting but derived no benefit. Our patient thus represents a rare instance in which ventriculo-peritoneal shunting provided a palliative effect.

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As a hypoglossal neurinoma is very rarely encountered, it often is misdiagnosed as another disease. The authors report a case of a dumbbell-shaped neurinoma of the hypoglossal nerve, presenting with atrophy of the tongue on the left side. The key to correctly identifying this type of neurinoma is the presence of hypoglossal nerve palsy and an enlargement of the hypoglossal canal. In this regard, MRI has proven to be a very useful diagnostic tool, since it provides valuable information about the localization and extent of the tumor. If the neurinoma is detected during its early stage, total surgical removal results in a favorable outcome.

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