Hereditary Musculoskeletal Disorders: Osteochondrodysplasias

Description

Osteochondrodysplasias are a group of hereditary disorders in which bone or cartilage grows abnormally, so that the skeleton develops abnormally. Many types of osteochondrodysplasia cause dwarfism. Achondroplasia is the most common of the many types of short-limbed dwarfism.
Achondroplasia is characterised by abnormal bone growth that results in short stature with disproportionately short arms and legs, a large head, and characteristic facial features. Intelligence and life span are usually normal, although an increased risk is present for death in infancy from compression of the spinal cord and/or upper airway obstruction. Achondroplasia is the most common form of inherited disproportionate short stature. It occurs in one in 15,000 to one in 40,000 live births.

Symptoms and Signs

Individuals with achondroplasia have short stature caused by rhizomelic shortening of the limbs, characteristic faces with frontal bossing and mid-face hypoplasia, exaggerated lumbar lordosis, limitation of elbow extension and rotation, bow legs , and trident appearance of the hands. Hyperextensibility of the knees and most other joints is common. Intelligence is usually normal. Average adult height for men with achondroplasia is 131 5.6 cm; for women, 124 5.9 cm. Obesity is a major problem in achondroplasia.
The most common medical complaint in adulthood is symptomatic spinal stenosis , which may require surgical intervention ( laminectomy ).

In infancy, mild to moderate hypotonia is usual, and acquisition of developmental motor milestones is often delayed. Infants have difficulty in supporting their heads due to both hypotonia and large head size. True megalencephaly occurs in individuals with achondroplasia, but intelligence is normal unless hydrocephalus or other central nervous system complications occur. The large head of the newborn with achondroplasia creates an increased risk of intracranial bleeding during vaginal delivery . Hydrocephalus may be caused by increased intracranial venous pressure due to stenosis of the sigmoid sinus at the level of the narrowed jugular foramina. Recurring otitis media is frequently a problem. As many as 7.5% of infants with achondroplasia die in the first year of life from obstructive apnea or central apnea.

Diagnosis and Treatment

Achondroplasia can be diagnosed by characteristic clinical and radiographic findings in most affected individuals. In individuals who may be too young to diagnose with certainty or in atypical cases, DNA-based testing can be used to detect a mutation in the FGFR3 gene (chromosomal locus 4p16). Such testing is 99% sensitive and is widely available. Molecular genetic testing can be helpful in the diagnosis of newborns and atypical cases and in prenatal testing.

Recommendations for management of children with achondroplasia were outlined by the American Academy of Pediatrics, Committee on Genetics. These recommendations of the committee are meant to supplement guidelines available for treating the child with average stature. The recommendations include, but are not limited to, the following measures:

• Monitoring height, weight, and head circumference using growth curves standardised for achondroplasia
• Measures to avoid obesity starting in early childhood
• Careful neurological examinations, with referral to a paediatric neurologist as necessary
• MRI or CT of the foramen magnum region for evaluation of severe hypotonia or signs of spinal cord compression
• Obtaining history for possible sleep apnea, with sleep studies as necessary
• Evaluation for low thoracic or high lumbar gibbus if truncal weakness is present
• Referral to a paediatric orthopaedist if bowing of the legs interferes with walking
• Management of frequent middle ear infections
• Speech evaluation by two years of age
• Careful monitoring of social adjustment

Treatment of obstructive sleep apnea may include adenotonsillectomy, weight reduction, continuous positive airway pressure (CPAP) by nasal mask, and trachesostomy for extreme cases. Improvement in disturbed sleep and some improvement in neurological function can result from these interventions.
Growth hormone (GH) therapy has been proposed as a possible treatment for the short stature of achondroplasia. Surgical limb lengthening procedures may also be used, although the complications are multiple.

Genetic counselling may be of use to those with this disorder.

The information in this page is presented in summarised form and has been taken from the following source(s):
1. Geneclinics at the University of Washington, Seattle: http://www.geneclinics.org/