Radiological Features of Achondroplasia

General Findings

• Stenosis at the foramen magnum
• Generalized spinal canal stenosis
• Hydrocephalus

CT Scan

• Foramen magnum stenotic: The rim of the foramen magnum is thick, resulting in a narrow foramen. The narrow foramen magnum means that the vertebral arteries are closer to the midline.
• Midline keel: There is a thick midline keel extending up from the foramen magnum.

3D CT angiography of foramen magnum in a child with achondroplasia: The foramen magnum is narrow, there is a deep midline keel in the occipital bone, the rim of the foramen magnum is thickened, and the distance between the vertebral arteries is less than normal.

MRI in Achondroplasia

• Loss of CSF space: The CSF spaces at the cervicomedullary junction are effaced.
• T2-weighted changes in spinal cord: T2-weighted signal change is present in more than 50% of cases at the time of surgical decompression (17).
• Instability rare: Cervical instability is rare in achondroplasia, either at presentation or as a result of decompression.

Sagittal T2-weighted MRI in a child with achondroplasia: There is stenosis at the foramen magnum with intramedullary signal change in the spinal cord. The rest of the spinal canal is also narrow, in keeping with achondroplasia.