Introduction

Every healthy woman of childbearing age dreads that time of the month when her periods begin. Now, picture adding to that already existing discomfort a sharp, recurring chest pain that appears right at the start of each cycle, only to fade away in a few days. This is the reality for a woman living with a rare condition called catamenial pneumothorax, where the lung collapses every month in time with menstruation. This can be a frightening and unsettling monthly scenario with no clue as to the reason behind it. If this sounds familiar, then don’t worry, as this article will walk you through what catamenial pneumothorax is and why it most often affects the right lung.

What is catamenial pneumothorax (CP)?

Definition

Catamenial pneumothorax (cat-uh-MEE-nee-uhl new-mo-THOR-axe) presents as a spontaneous recurring condition where air enters the space between the lung and chest wall, resulting in a collapsed lung (pneumothorax). It occurs in reproductive-aged women, usually in synchrony with menstruation.¹

The term “Catamenial” originates from a Greek word called “Katamenios,” which means monthly.²

When it happens

CP is generally seen in ovulating women aged 30-40 years. It’s rare in women who are pregnant, menopausal, or on birth control pills. It occurs within 72 hours before or after the start of the menses, and it affects the right lung in roughly 92% of these cases.²

Symptoms

Symptoms usually appear a few days before or after the start of menstruation and may include: 

• Chest pain radiating to the shoulder area
• Shortness of breath and difficulty in breathing (dyspnea)
• Dry cough with occasional crackling sound while breathing in^2,3

Link to thoracic endometriosis

Several studies have suggested a connecting link between CP and thoracic endometriosis, though the exact aetiology is not fully understood. 

Endometriosis is a condition where tissue similar to the uterine lining is seen growing outside the uterus. It can be seen anywhere in the body, but most commonly in the pelvic region and the chest cavity. When this tissue is seen in the chest cavity (thoracic cavity), it is called thoracic endometriosis, which is the most common site for endometriosis outside the pelvis. Now, spontaneous pneumothorax is a common finding in 72-73% of patients with thoracic endometriosis, which can happen during menses (catamenial) or at other times (non-catamenial).^1,2

Case studies suggest that thoracic endometriosis is associated with the majority of CP patients but not all. Therefore, their relationship can be explained as indefinite yet interlinked, as endometriosis-related pneumothorax can be catamenial or non-catamenial, and CP can be associated with thoracic endometriosis or not.²

Proposed theories

Researchers have proposed several possible mechanisms for CP:

Physiologic hypothesis

Prostaglandin F2 levels rise during menstruation. This causes blood vessels to narrow and airways to constrict, potentially rupturing the alveoli (small air sacs) in the lungs, causing air to leak into the pleural cavity and collapsing the lung (pneumothorax).

Metastatic theory

Endometrial tissue travels to the lungs through the blood or lymphatic system, damaging lung tissue and causing air leaks. This is supported by findings of endometrial tissue in both lungs and other body parts.

Transgenital-transdiaphragmatic theory

During menstruation, low cervical mucus allows air to enter through the vagina and uterus into the abdomen. It then moves up to the chest through tiny holes (defects) in the diaphragm (muscle below the lungs), collapsing the lungs.

Migration theory

Menstrual blood may flow backwards (retrograde menstruation) into the abdomen, which carries the endometrial tissue to the diaphragm. This tissue damages the diaphragm, creating defects that allow air to pass into the chest.²

How often does it affect the right lung?

A large number of documented case reports have shown that CP mostly involves the right lung (85-95%) when compared to the left side or bilateral involvement.^1,2 Surgeons performing video-assisted thoracic surgery (VATS) have frequently noticed small holes or fenestrations on the right diaphragm. In one study, all 8 women diagnosed with CP had right-sided involvement with visible defects.³

So, women presenting with repeated pneumothorax during their menstrual cycle are most likely to have right-sided lung disease. Recognising this pattern can help clinicians to diagnose faster and avoid delays in proper management. 

Why is the right lung usually affected?

The reason for this preference comes down to fluid flow, anatomy, and organ positioning. Here’s how.

Peritoneal fluid flow

Peritoneal fluid is a clear, straw-colored fluid found in the abdominal cavity. It acts as a lubricant and reduces friction between organs during digestion and normal bodily movements. Now, this fluid flows in a clockwise direction through the right paracolic gutter,² striking a barrier at the falciform ligament, causing most of the displaced endometrial cells to be concentrated in the right upper abdomen region near the liver. These cells may further pass through the diaphragmatic defects to reach the right lung, causing pneumothorax.^4,5

Diaphragmatic defects

Several studies have pointed out the right-sided preference of CP,² particularly a laparoscopic intraoperative study conducted on 46 women that showed most of the endometrial cells get implanted onto the right diaphragm.⁵ These implants undergo necrosis (damage or death), creating holes or defects in the diaphragm, which allows the circulating endometrial tissue to enter the pleural cavity, thereby causing alveolar damage, air leaks, and subsequently pneumothorax.²

