Heterozygous achondroplasia is the prototypical  and most common rhizomelic dwarfism syndrome. Although the disease is autosomal dominant in transmission, approximately 80 - 90 % of cases are sporadic, reflecting mutations. The genetic defect lies at the fibroblast growth factor-3 gene, but in a different location than thanatophoric dwarfism. This results in defective endochondral bone formation with resultant shortening of tubular bones. The shaft caliber is less affected because periosteal intramembraneous bone formation is not affected. 

The incidence of the disease is 1 in approximately 30,000 to 60,000 births. Females are affected more than males. 

The offspring of two achondroplasts may produce a hereditary form of the disease known as homozygous achondroplasia. This is a lethal form of the disease in the neonatal period. 

Patients with achondroplasia are of normal intelligence but may have delay in reaching pediatric milestones. Three percent have hydrocephalus. In addition, skull abnormalities may lead to a small foramen magnum, leading to spinal cord compression. Hearing problems may result from multiple ear infections in childhood due to skull abnormalities.

Hypochondroplasia is a milder form of achondroplasia which presents in late childhood.

REFERENCES

• Lemyre E, Azouz EM, Teebi AS, Glanc P, Chen MF. Bone dysplasia series. Achondroplasia, hypochondroplasia and thanatophoric dysplasia: review and update. Can Assoc Radiol J. 1999 Jun;50(3):185-97.
• Azouz EM, Teebi AS, Eydoux P, Chen MF, Fassier F. Bone dysplasias: an introduction. Can Assoc Radiol J. 1998 Apr;49(2):105-9.

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