Gigantism or giantism, (from Greek gigas, gigantas "giant") is a condition characterized by excessive height growth and bigness significantly above average height. As a medical term, gigantism is rarely used except to refer to the rare condition of pituitary gigantism due to prepubertal growth hormone excess. There is no precise definition of the degree of height that qualifies a person to be termed a "giant." The term has been typically applied to those whose height is not just in the upper 1% of the population but several standard deviations above mean for persons of the same sex, age, and ethnic ancestry. Typical adult heights of Americans of European descent to whom the term might be applied are 2.25 - 2.40 metres (7'6" - 8 feet). The term is not applied to those whose heights appear to be the healthy result of normal genetics and nutrition.
Pituitary growth hormone excess is the single condition that accounts for nearly all cases of pathologic extreme height. The excess growth hormone usually results from oversecretion by a group of somatotrope cells of the anterior pituitary gland (termed a "somatotrope adenoma"). These cells do not respond to normal controls of growth or function. They grow very slowly, so that for many years the only effects of such an adenoma are the secretion of excessive growth hormone. Over decades, such an adenoma may reach a large enough size (20 mm or more in diameter) to cause headaches, impair vision, or damage other pituitary functions. Many years of growth hormone excess can cause other problems as well.
The primary effect of growth hormone excess in childhood is excessive growth, but the extreme height is accompanied by a characteristic physique recognizable to an endocrinologist. The typical physique involves heavy, thick bones, with large hands and feet and a heavy jaw. Once puberty is complete and adult height is achieved, continued thickening of the skin and growth of the jaw results in a combination of features referred to as acromegaly.
If a physician suspects pituitary gigantism or acromegaly, the simplest diagnostic screening test is measurement of insulin-like growth factor 1 in the blood. This is usually quite elevated but levels must be interpreted in relation to age and pubertal status. Additional confirmatory testing may include magnetic resonance imaging (MRI) of the pituitary to look for a visible adenoma, and suppressibility of growth hormone levels by glucose. Treatment depends on the size of the adenoma and may involve removal by a neurosurgeon, drugs such as octreotide or bromocriptine, or radiation. Treatment is discussed in more detail in the acromegaly article.
Childhood pituitary gigantism is a rare condition, and those affected are often unusual enough to attain a degree of celebrity status (for example, André the Giant and The Great Khali). Acromegaly is the term used for the condition of growth hormone excess when it occurs in adults. Acromegaly is a far more common disease in adults than pituitary gigantism is in children.
Children who are excessively tall are often referred to as Giantigionists. The majority of children who seem excessively tall or large to their parents usually have a combination of simple familial tallness and childhood obesity.
Early onset of obesity results in above-average growth in mid-childhood, such that over half of overweight children have heights in the 70 - 99 percentile range at around 10 years of age. The adult heights achieved by these children are what would be expected from their families because the excess mid-childhood growth is offset by attenuation of the pubertal growth spurt.
Precocious puberty and a variety of conditions associated with excessive amounts of testosterone or estrogen in childhood will result in tallness by mid-childhood. However, the acceleration of bone maturation by the early rise of estradiol results in early completion of growth, and adult heights for these children may actually be below average for genetic potential.
Extra sex chromosomes (beyond the normal two) with therefore extra copies of the SHOX gene (beyond the normal two) usually results in enhancement of height growth. The most common of these karyotypes are 47,XXY (Klinefelter syndrome), 47,XYY, and 47,XXX. The added height increment is usually modest.
Hypogonadism is the condition of deficiency of sex hormones due to reduced function of the testes or ovaries at adolescence. When secretion of testosterone or estradiol remains below average throughout the teenage years, a taller adult height will be gradually achieved by extra growth of the arms and legs. This long-limbed tallness is termed "eunuchoid" tallness, but rarely adds more than 25 mm - 75 mm (1-3 in) to adult height. The extra growth is prevented if the child is given appropriate replacement of testosterone or estrogen from early adolescence.
A very rare but more extreme version of "eunuochoid" tallness occurs when a mutation of the estrogen receptor reduces the response of the bones to estradiol. Estradiol is a byproduct of testosterone in both males and females, and is the most potent accelerator of bone maturation and closure known. If a person fails to respond to estrogen, growth can continue until late-20s or longer, and the affected person can reach 8 feet or more in height. Estrogen resistance is the only other endocrine condition that can rival growth hormone excess in producing gigantism. In contrast, the tallness associated with the more common androgen insensitivity syndrome averages only a few inches, as estradiol is not produced directly but rather through conversion from androgens by aromatase.
Marfan syndrome is an uncommon genetic disease due to an inherited defect of connective tissue. In addition to moderate tallness, persons with this condition usually have a slender body build with unusually long fingers (arachnodactyly). Many can also develop a dislocaton of the lens of the eye or, more seriously, a progressive deterioration of the walls of the aorta which can result in sudden death in adulthood. It is usually inherited as an autosomal dominant trait.
Sotos syndrome resembles acromegaly in its mild distortion of facial growth. In addition to tallness, the chief characteristics are large head size, slow development, and autosomal-dominant inheritance.
There are about 50 even rarer genetic syndromes in which childhood growth is above average. These conditions are often associated with developmental delay or other more serious problems, and adult height may or may not be mildly increased.
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