What is dwarfism? What role do genetics play?

Quick Answer
“Dwarfism” in humans is a term used to describe adults who are less than 4 feet 10 inches in height. More than two hundred conditions are known to cause dwarfism, or short stature. Dwarfism is most often caused by genetic conditions, but it can also be related to endocrine malfunction, acquired conditions, or growth hormone deficiency. Individuals with dwarfism usually have normal intelligence and have an average life span. Little people can also give birth to children with normal stature. Dwarfism may result in multiple medical problems that can lead to death. The term “dwarf” is socially unacceptable by most people. Those with dwarfism prefer to be referred to as little people or as having short stature.
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Risk Factors

Having a parent with a form of dwarfism, such as achondroplasia, a parent who carries a mutated FGFR3 gene, or parents of advanced age can increase the risk of inheriting dwarfism. Other abnormalities such as damage or conditions of the pituitary gland, hormonal disorders, problems with absorption, malnutrition, kidney disease, or extreme emotional distress can increase the risk of developing dwarfism.

Etiology and Genetics

Dwarfism, of which there are several hundred forms, occurs in approximately one in every ten thousand births. Approximately 85 percent of little people are born to parents of average height. The most common type of dwarfism, achondroplasia, is an autosomal dominant trait, but in 80 percent of cases it appears in children born to normal parents as a result of mutations in the sperm or egg.

Dwarfisms in which body proportions are normal usually result from metabolic or hormonal disorders in infancy or childhood. Chromosomal abnormalities, pituitary gland disorders, problems with absorption, malnutrition, kidney disease, and extreme emotional distress can also interfere with normal growth. When body parts are disproportioned, the dwarfism is usually due to a genetic defect.

Skeletal dysplasias are the most common causes of dwarfism and are the major cause of disproportionate types of dwarfism. More than five hundred skeletal dysplasias have been identified. Chondrodystrophic dwarfism occurs when cartilage cells do not grow and divide as they should and cause defective cartilage cells. Most chondrodystrophic little people have abnormal body proportions. The defective cells occur only in the spine or only in the arms and legs. Short-limb dwarfism includes individuals with achondroplasia, diastropic dysplasia, and Hunter-Thompson chondrodysplasia.

Achondroplasia is the most common skeletal dysplasia and affects more than 70 percent of all dwarfs. It occurs in every 26,000 to 40,000 babies born of all races and ethnicities. Achondroplasia is caused by an autosomal dominant allele and is identified by a disproportionate short stature consisting of a long trunk and short upper arms and legs. Eighty percent of all cases of achondroplasia result from a mutation on chromosome 4 in a gene that codes for a fibroblast growth factor receptor. Achondroplasia is seen in both males and females, occurs in all races, and affects approximately one in every twenty thousand births. If one parent has achondroplasia and the other does not, then a child born to them would have a 50-percent chance of inheriting achondroplasia. On the other hand, if both parents have achondroplasia, their offspring have a 50 percent chance of inheriting achondroplasia, a 25-percent chance of being normal, and a 25-percent chance of inheriting the abnormal allele from each parent and suffering often fatal skeletal abnormalities. Children who do not inherit the defective gene will never have achondroplasia and cannot pass it on to their offspring, unless a mutation occurs in the sperm or egg of the parents. Geneticists have observed that fathers who are forty years of age or older are more likely to have children with achondroplasia as a result of mutations in their sperm.

Diastrophic dysplasia is a relatively common form of short-limb dwarfism that occurs in approximately one in 100,000 births and is identified by the presence of short arms and calves, clubfeet, and short, broad fingers with a thumb that has a hitchhiker type appearance. Infant mortality can be high as a result of respiratory complications, but if they survive infancy, short-limbed dwarfs have a normal life span. Orthopedic dislocations of joints are common. Scoliosis is seen especially in the early teens, and progressive cervical kyphosis and partial dislocation of the cervical spine eventually cause compression of the spinal cord. Diastrophic dysplasia is an inherited autosomal recessive condition linked to chromosome 5. Parents have a 25-percent chance that each additional child will get diastrophic dysplasia.

