Klinefelter's syndrome is a disorder in which male infants are born with an extra X chromosome (XXY).
Klinefelter's syndrome is relatively common. Most boys with Klinefelter's syndrome have normal or slightly decreased intelligence. Many have speech and reading disabilities and difficulties with planning. Most have problems with language skills. Lack of insight, poor judgment, and impaired ability to learn from previous mistakes often cause these children to get into trouble. Although their physical characteristics can vary greatly, most are tall with long arms and legs but are otherwise normal in appearance.
Puberty usually occurs at the normal time, but the testes remain small. At puberty, growth of facial hair is often sparse, and the breasts may enlarge somewhat (gynecomastia). Men and boys with the syndrome are usually infertile. Men with Klinefelter's syndrome develop diabetes mellitus, chronic lung disease, varicose veins, hypothyroidism, and breast cancer more often than other men.
Some affected boys have 3, 4, and even 5 X chromosomes along with the Y. As the number of X chromosomes increases, the severity of mental retardation/intellectual disability and physical abnormalities also increases. Each extra X is associated with a 15- to 16-point reduction in intelligence quotient (IQ), with language most affected, particularly expressive language skills.
The syndrome is usually first suspected at puberty, when most of the symptoms develop. Analysis of the chromosomes confirms the diagnosis. Many men are diagnosed during an infertility assessment (probably all men with Klinefelter's syndrome are sterile).
Boys with Klinefelter's syndrome usually benefit from speech and language therapy and eventually can do well in school. Some men benefit by taking supplemental testosterone for life. The hormone improves bone density, making fractures less likely, and stimulates development of a more masculine appearance.
Last full review/revision December 2008 by Gregory S. Liptak, MD, MPH