Pituitary dwarfism

Encyclopedia of Medicine by Tish Davidson

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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.