The most common cause of disproportionate short stature (dwarfism) - it is a type of skeletal dysplasia

Genetics

• The achondroplasia gene, fibroblast growth factor receptor (FGFR3), is on chromosome 4
• Achondroplasia results from either a sporadic mutation or inheritance of an abnormal copy of this gene
• The condition is inherited in an autosomal dominant pattern
• Homozygous gene mutations, meaning two abnormal gene copies with one from each parent, is fatal in the neonatal period. Therefore, patients with achondroplasia have one normal gene and one abnormal gene

Mutations in the FGFR3 gene causes abnormal function of the epiphyseal plates (growth plates). This restricts the bone growth in length, leading to short bones and short stature.

Features

• disporportiate short stature - average height is around 4ft
• limbs are most affected by reduced bone length
  • The femur and humerus (proximal limbs) are affected more than the bones of the forearm and lower leg.
• The spine length is less affected and patients have a normal trunk length.
• Intelligence and life expectancy are not affected by the condition.

Other features:

• Short digits
• Bow legs (genu varum)
• Disproportionate skull - The skull base grows and fuses via endochondral ossification, which is affected by achondroplasia and leads to a flattened mid-face and nasal bridge and foramen magnum stenosis. The cranial vault grows and fuses via membranous ossification, which is unaffected by achondroplasia and leads to a normal sized vault and frontal bossing (prominent forehead).
• Foramen magnum stenosis