Hypochondroplasia is inherited as an autosomal dominant trait, but most cases appear to be sporadic, presumably the result of a spontaneous mutation affecting FGFR-3, resulting in a milder dysplasia than achondroplasia. For unknown reasons, about 10% of patients are mentally retarded.

Clinical Manifestations. Birth weight and length may be low normal, and the short stature may not be recognized until the patient is 2 or 3 years of age. The typical appearance is a thick, stocky physique with a relatively long trunk and disproportionately short limbs, making the upper body segment longer than the lower body segment.

Head circumference is normal, although the forehead may be slightly prominent. The face is also normal with no midfacial hypoplasia or depression of the nasal bridge.

The limbs are short and stocky. Mild bowleg is common, but alignment tends to improve with age. Ligamentous laxity is usually mild, and range of motion in the elbow, especially extension and supination, is often limited. The hands are broad with short fingers but no trident formation.

The trunk commonly shows mildly exaggerated lumbar lordosis with a sacral tilt and a slightly protuberant abdomen. Aching knees, elbows, and ankles and low back pain are common in adulthood. Adult height ranges from 52 to 59 inches.

Neurologic complications, particularly compressive myelopathy or radiculopathy, are much less frequent than in achondroplasia.

Radiographic Findings. Characteristic findings permit differentiation from achondroplasia. The skull is essentially normal, except for a mild bossing of the forehead. Generalized shortening of the long bones with mild metaphyseal flaring is most notable at the knees. In children, the growth plates of the distal femurs may show a shallow, V-shaped indentation, but this is not as pronounced as the chevron-shaped notch seen in achondroplasia. The femoral necks are short and broad. The pelvis may be basically normal or mildly dysplastic (e.g., the greater sciatic notches are reduced in width and the ilia are square and shortened). In the lumbar spine, interpedicular distances lack the normal caudal widening, but these alterations are not as profound as in achondroplasia. The height of the vertebral bodies is normal, and the dorsal borders are only mildly scalloped.