NORD gratefully acknowledges Sarah Zhou, MD Candidate, McGill University School of Medicine, and Richard M. Pauli, MD, PhD, School of Medicine and Public Health, University of Wisconsin, for assistance in the preparation of this report.
Achondroplasia is the most commonly occurring abnormality of bone growth (skeletal dysplasia), occurring in approximately 1 in 20,000-30,000 live births. This genetic disorder is caused by a change (mutation) in the fibroblast growth factor receptor 3 (FGFR3) gene. Achondroplasia occurs as a result of a spontaneous genetic mutation in approximately 80 percent of patients; in the remaining 20 percent it is inherited from a parent. This genetic disorder is characterized by an unusually large head (macrocephaly), short upper arms (rhizomelic dwarfism), and short stature (adult height of approximately 4 feet). Achondroplasia does not typically cause impairment or deficiencies in mental abilities. If the bones that join the head and neck do not compress the brainstem or upper spinal cord (craniocervical junction compression), life expectancy is near normal.
This rare genetic disorder is characterized by distinctive features: short stature (usually under 4 feet 6 inches); an unusually large head (macrocephaly) with a prominent forehead (frontal bossing) and flat (depressed) nasal bridge; short arms and legs; prominent abdomen and buttocks (due to inward curve of the spine); and short hands with fingers that assume a “trident” or three-pronged position during extension.
Infants born with achondroplasia typically have a “dome-like” (vaulted) skull, and a very broad forehead. In a small proportion there is excessive accumulation of fluid around the brain (hydrocephalus). Low muscle tone (hypotonia) in infancy is typical of achondroplasia. Acquisition of developmental motor milestones may be delayed.
Achondroplasia results from specific changes (mutations) of a gene known as fibroblast growth factor receptor 3 (FGFR3).
For most patients, there is no apparent family history of the condition. Increased age of the father (advanced paternal age) may be a contributing factor in cases of sporadic achondroplasia.
Less commonly, familial cases of achondroplasia follow an autosomal dominant pattern of inheritance. Dominant genetic disorders occur when only a single copy of an abnormal gene is necessary to cause a particular disorder. The abnormal gene can be inherited from either parent or can be the result of a mutated (changed) gene in the affected individual. The risk of passing the abnormal gene from an affected parent to an offspring is 50% for each pregnancy. The risk is the same for males and females.
Achondroplasia appears to affect males and females in equal numbers. This disorder begins in the developing fetus and is one of the most common forms of skeletal dysplasia that causes dwarfism. The estimated frequency of achondroplasia has ranged from about one in 15,000 to one in 35,000 births.
Clinical and radiologic features of achondroplasia are well-characterized. Those with typical findings generally do not need molecular genetic testing to confirm the diagnosis. When clinical features raise suspicion in a newborn, X-ray (radiography) findings can be used to help confirm the diagnosis. However, if there is uncertainty, identification of the genetic variant of the FGFR3 gene by molecular genetic testing can be used to establish the diagnosis. Below is a list adapted from Pauli and Legare (2018) that provides clinical signs that may be used in the diagnosis of achondroplasia.
Recommendations for managing children with achondroplasia are outlined by the American Academy of Pediatrics Committee on Genetics, which are designed to supplement guidelines for children with average stature.
As outlined in Pauli and Legare (2018), the recommendations for the manifestations of achondroplasia include:
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Pauli RM, Botto LD (2018) Achondroplasia. In: Management of Genetic Syndromes. 4 ed. New York, NY: John Wiley & Sons. In press.
Miccoli M, Bertelloni S, Massart F. Height outcome of recombinant human growth hormone treatment in achondroplasia children: a meta-analysis. Horm Res Paediatr. 2016;86:27–34.
Pauli RM, Legare JM. Achondroplasia. 1998 Oct 12 [Updated 2018 May 10]. In: Adam MP, Ardinger HH, Pagon RA, et al., editors. GeneReviews® [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2018. Available from: https://www.ncbi.nlm.nih.gov/books/NBK1152/ Accessed July 26, 2018.
Bober MB, Bellus GA, Nikkel SM, et al. Hypochondroplasia. 1999 Jul 15 [Updated 2013 Sep 26]. In: Adam MP, Ardinger HH, Pagon RA, et al., editors. GeneReviews® [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2018.Available from: https://www.ncbi.nlm.nih.gov/books/NBK1477/ Accessed July 26, 2018.
Bacino C. Achondroplasia. UpToDate. topic last updated: Nov 15, 2017. https://www.uptodate.com/contents/achondroplasia Accessed July 12, 2018.
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