Ovarian remnant syndrome (ORS) is commonly seen in cats and dogs. It has been suggested that it is more common in cats due to the feline anatomy which makes it more difficult to break down the proper ligament and harder to exteriorize the ovary.
Etiology of Ovarian Remnant Syndrome in Small Animals
ORS occurs when there is functional ovarian tissue in a previously ovariohysterectomized (OHE) female. Suggested reasons include surgical error, failure to remove ectopic extraovarian tissue at the time of OHE, and autotransplantation of ovarian tissue.
Ovarian hypertrophy occurs following unilateral or partial ovariectomy in cats. With failure to remove all ovarian tissue, continued secretion of reproductive hormones results in clinical signs of proestrus and estrus and may lead to development of other pathological changes, including ovarian, mammary gland, or vaginal neoplasia or uterine stump pyometra.
Clinical Signs of Ovarian Remnant Syndrome in Small Animals
Affected females demonstrate normal signs of estrus, may allow copulation but do not become pregnant if bred. The estrous cycles usually show normal periodicity, but often a delay of months to years following OHE occurs before the estrous cycles appear.
Clinical signs of ORS typically mimic those of proestrus or estrus and include vulvar swelling, urine marking, behavioral changes (restlessness, increased activity and vocalization), with or without attraction of the male. Signs may first develop 7 days to 10 years after the OHE.
Other differential diagnoses for estrus behavior in a spayed female include exogenous estrogens, progestogens, and adrenal tumors (in dogs).
Diagnosis of Ovarian Remnant Syndrome in Small Animals
ORS can be diagnosed by confirmation of estrus using observation of mating behavior and on detection of serum progesterone concentrations > 2 ng/mL 2–3 weeks after ovulation in a spayed female. Vaginal cytologic findings consistent with estrus (> 90% cornified vaginal epithelial cells) is supportive.
Gonadotropin-releasing hormone (25 mcg/cat, IM) can be given and progesterone concentration checked 2 to 3 weeks later. Evidence of luteal tissue can be confirmed if the progesterone concentration is > 2 ng/mL.
Anti-Müllerian hormone (AMH) or luteinizing hormone (LH) concentration can also be used to diagnose the presence of functional ovarian tissue. A negative AMH test result is consistent with a spayed female, whereas a positive AMH test result indicates the presence of ovarian tissue. A positive LH test result is consistent with a spayed female as long as the test is not being performed during estrus or LH surge.
Treatment of Ovarian Remnant Syndrome in Small Animals
Treatment involves exploratory laparotomy and examination of both ovarian pedicles for retained or accessory ovarian tissue. Exploratory surgery is recommended within 3–6 weeks after induction of ovulation, at which time presence of corpus luteum make small remnants easier to identify. Most remnants are located at the ovarian pedicle. Any tissue removed should be submitted for histologic evaluation for confirmation of ovarian tissue.