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Hypochondroplasia.

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GeneReviews® [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2019.
1999 Jul 15 [updated 2013 Sep 26].

Author information

1
Alfred I duPont Hospital for Children, Wilmington, Delaware
2
Department of Pediatrics, University of Colorado Denver, Aurora, Colorado
3
Children’s Hospital of Eastern Ontario, Ottawa, Ontario
4
Southern California Permanente, Medical Group, Los Angeles, California

Excerpt

CLINICAL CHARACTERISTICS:

Hypochondroplasia is a skeletal dysplasia characterized by short stature; stocky build; disproportionately short arms and legs; broad, short hands and feet; mild joint laxity; and macrocephaly. Radiologic features include shortening of long bones with mild metaphyseal flare; narrowing of the inferior lumbar interpedicular distances; short, broad femoral neck; and squared, shortened ilia. The skeletal features are very similar to those seen in achondroplasia but tend to be milder. Medical complications common to achondroplasia (e.g., spinal stenosis, tibial bowing, obstructive apnea) occur less frequently in hypochondroplasia but intellectual disability and epilepsy may be more prevalent. Children usually present as toddlers or school-age children with decreased growth velocity leading to short stature and limb disproportion. Other features also become more prominent over time.

DIAGNOSIS/TESTING:

Hypochondroplasia is diagnosed by the recognition of characteristic clinical and radiologic findings that remain controversial. The diagnosis is difficult to make in children under age three years, as skeletal disproportion tends to be mild and many of the radiographic features are subtle during infancy. DNA-based testing is possible and about 70% of affected individuals are heterozygous for a pathogenic variant in FGFR3. However, there is evidence that locus heterogeneity exists, implying that pathogenic variants in other as-yet unidentified genes may result in similar, if not identical, phenotypes.

MANAGEMENT:

Treatment of manifestations: Management of short stature in hypochondroplasia is influenced by parental expectations and concerns; one approach is to address these concerns rather than trying to treat the child. Laminectomy relieves symptoms of spinal stenosis; about 70% of individuals experience relief of symptoms following decompression without laminectomy. Developmental milestones are followed closely during early childhood so that cognitive impairments are addressed with special educational programs. Epilepsy is treated in the standard fashion. Surveillance: Height, weight, and head circumference should be monitored using achondroplasia-standardized growth curves. The following should be performed at routine well-child visits: neurologic examination for signs of spinal cord compression, history for evidence of sleep apnea, physical evaluation for emerging leg bowing, and monitoring for social adjustment. MRI or CT examination of the foramen magnum is indicated if there is evidence of severe hypotonia, spinal cord compression, or central sleep apnea. Pregnancy management: Vaginal deliveries are possible, although for each pregnancy, pelvic outlet capacity should be assessed in relation to fetal head size; epidural or spinal anesthetic can be used, but a consultation with an anesthesiologist prior to delivery is recommended to assess the spinal anatomy; spinal stenosis may be aggravated during pregnancy.

GENETIC COUNSELING:

Hypochondroplasia is inherited in an autosomal dominant manner. The majority of new cases result from spontaneous pathogenic variants and the unaffected parents of a child with hypochondroplasia have an extremely low risk of having another affected child. An individual with hypochondroplasia who has a partner of average stature is at a 50% risk of having a child with hypochondroplasia. If an affected individual's partner also has hypochondroplasia (or another dominant form of skeletal dysplasia), genetic counseling becomes more complicated because of (1) the risk for inheriting two dominantly inherited skeletal dysplasias, (2) the high incidence of genetic heterogeneity, and (3) the lack of medical literature addressing these circumstances. Prenatal molecular genetic testing is available if the pathogenic variant(s) in the parent(s) with hypochondroplasia have been identified; however, requests for prenatal testing for conditions such as heterozygous hypochondroplasia are not common.

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