Klinefelter syndrome (synonyms: Malformation syndrome of sex chromosomes in male phenotype; anomaly of sex chromosomes in male phenotype; absence of sex chromosomes in male phenotype; karyotype 47,XYY; Klinefelter anomaly of sex chromosomes; Klinefelter-Reifenstein syndrome; Klinefelter syndrome; Klinefelter syndrome, karyotype 47,XXY; Klinefelter syndrome, male phenotype, with karyotype 46,XX; Klinefelter syndrome, male phenotype, with more than two X chromosomes; Male phenotype with gonosome mosaic; Male phenotype with karyotype 47,XYY; Male phenotype with structural abnormality of sex chromosomes; Male phenotype with structural abnormality of gonosomes; Mosaic of male sex chromosomes a. n.k.; XXXXY syndrome; XXY syndrome; XYY syndrome; ICD-10-GM Q98.-: Other abnormalities of gonosomes in male phenotype, not elsewhere classified) refers to a gonosome (sex chromosome) abnormality of the male sex that results in primary hypogonadism (gonadal hypofunction).
Klinefelter syndrome is one of the numerical chromosomal aberrations (chromosomal disorder). It is characterized in the majority of cases by a supernumerary X chromosome (47, XXY), in rare cases two or more X chromosomes may be present or in a part of the body cells a mosaic constellation (mos 47, XXY / 46, XY/47, XX) may be present.
Affected individuals are phenotypically (= appearance, which means here: Tall stature, small testes, gynecomastia (enlargement of the male mammary gland), azoospermia (absence of spermatozoa in semen)) and socially male. In late adolescence (period of life between late childhood and adulthood), they develop an increasing testosterone deficit (lack of male hormone).
Klinefelter syndrome is discovered prenatally (before birth) in about 10% of cases and in childhood and adulthood in about 25%. However, the majority remain undetected.
The prevalence (disease frequency) is about 80,000 boys/men in Germany, a large number of them are undiagnosed.
The incidence (frequency of new cases) is about 1-2 cases per 1,000 male newborns.
Course and prognosis: The success of testosterone substitution therapy (e.g. prevention of osteoporosis/bone loss) strongly depends on how early Klinefelter syndrome was detected. Klinefelter patients have a significantly increased morbidity rate (disease rate) for type 2 diabetes mellitus, bone fractures, varicosis (varicose veins), thrombosis, embolism, epilepsy, and other neurological and psychological disorders compared to the general male population. This leads to a reduced life expectancy of 11.5 years compared to the general male population. The clinical picture is often accompanied by male sterility.