Short-trunk dwarfism includes individuals with spondyloepiphyseal dysplasia, which results from abnormal growth in the spine and long bones that leads to a shortened trunk. It occurs in one of every 95,000 births. In spondyloepiphyseal dysplasia tarda, the lack of growth may not be recognized until five to ten years of age. Those affected have progressive joint and back pain and eventually develop osteoarthritis. Spondyloepiphyseal dysplasia congenita is caused by autosomal dominant gene mutations and is evidenced by a short neck and trunk, and barrel chest at birth. It is not uncommon for cleft palate, hearing loss, myopia, and retinal detachment to be present.

Morquio syndrome, which was first described in 1929, is classified as a mucopolysaccharidosis (MPS) disease caused by the body’s inability to produce enzymes that help to break down and recycle dead cells. Consequently, wastes are stored in the body’s cells.

Hunter-Thompson chondrodysplasia is a form of dwarfism caused by a mutation in growth factor genes. Affected individuals have shortened and misshapened bones in the lower arms, the legs, and the joints of the hands and feet. Fingers are shortened and toes are ball-shaped.

Growth hormone, a protein that is produced by the pituitary (“master”) gland, is vital for normal growth. Hypopituitarism results in a deficiency of growth hormone and afflicts between ten thousand and fifteen thousand children in the United States. In panhypopituitarism, the gland does not produce any hormones. The pituitary gland shuts down and growth is stunted.

Turner syndrome affects one in every two thousand female infants and is characterized by the absence of or damage to one of the X chromosomes in most of the cells in the body. Short stature and the failure to develop sexually are hallmarks of Turner syndrome. Learning difficulties, skeletal abnormalities, heart and kidney problems, infertility, and thyroid dysfunction may also occur. Turner syndrome can be treated with human growth hormones and by replacing sex hormones.

Symptoms

For inherited disorders at birth, a long trunk and shortened limbs will be noticeable. A child born with dwarfism may go on to exhibit delayed gross motor development and skills, breathing and neurologic problems, hydrocephalus (water on the brain), increased susceptibility to ear infections and hearing loss, weight problems, curvature of the spine (scoliosis), bowed legs, stiff arms, joint and back pain or numbness, and crowding of teeth. Portions of the face may be underdeveloped. Sleep apnea can develop as a result of compression of the spine. Adult height will be stunted (usually reaching 42-52 inches). Seeking proper medical care can help to relieve some of these symptoms and complications.

Screening and Diagnosis

Close monitoring by parents and doctors is necessary to record the constellation of symptoms for each unique case of dwarfism. Often an initial diagnosis can be made by observing physical characteristics. Magnetic resonance imaging (MRI) and computed tomography (CT) scans can illustrate spinal and other structural abnormalities before serious complications arise. Imaging techniques can also help to determine the type of dwarfism present. Molecular genetic testing can be done to detect a FGFR3 mutation. Genetic testing is 99-percent sensitive and available in clinical laboratories. The Human Genome Project continues to investigate genetic links to dwarfism. Prenatal counseling and screening for traits of dwarfism, along with genetic counseling and support groups, are avenues to pursue for family and individual physical, psychological, and social well-being and to make informed choices.

Treatment and Therapy

Some forms of dwarfism can be treated through state-of-the-art surgical and medical interventions such as bone-lengthening procedures, reconstructive surgery, and growth and sex hormone replacement.

Short stature is the one quality all people with dwarfism have in common. After that, each of the many conditions that cause dwarfism has its own set of characteristics and possible complications. Fortunately, many of these complications are treatable, so that people of short stature can lead healthy, active lives. Continued follow-up with the physician team is essential.

For example, some babies with achondroplasia may experience hydrocephalus (excess fluid around the brain). They may also have a greater risk of developing sleep apnea—a temporary stop in breathing during sleep—because of abnormally small or misshapen airways or, more likely, because of airway obstruction by the adenoids or the tonsils. Occasionally, a part of the brain or spinal cord is compressed. With close monitoring by doctors, however, these potentially serious problems can be detected early and surgically corrected.

Prevention and Outcomes

Genetic counseling as well as family and public education regarding dwarfism and growth problems can bring greater awareness of dwarfism to communities and allow parents to make good choices. Inherited dwarfism is not preventable, but some cases caused by malnutrition, injury, absorption, or kidney conditions may be prevented.

The type, symptoms, and severity of complications vary from person to person, but most little people have an average life span. With a sense of support, self-esteem, and independence, a person with dwarfism can lead a very satisfying and productive life.

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