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Achondroplasia is the most common skeletal dysplasia and the leading genetic cause of disproportionate short stature. It results from a mutation in the FGFR3 gene, leading to abnormal endochondral ossification while intramembranous ossification remains unaffected.

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Clinically, it presents with rhizomelic limb shortening, macrocephaly, and characteristic craniofacial and spinal abnormalities.

Etiopathology

Feature	Description
Gene involved	FGFR3 (Fibroblast Growth Factor Receptor 3)
Inheritance	Autosomal dominant (but ~80% cases are de novo mutations)
Pathogenesis	FGFR3 overactivity → inhibited chondrocyte proliferation and maturation
Affected bone growth	Endochondral ossification impaired (long bones, base of skull)
Unaffected	Intramembranous ossification (e.g., skull vault, clavicles)

Clinical Features

System	Key Manifestations
Growth & stature	Disproportionate short stature, rhizomelic limb shortening
Craniofacial	Frontal bossing, midface hypoplasia, large head (macrocephaly)
Limbs	Trident hand (separation between middle and ring fingers), short fingers
Spine	Thoracolumbar kyphosis, lumbar lordosis, narrowed spinal canal
Neurologic	Foramen magnum stenosis → hydrocephalus or compressive symptoms
Others	Otitis media, sleep apnea, dental crowding, bowing of legs

![The body phenotype is shown in individuals of different ages: Left to right – infancy, early childhood, childhood and adulthood. In all, note the rhizomelic shortening of the limbs, which are disproportionately short compared with the trunk. In the infant and young child macrocephaly is evident

Pauli, R.M. Achondroplasia: a comprehensive clinical review. Orphanet J Rare Dis 14, 1 (2019). https://doi.org/10.1186/s13023-018-0972-6](https://prod-files-secure.s3.us-west-2.amazonaws.com/2aa05644-4658-4c26-84d3-64c36b55fb6c/b61c5687-92bc-4c75-9c38-533d5973dfdc/13023_2018_972_Fig2_HTML.webp)

The body phenotype is shown in individuals of different ages: Left to right – infancy, early childhood, childhood and adulthood. In all, note the rhizomelic shortening of the limbs, which are disproportionately short compared with the trunk. In the infant and young child macrocephaly is evident

Pauli, R.M. Achondroplasia: a comprehensive clinical review. Orphanet J Rare Dis 14, 1 (2019). https://doi.org/10.1186/s13023-018-0972-6

Radiology

Region	Finding	Description
I. Skull	Frontal bossing	Prominent forehead due to normal vault formation
	Foramen magnum narrowing	Seen on sagittal CT/MRI
	Skull base	Shortened due to defective endochondral ossification
II. Spine	‣	Short, posteriorly scalloped vertebral bodies resiulting in reduced interpedicular distance caudally (unlike normal)
	‣	In thoracolumbar region
	Horizontal sacrum	Flattened base of spine
III. Pelvis	Narrow sciatic notch	Vertical iliac wings
	Squared iliac bones	Short and broad pelvis
	‣	Narrow sacrosciatic notch and flat acetabula
IV. Limbs	‣	Short proximal limbs (humerus, femur)
	‣	Widened metaphyses, irregular shape
	‣	Space between 3rd and 4th fingers
	Short tubular bones	With stubby appearance and normal mineralization

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Representative images:

Achondroplasia in newborns
Achondroplasia in children
Achondroplasia in adults </aside>

Differentials

Condition	Distinguishing Features
‣	Lethal, severe micromelia, cloverleaf skull
‣	Milder FGFR3 mutation, less craniofacial involvement
‣	Normal facial features, short stature, ligamentous laxity
Metaphyseal dysplasia	No skull involvement, only metaphyseal irregularities

Complications