Achondroplasia

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Achondroplasia: NEET PG High-Yield

Skeletal Dysplasia: Genetic Bone Disorder

1. Pathophysiology

The most common cause of dwarfism. It is an autosomal dominant disorder caused by a gain-of-function mutation in the FGFR3 (Fibroblast Growth Factor Receptor 3) gene, which inhibits chondrocyte proliferation and endochondral ossification.

2. Classic Clinical Features

  • Rhizomelic shortening: Disproportionate shortening of the proximal limbs (humerus/femur) compared to the distal limbs.
  • Craniofacial: Macrocephaly, prominent forehead (frontal bossing), and midface hypoplasia.
  • Spine: Exaggerated lumbar lordosis and spinal stenosis due to narrowing of the spinal canal.
  • Hand: “Trident hand” deformity (separation between the 3rd and 4th digits).

3. High-Yield NEET Pearls

  • Endochondral vs. Intramembranous: Achondroplasia affects endochondral ossification (long bones, base of skull); intramembranous ossification (flat bones, calvarium) is normal.
  • Spinal Stenosis: The most significant clinical complication in adulthood is symptomatic lumbar spinal stenosis.
  • Radiology: Progressive narrowing of the interpedicular distance of the lumbar spine (the “V-shaped” or “narrowing” appearance).
  • Paternal Age: Increased paternal age is strongly associated with the appearance of new, sporadic mutations (most cases are sporadic).