Achondrogenesis type 1A

Synonyms

3

Overview

Achondrogenesis is a group of severe disorders that affect cartilage and bone development. These conditions are characterized by a small body, short limbs, and other skeletal abnormalities. As a result of serious health problems, infants with achondrogenesis usually die before birth, are stillborn, or die soon after birth from respiratory failure. However, some infants have lived for a short time with intensive medical support.

Researchers have described at least three forms of achondrogenesis, designated as type 1A, type 1B, and type 2. The types are distinguished by their signs and symptoms, inheritance pattern, and genetic cause. However, types 1A and 1B are often hard to tell apart without genetic testing.

Achondrogenesis type 1A, which is also called the Houston-Harris type, is the least well understood of the three forms. Type 1A involves abnormal cartilage-forming cells (chondrocytes) and affected infants have extremely short limbs, a narrow chest, short ribs that fracture easily, and a lack of normal bone formation (ossification) in the skull, spine, and pelvis.

Symptoms

  • Facial abnormalities (flat face, protruding eyes and protruding tongue or only minor facial anomalies)
  • Short trunk and limbs
  • Short beaded ribs 
  • Thin skull bones (deficient ossification of the skull)
  • Bone formation is abn
  • Abnormal in the spine, pelvis and extremities, but the degree of the severity of skeletal involvement may be variable
  • Small thorax leads to underdevelop
  • Underdevelopment of lungs 
  • Death soon after birth
  • Abnormality of bone mineral density
  • Anteverted nares
  • Aplasia/Hypoplasia of the lungs
  • Frontal bossing
  • Hydrops fetalis
  • Long philtrum
  • Macrocephaly
  • Malar flattening
  • Micromelia
  • Narrow chest
  • Short Neck, Nose, Thorax
  • Skeletal dysplasia
  • Thickened nuchal skin fold

Causes

Achondrogenesis types 1A and 1B are rare genetic disorders; their incidence is unknown. Combined, achondrogenesis type 2 and hypochondrogenesis (a similar skeletal disorder) occur in 1 in 40,000 to 60,000 newborns.

Three forms of achondrogenesis, type 1A, type 1B and type 2, are distinguished by signs and symptoms, pattern of inheritance, and the results of imaging studies such as x-rays (radiology), tissue analysis (histology), and genetic testing. Type 1A and 1B achondrogenesis are both inherited in an autosomal recessive pattern.

Achondroplasia is a dominant trait, meaning that anybody with the genetic defect will display all the symptoms of the disorder. A parent with the disorder has a 50% chance of passing it on to the offspring. Although achondroplasia can be passed on to subsequent offspring, the majority of cases occur due to a new mutation (change) in a gene. Interestingly enough, the defect seen in achondroplasia is one of only a few defects known to increase in frequency with increasing age of the father (many genetic defects are linked to increased age of the mother).

Prevention

The only form of prevention is through genetic counseling, which could help parents assess their risk of having a child with achondroplasia.

Diagnosis

Diagnosis is often made at birth due to the characteristically short limbs, and the appearance of a large head. X-ray examination will reveal a characteristic appearance to the bones, with the bones of the limbs appearing short in length, yet broad in width. A number of measurements of the bones in x-ray images will reveal abnormal proportions

Prognosis

Achondroplasia is a disease which causes considerable deformity. However, with careful attention paid to the development of dangerous complications (nerve compression, hydrocephalus), most people are in good health, and can live a normal lifespan.

Treatment

No treatment will reverse the defect present in achondroplasia. All patients with the disease will be short, with abnormally proportioned limbs, trunk, and head. Treatment of achondroplasia primarily addresses some of the complications of the disorder, including problems due to nerve compression, hydrocephalus, bowed legs, and abnormal curves in the spine. Children with achondroplasia who develop middle ear infections (acute otitis media) will require quick treatment with antibiotics and careful monitoring in order to avoid hearing loss.

Resources

  • NIH
  • Genetics Home Reference