Achondroplasia

Definition

Achondroplasia is the most common cause of dwarfism, or significantly abnormal short stature.

Description

Achondroplasia is one of a number of chondodystrophies, in which the development of cartilage, and therefore, bone is disturbed. The disorder appears in approximately one in every 10,000 births. Achondroplasia is usually diagnosed at birth, owing to the characteristic appearance of the newborn.

Normal bone growth depends on the production of cartilage (a fibrous connective tissue). Over time, calcium is deposited within the cartilage, causing it to harden and become bone. In achondroplasia, abnormalities of this process prevent the bones (especially those in the limbs) from growing as long as they normally should, at the same time allowing the bones to become abnormally thickened. The bones in the trunk of the body and the skull are mostly not affected, although the opening from the skull through which the spinal cord passes (foramen magnum) is often narrower than normal, and the opening (spinal canal) through which the spinal cord runs in the back bones (vertebrae) becomes increasingly and abnormally small down the length of the spine.

Causes & symptoms

Achondroplasia is caused by a genetic defect. It 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).

People with achondroplasia have abnormally short arms and legs. Their trunk is usually of normal size, as is their head. The appearance of short limbs and normal head size actually makes the head appear to be oversized. The bridge of the nose often has a scooped out appearance termed "saddle nose." The lower back has an abnormal curvature, or sway back. The face often displays an overly prominent forehead, and a relative lack of development of the face in the area of the upper jaw. Because the foramen magnum and spinal canal are abnormally narrowed, nerve damage may occur if the spinal cord or nerves become compressed. The narrowed foramen magnum may disrupt the normal flow of fluid between the brain and the spinal cord, resulting in the accumulation of too much fluid in the brain (hydrocephalus). Children with achondroplasia have a very high risk of serious and repeated middle ear infections, which can result in hearing loss. The disease does not affect either mental capacity, or reproductive ability.

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.

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.

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.

Prevention

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

Key Terms

Cartilage

A flexible, fibrous type of connective tissue which serves as a base on which bone is built.

Foramen magnum

The opening at the base of the skull, through which the spinal cord and the brainstem pass.

Hydrocephalus

An abnormal accumulation of fluid within the brain. This accumulation can be destructive by pressing on brain structures, and damaging them.

Mutation

A new, permanent change in the structure of a gene, which can result in abnormal structure or function somewhere in the body.

Spinal canal

The opening that runs through the center of the column of spinal bones (vertebrae), and through which the spinal cord passes.

Vertebrae

The individual bones of the spinal column which are stacked on top of each other. There is a hole in the center of each bone, through which the spinal cord passes.

Further Reading

For Your Information

Books

• Hall, Bryan D. "Achondroplasia." In Nelson Textbook of Pediatrics, edited by Richard Behrman. Philadelphia: W.B. Saunders Co., 1996.
• Horton, W.A., and J.T. Hecht. "The Chondrodysplasias." In Connective Tissue and its Heritable Disorders, edited by R.M. Royce and B. Steinmann. Somerset, NJ: Wiley-Liss, 1993.
• Krane, Stephen M. and Alan L. Schiller. "Achondroplasia." In Harrison's Principles of Internal Medicine, edited by Anthony S. Fauci, et al. New York: McGraw-Hill, 1998.
• Whyte, Michael, P. "Achondroplasia." In Cecil Textbook of Medicine, edited by J. Claude Bennett and Fred Plum. Philadelphia: W.B. Saunders, 1996.

  Organizations

• Little People of America, c/o Mary Carten. 7238 Piedmont Drive, Dallas, TX 75227-9324. (214)388-9576 (Voice). (800)243-9273 (Helpline).

Gale Encyclopedia of Medicine. Gale Research, 1999.