Measurement and clinical significance of the posterior cranial fossa volume of patients with hemifacial spasm
Yamamoto, Y.; Kondo, A.; Hanakita, J.; Nishihara, K.; Kinuta, Y.; Nakatani, H.
No Shinkei Geka. Neurological Surgery 15(3): 243-248
1987
ISSN/ISBN: 0301-2603 PMID: 3600982 Document Number: 297808
The etiology of hemifacial spasm had long been obscure until 1962 when Gardner proved that this hyperdysfunction of the facial nerve was caused by mechanical compression of the facial nerve by vascular structures in the posterior cranial fossa. In 1977, Jannetta proposed a specific location at the root entry zone of the facial nerve; this area has consequently been considered to be especially vulnerable to minor trauma such as vascular compression. In patients with hemifacial spasm, the posterior cranial fossa cavity is commonly found to be small or shallow on plain craniogram; this anatomical change in the skull is regarded as pathognomonic for the facial nerve hyperdysfunction. To make a quantitative analysis of the posterior cranial fossa volume in these patients, the following method was used. In the preliminary study, a dry human skull with an artificial "tentorium" made of thick paper was prepared to decide the fundamental plane for volume measurements by CT scan. This plane included attachments of posterior clinoid ligaments, superior petrosal veins and lateral sinuses. When this fundamental plane was projected to the lateral view on CT scan, it appeared to be almost identical to the line connecting the tip of posterior clinoid process to the internal occipital protuberance (the fundamental line). A horizontal CT scan for an intracranial volume measurement was performed in a parallel fashion to this fundamental line, with a 5 mm slice for the infratentorial and a 10 mm slice for the supertentorial area. The intracranial area of each horizontal slice was calculated by computed planimeter. Finally the volume was obtained by multiplying the surface area of the intracranial part by the width (height) of each slice. The volume ratio of the posterior cranial fossa to the whole intracranial cavity was calculated. The area surrounded by the posterior clinoid process and/or the free edge of the tentorium was calculated as a part of the posterior fossa cavity. But the part beneath the apex of the tentorium was uncalculable by this method. To ascertain the accuracy of the measurement by CT scan, a dry human skull was packed tightly with clay and the volume ratio of the posterior cranial fossa to the whole intracranial cavity was obtained by weighing the packed clay from the each supra- and infratentorial cavities. The data were compared with data calculated by CT scan and agreed reasonably well. After this preliminary work, a clinical study was carried out by consecutive CT scan in 30 patients with hemifacial spasm and in 29 with non-hemifacial spasm (i.e. cerebrovascular diseases). The free edge of the tentorium was displayed by contrast enhancement. The volume ratio of the posterior cranial fossa to the whole cranial cavity was 12.69 .+-. 1.71 SD% in hemifacial spasm patients and 13.70 .+-. 1.30 SD% in non-hemifacial spasm patients, i.e. the posterior fossa volume is significantly smaller in patients with hemifacial spasm. These results indicate that "crowding of the posterior fossa" takes place in these patients; this condition makes encroachment by a compressing artery on a facial nerve and the occurrence of the facial nerve compression syndrome possible.