Klinefelter's syndrome history and symptoms
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There are many variances within the XXY population, just as within the 46,XY population. While it is possible to characterise XXY males with certain body types and physical characteristics, that in itself should not be the method of identification as to whether or not someone has XXY. The only reliable method of identification is karyotype testing. The degree to which XXY males are affected, both physically and developmentally, differs widely from person to person.
- As babies and children, XXY males may have weaker muscles and reduced strength. As they grow older, they tend to become taller than average. They may have less muscle control and coordination than other boys their age.
- During puberty, the physical traits of the syndrome become more evident; because these boys do not produce as much testosterone as other boys, they have a less muscular body, less facial and body hair, and broader hips. As teens, XXY males may have larger breasts, weaker bones, and a lower energy level than other boys.
- By adulthood, XXY males look similar to males without the condition, although they are often taller. In adults, possible characteristics vary widely and include little to no signs of affectedness, a lanky, youthful build and facial appearance, or a rounded body type with some degree of gynecomastia (increased breast tissue). Gynecomastia is present to some extent in about a third of affected individuals, a slightly higher percentage than in the XY population. About 10% of XXY males have gynecomastia noticeable enough that they may choose to have cosmetic surgery.
- Affected males are almost always effectively sterile, although advanced reproductive assistance is sometimes possible.
- Some degree of language learning impairment may be present, and neuropsychological testing often reveals deficits in executive functions although these deficits can often be successfully ameliorated through early intervention. 
- There may also be delays in motor development. XXY males may sit up, crawl, and walk later than other infants; they may also struggle in school, both academically and with sports.
- Abstract of Klinefelter, HF (1986). "Klinefelter syndrome: historical background and development". South Med J. 79 (9): 1089–1093. doi:10.1097/00007611-198609000-00012. PMID 3529433. provides information on microorchidism (small testes), hypogonadism (infertility/sterility and androgen hormone function) and gynecomastia. Bock, Robert (1993). "Understanding Klinefelter Syndrome: A Guide for XXY Males and Their Families". NIH Pub. No. 93-3202. Office of Research Reporting, NICHD. Retrieved 2007-03-28. Unknown parameter
|month=ignored (help) offers substantive information about body type and appearance until a more rigorous source is found/supplied.
- Bock, Robert (1993). "Understanding Klinefelter Syndrome: A Guide for XXY Males and Their Families, Adolescence section". NIH Pub. No. 93-3202. Office of Research Reporting, NICHD. Retrieved 2007-03-29. Unknown parameter
|month=ignored (help) describes statistical occurrence of gynecomastia and surgical treatment.
- Denschlag, Dominik, MD; Clemens, Tempfer, MD; Kunze, Myriam, MD; Wolff, Gerhard, MD; Keck, Christoph, MD (October 2004), "Assisted reproductive techniques in patients with Klinefelter syndrome: A critical review", Fertility and Sterility, 82 (4): 775–779, doi:10.1016/j.fertnstert.2003.09.085
- Graham, JM Jr; Bashir, AS; Stark, RE; Silbert, A; Walzer, S (June 1988), "Oral and written language abilities of XXY boys: implications for anticipatory guidance.", Pediatrics, 81 (6): 795–806, PMID 3368277
- Boone et al: (2001), "Neuropsychological profiles of adults with Klinefelter syndrome" in Journal of International Neuropsychological Society, #7, p 446-456.
- Boone KB; Swerdloff RS; Miller BL; et al. (2001). "Neuropsychological profiles of adults with Klinefelter syndrome". J Int Neuropsychol Soc. 7 (4): 446–56. doi:10.1017/S1355617701744013. PMID 11396547. Unknown parameter
- Samango-Sprouse, CA. "Expansion of the phenotypic profile of the young child with XXY." Pediatric Endocrinology Reviews. (2010): 160-68