Adenomyosis is a common gynecologic condition characterized by the abnormal presence of tissue that resembles the inner lining of the uterus, known as the endometrium. The uterus is composed of two main layers: the endometrium, which sheds monthly during menstruation, and the surrounding thick, muscular wall called the myometrium. In adenomyosis, the endometrial-like tissue embeds itself deep within the myometrium. Although this condition is common, it remains frequently underdiagnosed because many individuals experience no noticeable symptoms.
Understanding Adenomyosis
The mechanism of adenomyosis involves the invasion of endometrial-like glandular tissue into the myometrium. This misplaced tissue continues to function normally, responding to the body’s cyclical hormonal changes. During the menstrual cycle, the embedded tissue thickens, breaks down, and attempts to bleed within the uterine muscle.
Because the blood and tissue are trapped within the muscle wall, this process triggers a sustained inflammatory response. The surrounding myometrial smooth muscle cells react to this chronic irritation by growing larger (hypertrophy) and increasing in number (hyperplasia). This muscular reaction causes the uterus to become enlarged, often described as globular, and can result in the organ doubling or even tripling its usual size. The physical effects of the trapped blood and subsequent muscle enlargement are responsible for the most common symptoms, which include severe, painful cramping (dysmenorrhea) and abnormally heavy or prolonged menstrual bleeding (menorrhagia).
The Challenge of Determining Prevalence
Determining precisely how common adenomyosis is presents a significant challenge, leading to a wide range of prevalence estimates in medical literature. Historically, diagnosis could only be definitively confirmed through the histopathological examination of the uterus after a hysterectomy. This reliance on surgical specimens created a substantial selection bias, as only individuals who were highly symptomatic or were undergoing surgery for other uterine issues were included in the data pool.
These older studies based on post-hysterectomy analysis reported a prevalence that varied drastically, with figures ranging anywhere from 10% to over 57% of examined uteri. The variability was compounded by the lack of a universally standardized definition for histological diagnosis, with different pathologists using different criteria for the required depth of tissue invasion. This inconsistency made it difficult to compare findings across different research centers.
The advent of modern, non-invasive imaging has begun to provide a more accurate population-wide picture. Specialized imaging techniques, particularly high-quality transvaginal ultrasound (TVUS) and magnetic resonance imaging (MRI), allow for the visualization of the junctional zone, the interface between the endometrium and myometrium, without the need for surgery. MRI is useful for defining the junctional zone’s thickness and irregularities, which are characteristic signs of adenomyosis.
Population-based studies utilizing medical coding suggest a much lower overall prevalence, with estimates falling around 0.8% to 1.03% in the general female population. However, studies focusing on symptomatic patients show much higher rates, with reports indicating that approximately 21% of symptomatic women undergoing ultrasound screening are diagnosed with the condition. The prevalence is highest in specific sub-groups; for example, in women with subfertility, the rate of isolated adenomyosis is approximately 10%. Furthermore, for individuals who also have endometriosis, the co-occurrence of adenomyosis can be substantially higher, sometimes reaching over 70%.
Factors Influencing Occurrence
Several demographic and health factors are associated with an increased likelihood of developing adenomyosis. Diagnosis is most frequently made in women during their late reproductive years, typically between the ages of 35 and 50. This pattern may be linked to the cumulative exposure to estrogen over many menstrual cycles, which fuels the growth and activity of the endometrial-like tissue.
A history of childbirth, known as parity, is another significant factor, with many diagnosed individuals having given birth at least once. One theory suggests the barrier between the endometrial lining and the myometrium may be compromised during embryo implantation or during recovery after delivery. This trauma could facilitate the invasion of the lining tissue into the muscle wall.
Prior surgical procedures performed on the uterus also increase the risk. Operations that create an incision through the uterine wall, such as a Cesarean section, fibroid removal (myomectomy), or dilation and curettage (D&C), are thought to disrupt the delicate junctional zone. This disruption provides a pathway for the endometrial tissue to migrate into the underlying myometrium.
Adenomyosis frequently coexists with other gynecologic conditions, indicating a shared underlying risk profile. Around half of individuals diagnosed with adenomyosis also have uterine fibroids. Additionally, a significant proportion of those with adenomyosis also have endometriosis, a related condition where endometrial-like tissue grows outside the uterus entirely.