Dwarfism

Definition

Pituitary dwarfism is a condition of growth retardation in which patients are very short, but have normal body proportions. Some children who have this condition go through delayed, but normal puberty and have normal reproductive capabilities. Others never become sexually mature.

Description

Pituitary dwarfism is caused by a dysfunction of the pituitary gland. The pituitary gland is a pea-sized mass of tissue at the base of the brain. Functionally, it is divided into two parts, the anterior pituitary and the posterior pituitary. The anterior pituitary secretes growth hormone (GH), thyroid-stimulating hormone (TSH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), adrenocorticotropic hormone (ACTH), and prolactin. The posterior pituitary releases antidiuretic hormone (ADH) and oxytocin.

There are two types of pituitary dwarfism:

• Panhypopituitarism due to a deficit of all of the anterior pituitary hormones. There is proportionate, generalized slow growth and patients do not go through puberty. This accounts for about 2/3 of pituitary dwarfism cases.
• Pituitary dwarfism due to an isolated deficiency of GH. This accounts for about 1/3 of cases. These perfectly proportioned patients do mature sexually and may reproduce.

There is a subgroup of the second type of pituitary dwarfism listed above. In some cases there is normal or even high GH secretion, but there is a hereditary inability to form somatomedin-C (also called insulin-like growth factor-1 or IGF-1) in response to the GH. African pygmies are an example of this type of pituitary dwarfism. At least four somatomedins are produced by the liver in response to GH and are responsible for the actual growth of bone and tissues.

In both types of pituitary dwarfism, height is stunted, but so is growth in the rest of the body, resulting in a perfectly proportioned little person. For this reason, pituitary dwarfism is sometimes called proportionate dwarfism.

Pituitary dwarfism is a rare disease. It occurs only in children. Low hormonal output from the pituitary in adults produces different disorders. There are many reasons why children do not attain heights within a normal range. These include normal hereditary factors (short parents), poor nutrition, inherited and congenital skeletal diseases, and diseases of the kidney and heart. Only about 15% of serious growth retardation is caused by failures in the endocrine system.

Pituitary dwarfism with sexual maturity does run in families and has been linked to an inherited recessive gene. However, in most cases where children fail to attain a normal height, it is not possible to identify a specific pituitary or genetic disorder. Children who are born with cleft palates, who suffer serious head trauma, severe environmental deprivation, tumors of the pituitary gland, brain infections, or bleeding in the brain, are all more likely to have pituitary dwarfism. Endocrinologists (doctors who specialize in the functioning of hormones) have the most experience in diagnosing and treating this disorder.

Causes & symptoms

An absence of GH hormone is at the center of most cases of pituitary dwarfism. Human GH, also called somatotropin, is produced in the anterior pituitary in response to the growth hormone-releasing hormone (GHRH). GHRH is manufactured by the hypothalamus. The hypothalamus is part of the base of the brain.

At intermittent intervals, GHRH arrives at the anterior pituitary and stimulates it to release GH into the circulatory system. GH is then carried to the liver and the hormone causes the release of IGF-1. IGF-1 then acts directly on the ends of the long bones of the body causing them to grow. This process continues roughly through adolescence until the adult height is reached. Growth can be interrupted if any of these hormones, or the cells they act on, are absent or defective.

When there is a deficit of only GH, children grow very slowly and reach sexual maturity long after their peers. These children are below the third percentile in height, and their rate of growth is less than 1.5 in (4 cm) per year. By age 18, boys reach an average height of 51 in (130 cm) compared with a normal average height of 64 in (162 cm). These children usually have bone ages that are more than two years behind their chronological ages. GH deficiency is hereditary in about 10% of pituitary dwarfism cases. It is also sometimes seen in children with midline defects such as cleft palate.

Children who are deficient in additional hormones secreted by the pituitary gland grow very slowly, but also have many other medical disorders. These include problems in metabolic regulation and water balance, and failure to develop secondary sexual characteristics. Children who are deficient in multiple pituitary hormones may have a pituitary tumor or may have suffered a serious head injury or brain infection.

Severe emotional deprivation coupled with abuse, neglect, and isolation can sometimes cause the hypothalamus to stop producing GHRH. Normal growth then stops, but the growth rate quickly returns to normal when the child is removed from the deprived environment.

Diagnosis

Careful recording of a child's height and weight at regular time periods will show if a child's growth rate is below normal. Initially, the doctor might do an x ray of the hand to determine the child's bone age and compare it to the child's chronological age. Children with pituitary dwarfism have bones that look less mature than one would expect for their age. However, this only indicates a pituitary problem, and extensive testing by specialists is needed to make a diagnosis of pituitary dwarfism.

