Limitations of techniques

Both false-positive and false-negative diagnoses may occur with antenatal ultrasonography of skeletal dysplasias. Heterozygous disease may not be recognized until late in the second trimester (>24-28 wk), because ultrasonograms are normal early in the course of the disease.

Ultrasonography remains operator dependent. Conventional radiography is good for depicting skeletal pathology, but it is poor at providing information on the brain and spinal cord.

Myelography is invasive; however, this technique has traditionally been the most useful examination in delineating the level of compression of the spinal cord or the cauda equina. However, in patients with achondroplasia, myelography is difficult because of stenosis of the lumber spine. When myelography is performed near the lumber root, there is a risk that the neurologic deficit will increase; this is particularly so in cases of kyphosis. Therefore, when myelography is necessary, the study is best performed by means of cisternal puncture; however, this step makes the procedure even more invasive than it otherwise would be.

CT scanning exposes the patient to ionizing radiation and is limited in its capacity to portray the spinal cord and to depict structures of the posterior fossa. With this modality, infants and young children may need sedation or general anesthesia.

MRI is frequently used to evaluate the brain and spinal cord in patients with achondroplasia. This modality can accurately depict anatomic encroachment on the central nervous system (CNS). However, MRI is expensive and creates problems for patients with claustrophobia, and it cannot be performed in patients with cardiac pacemakers or in patients who have certain surgical clips or other foreign objects in the body. As with CT scanning, young children may need sedation or general anesthesia to undergo MRI.