Achondroplasia, the most common type of dwarfism in humans, is a hereditary disease characterized by short limbs, small face with normal trunk. Fingers are widely opposed and equal length ("trident hands").3 Although achondroplasia literally means "without cartilage formation," the problem is not in forming cartilage but in converting it to bone, particularly in the long bones of the arms and legs.4
It is an autosomal dominant disease caused by a mutation in the gene that codes for fibroblast growth factor receptor 3 (FGFR3). The FGFR3 gene provides instructions for making a protein that is involved in the development and maintenance of bone and brain tissue. This protein limits the formation of bone from cartilage (a process called ossification), particularly in the long bones. Two specific mutations in the FGFR3 gene are responsible for almost all cases of achondroplasia. Researchers believe that these mutations cause the protein to be overly active, which interferes with skeletal development and leads to the disturbances in bone growth seen with this disorder.
Achondroplasia can be diagnosed by characteristic clinical and radiographic findings in most affected individuals. In individuals who may be too young to diagnose with certainty or in individuals with atypical findings, molecular genetic testing can be used to detect a mutation in the FGFR3 gene. Such testing detects mutations in 99% of affected individuals and is available in clinical laboratories.2
The average height of an adult male with achondroplasia is 131 centimeters (4 feet, 4 inches), and the average height for adult females is 124 centimeters (4 feet, 1 inch). enlarged head (macrocephaly) with a prominent forehead. Fingers are typically short and the ring finger and middle finger may diverge, giving the hand a three-pronged (trident) appearance. People with achondroplasia are generally of normal intelligence.4
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Health Problems Associated with Achondroplasia
Health problems commonly associated with achondroplasia include episodes in which breathing slows or stops for short periods (apnea), obesity, and recurrent ear infections. In adulthood, individuals with the condition usually develop a pronounced and permanent sway of the lower back and bowed legs. Older individuals often have back pain, which can cause difficulty with walking.
Over eighty percent of individuals with achondroplasia have parents with normal stature and have achondroplasia as the result of a de novo gene mutation. Such parents have a low risk of having another child with achondroplasia. An individual with achondroplasia who has a reproductive partner with normal stature has a 50% risk in each pregnancy of having a child with achondroplasia. When both parents have achondroplasia, the risk to their offspring of having normal stature is 25%; of having achondroplasia, 50%; and of having homozygous achondroplasia (a lethal condition), 25%. Prenatal molecular genetic testing is available.4
- Sharon L. Vanderlip D.V.M. The Shih Tzu Handbook, Barron's Educational Series 2005, p. 120
- Clair A Francomano, MD, FACMG, Achondroplasia, Jan. 2006, gentests.org
- Brian Funaki, MD, Achondroplasia, CHORUS, September 2006
- Achondroplasia, Genetic Home Reference, December 14, 2007