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.
- 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.
- 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.
- 1. Kao SC, Waziri MH, Smith WL et-al. MR imaging of the craniovertebral junction, cranium, and brain in children with achondroplasia. AJR Am J Roentgenol. 1989;153 (3): 565-9. AJR Am J Roentgenol (abstract) - Pubmed citation
- 2. Wang H, Rosenbaum AE, Reid CS et-al. Pediatric patients with achondroplasia: CT evaluation of the craniocervical junction. Radiology. 1987;164 (2): 515-9. Radiology (abstract) - Pubmed citation
- 3. Albright AL, Adelson PD, Pollack IF. Principles and practice of pediatric neurosurgery. Thieme Medical Pub. (2007) ISBN:1588903958. Read it at Google Books - Find it at Amazon
- 4. Hirabuki N, Watanabe Y, Mano T et-al. Quantitation of flow in the superior sagittal sinus performed with cine phase-contrast MR imaging of healthy and achondroplastic children. AJNR Am J Neuroradiol. 2000;21 (8): 1497-501. AJNR Am J Neuroradiol (full text) - Pubmed citation
Synonyms & Alternative Spellings
|Synonyms or Alternative Spelling||Include in Listings?|
|Base of skull in achondroplasia||✓|
|Foramen magnum stenosis in achondroplasia||✓|