Clinical characteristics.

Achondroplasia is the most common process resulting in disproportionate small stature. Affected individuals have short arms and legs, a large head, and characteristic facial features with frontal bossing and midface retrusion (formerly known as midface hypoplasia). 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.

Diagnosis/testing.

Achondroplasia can be diagnosed by characteristic clinical and radiographic findings in most affected individuals. In individuals in whom there is diagnostic uncertainty or atypical findings, molecular genetic testing can be used to detect a mutation in FGFR3, the only gene known to be associated with achondroplasia. Such testing detects mutations in 99% of affected individuals.

Management.

Treatment of manifestations: Ventriculoperitoneal shunt may be required for increased intracranial pressure; suboccipital decompression as indicated for signs and symptoms of craniocervical junction compression; adenotonsillectomy, positive airway pressure, and, rarely, tracheostomy to correct obstructive sleep apnea; aggressive management of middle-ear dysfunction; evaluation by an orthopedist if progressive bowing of the legs arises; surgery to correct spinal stenosis in symptomatic adults; and educational support in socialization and school adjustment.

Surveillance: Monitor height, weight, and head circumference in childhood using growth curves standardized for achondroplasia; evaluation of developmental milestones throughout infancy and childhood; baseline CT scan of the brain in infancy; monitor for signs and symptoms of sleep apnea; monitor for middle ear problems or evidence of hearing loss in childhood; clinical assessment for kyphosis and bowed legs, with radiographic evaluation and referral to an orthopedist, if necessary; in adults, clinical history and neurologic examination to screen for spinal stenosis every three to five years.

Agents/circumstances to avoid: Activities in which there is risk of injury to the craniocervical junction, such as collision sports; use of a trampoline; diving from diving boards; vaulting in gymnastics; and hanging upside down from the knees or feet on playground equipment.

Pregnancy management: Pregnant women with achondroplasia must undergo Caesarian section delivery because of small pelvic size.

Genetic counseling.

Achondroplasia is inherited in an autosomal dominant manner. Around 80% of individuals with achondroplasia have parents with average stature and have achondroplasia as the result of a de novo gene mutation. Such parents have a low risk of having another child with achondroplasia. An individual with achondroplasia who has a reproductive partner with average stature has a 50% risk in each pregnancy of having a child with achondroplasia. When both parents have achondroplasia, the risk to their offspring of having average stature is 25%; of having achondroplasia, 50%; and of having homozygous achondroplasia (a lethal condition), 25%. Prenatal testing for pregnancies at increased risk is possible once the disease-causing mutation has been identified in the family.