Equis ISSN 2398-2977

Uterus: cysts

Synonym(s): Uterine cysts

Contributor(s): Madeleine L H Campbell, Charles Cooke


  • Endometrial cysts may be luminal or extra-luminal. 
  • They are unlikely to be significant for fertility unless they are extremely widespread, exceptionally large, inhibit embryonic mobility, or happen to be positioned exactly at the base of an uterine horn where an embryo wishes to fix.
  • However, uterine cysts can cause a significant problem for the clinician when it comes to pregnancy diagnosis, since the ultrasonographical appearance of some cysts is very similar to, and can be confused with, an embryo or twin embryo   Twinning  .
  • Cause: degenerative change of endometrium.
  • Signs: often none; early embryonic death, abortion, subfertility.
  • Diagnosis: ultrasonography, hysteroscopy, biopsy.
  • Treatment: often not necessary; manual removal, laser ablation.
  • Prognosis: depends on severity.



  • A degenerative change of the endometrium   Endometrium: biopsy  . 
  • Cysts may originate from uterine glands or from lymphatic lacunae. 
  • Glandular cysts are situated in the lamina propria, and may be caused by the constricting effect of periglandular fibrosis   Endometrium: fibrosis  which, combined with a lack of myometrial tone, results in cystic distension of uterine glands. It is possible that the formation of glandular endometrial cysts in the mare occur in response to elevated plasma estrogen levels, analogous to the formation of cystic endometrial hyperplasia in dogs and women. 
  • Lymphatic cysts are formed as the result of collection(s) of lymphatic fluid in the endometrium.
  • Lymphatic fluid may collect due to obstruction of a lymph channel, or as a result of pooling of lymphatic fluid in a gravity-dependent part of the uterus.
  • Lymphatic cysts within the myometrium have been described.

Predisposing factors

  • Age (more common in mares >10 years).
  • Incidence increases with parity.


  • Endometrial cysts reflect uterine senility, and associated subfertility. 
  • They can occur anywhere in the uterus or cervix, but occur most commonly at the bifurcation of the uterine horns and body, which can impede embryonic movement (thus causing failure of maternal recognition of pregnancy), or can interfere with nidation (thus resulting in insufficient exchange of nutrients and oxygen, and early embryonic death).
  • Superficially located cysts can impede the development of placental microcotyledons, which may result in inadequate placentation and abortion. However, many mares with endometrial cysts, even in cases in which the cysts are numerous and appear widespread, conceive and carry foals to term successfully.


  • Cysts occur initially as small, discrete structures and increase in size gradually (over months to years). In the absence of treatment, they persist indefinitely.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Bartmann C P, Köllmann M, Schiemann et al (2008) Hysteroscopic removal of uterine cysts in mares I - Hysteroscopy and surgical procedures. Pferdeheilkunde 24 (1), 31-34 VetMedResource.
  • Ferreira J C, Gastal E L & Ginther O J (2008) Uterine blood flow and perfusion in mares with uterine cysts: effect of the size of the cystic area and age. Reproduction 135 (4), 541-550 PubMed.
  • Köllmann M, Bartmann C P, Schiemann V et al (2008) Hysteroscopic removal of uterine cysts in mares II - Follow-up and long term fertility analysis with regard to patho-histological findings. Pferdeheilkunde 24 (1), 35-37 VetMedResource.
  • Rambags B P B & Stout T A E (2005) Transcervical endoscope guided emptying of a transmural uterine cyst in a mare. Vet Rec 156, 679-686 PubMed.
  • Stanton M, Steiner J & Pugh D (2004) Endometrial cysts in the mare. J Eq Vet Sci 24 (1), 14-19 VetMedResource
  • Bracher V, Mathias S & Allen W R (1992) Videoendoscopic evaluation of the mares uterus II. Findings in subfertile mares. Equine Vet J 24, 279-284 PubMed.
  • Brook D & Frankel K (1987) Electrocoagulative removal of endometrial cysts in the mare. Equine Vet Sci 7, 77-81 VetMedResource.
  • Kenney R M (1978) Cyclic and pathologic changes of mare endometrium as detected by biopsy, with a note on early embryonic death. JAVMA 172, 241262 PubMed.