Adenomyosis is a common gynecological condition characterized by the abnormal presence of tissue, which normally lines the inside of the uterus (the endometrium), growing into the muscular wall of the uterus (the myometrium). This misplaced tissue causes the uterine wall to thicken and become disorganized. Transvaginal ultrasound is the primary, non-invasive imaging tool used by clinicians to diagnose this condition because it offers a clear view of the uterine architecture. Understanding the specific features that doctors look for helps clarify the diagnosis.
Understanding Adenomyosis: The Anatomical Context
The uterus has two main layers: the inner lining (endometrium) and the surrounding muscle wall (myometrium). These layers are separated by a distinct boundary called the junctional zone. Adenomyosis occurs when endometrial glands and stroma penetrate this zone and embed themselves deep within the myometrium. Once inside the muscle wall, this ectopic tissue thickens and bleeds during the menstrual cycle. The body responds by stimulating the surrounding smooth muscle tissue to grow (hypertrophy), which causes the myometrium to become enlarged and stiff, resulting in the distorted appearance seen on imaging.
Key Ultrasound Features of Adenomyosis
The diagnosis of adenomyosis relies on recognizing a collection of specific visual features, not a single sign. The most frequently observed feature is myometrial heterogeneity, describing the muscle tissue as having an irregular, non-uniform texture. Instead of a smooth, consistent appearance, the myometrium looks mottled or “moth-eaten” due to the disordered mix of muscle and infiltrating tissue. This lack of homogeneity is considered a sensitive indicator of the condition.
Another common sign is uterine asymmetry and enlargement, where the uterus appears globally bulky or globular. This enlargement is often not uniform, meaning one wall, frequently the posterior wall, is significantly thicker than the opposing wall. This asymmetrical thickening results directly from muscle hypertrophy reacting more intensely in one area.
The most specific sonographic features are myometrial cysts, sometimes referred to as lacunae. These present as small, anechoic (fluid-filled) pockets measuring between one and five millimeters in diameter located within the muscle wall. These tiny cysts represent the displaced endometrial glands and are a highly characteristic marker for adenomyosis.
The disruption of the inner lining is evident through subendometrial striations or lines. These are echogenic (bright) linear streaks that radiate outward from the inner lining into the myometrium. This visual sign is sometimes described as a “venetian blind” or “rain shower” shadowing pattern. Furthermore, the normally distinct border between the inner lining and the muscle wall often becomes ill-defined, making the junctional zone difficult to identify and measure.
Differentiating Adenomyosis from Other Conditions
Distinguishing adenomyosis from other uterine conditions, particularly uterine fibroids (leiomyomas), is a common challenge. Fibroids are benign tumors typically seen on ultrasound as well-defined, discrete, solid masses with clear borders. In contrast, adenomyosis is characterized by its diffuse nature, lacking the clear, encapsulating border of a fibroid.
When a focal area of adenomyosis, called an adenomyoma, is present, its margins remain ill-defined, blending into the surrounding myometrium. This contrast is reinforced when using Color Doppler ultrasound, which detects blood flow patterns. Fibroids usually show blood flow that circles the mass circumferentially, whereas adenomyosis often displays tortuous vessels that penetrate directly into the disorganized tissue.
In cases where ultrasound findings are ambiguous, or when both adenomyosis and fibroids coexist, Magnetic Resonance Imaging (MRI) is used to provide greater clarity. MRI offers superior soft tissue contrast, which helps visualize the junctional zone. On MRI, a junctional zone thickness exceeding 12 millimeters is highly suggestive of adenomyosis, helping to confirm the diagnosis when ultrasound is inconclusive. While transvaginal ultrasound is accurate and cost-effective, MRI serves as the definitive clarifying tool for complex findings.