Thoracic endometriosis is a condition in which tissue resembling the uterine lining grows on or within structures of the chest, including the diaphragm, the pleural membranes that line the lungs, the lung tissue itself, and occasionally the airways. It is grouped under the umbrella term thoracic endometriosis syndrome, which encompasses several distinct clinical presentations that tend to cycle with menstruation. The condition is considered rare, though many clinicians suspect it is significantly underdiagnosed because chest symptoms are not routinely connected to a gynecological cause.
What Thoracic Endometriosis Actually Looks Like
The hallmark of thoracic endometriosis syndrome is symptoms that recur in sync with the menstrual period, typically within about 72 hours before or after bleeding starts. The most common presentation by far is catamenial pneumothorax, a collapse of the lung that happens around menstruation.1PubMed Central. Catamenial pneumothorax Less frequent presentations include catamenial hemothorax (blood collecting in the space around the lung), catamenial hemoptysis (coughing up blood timed to the cycle), and pulmonary nodules made of endometrial-like tissue.2PubMed Central. Thoracic Endometriosis Syndrome: A Review of Diagnosis and Management Lung nodules are the least common of these, accounting for roughly 6% of thoracic endometriosis cases.3PubMed Central. Catamenial hemoptysis: A case report of pulmonary endometriosis
Some patients experience cyclical chest pain without a clear pneumothorax or effusion, which can make the condition even harder to pin down. In one reported case, a 24-year-old woman presented with cyclical chest pain, shortness of breath around her menses, and a right-sided pleural effusion whose aspirate was chocolate-colored, a finding strongly suggestive of old blood from endometrial implants.4PubMed Central. Thoracic endometriosis syndrome posing diagnostic and therapeutic challenges in a low-resource setting: a case report That dark brown fluid is a useful clinical clue, though it is not always present.
Why It Overwhelmingly Affects the Right Side
One of the most striking features of thoracic endometriosis is its lopsidedness. In an analysis of 110 cases, the right side of the chest was involved in more than 90% of all manifestations except for lung nodules.5PubMed. Thoracic endometriosis syndrome: new observations from an analysis of 110 cases This right-sided preference is one of the condition’s defining quirks, and it is a strong diagnostic hint for clinicians.
The leading explanation involves anatomy. The peritoneal fluid in the abdomen circulates in a clockwise pattern, driven partly by gut contractions and the motion of the diaphragm during breathing. This flow carries loose particles, including potentially shed endometrial cells, preferentially toward the right side of the diaphragm. The diaphragm on the right side also contains small natural gaps and weak points, particularly in the tendinous center, that may allow cells or air to pass through into the chest cavity. When surgeons look at the diaphragm during surgery for catamenial pneumothorax, they often find small holes or fenestrations at the tendon, along with red, brown, or blueberry-colored spots and nodules on the diaphragmatic and pleural surfaces.6PubMed Central. Catamenial pneumothorax: a rare entity? Report of 5 cases and review of the literature
How Endometrial Tissue Reaches the Chest
Researchers have proposed several theories, and the reality is probably a combination of mechanisms depending on which part of the chest is affected.
The migration theory, sometimes called the transdiaphragmatic theory, is the most widely cited explanation for diaphragmatic and pleural disease. In this model, endometrial cells shed during menstruation travel retrograde through the fallopian tubes into the pelvic cavity, then ride peritoneal fluid currents up to the diaphragm. Once there, they can implant on the diaphragmatic surface, erode through existing gaps, or create new fenestrations. This theory aligns neatly with the right-sided predominance and the diaphragmatic holes that surgeons frequently find.
A second theory, coelomic metaplasia, suggests that cells lining the chest cavity transform into endometrial-type cells because the pleura and the endometrium share a common embryonic origin. However, this theory has trouble explaining why the disease so heavily favors the right side and why endometrial tissue sometimes shows up deep inside the lung itself.7The Annals of Thoracic Surgery. Thoracic Endometriosis Syndrome
A third possibility, the vascular or lymphatic spread theory, posits that endometrial cells enter blood vessels or lymphatic channels and are carried to the lungs much like cancer cells spread during metastasis. This explanation works best for disease found within the lung tissue itself, where the transdiaphragmatic route would be an unlikely path. Most experts suspect that pleural and diaphragmatic disease follows the migration route, while parenchymal lung disease and airway involvement may reflect vascular or lymphatic spread.
The Overlap With Pelvic Endometriosis
Because thoracic endometriosis is generally understood as an extension of pelvic endometriosis, clinicians often look for disease in the pelvis too. The overlap is not as airtight as you might expect, though. In one study of 16 patients with thoracic endometriosis, pelvic endometriosis was confirmed in just over half, and among those with confirmed pelvic disease, the vast majority had advanced-stage endometriosis.8PubMed. Thoracic Endometriosis Syndrome: Association With Pelvic Endometriosis and Fertility Status That said, several patients in the same study never underwent laparoscopy to rule pelvic disease in or out, so the true co-occurrence rate remains murky. Thoracic endometriosis can also surface in patients with no known pelvic disease, which is one reason the coelomic metaplasia theory has not been abandoned entirely.
