Achondroplasia is a congenital disorder due to deficient cartilaginous development, hence it is usually seen in bones growing with enchondral ossification such as long bones and skull base, while bones growing via membranous ossification grow normally, such as the vault of the skull. This discrepancy in growth results in the typical skull appearance including narrowed foramen magnum compressing the cervico-medullary junction and CSF outflow with consequent hydrocephalus, as in this patient.
Typical CNS findings that are seen in this patient include:
- large skull vault and prominent forehead
- depressed nasal bridge
- small skull base and narrow foramen magnum
- widened suprasellar cistern
- vertical straight sinus
- supratentorial hydrocephalus with normal 4th ventricle
The long bones are typically shortened due to arrested growth with widened metaphysis. The ribs show similar changes in the form of widening of the anterior segments at the cartilaginous junction with the sternum. The pelvic bones are small resulting in small pelvic outlet and trident appearance of the pelvis. Overall there is generalized dwarfism. Achondroplasia is the commonest cause of limb shortening and dwarfism.