Hypochondroplasia

Hypochondroplasia is characterized by challenges in converting cartilage into bone. The ossification is primarily affected in the long bones of the arms and legs, resulting in distinct physical characteristics and shorter stature.

Characteristics

All individuals with hypochondroplasia have short stature, in some cases it may not be evident until the age of 2 or 3, with adult males averaging around 145 centimeters (4 feet, 9 inches) and adult females averaging approximately 135 centimeters (4 feet, 5 inches) in height. They may also present with broad, short hands and feet. Other characteristic features include a large head (macrocephaly), limited range of motion at the elbows, a sway of the lower back (lordosis), scoliosis and bowed legs. Affected individuals may also have a small increased risk of a seizure disorder known as temporal lobe epilepsy. Hypochondroplasia shares striking skeletal similarities with achondroplasia, albeit typically milder, without skull involvement and while it presents fewer medical issues commonly associated with achondroplasia, it may exhibit a higher prevalence of deficits in mental capacity.

Cause

The vast majority of cases of hypochondroplasia are caused by variants in the FGFR3 gene. This gene provides instructions for making a protein that is involved in the development and maintenance of bone and brain tissue. Although it remains unclear how FGFR3 gene variants lead to the features of hypochondroplasia, researchers believe that these genetic changes cause the protein to be overly active. The overactive FGFR3 protein likely interferes with skeletal development and leads to the disturbances in bone growth that are characteristic of this disorder.

Diagnosis

A diagnosis is made with a complete medical history, physical examination and X-rays of the spine and lower extremities. Genetic testing through amniocentesis or chorionic villus sampling (CVS) can also confirm a diagnosis.

Management

Orthopedic conditions common among patients with hypochondroplasia are much milder than those with achondroplasia and include:

Kyphosis: An outward curvature of the lower spine may be present but usually does not require surgery and can be avoided by keeping an infant’s spine supported and letting a baby achieve milestones like rolling over, sitting and standing on their own.

Scoliosis: The occurrence of scoliosis in hypochondroplasia tends to be more prevalent than spinal stenosis and can range from being very mild, requiring no intervention, to more severe in which bracing and/or surgery would be required.

Spinal Stenosis: Like achondroplasia, narrowing of the spinal canal may be present but usually not severe enough to require surgery. Stenosis can occur in infants and young children as a narrowing at the bottom of the skull (foramen magnum stenosis) and in older children and adults as a narrowing of the lower spinal canal (thoracic or lumbar stenosis).

Genu Varum: An outward bowing of the legs (genu varum) typically appears and becomes pronounced during weight bearing. This condition often improves during childhood or is so mild that no intervention is usually needed.

Sleep Apnea: This is usually mild but can be treated with surgery to remove tonsils and adenoids (adenotonsillectomy).

Support and Resources

Supportive Community: Support our community of individuals, families, and medical professionals who understand and support those living with Hypochondroplasia.

Educational Materials: Access a wealth of educational resources, including articles, videos, and guides to help you navigate life with Hypochondroplasia.

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