Klinefelter

Klinefelter syndrome, also known as Klinefelter, is a chromosomal condition that affects males and is characterized by the presence of at least one extra X chromosome. The most common karyotype is 47,XXY, arising from nondisjunction during parental meiosis. A minority of individuals have mosaic forms such as 46,XY/47,XXY. The condition occurs in about 1 in 500 to 1,000 male births.

Typical features include taller stature with relatively long limbs, reduced muscle tone, gynecomastia, and small testes with lowered testosterone levels. Many individuals have normal intelligence, but language-based learning difficulties, later first language acquisition delays, and mild social or executive function challenges may occur. Puberty may be delayed or arrested, and infertility is common due to impaired spermatogenesis; most affected individuals are unable to have biological children. Some men have partial or complete testosterone deficiency, leading to reduced libido, mood changes, and osteoporosis risk.

Diagnosis is usually made in adolescence or adulthood when symptoms are noticed or during infertility evaluation and is confirmed by karyotype testing of peripheral blood.

Management is multidisciplinary. Testosterone replacement therapy can improve secondary sexual characteristics, muscle mass, bone density, and energy; it does not restore fertility but may improve quality of life. Speech and language therapy, reading support, and educational interventions can help with learning difficulties. Assisted reproductive techniques using retrieved sperm may offer a path to biological fatherhood in some mosaic or milder cases. Regular medical follow-up addresses bone health, metabolic risk factors, and mood.

Named after Harry Klinefelter, who first described the condition in 1942.