Ovarian Remnant Syndrome

Ovarian remnant syndrome (ORS) is a clinical condition in female dogs and cats characterized by the persistence of functional ovarian tissue after an intended ovariohysterectomy (OHE) or ovariectomy (OVE). This residual tissue, often a small fragment of the ovary, continues to exhibit cyclic hormonal activity, leading to signs of estrus, behavioral changes, and potential complications such as pyometra in the remaining uterine stump. The syndrome is a well-recognized surgical complication that can occur weeks to years after the initial spay procedure. The retained ovarian tissue may be located in the original ovarian pedicle, within the omental adhesions, or rarely in ectopic locations. The condition is diagnosed based on clinical signs, hormonal assays, and imaging, and is definitively treated by surgical excision of the remnant tissue. The surgical approach requires meticulous exploration of the abdominal cavity, particularly the ovarian pedicles and surrounding tissues, to identify and remove all functional ovarian tissue.

The primary etiology of ovarian remnant syndrome is iatrogenic, resulting from incomplete excision of ovarian tissue during ovariohysterectomy or ovariectomy. This can occur due to several factors: (1) surgical error, such as failure to identify and remove the entire ovary, especially when the ovarian pedicle is not adequately visualized or when the ovary is small and atrophic; (2) anatomical variations, including a long ovarian bursa or excessive perivascular fat that obscures the ovary; (3) surgical technique, such as using a clamp that crushes the ovary and leaves a portion behind, or performing the surgery via a small incision that limits exposure; (4) hemorrhage or adhesions that obscure the surgical field; (5) surgeon inexperience; and (6) in rare cases, ectopic ovarian tissue that is not located in the normal anatomical position. Additionally, in cats, the presence of accessory ovarian tissue has been reported. The retained tissue retains its blood supply and hormonal function, leading to cyclic estrus and associated clinical signs.

Ovarian remnant syndrome is reported in both dogs and cats, with a higher incidence in cats. The exact prevalence is unknown, but it is considered a relatively uncommon complication of spay surgery. In dogs, certain breeds may be overrepresented due to anatomical variations, such as deep-chested breeds where the ovaries are located more cranially and are more difficult to access. In cats, the condition is more frequently diagnosed because of the small size of the ovaries and the common use of a small incision for OHE. The age at diagnosis varies, with clinical signs typically appearing within months to years after the initial surgery. There is no sex predilection as the condition occurs in females. The risk of ORS is higher in surgeries performed by less experienced surgeons, in emergency settings, or when the surgery is performed during the anestrous period when the ovaries are small and less conspicuous. Additionally, the use of a flank approach in cats may increase the risk of incomplete removal due to limited visualization.

The pathophysiology of ovarian remnant syndrome involves the persistence of functional ovarian tissue that responds to the normal hormonal feedback mechanisms. The remnant tissue, which may be a small fragment of the ovarian cortex, retains its ability to produce estrogen and progesterone in response to follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the pituitary gland. During the follicular phase, the remnant tissue produces estrogen, leading to clinical signs of proestrus and estrus, such as vulvar swelling, serosanguinous discharge, and attraction of males. After ovulation, the remnant tissue forms a corpus luteum and produces progesterone, which can lead to signs of diestrus, including behavioral changes and mammary gland development. In some cases, the retained ovarian tissue may undergo cystic changes or neoplastic transformation, although this is rare. The hormonal stimulation can also cause endometrial hyperplasia in the uterine stump, predisposing to stump pyometra if the uterine body was not completely removed. The cyclic hormonal activity can be detected through vaginal cytology, hormone assays, and imaging studies.

Several factors predispose to the development of ovarian remnant syndrome. Intrinsic factors include anatomical variations such as a long ovarian pedicle, excessive fat within the ovarian bursa, or a small ovary that is difficult to visualize. In cats, the ovaries are small and may be easily missed, especially if the surgeon is not experienced. Extrinsic factors include surgical technique, such as making an incision that is too small, inadequate hemostasis leading to hemorrhage that obscures the surgical field, and failure to properly identify the ovary before ligation. The timing of surgery is also a factor; performing OHE during anestrus when the ovaries are small and inactive increases the risk of leaving a remnant. Additionally, emergency surgeries, such as those performed for pyometra, may be rushed, increasing the risk of incomplete removal. Surgeon experience is a significant factor, with higher rates of ORS reported in surgeries performed by less experienced veterinarians or in training settings.

The clinical signs of ovarian remnant syndrome are primarily related to the hormonal activity of the retained ovarian tissue. The most common sign is the recurrence of estrus cycles, which may occur within weeks to months after the spay. Signs of estrus include vulvar swelling, serosanguinous vaginal discharge, increased attractiveness to males, and behavioral changes such as restlessness, increased vocalization, and urine marking. In some cases, the animal may exhibit signs of pseudopregnancy, including mammary gland enlargement and lactation. If the uterine stump is present, there is a risk of stump pyometra, which can present with lethargy, anorexia, polyuria, polydipsia, and a purulent vaginal discharge. On physical examination, the vulva may be swollen, and the vaginal mucosa may appear hyperemic. Palpation of the abdomen may reveal a mass if a granuloma or neoplasm has formed around the remnant. Systemic signs are usually absent unless complications such as pyometra or peritonitis occur.

