Dwarfism is a condition of short stature. It is defined by the advocacy group Little People of America (LPA) as an adult height of 4 feet 10 inches or under, as a result of a medical or genetic condition. Although other groups may extend the criteria for certain forms of dwarfism to 5 feet, the average height of an adult with dwarfism is 4 feet.
There are two main categories of dwarfism -- disproportionate and proportionate. Disproportionate dwarfism is characterized by an average-size torso and shorter arms and legs or a shortened trunk with longer limbs. In proportionate dwarfism, the body parts are in proportion but shortened.
Dwarfism can be caused by any of more than 200 conditions. Causes of proportionate dwarfism include metabolic and hormonal disorders such as growth hormone deficiency.
The most common types of dwarfism, known as skeletal dysplasias, are genetic. Skeletal dysplasias are conditions of abnormal bone growth that cause disproportionate dwarfism.
Achondroplasia. The most common form of dwarfism, achondroplasia occurs in about one out of 26,000 to 40,000 babies and is evident at birth. People with achondroplasia have a relatively long trunk and shortened upper parts of their arms and legs. Other features of achondroplasia include:
- a large head with a prominent forehead
- a flattened bridge of the nose
- protruding jaw
- crowded and misaligned teeth
- forward curvature of the lower spine
- bowed legs
- flat, short, broad feet
Spondyloepiphyseal dysplasias (SED). A less common form of dwarfism, SED affects approximately one in 95,000 babies. Spondyloepiphyseal dysplasia refers to a group of conditions characterized by a shortened trunk, which may not become apparent until a child is between 5 and 10 years old. Other features can include:
- club feet
- cleft palate
- severe osteoarthritis in the hips
- weak hands and feet
- barrel-chested appearance
Diastrophic dysplasia. A rare form of dwarfism, diastrophic dysplasia occurs in about one in 100,000 births. People who have it tend to have shortened forearms and calves (this is known as mesomelic shortening).
Other signs can include
- deformed hands and feet
- limited range of motion
- cleft palate
- ears with a cauliflower appearance
Skeletal dysplasia is caused by a genetic mutation. The gene mutation can occur spontaneously or can be inherited.
Diastrophic dysplasia and usually spondyloepiphyseal dysplasias are inherited in a recessive manner. This means a child must receive two copies of the mutated gene -- one from the mother, one from the father -- to be affected.
Achondroplasia, on the other hand, is inherited in a dominant manner. That means a child needs only one copy of the mutated gene to have that form of skeletal dysplasia. There is a 25% chance that a child born to a couple in which both members have achondroplasia will be of normal height. But there is also a 25% chance the child will inherit both dwarfism genes, a condition known as double-dominant syndrome. This is a fatal condition that usually results in miscarriage.