As the skull base forms by endochondral ossification whereas the skull vault by membranous ossification, there is a marked discrepancy in relative size as the skull vault, brain and cord grow normally whereas the skull base remains small.
As a result of this size discrepancy, there is stenosis of the foramen magnum, often with basilar invagination. Compression of the medulla may result in quadriparesis, sleep apnea, and even sudden death. Interruption to normal cerebrospinal fluid (CSF) flow can lead to hydrocephalus.
- small/non-existent cisterna magna
- enlarged suprasellar cistern +/- empty sella due to normal brain and brain stem size but a short clivus
- enlarged lateral ventricles
- enlarged third ventricle
- +/- enlarged fourth ventricle
- enlarged CSF space between and over the cerebral hemispheres
- occasional transependymal edema
- more vertical course of the straight sinus
- increased tentorial angle (55-70 degrees)
- small jugular foramen which can result in elevated venous pressures and reduced flow in the superior sagittal sinus 4
- foramen magnum narrowed
- obliteration of the CSF space around the cervicomedullary junction
- increased intramedullary T2 signal sometimes seen, representing myelomalacia and gliosis
Treatment and prognosis
Surgical decompression may be required, with resection of the posterior aspect of the foramen magnum, the posterior arch of C1 and duroplasty.
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