Data from large population-based studies of chromosome abnormalities at birth have demonstrated that the KS occurs in approx 1 in 400-600 male births (30-32). In adults, overall age-matched mortality is roughly doubled (33,34). Most of this increase is from non-neoplastic diseases of the cardiovascular, respiratory, and digestive systems, and diabetes; however, the incidence of breast cancer and autoimmune disease is also increased (35).
The risk of breast cancer is 20-fold higher in men with KS, who account for 4% of all breast cancer cases in men. Fortunately, breast cancer in men is rare, so the absolute risk of breast cancer in an individual with KS is well under 1%, and screening with mammography is not generally recommended (36). There is also an increased risk of extragonadal germ cell tumors in individuals with KS (37). Affected patients are young and frequently present with advanced disease, often of the mediastinum. It is believed that these neoplasms arise from primordial germ cells that failed to completely migrate to the fetal testes during embryogenesis. These germ cell precursors may then undergo malignant transformation, partly in response to the elevated gonadotropin levels seen in individuals with KS. Fortunately, these tumors are rare even in individuals with KS, and most of these tumors respond well to chemotherapy.
There may be an increased risk for autoimmune disease in individuals with KS. Systemic lupus erythematosis, rheumatoid arthritis, ankylosing spondylitis, and even rare vascular disorders, such as Takaysu's arteritis, have been reported in patients with KS, although the true relative increase in risk is difficult to calculate because of the small number of affected individuals (38-40). It has been speculated that this possible increased risk of autoimmune disease results from an increase in the estrogen to testosterone ratio, because it is believed that elevated circulating estrogen levels may place premenopausal women at increased risk of autoimmune disease (41). Alternative explanations for this increased risk of autoimmune disease in individuals with KS include abnormalities in the T-lymphocyte subsets, including a reduced number of T-suppressor cells (CD8+), because this is occasionally found in women with autoimmune disease. It is interesting to note that androgen therapy may both improve the clinical symptoms of autoimmune disease and correct some of the laboratory features of autoimmunity (42).
Diabetes mellitus is more common in individuals with KS than in the general population (43,44). Fortunately, the severity of illness is most frequently mild. The mechanism of diabetes in individuals with KS probably results from increased resistance to insulin action (45).
Taurodontism (an enlargement of the pulp in molar teeth) is seen in almost half of individuals with KS (46). Taurodontism is diagnosed by dental X-rays and predisposes to premature dental caries and tooth loss. Good dental hygiene and frequent cleaning may be protective.
Early varicose veins are a feature of KS, with 20% of men having severe varicosities at a relatively early age (47). In addition to lower extremity venous ulcers, an increased risk of deep-vein thrombosis has been noted (48,49). There is speculation that a defect in venous basement membrane connective tissue is present in some individuals with KS.
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