Preferred examination

Prenatal diagnosis of achondroplasia can be achieved with ultrasonography. Antenatal, targeted ultrasonography is indicated in at-risk pregnancies. Conventional radiography remains the preferred modality for initial investigation for both children and adults.

Myelography, computed tomography (CT) scanning, CT myelography, and magnetic resonance imaging (MRI) are added when indicated, as with compressive cord symptoms at the craniocervical and thoracolumbar junctions. Both CT scanning and MRI can be used to examine the size of the foramen magnum, which is an important determinant of compressive myelopathy of the upper cervical cord. MRI has the added advantage of depicting posterior cranial fossa anatomy and other abnormalities, such as syringomyelia and hydrocephalus.

MRI is the modality of choice in cases of suspected spinal stenosis; disk lesions; and compromise of the exiting nerve root in the lumbar region. Radiologic studies are indicated if the head circumference increases disproportionately or if symptoms of hydrocephalus develop.

Ultrasonography provides a noninvasive and fairly reliable method of assessing the ventricles of infants before the fontanels close. Ultrasonography may be supplemented with CT scanning and/or MRI of the head to monitor for compression of the foramen magnum.