Adenomyosis: What Changes Inside the Uterine Muscle?

Adenomyosis makes much more sense once you picture the uterine wall properly.

The endometrium is the inner lining of the uterus. Its functional layer changes across the menstrual cycle and is shed during menstruation if pregnancy does not occur. Surrounding it is the myometrium, the thick smooth-muscle wall of the uterus.

In adenomyosis, endometrial glands and supporting stroma are found within the myometrium, where they are not normally meant to be. The surrounding muscle often becomes hypertrophied and remodelled. Instead of the endometrium and myometrium behaving as two clearly organised tissues, the boundary between them becomes disrupted.

That one anatomical change helps explain most of the condition. Tissue within the muscle remains hormone-responsive. The myometrium becomes inflamed and structurally altered. Uterine contractility can change. Some patients develop heavy menstrual bleeding, severe period pain, chronic pelvic pain or a feeling of pelvic heaviness, while others have very few symptoms.

What is actually changing inside the uterine muscle?

Adenomyosis is a benign uterine condition, but it is not simply a piece of endometrium sitting in the wrong place. The surrounding myometrium reacts to the presence of ectopic endometrial-type glands and stroma. Smooth-muscle cells can enlarge, the tissue can become fibrotic and the affected uterus may become bulky or globular.

The exact cause is still not settled. One longstanding explanation is that tissue from the basal endometrium invaginates into the underlying myometrium through a disrupted endometrial-myometrial interface. Other theories include abnormal tissue repair, stem-cell activity and de novo development of endometrial-type tissue within the muscle. Current research increasingly points towards several mechanisms acting together rather than one single cause.

The inner part of the myometrium next to the endometrium is often called the junctional zone. It is important because this region appears to be involved in normal uterine contractions and reproductive function, and imaging changes here are commonly described in adenomyosis. Disruption or irregularity of this zone is one of the features specialists may look for on ultrasound or magnetic resonance imaging.

Adenomyosis is also heterogeneous. It may be diffuse, involving a broad area of the uterine wall, or focal, where changes are concentrated in one region. A more localised mass-like form is sometimes called an adenomyoma. This is one reason two people with the same diagnosis can have very different symptoms and scan appearances.

Hormones are part of the story too. Adenomyotic tissue is hormone-responsive, and studies describe altered oestrogen signalling, progesterone resistance, inflammation, fibrosis and increased nerve and blood-vessel growth within affected tissue. These mechanisms are still being studied, but they help explain why the condition can produce both bleeding and pain rather than behaving like a simple structural lump.

Why can adenomyosis cause so much pain and bleeding?

Severe dysmenorrhoea is one of the classic symptoms, but there is not one single pain mechanism. The uterine muscle is altered, inflammatory signalling is increased and uterine contractions may become less coordinated. Increased sensory nerve fibres and local inflammatory mediators may also contribute to pain.

This helps explain why the pain can feel different from ordinary menstrual cramping. Some patients describe deep central pelvic pain, severe cramping during menstruation, aching between periods or pain during sex. An enlarged uterus can also produce a sensation of pressure, fullness or heaviness in the pelvis.

Heavy menstrual bleeding is also common, although the precise mechanism is complex. Proposed explanations include abnormal contractility of the myometrium, altered vascularity, inflammatory changes, disruption of the endometrial-myometrial interface and impaired local haemostasis. In real life, the result may be prolonged bleeding, flooding, large clots or a cycle that is regular in timing but extremely heavy.

This is where the history matters more than a textbook definition. If a patient says their periods are “heavy”, ask what that means for them. Are they changing pads, tampons or menstrual products very frequently? Are they bleeding through clothes or bedding? Waking repeatedly at night? Avoiding leaving the house? Missing work, university or caring responsibilities?

NICE defines heavy menstrual bleeding by its effect on physical, social, emotional and material quality of life rather than by one fixed measured volume. NICE also recommends a full blood count for people presenting with heavy menstrual bleeding, in parallel with treatment. Repeated heavy bleeding can lead to iron deficiency and iron-deficiency anaemia, so fatigue, dizziness, palpitations, exertional breathlessness and reduced exercise tolerance are clinically relevant symptoms, not side issues.

Adenomyosis can also exist without obvious symptoms. Modern imaging has changed the way the condition is recognised, and it is increasingly diagnosed in younger people rather than only being discovered after hysterectomy later in reproductive life.

