Prevalence of Menstrual Dysfunction in Athletes
The prevalence of menstrual dysfunction among women in the general population who are not pregnant, lactating, or postmenopausal is estimated to be 2% to 5%, whereas the range is 6% to 79% among female athletes [2,36]. This wide range of prevalence estimates seen in athletes can be largely explained by meth-odologic differences among the various studies that have attempted to measure menstrual dysfunction. Some of these methodologic issues are described.
- Differences in the athletic population studied, including the type of sport (ie, endurance versus esthetic versus strength/power; individual versus team sports), the level of competition (ie, elite versus recreational versus collegiate), and the age of the athlete. Small, nonrandomized studies that sample a single sport or athletes in similar types of sports may produce biased estimates of the incidence of menstrual dysfunction. For example, it is well known that menstrual dysfunction is common among distance runners [37-39]; if this population is used to represent the general female athlete population, it would likely produce an overestimation of prevalence. Conversely, if the sample is limited to female basketball or volleyball players (groups with a lower incidence of menstrual dysfunction), an underestimation of prevalence is likely to occur. To date, few studies have examined the range of menstrual disturbances in a large, heterogeneous group of female athletes.
- Failure to control for oral contraceptive use. Early prevalence studies in particular did not account for oral contraceptive use, or if they did, they did not indicate the rationale for use, which could confound prevalence estimates [34,40]. Many female athletes take oral contraceptives to regulate their menstrual cycle; if this is not taken into account, it could confound (ie, underestimate) the true prevalence of menstrual dysfunction.
- Assessment of menstrual dysfunction. Most prevalence studies have used self-report menstrual history questionnaires to ascertain menstrual dysfunction. Such questionnaires rely heavily on the honesty and accuracy of the individuals completing them and are subject to response bias. Even assuming honest responses, self-report may underestimate the incidence of menstrual dysfunction because many subclinical menstrual disturbances have no overt symptoms. Even studies that have attempted to verify self-report menstrual disturbances via measures of endocrine hormones generally investigated only a single menstrual cycle. Research by De Souza and colleagues  showed that data based on a single cycle grossly underestimate the actual incidence of menstrual disturbances.
- Definitions of ''menstrual dysfunction" used. As previously indicated, researchers have used a variety of definitions for the different menstrual disturbances seen in athletes. which can have a great impact on the estimated prevalence. The more liberal the definitions used, the greater the prevalence. Johnson and associates  defined amenorrhea as one or fewer menstrual periods in 6 months and found that 6% of the athletes were amenorrheic. Fo-gelholm and Hiilloskorpii  reported that only 1% of athletes had amenor-rhea, whereas the spectrum of menstrual dysfunction (including primary amenorrhea, secondary amenorrhea, and oligomenorrhea) among athletes not using oral contraceptives ranged from 32% to 37% (depending on the sport type examined—esthetic, speed, endurance, weight-dependent, or ballgame). These authors did not provide a clinical definition for the menstrual disturbances they examined; it is unclear what criteria were used for the various menstrual disturbances they examined. Beals and Manore  found that 31% of collegiate athletes studied reported menstrual irregularity (described as cycles not occurring every 28-34 days), whereas 1% had no menstrual periods, 12% had fewer than 6 menstrual periods over the past year, and 8% had more than 12 menstrual periods over the past year. Dusek  found that 30% of a sample of 72 ballet dancers, runners, basketball players, and volleyball players experienced amenorrhea, defined as no menstruation for more than 3 months postmenarche. Finally, Torstveit and Sundgot-Borgen  reported that 31.4% of female athletes had menstrual dysfunction, which included primary amenorrhea (defined as absence of menarche by age 16 years), secondary amenorrhea (defined as an absence of three consecutive menstrual cycles), oligomenorrhea (defined by the authors as cycles of >35 days), and shortened luteal phase (defined by the authors as cycles of <22 days). These authors did not break the prevalence estimates down by menstrual dysfunction category.
Despite differences in the definitions used for menstrual dysfunction among the above-cited studies, without exception, all found that menstrual dysfunction was most evident among athletes participating in sports that emphasize leanness.
The estimated prevalence of delayed menarche among young women in the United States is less than 1% . In contrast, Beals and Manore  found that 7.4% of a sample of 425 collegiate athletes (representing 15 different sports) reported not menstruating until after age 16 (as primary amenorrhea was previously defined), and 22.2% of athletes in esthetic sports (ie, cheerleading, diving, and gymnastics) reported primary amenorrhea.
The prevalence of oligomenorrhea also seems to be significantly higher among female athletes than the general female population . Klentrou and Plyley  found that 61% of elite rhythmic gymnasts from Greece and Canada (n = 45) regularly experienced menstrual cycles longer than 35 days. Using a slightly different definition of oligomenorrhea (ie, more than three but fewer than nine cycles in 3 months), Burrows and coworkers  found the incidence among a group of English distance runners to be 21%. In a similar population (ie, English distance runners), Rosetta and colleagues  found a 40% total incidence of short (<21 days) and long (>35 days) cycles.
The lack of overt symptoms makes identifying luteal suppression or anovu-lation and consequently accurately assessing their prevalence among active women difficult. Nonetheless, both menstrual disorders are hypothesized to be common among female athletes. In regularly menstruating, recreational runners, the total incidence of luteal suppression and anovulation was 78% . Similarly, Loucks and colleagues  found an 80% occurrence of luteal suppression in at least 1 of 3 consecutive months among a small group (n = 9) of "athletic" women.
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