Achondroplastic base of skull abnormalities

Achondroplasia is an autosomal dominant disorder that is the most common cause of short limb dwarfism. (For a general discussion, see the generic article on achondroplasia.)

As the skull base forms by enchondral 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 medulla may result in quadriparesis, sleep apnoea, and even sudden death. Interruption to normal CSF flow can produce hydrocephalus. 

Radiographic features

Cranial content

• 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 oedema
• 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.

Cervico-medullary junction

• foramen magnum narrowed
• obliteration of the CSF space around CMJ
• 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.