To investigate overcrowding in the posterior cranial fossa as the pathogenesis of adult type Chiari malformation, we morphometrically studied the brainstem and cerebellum within the posterior cranial fossa as well as the occipital bone while taking into consideration their embryological development. Compared with the control group, the Chiari group had a significantly larger “volume ratio” as PFBV/PFCV, the three occipital enchondral parts (the basiocciput, exocciput and supraocciput) were significantly smaller and the tentorium was much steeper. There was no significant difference in the posterior fossa brain volume or in the axial lengths of the hindbrain (the brainstem and cerebellum). These results suggest that in adult type Chiari malformation an underdeveloped occipital bone, possibly due to underdevelopment of the occipital somite originating from the paraxial mesoderm, induces overcrowding in the posterior cranial fossa which contains the normally developed hindbrain, and secondary the cerebellar tonsils herniate into the spinal canal.

To investigate overcrowding in the posterior cranial fossa as the pathogenesis of adult-type Chiari malformation, the authors studied the morphology of the brainstem and cerebellum within the posterior cranial fossa (neural structures consisting of the midbrain, pons, cerebellum, and medulla oblongata) as well as the base of the skull while taking into consideration their embryological development. Thirty patients with Chiari malformation and 50 normal control subjects were prospectively studied using neuroimaging. To estimate overcrowding, the authors used a "volume ratio" in which volume of the posterior fossa brain (consisting of the midbrain, pons, cerebellum, and medulla oblongata within the posterior cranial fossa) was placed in a ratio with the volume of the posterior fossa cranium encircled by bony and tentorial structures. Compared to the control group, in the Chiari group there was a significantly larger volume ratio, the two occipital enchondral parts (the exocciput and supraocciput) were significantly smaller, and the tentorium was pronouncedly steeper. There was no significant difference in the posterior fossa brain volume or in the axial lengths of the hindbrain (the brainstem and cerebellum). In six patients with basilar invagination the medulla oblongata was herniated, all three occipital enchondral parts (the basiocciput, exocciput, and supraocciput) were significantly smaller than in the control group, and the volume ratio was significantly larger than that in the Chiari group without basilar invagination. These results suggest that in adult-type Chiari malformation an underdeveloped occipital bone, possibly due to underdevelopment of the occipital somite originating from the paraxial mesoderm, induces overcrowding in the posterior cranial fossa, which contains the normally developed hindbrain. Basilar invagination is associated with a more severe downward herniation of the hindbrain due to the more severely underdeveloped occipital enchondrium, which further exacerbates overcrowding of the posterior cranial fossa.

The authors present four cases of vascular lesions and 10 cases of tumors involving the cavernous sinus. They were operated on via a combined orbitozygomatic infratemporal epidural and subdural approach. With this approach, multisided exposure of the cavernous sinus can be achieved via the shortest possible distance with minimal retraction of the neural structures in and around the cavernous sinus. In one patient the carotid artery had been occluded previously, but in the other 13 patients it was preserved. There was no mortality, and all patients except one returned to work within 6 months after surgery.

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The microsurgical anatomy of the jugular foramen was studied in 10 fixed cadavers, each cadaver consisting of the whole head and neck. Five of the cadavers were injected with latex. The jugular foraminal region was exposed using the infratemporal fossa type A approach of Fisch and Pillsbury in five cadavers (10 sides) and the combined cervical dissection-mastoidectomy-suboccipital craniectomy approach in five cadavers (10 sides). The right foramen was larger than the left in seven cases (70%), equal in two cases (20%), and smaller in one case (10%). The dura covering the intracranial portal of the foramen had two perforations, a smaller anteromedial perforation through which passed the ninth cranial nerve (CN IX), and a larger posterolateral perforation, through which passed the 10th and 11th cranial nerves (CNs X and XI) and the distal sigmoid sinus. The perforations were separated by a fibrous septum in 16 specimens (80%). After exiting the posterior fossa, CNs IX, X, and XI all lay anteromedial to the superior jugular bulb (SJB) within the jugular foramen. The inferior petrosal sinus (IPS) entered the foramen between CNs IX and X in most cases; however, in 10% of our cases it entered the foramen between CNs X and XI, and in 10% it entered the foramen caudal to CN XI. The IPS terminated in the SJB in 90% of our cases; in 40%, the IPS termination consisted of multiple channels draining into both the SJB and internal jugular vein. This study shows that the arrangement of the neurovascular structures within the jugular foramen does not conform to the hitherto widely accepted notion of discrete compartmentalization into an anteromedial pars nervosa containing CN IX and the IPS and a posterolateral pars venosa containing the SJB, CNs X and XI, and the posterior meningeal artery.