Cartilage-Hair Hypoplasia (CHH) is a rare genetic disorder characterized by short-limbed dwarfism. As a form of skeletal dysplasia, it affects bone and cartilage growth. The condition’s name highlights two of its most common features: underdeveloped cartilage and fine, sparse hair. However, its effects extend beyond the skeletal system and hair, involving multiple body systems.
Core Physical and Clinical Features
The most apparent characteristic of CHH is dwarfism where the arms and legs are disproportionately short compared to the trunk. This results from metaphyseal chondrodysplasia, an abnormality in the growth plates of the long bones. Many individuals also exhibit joint hypermobility, particularly in the hands and feet. Other skeletal findings can include short ribs, spinal abnormalities, and bowed legs.
A defining feature of CHH is fine, sparse, and light-colored hair. This characteristic, which gives the condition part of its name, is a consistent finding. The hair can be fragile, and eyebrows may also be thin. While cosmetic, this feature is a primary indicator that helps distinguish CHH from other forms of dwarfism.
Many individuals with CHH have some degree of immunodeficiency. This weakened immune system is due to a deficiency in T-cells, a type of white blood cell that fights infection. Consequently, people with the condition are more susceptible to recurrent infections, like respiratory infections and severe chickenpox. The severity of the immunodeficiency varies, ranging from mild issues to life-threatening infections.
Other health conditions are associated with CHH, including an increased risk for certain gastrointestinal issues like Hirschsprung disease. Anemia, a lack of healthy red blood cells, is another common finding. Individuals with CHH also have a higher predisposition to developing cancers such as lymphoma and basal cell carcinoma.
Genetic Origins and Inheritance
Cartilage-Hair Hypoplasia is caused by mutations in the RMRP gene. This gene provides instructions for making a component of the enzyme RNase MRP. This enzyme is involved in several cellular processes, including processing ribosomal RNA for cell growth and mitochondrial DNA replication.
CHH follows an autosomal recessive inheritance pattern. An individual must inherit two mutated copies of the RMRP gene, one from each parent, to have the condition. Parents who carry one mutated copy are known as carriers and do not show symptoms. When two carriers have a child, there is a 25% chance with each pregnancy that the child will have the disorder.
The prevalence of CHH is not uniform, being found more frequently in specific groups due to the founder effect. This occurs when a small group, some carrying a specific genetic mutation, establishes a new community. The condition is more common among the Old Order Amish and people of Finnish descent, where RMRP gene mutations are higher than in the general population.
Diagnosis and Medical Management
Diagnosis of Cartilage-Hair Hypoplasia begins with observing its characteristic features, such as short-limbed dwarfism, fine hair, and recurrent infections. Skeletal X-rays are used to identify abnormalities in the long bones and ribs. The diagnosis is confirmed through genetic testing that identifies mutations in the RMRP gene.
Managing CHH requires a multidisciplinary team of specialists. Orthopedic care is central to managing skeletal aspects, including monitoring bone growth, joint stability, and spinal alignment. Surgical interventions may be considered for issues like severe bowed legs that cause pain or difficulty walking.
Immunological care is another focus, involving preventative measures and prompt treatment for infections. For severe immunodeficiency, a hematopoietic stem cell transplant may be considered to rebuild the immune system. Regular surveillance for cancer is also recommended, with routine screenings for skin cancer and lymphoma.
Long-Term Outlook and Daily Life
With consistent medical care, most individuals with Cartilage-Hair Hypoplasia have a normal or near-normal life expectancy. The primary factors that influence this outlook are the severity of the immunodeficiency and the potential development of malignancies.
The average adult height for males with CHH is between 3 feet 7 inches and 4 feet 11 inches, while females reach heights between 3 feet 5 inches and 4 feet 6 inches. Despite their short stature, individuals with CHH have normal intelligence and achieve typical developmental milestones.
Many people with CHH lead full and productive lives. While joint issues or skeletal differences may require physical adaptations, these challenges do not preclude an active lifestyle. Supportive networks and adaptive strategies contribute to a high quality of life as the focus of care shifts to maintaining health and function.