Klinefelter syndrome is a disorder in which male infants are born with an extra X chromosome (XXY).
Klinefelter syndrome is relatively common and occurs in about 1 in every 1,000 male births. Most boys inherit the extra X chromosome from their mother. Most boys with Klinefelter 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. They may also have wider hips but otherwise have a relatively normal 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). Males with the syndrome are usually infertile. Men with Klinefelter syndrome tend to 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 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. However, many men are diagnosed only during an infertility assessment. Earlier in life, Klinefelter syndrome is usually only diagnosed incidentally when chromosome analysis is done for another reason. For example, amniocentesis (see Amniocentesis) may have been done to obtain cells from the fetus for a karyotype analysis or chorionic villus sampling (see see Chorionic Villus Sampling) may have been done if the mother was 35 years old or older.
Boys with Klinefelter syndrome usually benefit from speech and language therapy and eventually can do well in school. Males with Klinefelter syndrome benefit by taking supplemental testosterone for life beginning at puberty. The hormone improves bone density, making fractures less likely, and stimulates development of a more masculine appearance. Hormone replacement therapy may help lessen some behavior problems.
Last full review/revision April 2013 by Nina N. Powell-Hamilton, MD