Getting a Diagnosis
Thoracic endometriosis is notoriously difficult to diagnose because the symptoms mimic other, more common chest conditions. A young woman showing up in an emergency room with a collapsed lung is far more likely to be worked up for spontaneous pneumothorax than for endometriosis, and the cyclical pattern may not be recognized until multiple episodes have occurred. Catamenial pneumothorax accounts for roughly 3 to 6% of recurrent spontaneous pneumothorax cases and about a third of all spontaneous pneumothorax in women of reproductive age, yet it often goes unrecognized for years.9International Journal of Respiratory and Pulmonary Medicine. Catamenial Pneumothorax
A quality-of-life study found that among 15 patients with thoracic endometriosis, there was a substantial delay to diagnosis, and participants reported significant physical and psychological burdens from the condition.10Obstetrics & Gynecology. Thoracic Endometriosis Syndrome and Quality of Life Part of the problem is that thoracic endometriosis sits at the intersection of two specialties, gynecology and thoracic surgery, and neither may think of the other’s diagnoses first.
Imaging
CT scans are typically the first imaging study performed for chest symptoms. In the context of thoracic endometriosis, CT is useful mainly for ruling out other causes, such as tumors or infection, rather than confirming the diagnosis directly. Its specificity for endometriosis is poor, though some CT findings in a woman with the right clinical story can be suggestive.11PubMed. Thoracic endometriosis syndrome: CT and MRI features
MRI is better at characterizing soft tissue and can detect signal changes consistent with blood products within diaphragmatic or pleural lesions, which is far more helpful for identifying endometriotic implants.12PubMed. Thoracic endometriosis syndrome: imaging findings and the value of a dedicated MRI protocol A dedicated MRI protocol designed to evaluate the diaphragm and chest wall is gaining traction at specialized centers, though it is not yet standard everywhere. Imaging timed to the menstrual phase when lesions are most active can improve the yield.
Surgical and Pathological Confirmation
Video-assisted thoracoscopic surgery (VATS) remains the gold standard for diagnosis. During VATS, the surgeon can directly visualize and biopsy the characteristic lesions on the diaphragm and pleural surfaces. These may appear as small or large holes in the diaphragm, or as red, brown, or dark blueberry-colored spots and nodules.13PubMed Central. Catamenial pneumothorax: a rare entity? Report of 5 cases and review of the literature
Biopsied tissue is examined under the microscope for the hallmark finding: endometrial glands and stroma in a location where they do not belong. When the tissue sample does not contain classic glandular structures, immunohistochemical staining for estrogen receptors, progesterone receptors, and the marker CD10 can help confirm the diagnosis. In cases where only endometrial stroma is present without clear glands, CD10 positivity in an aggregated staining pattern may be necessary to make the call.14PubMed Central. Diagnosis of thoracic endometriosis with immunohistochemistry Positive staining for estrogen and progesterone receptors in lung tissue has also been used to confirm the diagnosis in cases that were initially ambiguous.15PubMed Central. Pathological diagnosis of thoracic endometriosis
Treatment Options
Managing thoracic endometriosis typically involves some combination of surgery and hormonal therapy, and the evidence increasingly suggests that both together work better than either alone.
Surgery
VATS is both diagnostic and therapeutic. During the procedure, the surgeon can resect visible implants, repair diaphragmatic defects, and perform pleurodesis (a procedure that seals the lung to the chest wall to prevent future pneumothorax). In a French multicenter study, diaphragmatic nodules were resected in 38 cases and destroyed in 12. When diaphragmatic defects needed repair, simple suture was sufficient in the majority of patients, though a small number required mesh.16PubMed. Surgical management of diaphragmatic and thoracic endometriosis: A French multicentric descriptive study
Surgery addresses the immediate problem effectively, but it does not change the hormonal environment that drives the disease. Without additional treatment, the underlying cause of endometrial implant growth remains, which is why recurrence is a real concern.
Hormonal Therapy
Hormonal treatment aims to suppress the menstrual cycle and the estrogen that fuels endometrial tissue growth. Options include GnRH agonists (which temporarily shut down ovarian hormone production), progestins like dienogest, and continuous combined oral contraceptive pills. In one reported approach, patients treated with a course of GnRH agonist followed by long-term dienogest maintenance saw their hemoptysis symptoms resolve, CT-visible lung lesions shrink significantly, and no recurrence at extended follow-up.17PubMed Central. Case Report: Long-term maintenance of GnRH-a combined with dienogest for thoracic endometriosis syndrome
The case from a low-resource setting described earlier illustrates another approach: after drainage of the pleural effusion, the patient was placed on continuous combined oral contraceptive pills, which resolved her symptoms.18PubMed Central. Thoracic endometriosis syndrome posing diagnostic and therapeutic challenges in a low-resource setting: a case report The choice of agent often depends on what is available, what side effects the patient tolerates, and whether she has fertility goals, since most hormonal options prevent conception while in use.
