The breathlessness that keeps time
Why cyclical chest symptoms are read as asthma — and when to think of the diaphragm

A woman in a chest clinic, breathless and in pain, whose lung-function tests come back normal and whose inhaler does nothing, is one of the few patients whose diagnosis is written in her menstrual history rather than her chest. The pattern has a name — thoracic endometriosis — and it reaches a surgeon, on average, years too late.

An asthma that never quite fits

Most women who eventually reach me with thoracic endometriosis have carried another label first, and more often than not that label is asthma. It is an understandable mistake. The symptoms — breathlessness, chest tightness, a cough — are the symptoms of asthma, and asthma is common while this is rare. But the asthma label tends not to fit, and the ways it fails to fit are consistent. The inhalers do little; adding a nebuliser does little more. Allergy testing is negative. Spirometry and lung-function tests, done properly, are normal. And the symptoms do not follow the things that set off asthma — exercise, cold air, pollen — but something an asthma review never asks about: the menstrual cycle. When a treatment that should work does not, it is usually the diagnosis, not the dose, that needs questioning.

The one question that separates them

The single discriminating question is whether the symptoms keep time with the period. Thoracic endometriosis is caused by tissue like the lining of the womb growing in the chest — on the diaphragm, on the lining of the lung, occasionally within the lung itself — and because that tissue answers to the same monthly hormones as the womb, the symptoms it causes are cyclical. They cluster within about three days of a period starting and ease between periods. They are overwhelmingly right-sided, because the route the disease takes into the chest is up the right side of the diaphragm. The presentations run from cyclical chest pain and breathlessness, through right shoulder-tip pain, to coughing blood with a period, to the most dramatic — a lung that collapses every month, the catamenial pneumothorax, which is the commonest single presentation and is itself repeatedly mistaken for an ordinary spontaneous one. A woman of reproductive age with recurrent right-sided symptoms that track her cycle has, in effect, already given the diagnosis — and one who already carries a diagnosis of endometriosis has given most of it, since the thoracic disease almost always sits on a background of pelvic disease that came first.

Why it lands in the wrong clinic

If the clue is so simple, why does it take so long? In the UK, endometriosis now takes an average of around nine years to diagnose (Endometriosis UK's 2025 survey; a University of York review the year before put the global figure at 6.6 years, with the UK among the longest). For the thoracic form the delay is longer still, and the reason is structural rather than anyone's failing: the chest symptoms are seen by respiratory and emergency teams, the periods are the territory of gynaecology, and the link between them — the timing — falls in the gap where no one clinician is looking at the whole patient. Worse, the chest tests are actively reassuring. A normal chest X-ray, a normal CT and normal spirometry all point away from the diagnosis, when in fact none of them excludes it. CT is good for ruling out other disease but not for the small deposits themselves; even MRI, the better test for the diaphragm, misses deposits under a few millimetres. A normal scan in this condition is not a normal diaphragm.

Why recognising it matters — the disease is on the diaphragm

Recognition is not an academic point, because what is found, once someone looks, is not ordinary thoracic disease. It is disease on the diaphragm, and the diaphragm is where this condition is both most treatable and most often mistreated. The visible holes — the fenestrations through which air passes from the abdomen into the chest — are the obvious target, but in my experience they are not always the whole story. There are weaker areas, harder to see, where the diaphragm pulls away from the chest wall, and a repair that closes the visible holes but misses these can fail. The correct repair is to remove the deposits and repair the diaphragm itself — directly for small defects, with a patch for larger ones — rather than simply lay a mesh over the holes, which does not seal them. And surgery is rarely the whole treatment: continuing hormonal suppression for some months afterwards is what makes the result last, which is why this is managed jointly with gynaecology from the outset rather than handed between teams. None of this is exotic, but it is specific, and it is the part most easily got wrong by a service that meets the condition rarely.

What I would ask of a chest clinic

The ask is small and costs nothing. The single strongest flag is a diagnosis the patient often already carries: in a woman known to have endometriosis, new chest symptoms should put thoracic disease on the differential at once, and cyclical ones near the top of it. Beyond that, for any woman of reproductive age presenting with breathlessness, chest pain, a pleural effusion or a pneumothorax — particularly when an asthma label has never quite delivered — the question to add is simply whether the symptoms keep time with her period. If they do, and especially if they are right-sided and recurrent, the diagnosis is worth pursuing rather than managing the symptoms for another few years. A normal CT does not close the door; the gynaecological history is often more informative than the scan. The patients this reaches are not numerous, but for the ones it does, the difference between recognising the pattern and missing it is measured in years.

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