Cartilage Hair Hypoplasia (CHH) is also known as metaphyseal dysplasia, McKusick type. The disorder was recognized as a clinical entity in 1965, when Victor McKusick and colleagues described the condition in an Amish population. The term “metaphyseal” relates to the metaphysis, which is the wide region located at the ends of long bones. The name “Cartilage Hair Hypoplasia” was used because of the characteristic features: fine, sparse hair and cartilage abnormalities.
It is more common in families with Old-world Amish and Finnish ancestors. Among Amish people, the incidence is approximately 1.5 in 1000 live births, and in Finland, it is 1 in 18,000 to 23,000.
Learn more about treatment options.
This dysplasia is caused when an individual has mutations in both copies of their RMRP gene. This gene encodes the RNA component of the ribonuclease mitochondrial RNA processing complex.
Cartilage Hair Hypoplasia is typically inherited in an autosomal recessive manner.
Talking to a genetic counselor can help families understand how CHH is inherited.
The physical characteristics of cartilage hair hypoplasia include a short limbed form of disproportionate short stature with fine, sparse hair. Intelligence is typically average.
The major radiographic features in infancy include shortened tubular bones. Slight anterior angulation of the sternum is also characteristic. The radiographic features in children and adults include short, flared and irregular metaphyses of tubular bones. These changes are typically more prominent in the knee region than at the hips. Coxa vara has been noted, but genu varum is more common. The fibula is often disproportionately long, most notably at the ankle, and can lead to ankle varus.
In infancy, diagnosis of cartilage hair hypoplasia can be difficult. It is often not until 9 to 12 months of age that the features become apparent. X-rays provide the greatest insight. Widened metaphyses, short long bones, elongated fibulae, and anterior angulation of the sternum are all signs of CHH. Hair hypoplasia can also contribute a clue to the diagnosis; but having typical hair does not exclude the diagnosis of CHH.
Testing of the RMRP gene can confirm the diagnosis molecularly. Important to note that this gene is non-coding, therefore mutations could be missed in whole exome sequencing.
Newborn screening for severe combined immunodeficiency (SCID) could detect some individuals with CHH.
Scoliosis is typical of cartilage hair hypoplasia. Depending on the degree of curvature, it can be managed through observation, bracing, or surgery.
Intestinal malabsorption can occur in early childhood. Hirschsprung disease occurs in approximately 10% of infants with CHH. Surgical intervention is often necessary.
About half of younger children with CHH experience recurrent infections, especially respiratory tract infections. Children with CHH have been reported to be unusually susceptible to chicken pox, and have reactions to the varicella live vaccine.
About 10% of individuals with CHH have been known to develop autoimmune conditions as well.
Individuals have a predisposition to cancer, especially non-Hodgkin’s lymphoma and basal cell carcinoma.
Anemia can occur in varying degrees in childhood.
It is vital to keep a close watch for the possibility of serious infection or malignancy. Varicella and other live virus vaccinations should be avoided until consultation with immune specialist.
Nemours has a number of service regions. Selecting your region will help us show you the right contact information and the most relevant content for you.