Hypochondroplasia

Disproportionate short stature with mild rhizomelia but relatively normal facial features suggests hypochondroplasia, which is a common autosomal-dominant skeletal dysplasia caused by a gain-of-function mutation in FGFR3 - the same gene as achondroplasia, making them allelic disorders, but with a milder phenotype. Because it is mild and the facies are subtle or normal, it is typically diagnosed postnatally, in childhood, and is often under-recognised, sometimes presenting simply as a short child with mildly abnormal proportions. The key radiographic clue is shared with achondroplasia but milder: failure of the normal caudal widening of the lumbar interpedicular distance - the gap between the pedicles should increase from L1 to L5, and here it fails to widen - together with a short, broad, squared pelvis and mildly short tubular bones. I would confirm the diagnosis with FGFR3 molecular testing. It is distinguished from achondroplasia, which shares the gene but is more severe, with frontal bossing, midface hypoplasia, usually prenatal or birth diagnosis, and the important infantile complication of foramen magnum stenosis; the milder hypochondroplasia phenotype generally lacks the severe canal and foramen problems. I would also consider idiopathic short stature and SHOX-related short stature in the differential. Management is largely supportive and surveillance-based - monitoring growth, limb alignment, treating significant genu varum, and watching the spine - within a multidisciplinary skeletal-dysplasia service, and with FGFR3-pathway therapies emerging for achondroplasia, accurate, timely diagnosis is increasingly relevant.