Endometrial cysts (fluid-filled structures on or within the endometrium, or uterine lining) occur in up to 22% of all mares, and up to 55% of older mares, said Karen Wolfsdorf, DVM, Diplomate ACT, of Hagyard-Davidson-McGee (HDM) Veterinary Associates, in a presentation on this topic at the recent HDM Bluegrass Equine Reproduction Symposium.

While their impact on fertility isn't completely clear, some studies have found that early embryonic loss (22-44 days) is higher in mares with these cysts. Wolfsdorf explained that numerous or large cysts can impede the movement of the embryo around the uterus, preventing maternal recognition of pregnancy at day 11 prior to fixation, and can prevent the embryo from contacting the endometrium to receive nutrients. "If they are superficial, large, and extensive, they may reduce the total area of placental exchange, which can be detrimental in an older mare with an already compromised placenta," she said.

Additionally, Wolfsdorf noted that cysts can sometimes be mistaken for early embryonic vesicles, necessitating repeated examinations to detect a change in vesicle size or embryonic heartbeat for pregnancy identification.
After discussing the various types of cysts, Wolfsdorf described procedures commonly used at HDM for removing large pedunculated cysts (large cysts extending from the uterine wall via a thinner section like a stalk) and small visible non-pedunculated ones. "Cyst removal should be delegated to those mares with poor reproductive histories that have large cysts that may obstruct embryo movement or those mare with numerous small cysts that may prevent early embryonic growth or severely compromise the placenta," she stated.

Endometrial cyst Lasering cyst

Endometrial cysts can be very small and nearly flush with the surface of the endometrium, or they can project out into the uterus (above left). They are visible on ultrasound as dark, fluid-filled cavities (left). The larger ones can sometimes be punctured with a laser (above) to allow drainage and removal.
See endoscopic video of lasering a cyst.

She removes the first type when the mare is in heat (allowing access via the relaxed cervix) using a small looped wire inserted through a steel catheter. "Once the snare is around the base, gradual sawing motion will cut the cyst from the endometrium," she explained, usually "without rupturing the cyst and with minimal removal of or damage to the underlying endometrium."

Small non-pedunculated cysts can be removed with a hysteroscopic laser, which is used to heat and puncture the cyst and thus allow immediate visible drainage of the fluid contents, she said. This does not work as well for larger cysts, because the larger amount of heat that is required to destroy them can damage the uterus. "Care must be taken to not laser blood vessels on the cyst's surface, which makes visibility difficult," she added. "Also, with a multilobular cyst (one with several 'chambers'), you must puncture all the chambers to fully drain the cyst. Since the uterus is distended with fluid to make the cysts visible, the lymphatic fluid drained from the cysts is flushed out of the endometrium at the end of the procedure. Post-laser treatment includes prostaglandin to bring the mare back into heat and uterine lavage for the next couple of days to remove the remaining debris.

"Most cysts do not cause a problem, but are usually signs of an underlying one," Wolfsdorf concluded. "These mares are usually older mares with poor uterine clearance and benefit tremendously by simply using uterine lavage and oxytocin in their breeding regimen."

About the Author

Christy M. West

Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.

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