Achondrogenesis

Achondrogenesis is a type of osteochondrodysplasia, an intrinsic abnormality of cartilage and/or bone formation. Achondrogenesis is characterized by normal or poor mineralization of the skull, poor or absent mineralization of the spine, severe pulmonary hypoplasia, micromelia, and abnormalities of the pelvis. Two types of achondrogenesis have been described. Type I achondrogenesis is further divided into subtypes A and B.

	Type IA (Houston-Harris)	Type IB (Parenti-Fraccaro)	Type II (Langer-Saldino)
Skull	Large Poorly ossified		Large Normal ossification
Mandible	Micrognathia
Spine	Completely unossified	Pedicles may be ossified	Poor ossification
Thorax	Hypoplastic with severe pulmonary hypoplasia
Ribs	Short and flared Multiple fractures	Short and flared No fractures
Pelvis	Arched ileum hypoplastic ischium	Crenated ileum	Hypoplastic ileum with medial spike
Femurs	Proximal: Metaphyseal spikes	Distal: Metaphyseal abnormalities	Flared metaphyses

All three types are lethal. Diagnosis can be made as early as 12 weeks. Type II achondrogenesis gives rise to the floating head appearance: A normally ossified skull is seen against a background of poor spine mineralization.

References

• STATdx
• Whitley CB, Gorlin RJ. Achondrogenesis: new nosology with evidence of genetic heterogeneity. Radiology. 1983 Sep;148(3):693-8.