Recurrence and the Case for Combined Treatment
Recurrence is the most frustrating aspect of thoracic endometriosis. A meta-analysis and review found that symptoms recurred in about 27% of patients, even with treatment.19PubMed. Treatment of Thoracic Endometriosis Syndrome: A Meta-Analysis and Review The same analysis noted that roughly 61% of patients received postoperative hormonal therapy, though usage varied widely across studies.
The evidence for adding hormones after surgery is growing stronger. A study published in the European Journal of Cardio-Thoracic Surgery found that postoperative hormonal therapy was significantly associated with reduced recurrence of endometriosis-related pneumothorax on both single-variable and multivariable analysis, with treated patients showing markedly lower recurrence rates.20PubMed. Postoperative hormonal therapies reduce the recurrence of thoracic endometriosis-related pneumothorax A smaller study showed the same trend, with hormone-treated patients recurring less often, though the difference did not reach statistical significance in that cohort, likely because the sample size was too small to detect it.21Journal of Chest Surgery. Recurrence-Free Survival after Postoperative Hormone Therapy for Catamenial Pneumothorax
The practical takeaway is that surgery alone, while it can fix the immediate crisis, leaves a woman vulnerable to recurrence. Adding hormonal suppression afterward is the current best approach for reducing that risk, and the data increasingly support making it a standard part of care.
Fertility Considerations
Thoracic endometriosis creates a thorny situation for women who want to become pregnant. The hormonal treatments that keep the disease quiet also prevent conception, and the fertility treatments that help with pregnancy can potentially stir the disease up.
Ovarian stimulation, a standard part of in vitro fertilization (IVF), increases estrogen levels dramatically. In at least one reported case, a woman undergoing controlled ovarian stimulation for IVF developed bilateral hemorrhagic pleural effusions that were initially attributed to ovarian hyperstimulation syndrome but turned out to be caused by previously unrecognized thoracic endometriosis.22PubMed Central. Thoracic endometriosis unmasked by ovarian hyperstimulation for in vitro fertilization The surge of estrogen during stimulation can activate dormant implants in the chest and unmask disease that was otherwise silent.
More recently, research has looked at whether fertility treatment can be done safely in women with a known history of catamenial pneumothorax. In one study, over a third of the women experienced their first pneumothorax episode within four months of undergoing assisted reproductive technology. Encouragingly, though, no thoracic recurrences were observed after surgical treatment of the endometriosis, even when those women went through subsequent rounds of ovarian stimulation.23PubMed. Assisted Reproductive Technology in Women With Endometriosis and a History of Catamenial Pneumothorax: Reproductive Outcomes and Safety Considerations This suggests that with proper surgical management beforehand, fertility treatment can proceed with a reasonable safety profile, though close monitoring of the chest is essential.
The quality-of-life data reinforce how central fertility concerns are for this population. In one survey, fertility was the domain where patients with thoracic endometriosis reported the greatest impact, with 67% scoring it as the area most affected by their disease.24Obstetrics & Gynecology. Thoracic Endometriosis Syndrome and Quality of Life
Postmenopausal and Atypical Presentations
Thoracic endometriosis is overwhelmingly a condition of reproductive-age women, with a peak incidence between 30 and 35 years.25International Journal of Respiratory and Pulmonary Medicine. Catamenial Pneumothorax But it does not vanish entirely after menopause. Rare cases of active thoracic endometriosis have been documented in postmenopausal women, and the natural progression and optimal treatment for these cases remain poorly understood.26PubMed Central. Rare Clinical Presentation of Postmenopausal Endometriosis: A New Perspective In theory, without ovarian estrogen production, endometriotic tissue should become inactive. In practice, low levels of estrogen produced by fat tissue, adrenal glands, or hormone replacement therapy may be enough to sustain implants in some individuals.
Postmenopausal thoracic endometriosis is also clinically important because it enters a very different differential diagnosis. When a postmenopausal woman presents with a lung nodule or pleural effusion, the primary concern is usually cancer, not endometriosis. The possibility that the lesion is endometrial-related may never be considered unless the pathologist identifies the tissue correctly. This underscores why immunohistochemical staining for hormone receptors and CD10 is so valuable. Without it, a biopsy showing only non-specific stromal tissue could easily be mischaracterized.
The Multidisciplinary Challenge
Thoracic endometriosis sits in the gap between gynecology and thoracic surgery, and neither field has historically owned it. A thoracic surgeon may see the recurrent pneumothorax and perform pleurodesis without recognizing or treating the underlying endometriosis. A gynecologist managing pelvic endometriosis may not think to ask about chest symptoms. The result is fragmented care that can leave the disease cycling for years.
Specialized endometriosis centers that include both thoracic surgeons and gynecological endometriosis surgeons are increasingly recognized as the ideal setting for managing these patients. In such centers, a single surgical session can address diaphragmatic and pleural disease through VATS while simultaneously treating any pelvic disease laparoscopically. Coordinated postoperative hormone management is planned from the start, and fertility counseling can be integrated into the treatment timeline. This kind of team-based approach is still not widely available, but it represents the direction the field is moving. For patients who have access, seeking out a center with specific experience in thoracic endometriosis can meaningfully change the trajectory of their care.