A GH stimulation test is also done when a child's growth is seriously retarded. This is a blood test that measures changes in the level of GH after the endocrine system is stimulated with the amino acid arginine. A baseline blood sample is drawn, then additional blood samples are taken after arginine is injected. The results are available in two days. If the level of GH does not increase with arginine stimulation, pituitary dwarfism is suspected.

Once GH deficiency has been diagnosed, additional blood and urine tests are done to determine the levels of other pituitary hormones. Tests can also measure the level of IGF-1, but it is often impossible to determine the exact reason why children fail to grow. Additional tests that may aid the doctor in diagnosis include a computed tomography scan (CT scan) of the head, magnetic resonance imaging (MRI) of the head, and x rays of the skull.

Treatment

Treatment for pituitary dwarfism focuses on replacement GH therapy for children with documented GH deficiency. Until the mid-1980s, the only source of human GH was from pituitary glands collected from human corpses. Animal GH did not produce an effect in humans. The treatment was extremely expensive and recipients of the replacement hormones ran the risk of developing Creutzfeldt-Jakob disease, a fatal neurological infection.

Today, synthetic human GH has been created using recombinant DNA technology. GH derived from human sources has been outlawed. The synthetic GH does not transmit Creutzfeldt-Jakob disease. Treatment, however, is still expensive.

The most effective treatment schedule of GH replacement in children has not been fully established. Growth of 4-6 in (10-15 cm) frequently occurs in the first year of treatment. After that, the rate tends to slow down, and the dose of GH may need to be increased. Replacement GH therapy must be started early, while the long bones can still grow. For this reason, replacement GH therapy is ineffective in adults.

When other pituitary hormones are also missing, treatment becomes a complex balancing act, where combinations of GH and the other missing hormones are replaced. These combinations try to alleviate the patient's symptoms of short stature, metabolic and fluid imbalances, and sexual immaturity. As the child grows, the dosages of replacement hormones are frequently changed to reflect the child's changing metabolic state.

Prognosis

Untreated pituitary dwarfs who lack only GH grow to heights under 5 ft, but are capable of normal reproduction. With childhood replacement of GH, normal growth rates and heights are usually achieved. The long term effectiveness of GH replacement therapy is still being studied. Research in this area continues, and gene replacement therapy may be a possible solution to some forms of pituitary dwarfism.

Untreated pituitary dwarfs who lack multiple pituitary hormones often die. The success of multiple hormone replacement therapy depends on which hormones are absent, the severity of the deficit, and the age at which replacement begins. These children usually have serious medical problems throughout their lives.

Prevention

Most cases of pituitary dwarfism are not preventable. The exception is pituitary dwarfism brought about by environmental conditions. Preventing children from living in severely neglectful conditions will prevent this cause of dwarfism.

Key Terms

Adrenocorticotropic hormone (ACTH)

An anterior pituitary hormone that acts on the cells of the adrenal cortex, causing these cells to produce male sex hormones and hormones that control water and mineral balance in the body.

Antidiuretic hormone (ADH)

A posterior pituitary hormone (also called vasopressin) that acts on the kidneys to help control the fluid balance in the body.

Arginine

An amino acid that is an essential component of many proteins made by the body. Injecting arginine into the circulatory system will cause a temporary rise in the production of growth hormone in normal individuals.

Follicle-stimulating hormone (FSH)

An anterior pituitary hormone that, in females, causes the release of estrogens and, in males, is one of the hormones that stimulates sperm production.

Hormone

A substance secreted by one organ in the body (such as the pituitary gland) that is carried by the circulatory system to another place in the body, to produce an effect on a specific organ or tissue.

Luteinizing hormone (LH)

An anterior pituitary hormone that, in females, helps regulate the menstrual cycle and triggers ovulation and, in males, stimulates the cells of the testes to secrete testosterone.

Oxytocin

A posterior pituitary hormone that stimulates the uterus to contract during birthing and causes the breasts to release milk when a baby begins to suckle.

Prolactin

An anterior pituitary hormone that prepares the breasts to produce milk during pregnancy.

Thyroid-stimulating hormone (TSH)

An anterior pituitary hormone (also called thyrotropin) that stimulates the thyroid gland to produce hormones that regulate the rate of cellular metabolism.

Further Reading

For Your Information

Organizations

• Human Growth Foundation. 7777 Leesburg Pike, Suite 202S, Falls Church, VA 22043. (800) 451-6434 or (703) 883-1773. http://www.genetic.org.
• Little People's Research Fund, Inc. 80 Sister Pierre Dr., Towson, MD 21204-7534. (800) 232-5773 or (410) 494-0055.
• MAGIC Foundation for Children's Growth. 1327 N. Harlem Ave., Oak Park, IL 60302. (800) 3 MAGIC 3 or (708) 383-0808. http://www.magicfoundation.org.

Gale Encyclopedia of Medicine. Gale Research, 1999.