Differential diagnoses for ovarian remnant syndrome include: (1) Exogenous hormone administration, such as estrogen or progesterone therapy, which can cause similar signs of estrus; (2) Adrenal gland tumors that produce sex hormones, leading to signs of estrus; (3) Uterine stump pyometra, which can cause vaginal discharge and systemic signs; (4) Vaginitis, which may cause discharge but is not associated with estrus cycles; (5) Foreign body or neoplasia of the reproductive tract; (6) In cats, the presence of an ovarian remnant may be confused with a retained testicle in a male cat if the history is unclear; (7) Behavioral issues that mimic estrus, such as attention-seeking or territorial marking. Definitive diagnosis is based on hormonal testing and imaging.

The diagnostic algorithm for ovarian remnant syndrome begins with a thorough history and physical examination, focusing on the reproductive tract. If the animal is showing signs of estrus, vaginal cytology can be performed to confirm the presence of cornified epithelial cells, indicating estrogen stimulation. Hormonal assays are the most reliable diagnostic tools: serum progesterone levels can be measured during diestrus (typically 1-2 months after estrus) to confirm the presence of luteal tissue; a progesterone level >2 ng/mL is indicative of an ovarian remnant. Alternatively, a GnRH stimulation test can be performed, where baseline LH and estradiol are measured, followed by administration of GnRH and repeat measurement; a significant increase in estradiol or progesterone confirms functional ovarian tissue. Imaging studies, such as abdominal ultrasonography, may identify the remnant as a small hypoechoic structure near the kidney or within the omentum, but it is not always visible. Exploratory laparotomy is both diagnostic and therapeutic, allowing direct visualization and removal of the remnant. In cases where the remnant is not easily found, a thorough exploration of the entire abdominal cavity, including the omentum and mesentery, is necessary.

Laboratory findings in ovarian remnant syndrome are typically unremarkable unless there is a concurrent condition such as pyometra. Complete blood count may show leukocytosis with a left shift if pyometra is present. Serum biochemistry may reveal elevated globulins or hyperglobulinemia in cases of chronic inflammation. Hormonal assays are the most specific laboratory tests: serum progesterone levels >2 ng/mL during diestrus confirm the presence of luteal tissue. Serum estradiol levels may be elevated during estrus, but they are less reliable due to fluctuations. Vaginal cytology can be performed to assess the stage of the estrous cycle; the presence of >90% superficial cells indicates estrus. In cases of stump pyometra, cytology of the vaginal discharge may show neutrophils and bacteria. Culture and sensitivity of the discharge can guide antimicrobial therapy.

Imaging plays a crucial role in the diagnosis and surgical planning of ovarian remnant syndrome. Abdominal radiography is often unremarkable but may reveal a soft tissue mass if a granuloma or neoplasm is present. Ultrasonography is the most useful imaging modality; the remnant ovary may be identified as a small, oval, hypoechoic structure with a distinct cortex and medulla, often located near the caudal pole of the kidney or within the omental fat. Color Doppler may demonstrate blood flow to the remnant. In some cases, the remnant may be cystic or contain follicles. Computed tomography (CT) can provide more detailed anatomical information, especially in obese animals or when the remnant is small and difficult to locate. CT with contrast may enhance the remnant, aiding in identification. Magnetic resonance imaging (MRI) is rarely used but can be helpful in complex cases. Intraoperative ultrasonography can be used to guide surgical exploration. In cases where the remnant is not visible on imaging, exploratory surgery is indicated.

Cytological and histopathological examination of the ovarian remnant is essential for confirming the diagnosis and ruling out neoplastic transformation. Fine-needle aspiration of a suspected remnant may yield ovarian stromal cells, follicular cells, or luteal cells. Histopathology of the excised tissue typically shows normal ovarian architecture, including follicles at various stages of development, corpora lutea, and stroma. In some cases, the remnant may show cystic follicles or luteinized cysts. If neoplastic changes are present, such as granulosa cell tumors, histopathology will reveal characteristic features, including Call-Exner bodies and nuclear palisading. Immunohistochemistry may be used to differentiate ovarian tissue from other tissues, with positive staining for inhibin and vimentin. The surgical margins should be evaluated to ensure complete excision.

