Increased Sacral Uptake on a Bone Scan with SPECT/CT in a Patient with Achondroplasia: Normal or Abnormal?

DISCUSSION

Achondroplasia is the most common cause of dwarfism. About 75% of cases are due to sporadic mutations, often associated with advanced maternal age, but it can also be inherited as an autosomal dominant genetic disorder. Achondroplasia occurs because of an abnormality of cartilage formation secondary to a mutation in the gene for fibroblast growth factor receptor 3 (1). The condition can be diagnosed with prenatal ultrasound or on the basis of clinical and radiographic findings; genetic testing is not typically needed (2). The main radiographic findings involve the skull, extremities, spine, and pelvis. Narrowing of the spinal canal is the pathologic hallmark (3). Platyspondyly (flattened vertebral bodies) can be seen, and decreasing interpedicular spaces caudally from L1 to L5 can lead to spinal stenosis and lower back pain. The skull often demonstrates a prominent forehead and depressed nasal bridge. The skull base is disproportionately small because it forms by endochondral ossification, whereas the skull vault is normal because it forms by membranous ossification. This disproportionate growth can lead to a small cisterna magna, empty sella, and a narrowed foramen magnum, which may necessitate cervicomedullary decompression (4,5). The extremities are disproportionately shortened, especially proximally—known as rhizomelic shortening—and are often widened because of unopposed appositional bone growth. This is the most likely explanation for the faint uptake in the proximal humeri in Figure 1. The pelvis can demonstrate a champagne glass appearance secondary to anteversion, squared-off and shortened ilia, a low anterior superior iliac spine, and a high posterior superior iliac spine. The pelvis itself is typically small, and horizontally oriented acetabula are frequently seen.