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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.

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Causes of Cartilage Hair Hypoplasia

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.

How Is Cartilage Hair Hypoplasia Inherited?

Cartilage Hair Hypoplasia is typically inherited in an autosomal recessive manner.

Talking to a genetic counselor can help families understand how CHH is inherited.

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Physical Characteristics of Cartilage Hair Hypoplasia

The physical characteristics of cartilage hair hypoplasia include a short limbed form of disproportionate short stature with fine, sparse hair. Intelligence is typically average.

Face and Skull

  • Fine, sparse and light hair
  • Sparse eyebrows and eyelashes

Trunk, Chest and Spine

  • Anterolateral chest deformity
  • Prominent sternum (chest bone)
  • Lumbar lordosis (sway back)

Arms and Legs

  • Rhizomelic shortening of the limbs
  • Limited elbow extension, but otherwise loose joints
  • Short fingers and toes
  • Mild bowing of the legs
  • Flat feet

X-Ray Characteristics

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.

There is minimal craniocaudal widening of interpediculate distance in lumbar spine. Flaring and cupping at the costochondral junction of ribs is also characteristic.

The metacarpals, metatarsals, and phalanges are short, with metaphyseal cupping and cone-shaped epiphyses.

How Cartilage Hair Hypoplasia Is Diagnosed

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.

Related Musculoskeletal Problems

Bowing of Legs

Mild bowing of the legs (genu varum) is characteristic and may require surgical treatment. Subluxation of the hips also may require surgery.

Scoliosis

Scoliosis is typical of cartilage hair hypoplasia. Depending on the degree of curvature, it can be managed through observation, bracing, or surgery.

Other Health Issues That Can Develop

Gastrointestinal

Intestinal malabsorption can occur in early childhood. Hirschsprung disease occurs in approximately 10% of infants with CHH. Surgical intervention is often necessary.

Immunodeficiency

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.

Malignancy

Individuals have a predisposition to cancer, especially non-Hodgkin’s lymphoma and basal cell carcinoma.

Anemia

Anemia can occur in varying degrees in childhood.

What should I watch for with cartilage hair hypoplasia?

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.

References

  1. McKusick, V. A.; Eldridge, R.; Hostetler, J. A.; Egeland, J. A.; Ruangwit, U. Dwarfism in the Amish. II. Cartilage-hair hypoplasia. Bull. Johns Hopkins Hosp. 116: 285-326, 1965.
  2. Mäkitie, Outi. Cartilage-Hair Hypoplasia: Clinic radiological and genetic study of an inherited skeletal dysplasia. University of Helsinki, Finland. 1992.
  3. Riley P Jr, Weiner DS, Leighley B, Jonah D, Morton DH, Strauss KA, Bober MB, Dicintio MS. Cartilage hair hypoplasia: characteristics and orthopaedic manifestations. J Child Orthop. 2015 Apr;9(2):145-52.
  4. Vakkilainen S, Taskinen M, Mäkitie O. Immunodeficiency in cartilage-hair hypoplasia: Pathogenesis, clinical course and management. Scand J Immunol. 2020 Oct;92(4):e12913.
  5. Mäkitie O, Vakkilainen S. Cartilage-Hair Hypoplasia – Anauxetic Dysplasia Spectrum Disorders. 2012 Mar 15 [Updated 2020 Aug 6]. In: Adam MP, Ardinger HH, Pagon RA, et al., editors. GeneReviews® [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2021. Available from: https://www.ncbi.nlm.nih.gov/books/NBK84550/