CHAPTER 72
Achondroplasia
Introduction/Etiology/Epidemiology
• Achondroplasia is the most common skeletal dysplasia, with an incidence of 1 in 30,000 live births.
• Mutations of fibroblast growth factor receptor 3 (FGFR3) on chromosome 4 result in the inhibition of chondrocyte growth and proliferation, with underdevelopment and shortening of bones formed by endochondral ossification. Articular cartilage is unaffected.
• Patients with achondroplasia display manifestations in the spine, upper and lower extremities, and face.
• Inheritance is autosomal dominant.
• More than 80% of cases are sporadic.
• Advanced paternal age (older than 35 years) is associated with sporadic mutations.
Signs and Symptoms
• The primary feature of achondroplasia is short stature, defined as a height at least 2 standard deviations (SD) below the population mean.
— Short stature is caused by rhizomelic (proximal extremity) shortening—the humerus and femur are affected more than the forearm and tibia. Trunk length is in the lower range of normal.
— Patient length on average is −1.5 SD at birth, −4.4 SD at 1 year, and −5.0 SD at 2 years.
— The mean final adult height for those with achondroplasia is 6 to 7 SD below normal; the overall mean adult height for patients with achondroplasia is 52 inches (range, 46–57 inches) in men and 49 inches (range, 44–54 inches) in women.
• There may be frontal bossing and midface hypoplasia.
• During infancy, patients may have foramen magnum stenosis.
— Stenosis may present with sleep apnea, excessive snoring, signs of chronic brainstem compression (ie, lower cranial nerve dysfunction, swallowing difficulty, hyperreflexia, hypotonia, weakness, developmental delay, and clonus), or death
• Most neonates develop thoracolumbar kyphosis as sitting begins.
— When sitting begins, these newborns slump forward because of their large heads and poor trunk control.
— Repeated slumping can increase the kyphosis and result in anterior wedging of the vertebral bodies.
— Examination reveals a gibbus or prominence of the thoracolumbar junction when the neonate is in the sitting position. An infant with a “bump on their back” from a kyphotic thoracolumbar spine may prompt further evaluation, including radiographic and genetic evaluation, because many patients with achondroplasia or other skeletal dysplasias may present this way.
— Thoracolumbar kyphosis in patients with achondroplasia is defined by the presence of more than 20 degrees of kyphosis at the thoracolumbar junction.
— Most kyphoses resolve within 1 year after walking begins.
— Persistent thoracolumbar kyphosis occurs in 30% of patients and may result in deformity progression and neurologic symptoms, including paresthesias, incontinence, and the inability to walk.
• Genu varum
— The primary manifestation in the lower extremity
— May be asymptomatic or associated with knee pain or instability
— A lateral thrust may be evident with walking.
• Lumbosacral hyperlordosis
— May be seen during childhood in up to 80% of patients
— Secondary to increased pelvic tilt while standing
— Presents as a prominent abdomen and buttocks with hip flexion contractures
• Symptomatic spinal stenosis
— Occurs as a result of endochondral ossification defects along the entire spinal column, producing short, thickened pedicles and a narrowing of the interpediculate distance from L1 to L5 (Figure 72-1). Mismatch between the smaller spinal canal and the normal-sized neural elements increases risk for spinal stenosis.
— Occasionally develops during adolescence, but more often in adulthood
— Symptoms include leg pain; numbness, weakness, or frequent squatting to relieve pain as in neurogenic claudication; and neurologic incontinence.
• Flexion contractures of the elbow and subluxated radial heads
• Trident hand
— Extra space between first and second rays and third and fourth rays, with all fingers of nearly equal length
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