There are more than 200 different conditions that cause dwarfism, which can be divided into two broad categories called proportionate dwarfism and disproportionate dwarfism. A person with proportionate dwarfism, which is most commonly caused by disorders in hormone production, is smaller than average but has the same relative physical proportions as someone without dwarfism. In disproportionate dwarfism, which is usually caused by disorders affecting bone or cartilage development, the relative sizes of different parts of the person's body are different from those of a nondwarf. For instance, a person with disproportionate dwarfism may have a normal-sized torso but unusually short limbs. Prominent types of dwarfism include hypochondroplasia, diastrophic dysplasia, and growth hormone deficiency.
All types of dwarfism cause short stature, usually defined as a height of 4 feet 10 inches (about 1.5 m) or less, but they otherwise vary widely in their specific effects. Most dwarfism is genetic, either inherited from a parent or as the result of a new genetic mutation. Dwarfism can also be the result of other health problems in childhood, including congenital hypothyroidism, growth hormone deficiency, or the stress caused by severe ongoing psychological trauma. Short stature can also be the result of normal heredity in the children of short parents rather than any disorder or health problem, but this is not usually classified as dwarfism.
Achondroplasia, which is by far the most common form of dwarfism and accounts for 70 percent of cases, is a genetic disorder that causes overproduction of a protein involved in the regulation of bone growth called fibroblast growth factor receptor three. The excess interferes with bone development, resulting in disproportionately short arms and legs, an average adult height of 4 feet 3.5 inches (about 1.3 m) in men and 4 feet 0.5 inches (about 1.2 m) in women, and characteristic skeletal features including shortened fingers and an unusually prominent forehead. Small children with achondroplasia have an increased risk of death from central or obstructive apnea and often suffer from hypotonia and delayed development of motor skills, but people with achondroplasia have normal intelligence and can live out the same lifespan as people without the condition.
Another form of dwarfism caused by a mutation in the fibroblast growth factor receptor three gene is hypochondroplasia, also called atypical achondroplasia. Hypochondroplasia produces skeletal abnormalities similar to those caused by achondroplasia, but they are usually less pronounced. Children with hypochondroplasia are less likely to suffer from the delayed motor development and respiratory problems common in children with achondroplasia. Hypochondroplasia may cause an increased risk of mild mental retardation, although this is still controversial among researchers and most people with hypochondroplasia have unimpaired intelligence.
Diastrophic dysplasia is another type of disproportionate dwarfism caused by problems with bone and cartilage development. Diastrophic dysplasia results in significantly shortened arms and legs, spinal deformities such as scoliosis, and problems with joint mobility. About half of children with diastrophic dysplasia are born with a cleft palate, and deformities of the ears are common. Intelligences is unaffected, but people with diastrophic dysplasia often have mobility and motor control problems. Diastrophic dysplasia is similar to a condition called atelosteogenesis, type two, but the latter is much more severe and causes stillbirth or death in infancy.
Growth hormone deficiency is a common cause of proportionate dwarfism. If a child's body does not produce enough of the proteins necessary for growth, growth will slow down and developmental milestones in motor control may be delayed. Growth hormone deficiency can result in an adult height as short as 4 feet (1.2 m). Unlike the types of dwarfism caused by skeletal problems, growth hormone deficiency can be treated in childhood by injecting additional human growth hormones to compensate for the body's own low production.
Insufficient thyroid levels in infancy, called congenital hypothyroidism, can cause dwarfism if left untreated. Adult height in people with untreated congenital hypothyroidism can be as low as 3 feet 4 inches (about 1 m). They can also suffer from mental retardation, infertility, and diminished muscle tone and motor control. Thyroid deficiency can be caused by a genetic defect or by dietary deficiencies such as a lack of iodine.
Most people with dwarfism have normal intelligence, but some conditions that cause dwarfism also cause mental retardation. Besides congenital hypothyroidism, these types of dwarfism include Turner syndrome and some cases of Noonan syndrome. Some types of dwarfism can result in a significantly shortened lifespan due to symptoms such as heart defects, seizures, and breathing problems. For example, Ellis–van Creveld syndrome causes congenital heart defects and respiratory problems that cause about half of sufferers to die in infancy, although those who live to adulthood can live out a normal lifespan. Primordial dwarfism, a term referring to several forms of proportionate dwarfism that begin in the womb, is almost always fatal within the first three decades of life.