The definitive treatment for ovarian remnant syndrome is surgical excision of the remnant tissue. Preoperative stabilization is necessary if the animal has concurrent pyometra or systemic illness. The surgical approach is a midline celiotomy, extending from the umbilicus to the pubis. The abdominal cavity is thoroughly explored, with particular attention to the ovarian pedicles, which are located caudal to the kidneys. The remnant may be embedded in omental adhesions or granulation tissue. The surgeon should carefully dissect the remnant from surrounding tissues, ligating the vascular supply with absorbable suture (e.g., polydioxanone or polyglactin 910, size 3-0 or 4-0). The remnant should be submitted for histopathology. If the uterine stump is present and there is evidence of stump pyometra, it should be removed as well. Postoperative care includes pain management with opioids (e.g., buprenorphine 0.01-0.02 mg/kg IV or IM q8-12h) and NSAIDs (e.g., carprofen 2.2 mg/kg PO q12h for 3-5 days). Antibiotics are indicated if there is evidence of infection. The prognosis is excellent after complete excision, with resolution of clinical signs.

The prognosis for ovarian remnant syndrome is excellent following complete surgical excision of the remnant tissue. Most animals show resolution of estrus signs within days to weeks after surgery. The recurrence rate is low if the remnant is completely removed. However, if the remnant is not found or incompletely excised, clinical signs may persist. Complications such as stump pyometra or peritonitis can affect the prognosis, but with appropriate treatment, the outcome is generally good. Long-term prognosis is excellent, with no adverse effects on the animal's health or lifespan. Negative prognostic indicators include incomplete excision, neoplastic transformation, or concurrent disease.

Postoperative follow-up for ovarian remnant syndrome includes monitoring for resolution of clinical signs. The animal should be re-examined 10-14 days after surgery for suture removal and assessment of wound healing. Hormonal assays (e.g., serum progesterone) can be performed 1-2 months after surgery to confirm the absence of functional ovarian tissue. If the animal was in estrus at the time of surgery, a follow-up vaginal cytology may be performed to confirm the absence of cornified cells. Long-term follow-up is generally not required, but owners should be advised to monitor for any recurrence of estrus signs, which would indicate incomplete excision. In cases where a uterine stump was removed, the animal should be monitored for signs of urinary incontinence, which can occur due to disruption of the pelvic nerve supply.

Clinical pearls: (1) Always perform a thorough exploration of the ovarian pedicles during OHE, especially in cats, where the ovaries are small; (2) If the ovary is not easily visualized, extend the incision or use a spay hook to exteriorize the uterine horn and trace it to the ovary; (3) During surgery for ORS, use a systematic approach: start at the uterine stump, follow the broad ligament to the ovarian pedicle, and inspect the omentum and mesentery; (4) Consider using a GnRH stimulation test preoperatively to confirm the presence of functional ovarian tissue; (5) Submit all excised tissue for histopathology to confirm the diagnosis and rule out neoplasia. Pitfalls: (1) Failure to identify the remnant due to inadequate exploration, leading to persistent clinical signs; (2) Inadvertent damage to the ureter or kidney during dissection; (3) Hemorrhage from the ovarian pedicle if ligation is not secure; (4) Leaving a portion of the remnant behind due to fragmentation; (5) Not considering ectopic ovarian tissue, which may be located in the omentum or mesentery.

Perioperative drug protocols for ovarian remnant syndrome surgery are based on Plumb's Veterinary Drug Handbook. Prophylactic antimicrobials: cefazolin 22 mg/kg IV at induction and every 90 minutes during surgery. Postoperative analgesics: opioids such as buprenorphine 0.01-0.02 mg/kg IV or IM q8-12h, or hydromorphone 0.05-0.1 mg/kg IV or IM q4-6h. NSAIDs: carprofen 2.2 mg/kg PO q12h for 3-5 days, or meloxicam 0.1 mg/kg PO q24h. Local anesthesia: a line block with bupivacaine 1-2 mg/kg (maximum 2 mg/kg) at the incision site. If pyometra is present, antibiotics should be based on culture and sensitivity, with empirical therapy using amoxicillin-clavulanate 12.5-25 mg/kg PO q12h or enrofloxacin 5-10 mg/kg PO q24h. For hormonal suppression preoperatively, if needed, a GnRH agonist such as deslorelin (4.7 mg implant SC) can be used, but this is not commonly necessary.

Evidence-based literature on ovarian remnant syndrome is limited to retrospective studies and case series. A study by Okkens et al. (1981) reported that ORS is a rare complication of OHE, with an incidence of less than 1% in dogs and cats. Another study by DeTora and McCarthy (2011) reviewed 20 cases of ORS in cats and found that the most common clinical sign was recurrence of estrus, and that surgical excision was successful in all cases. A retrospective study by Ball et al. (2010) evaluated the use of ultrasonography in diagnosing ORS and found that it had a sensitivity of 80% and specificity of 90%. The use of hormonal assays, particularly progesterone measurement, is well-documented as a reliable diagnostic tool. A study by Johnston et al. (2001) recommended measuring progesterone 1-2 months after estrus to confirm the presence of luteal tissue. Surgical exploration remains the gold standard for diagnosis and treatment. There are no consensus guidelines from ACVS or ECVS specifically for ORS, but general principles of meticulous surgical technique and thorough exploration are emphasized.