There is also an association between adenomyosis and subfertility. Research has linked the condition with impaired implantation, lower pregnancy and live birth rates in some assisted-reproduction studies, and higher miscarriage rates. The relationship is not simple, though. Adenomyosis frequently coexists with endometriosis or fibroids, and disease location and severity vary. A diagnosis of adenomyosis does not mean that pregnancy is impossible, and fertility discussions should be individual rather than deterministic.

Adenomyosis, endometriosis and fibroids are not the same thing

These conditions often appear together in teaching because their symptoms overlap, but anatomically they are different.

In adenomyosis, endometrial glands and stroma are present within the myometrium. In endometriosis, endometrium-like tissue is found outside the uterus, for example on the ovaries, pelvic peritoneum or other pelvic structures. A person can have both conditions at the same time.

Fibroids are different again. They are benign tumours arising from uterine smooth muscle. A fibroid tends to form a more discrete mass, whereas adenomyosis often produces a diffuse change in the texture and architecture of the myometrium, although focal adenomyosis can mimic a mass.

This distinction matters when reading scan reports. “Bulky heterogeneous myometrium” or “myometrial cysts” points the reader in a different direction from a well-defined intramural fibroid. Neither is diagnosed by symptom pattern alone.

How is adenomyosis diagnosed now?

Historically, adenomyosis was often confirmed only when the uterus was examined under a microscope after hysterectomy. That created the impression that it was mainly a condition of older people who had completed their families.

Modern pelvic imaging has changed that. A diagnosis can now often be made without surgery.

For someone with heavy menstrual bleeding and either significant dysmenorrhoea or a bulky, tender uterus that suggests adenomyosis, NICE recommends transvaginal ultrasound as the preferred first-line investigation. If transvaginal ultrasound is declined or unsuitable, transabdominal ultrasound or magnetic resonance imaging may be considered, with the limitations explained.

Ultrasound is not looking for one single “adenomyosis lesion”. Specialists assess the pattern of the myometrium. The revised Morphological Uterus Sonographic Assessment, or MUSA, terminology separates ultrasound findings into direct and indirect features.

Direct features are signs that more directly suggest ectopic endometrial tissue within the muscle, such as myometrial cysts, hyperechogenic islands and echogenic subendometrial lines or buds. Indirect features reflect the way the surrounding myometrium has changed, including a globular uterus, asymmetric myometrial thickening, fan-shaped shadowing and an irregular or interrupted junctional zone.

You do not need to memorise the MUSA criteria as a student nurse. The useful takeaway is that adenomyosis is often diagnosed from a pattern of myometrial changes, not one big abnormality. Recent studies also show that no single ultrasound feature is perfectly reliable on its own, which is why the overall scan, symptoms and clinical context are interpreted together.

Magnetic resonance imaging can be useful when ultrasound is inconclusive, technically difficult or when more detailed mapping is needed. It can show thickening or irregularity of the junctional zone and areas of altered signal within the myometrium. It is not automatically the first investigation simply because it is a more advanced imaging modality.

Hysteroscopy, by contrast, looks inside the uterine cavity. It can be very useful when polyps, submucosal fibroids or endometrial pathology are suspected, but adenomyosis is primarily a disease of the myometrium, so hysteroscopy does not replace appropriate imaging for suspected adenomyosis.

Treatment is about the symptom burden and the person's priorities

There is no single treatment that suits everyone with adenomyosis. Some people need no treatment at all. Others are mainly troubled by pain, heavy bleeding, anaemia, fertility concerns or a combination of these.

For heavy menstrual bleeding with suspected or diagnosed adenomyosis, NICE recommends considering a levonorgestrel-releasing intrauterine system (LNG-IUS) as first-line treatment when it is suitable and acceptable to the patient. It works mainly by suppressing the endometrium and reducing menstrual blood loss, and many patients also experience less dysmenorrhoea.

If an LNG-IUS is declined or unsuitable, NICE lists other options including tranexamic acid, non-steroidal anti-inflammatory drugs (NSAIDs), combined hormonal contraception and cyclical oral progestogens. Progestogen-only contraception may also suppress menstruation and can be useful for some people with heavy bleeding.

These treatments control symptoms rather than removing adenomyosis itself. A patient may improve markedly while using hormonal suppression, then notice symptoms return if treatment is stopped. That is not a treatment failure in the same way that an antibiotic failing to clear an infection would be. Adenomyosis is a chronic structural and hormone-responsive condition.

If symptoms remain severe, treatment is unacceptable or medical management has not worked, referral to specialist gynaecology care is appropriate. Options depend on age, symptom pattern, imaging findings, whether the uterus needs to be preserved and reproductive plans.

