Achondroplasia is the most common form of dwarfism and skeletal dysplasia.
The name of the condition is not strictly accurate because cartilage does develop and grow both at the physes and other locations. However, the bone that is formed by endochondral means is most underdeveloped in length, resulting in disproportionate short stature.
The cause of achondroplasia is a point mutation the gene that encodes fibroblast growth factor receptor-3.
The gene for this receptor is on the short arm of chromosome 4.
Achondroplasia is transmitted as an autosomal dominant condition, however in 80% of patients; the disorder is because of a spontaneous mutation.
The risk of having a child with achondroplasia increases with increasing paternal age.
The striking clinical feature of achondroplasia is short stature and it is apparent at birth.
The facial appearance is characterized by frontal bossing and midface hypoplasia. Macrocephaly is seen and the skull appears flattened in the anteroposterior plane and broad when viewed from the front. Dentition is normal.
The trunk length is within the lower range of normal, whereas the extremities are much shorter than normal. The proximal segments of the extremities, the humeri and femurs are the most foreshortened. The fingertips usually reach only to the tops of the greater trochanters and this leads to difficulties in personal care.
The digits of the hand have an extra space between the third and fourth rays so that the digits are separated into three groups, including the thumb, the trident hand.
Kyphosis at the thoracolumbar junction is common, especially in infancy. The condition usually improves with the increasing age of the child. Lumbar lordosis increases.
Ligamentous laxity is common at the knees and ankles with varus alignment.
Intelligence is normal.
The characteristic findings are shortening of the tubular long bones, with a relative increase in bony diameter and density. The metaphyses are widened and flared, but the epiphyses are uninvolved. The growth plates are U shaped or V-shaped, best seen at the distal femur.
The pelvis is characteristically broad and flat, with squared iliac wings. The sciatic notches are small and the acetabula are horizontal.
The spinal canal narrows and interpedicular distances decrease from L1 to L5.
Prenatal ultrasonography does reveal decreased femoral length for the gestational age.
If Achondroplasia seems likely based on ultrasonographic findings, studies on the FGFR gene may confirm the diagnosis.
Craniocervical stenosis – Sudden death has been reported in infants younger than 1 year of age and cervical cord compression has been suggested as the cause.
Presenting symptoms are hypotonia and sleep apnea.
Treatment of central sleep apnea is neurosurgical decompression consisting of foramen magnum decompression, suboccipital craniectomy, and C1 laminectomy.
Hydrocephalus – Does occur on rare occasions and is usually the communicating type.
Thoracolumbar kyphosis – Developmental problem that becomes evident in the slightly older infants and most noticeable when an infant is placed in a sitting position.
As the child learns to walk, muscle tone and trunk control improve and kyphosis usually resolves without treatment.
Spinal stenosis – Interpedicular narrowing in the lumbar spine results from abnormal growth of the pedicles. One-third of patients will present with neurogenic claudication by age 15 years.
Treatment is by surgical posterior decompression.
Angular deformities of the lower extremities – Genu varum and tibia vara are more common than valgus deformity.
Proximal fibular epiphysiodesis has been recommended for the treatment of tibia vara and fibular overgrowth.
Surgical treatment of genu varum consists of proximal tibial and fibular osteotomy with fixation.
Short stature – Limb lengthening