Right hemidiaphragm anatomy

Multiple nodules, or holes, have been found on and around the tendinous portion of the right hemidiaphragm, often revealed through VATS. These defects, which can be acquired or congenital, act as natural passageways for air or endometrial tissue from the abdominal cavity to enter the lung space.¹

The “piston effect” of the liver

The liver, a large, solid organ under the right diaphragm, is less compressible than the stomach and spleen on the left. As a result, when pressure builds up in the abdomen (during coughing or normal bodily movements), the liver pushes upwards like a “piston,” driving air or endometrial tissue through the diaphragmatic defects into the right pleural cavity.⁴

How do hormonal and biological factors affect CP?

Hormones like estrogen and progesterone rise and fall throughout the menstrual phase. The fluctuating levels of these hormones may impact the timing and the recurrence of CP, but they don’t dictate which side gets affected. A research study reveals the presence of positive estrogen and progesterone receptors in the lung and diaphragm tissue of CP patients, proving that these sites directly respond to these hormonal shifts.⁷

Prostaglandins are lipids with hormone-like effects that play a key role in menstruation. They cause the uterus to contract, which sheds the endometrium (uterine lining), leading to menstrual bleeding. Prostaglandin levels, especially PGF2, increase during menstruation, constricting blood vessels and narrowing airways, which can lead to alveolar rupture and lung collapse.

These hormonal changes can also cause rupture of blebs, which are small air- or fluid-filled blisters or pustules on the lungs, causing them to collapse.

The lung collapses associated with CP usually occur within a narrow timeframe of 24 hours before or 72 hours after the start of menses, emphasising a close link between hormonal changes and lung involvement.²

Why are some CP cases left-sided or bilaterally involved?

Most of the cases are right-sided (85-95%), as elucidated above, but a few cases have been reported to affect the left side or involve both the lungs (bilateral).² The following theories might throw light on the reason behind this.

Air passage theory

Literature reviews from different studies have implied that the absence of a cervical mucus plug during menses may allow air to enter the fallopian tubes through the cervix. This air further moves up to the lungs through the diaphragmatic defects to cause CP. Now, this could be an explanation for all cases, irrespective of the side involved.^2,6,8

Coelomic metaplasia

Another possible explanation could be that, at the embryonic stage of development, three parts of the body, which are the pleura (lining of the lungs), peritoneum (lining of the abdomen), and the gonads (ovaries), all originate from the same tissue known as coelomic epithelium. So, when estrogen rises during menses, this may cause the pleura and peritoneum to transform into endometrial-like cells, which break down, causing lung damage and pneumothorax.⁸

Microembolization theory

During menstruation, some of the displaced endometrial tissue may travel through the blood or lymphatic fluid and get attached to the pleura or lungs, causing damage and collapse.²

In short, CP cases have increasingly shown a tendency to involve the right lung, probably due to anatomy and fluid dynamics. The alternate pathways may explain left-sided or bilateral CP, showing that menstrual timing is the key clue to diagnosing CP regardless of the affected side.

Diagnosis and treatment

Diagnosis mainly depends on the woman’s age (whether ovulating or not), the timing of clinical symptoms presenting in sync with menses, and the history of any previous episodes or surgery involving endometriosis, infertility, or any uterine abnormalities.²

If CP is suspected, then a definitive diagnosis can be made through

• Imaging: Chest x-ray (most preferred), CT, and MRI may reveal single or multiple diaphragmatic defects, thoracic spots or nodules (possible endometrial implants), or bleb or bullae formation seen on the lungs
• Cancer antigen 125 (CA125): This serves as a promising biomarker that may aid in the early identification of CP. While not definitive, elevated levels of CA125 can be suggestive of thoracic endometriosis and warrant further investigation
• VATS: This is a minimally invasive surgical procedure where surgeons can directly visualise and repair diaphragmatic defects and also remove lesions

Treatment mainly involves a combination of 

• Hormonal therapy involving the administration of a gonadotropin-releasing hormone (GnRH) analogue to suppress the endometrial tissue growth and prevent recurrences
• Surgery to repair diaphragmatic defects and remove all visible lesions, including blebs, bullae, and endometrial lesions^1,2

Summary 

Catamenial pneumothorax is rare but highly patterned, most often affecting women in their 30s and 40s who experience recurrent chest pain or lung collapse in sync with their menstruation. It commonly involves the right lung, probably because of anatomical differences, liver pressure, and peritoneal fluid flow. Hormonal changes trigger the timing of events, while structural changes determine the involved side. Recognising this right-sided pattern of catamenial pneumothorax can speed up diagnosis, target the right area for surgical repair, and ease anxiety in women suffering from repeated, unexplained episodes.