Hysterectomy is the definitive treatment because the uterus containing the adenomyosis is removed. It is major surgery, however, and permanently removes the ability to carry a pregnancy. NICE recommends a full discussion of implications, alternatives, risks and patient expectations before a hysterectomy decision is made.

Other uterine-sparing procedures may be considered in selected cases. NICE supports uterine artery embolisation for adenomyosis under normal clinical governance arrangements, noting that it provides short- and medium-term symptom relief for a substantial proportion of patients. It is also important to explain that symptoms may persist or recur, further treatment may be needed and the effects on future fertility remain uncertain.

Endometrial ablation may also be discussed for heavy menstrual bleeding in specialist care, but it is worth understanding the anatomical limitation: ablation treats the endometrial surface, while adenomyosis extends into the myometrium. NICE has specifically identified uncertainty about how well endometrial ablation performs when adenomyosis is present.

For someone who wishes to conceive, management can become more specialised. The evidence around fertility-directed medical or surgical treatment is still developing, and decisions are usually made with gynaecology or fertility specialists rather than using one routine pathway for everyone.

What matters in nursing assessment

Adenomyosis is a good example of why “period problems” should not be dismissed as minor symptoms simply because menstruation itself is normal.

When taking a history, ask about the pattern of bleeding, how long it lasts, flooding, clots, dysmenorrhoea, pelvic pain between periods, pain during sex, abdominal or pelvic pressure, contraception, pregnancy possibility and previous gynaecological diagnoses. Ask about symptoms of anaemia and what the bleeding or pain is doing to sleep, study, work, exercise, relationships and daily routines.

It is also useful to ask what the patient wants from treatment. Reducing pain, stopping flooding, correcting anaemia, avoiding hormones, preserving the uterus and trying for pregnancy are not interchangeable priorities. The same scan result can lead to very different treatment decisions depending on what matters to the person living with it.

That is the clinical point to carry forward: adenomyosis is not simply “heavy periods caused by tissue in the muscle”. It is a heterogeneous condition that alters the structure and behaviour of the uterine wall, and its significance depends on the symptoms, reproductive context and impact on the person's life.

Summary

Adenomyosis occurs when endometrial glands and stroma are present within the myometrium. The surrounding smooth muscle remodels and may hypertrophy, producing diffuse or focal changes in the uterine wall. The condition can cause heavy menstrual bleeding, dysmenorrhoea, chronic pelvic pain, dyspareunia and pelvic pressure, although some people are asymptomatic.

The cause is not fully understood. Current research implicates disruption of the endometrial-myometrial interface, altered hormone signalling, inflammation, fibrosis and changes in uterine contractility.

Transvaginal ultrasound is the preferred first-line imaging test in UK guidance when adenomyosis is suspected in someone with heavy menstrual bleeding and significant dysmenorrhoea or a bulky tender uterus. Treatment is symptom-led and preference-sensitive. An LNG-IUS is a first-line option for heavy menstrual bleeding when suitable, with other medical and specialist procedural options available. Hysterectomy is definitive, while uterine-sparing treatment requires more individualised discussion.

References

Harmsen, M.J. et al. (2022) ‘Consensus on revised definitions of Morphological Uterus Sonographic Assessment (MUSA) features of adenomyosis: results of modified Delphi procedure’, Ultrasound in Obstetrics & Gynecology, 60(1), pp. 118–131. Available at: https://pubmed.ncbi.nlm.nih.gov/34587658/

National Institute for Health and Care Excellence (NICE) (2018, updated online) Heavy menstrual bleeding: assessment and management (NG88). Available at: https://www.nice.org.uk/guidance/ng88

National Institute for Health and Care Excellence (NICE) (2013) Uterine artery embolisation for treating adenomyosis (HTG324). Available at: https://www.nice.org.uk/guidance/htg324

NHS (2026) Adenomyosis. Available at: https://www.nhs.uk/conditions/adenomyosis/

Selntigia, A. et al. (2024) ‘Adenomyosis: An Update Concerning Diagnosis, Treatment, and Fertility’, Journal of Clinical Medicine, 13(17), 5224. Available at: https://pubmed.ncbi.nlm.nih.gov/39274438/

Moawad, G. et al. (2023) ‘Adenomyosis: An Updated Review on Diagnosis and Classification’, Journal of Clinical Medicine, 12(14), 4828. Available at: https://pubmed.ncbi.nlm.nih.gov/37510943/