Achondroplasia is the most common cause of disproportionate short stature. Affected individuals have rhizomelic shortening of the limbs, macrocephaly, and characteristic facial features with frontal bossing and midface retrusion. In infancy, hypotonia is typical, and acquisition of developmental motor milestones is often both aberrant in pattern and delayed. Intelligence and life span are usually near normal, although craniocervical junction compression increases the risk of death in infancy. Additional complications include obstructive sleep apnea, middle ear dysfunction, kyphosis, and spinal stenosis.

Paget disease is a metabolic bone disease characterized by focal abnormalities of increased bone turnover affecting one or more sites throughout the skeleton, primarily the axial skeleton. Bone lesions in this disorder show evidence of increased osteoclastic bone resorption and disorganized bone structure. See reviews by Ralston et al. (2008) and Ralston and Albagha (2014). For a discussion of genetic heterogeneity of Paget disease of bone, see 167250.