Why do catamenial pneumothoraces almost always occur on the right side?
Catamenial pneumothoraces (i.e., pneumothoraces that occur during menstruation) are not particularly common. But, if any reader has seen one it almost certainly occurred on the right side. For this episode we discussed this clear preference and its fascinating explanation.
In 1958 a woman presented with a 12 month history of monthly right-sided pneumothoraces. More specifically, each month a pneumothorax would occur during her menstrual cycle. And in each instant it was on the right. Finally, after a full year, she underwent a thoracotomy and was found to have endometriosis involving the right hemidiaphragm. There seemed to be a link between endometriosis in the chest and her recurrent episodes of pneumothorax.
Some Basics
In order to understand this phenomenon it is necessary to review some basics of endometriosis. Endometriosis is extra-uterine implantation of endometrial tissue.
There are 3 main theories offered to explain the mechanism of spread of endometrial tissue.
- Retrograde menstruation leading to abdominopelvic spread
- Blood/lymph-borne deposition
- Metaplasia of local tissue
Retrograde menstruation is considered to be the predominant mechanism but all three likely play some role.
Why the Thorax…
While the pelvis and abdomen are the most common sites for implantation, thoracic implantation is a known site as well. In fact, the thoracic cavity is the most common non-abdominal site for endometriosis to arise. Implants can occur in the lung parenchyma or on the pleura. And it has been confirmed as the primary cause of up to 90% of catamenial peumothoraces.
As to why thoracic endometriosis leads to pneumothorax, the endometrial implants undergo changes during the secretory phase of the menstrual cycle in a process called decidualization. These changes involve fibroblasts in the endometrium transforming into secretory cells and vascular changes. This disrupts the pleura and can lead to pneumothorax and even hemothorax.
…and Why the Right Side?
Amazingly, 90% of catamenial pneumothoraces arise in the right chest. Most people with thoracic endometriosis have abdominopelvic involvement so thoracic endometriosis is thought to result from transit of endometrial cells from the abdomen and across the diaphragm.
Amazingly, there is a directionality to flow of fluid in the abdominal cavity: fluid flows in a clockwise fashion. If starting in the pelvis, it goes up the right paracolic gutter to Morrison’s pouch and the right subdiaphragmatic space. This was first demonstrated in 1970 by the use of peritoneography (i.e., injection of radiographic contrast dye into the abdomen and observing what happens on x-ray).
A number of questions remain. Why does peritoneal fluid flow upward? You would expect it to follow gravity and flow away from the diaphragm toward the pelvis. Why up the right-side of abdomen?
The upward flow results from a combination of 1) diaphragmatic action leading to pump action and 2) peristalsis from the ascending colon. The pump action of the diaphragm was established in 1948 when pressure gradients in the abdomen were studied in an aptly named paper called “Intraperitoneal pressure in the human”. The study found that with respiration the diaphragm creates a gradient between upper and lower abdomen, with lowest pressures just under the diaphragm. This makes sense because this is exactly what the diaphragm does: create pressure gradients in the chest to allow us to breath. This draws fluid up from the pelvis to the diaphragm. Peristalsis also likely plays a role with the upward motion in the ascending colon helping draw fluid upward.
As to the clockwise flow, the explanation is purely anatomical. There are two structures that force fluid to go right to left, or clockwise, within the abdomen. The first is the falciform ligament, the fibrous structure that connects the anterior portions of the liver to the abdominal wall.
The second is the colon. More specifically the transverse and sigmoid portions of the colon. These are the parts of the colon that reside in the peritoneum and “jut out” more than the ascending colon. In effect, these structures block the flow of fluid across the abdomen and force fluid being pumped up from the pelvis to preferentially contact the right hemidiaphragm over the left.
To summarize, endometrial cells are swept up in a clockwise intra-abdominal current, first contacting the right diaphragm, and going across it and into the right pleural cavity. The combination of the diaphragm pump, the falciform ligament, and the transverse and sigmoid colon segments all lead to endometrial cells preferentially contacting the right hemidiaphragm. They can then transit into the right thoracic cavity and implant on the pleura. During menstruation this then predisposes to pneumothorax as well as hemothorax, and explains why these phenomena are almost always right-sided.
A similar physiology explains the right-sided predilection for hepatic hydrothoraces. One additional note with this condition is that defects within the diaphragm may also play a role. These defects are typically within the tendenous portion of the diaphram. And because the tendon is larger on the right, one theory posits that these defects – and the resulting hepatic hydrothorax – are thus more common on the right.
Take Home Points
- Catamenial pneumothorax results from thoracic endometriosis and is almost always right-sided
- Endometriosis can lead to pleural disruption, causing pneumothorax specifically during menstruation
- Right-sided propensity results from clockwise peritoneal fluid flow, leading to a predilection for contact of endometrial cells with the right hemidiaphragm preferentially
- This clockwise peritoneal flow results from a combination of pump action by the diaphragm and ascending colon peristalsis, which lead to upward flow in the abdomen, and anatomic limitation by the falciform ligament and transverse and sigmoid colon segments
Learning Objectives
- Understand that catamenial pneumothorax results from thoracic endometriosis and is almost always right-sided
- Learn that endometriosis can lead to pleural disruption, causing pneumothorax specifically during menstruation
- Realize that right-sided propensity of catamenial pneumothoraces results from clockwise peritoneal fluid flow, leading to a predilection for contact of endometrial cells with the right hemidiaphragm preferentially
- Learn that clockwise peritoneal flow results from a combination of pump action by the diaphragm and ascending colon peristalsis – which lead to upward flow in the abdomen – and anatomic limitation by the falciform ligament and transverse and sigmoid colon segments
CME/MOC
Click here to obtain AMA PRA Category 1 Credits™ (1.00 hours), Non-Physician Attendance (1.00 hours), or ABIM MOC Part 2 (1.00 hours).
Listen to the episode
https://oembed.libsyn.com/embed?item_id=20799779
Credits & Citation
◾️Episode written by Avi Cooper
◾️Show notes written by Tony Breu and Avi Cooper
◾️Audio edited by Clair Morgan of nodderly.com
Cooper AZ, Abrams HR, Breu AC. The curious laterality of catamenial pneumothorax The Curious Clinicians Podcast. October 13, 2021
Image credit: https://www.ajconline.org/article/S0002-9149(97)00936-3/fulltext
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