Ovarian Remnant Syndrome (ORS)

Ovarian remnant syndrome (ORS) is a clinical condition in female dogs and cats characterized by the recurrence of estrous cycles and associated clinical signs (e.g., vulvar swelling, serosanguinous discharge, attraction of males) following an ostensibly complete ovariohysterectomy (OHE) or ovariectomy (OVE). The syndrome arises from the presence of functional ovarian tissue that was inadvertently left in situ during the surgical procedure, either as a small fragment of the ovary or as an accessory ovarian tissue. This residual tissue retains the capacity to undergo follicular development, ovulation, and luteinization, thereby producing cyclic elevations in serum estradiol and progesterone concentrations that mimic a normal estrous cycle. The condition is a well-recognized complication of elective sterilization surgery and is a common reason for referral to veterinary theriogenologists. The clinical presentation, diagnostic approach, and management of ORS are distinct from other causes of post-OHE estrus, such as exogenous hormone administration or neoplasia of the reproductive tract. Accurate diagnosis and surgical or medical intervention are essential to resolve clinical signs and prevent potential complications, including pyometra in the uterine stump or mammary neoplasia. The syndrome is classified as an iatrogenic disorder, and its incidence is influenced by surgical technique, surgeon experience, and the timing of surgery relative to the estrous cycle.

The primary etiology of ovarian remnant syndrome is the inadvertent retention of ovarian tissue during ovariohysterectomy or ovariectomy. This can occur due to several surgical factors: (1) incomplete visualization of the ovarian pedicle, particularly in obese animals or those with excessive periovarian fat; (2) improper placement of ligatures or hemostatic clips, leading to partial transection of the ovary; (3) failure to remove the entire ovary when the ovarian bursa is not fully exteriorized; (4) accidental tearing of the ovarian tissue during manipulation, leaving small fragments attached to the mesovarium or omentum; (5) the presence of accessory ovarian tissue, which is a congenital anomaly where ectopic ovarian tissue is located along the urogenital ridge; and (6) surgical inexperience or a rushed procedure. Additionally, the timing of surgery in relation to the estrous cycle may influence the risk, as the ovarian tissue is more vascular and friable during proestrus and estrus, increasing the likelihood of fragmentation. In some cases, the remnant may be located in the omentum, mesometrium, or retroperitoneal space, making it difficult to identify at the time of the initial surgery. The residual tissue is functional and responds to endogenous gonadotropins, leading to cyclic hormonal activity. In rare instances, ORS may also result from incomplete removal of an ovarian tumor or from the implantation of ovarian tissue during surgery, although these are less common.

Ovarian remnant syndrome is reported in both dogs and cats, with a higher incidence in cats than in dogs, likely due to the smaller size of the feline ovary and the technical difficulty of complete removal. The condition is more commonly diagnosed in animals that underwent OHE or OVE at a young age, as the ovarian tissue is small and may be inadvertently left behind. There is no strong breed predisposition, but certain breeds with a high body condition score (e.g., Labrador Retrievers, Golden Retrievers) may be at increased risk due to the presence of excessive periovarian fat, which obscures the surgical field. The incidence of ORS is estimated to be between 0.3% and 2% of all OHE procedures, but this may be an underestimate due to underreporting. The condition is more frequently recognized in animals that present with signs of estrus within weeks to months after surgery, although some cases may remain asymptomatic for years. The risk of ORS is higher in surgeries performed by less experienced surgeons, in emergency settings, or when the surgical approach is limited (e.g., flank incision in cats). Additionally, the use of laparoscopic ovariectomy may be associated with a lower incidence of ORS if performed by a skilled surgeon, but incomplete removal can still occur. There is no sex predilection, as the condition is exclusive to females. Parity and breeding status do not appear to influence the risk, but animals that were in estrus at the time of surgery may have a higher risk due to increased ovarian vascularity and friability.

The pathophysiology of ovarian remnant syndrome revolves around the functional activity of residual ovarian tissue. The remnant tissue contains primordial, primary, secondary, and antral follicles, as well as stromal cells, which are responsive to the pituitary gonadotropins, follicle-stimulating hormone (FSH) and luteinizing hormone (LH). In the absence of the negative feedback from ovarian steroids (estradiol and progesterone), the hypothalamus and pituitary gland continue to secrete GnRH and gonadotropins, driving the remnant tissue to undergo cyclic follicular development. During proestrus and estrus, the developing follicles produce increasing amounts of estradiol, which leads to the characteristic clinical signs of estrus, including vulvar swelling, serosanguinous vaginal discharge, and behavioral changes. The estradiol surge triggers an LH surge, which induces ovulation, and the ruptured follicles luteinize to form corpora lutea. The corpora lutea then produce progesterone, which maintains the luteal phase for a duration similar to that of an intact animal (approximately 60 days in the dog and 40 days in the cat). If pregnancy does not occur, the corpora lutea regress, and the cycle repeats. The presence of functional ovarian tissue also has systemic effects, including the potential for endometrial hyperplasia and the development of pyometra in the uterine stump, if a portion of the uterus was left in situ. Additionally, the cyclic production of estradiol and progesterone may increase the risk of mammary neoplasia, as these hormones are known to promote mammary tumorigenesis. The remnant tissue may also undergo pathological changes, such as the formation of ovarian cysts or tumors, which can lead to persistent hormonal production and clinical signs.

Several factors predispose to the development of ovarian remnant syndrome. Intrinsic factors include: (1) congenital presence of accessory ovarian tissue, which is rare but can be located along the urogenital ridge; (2) obesity, which increases the amount of periovarian fat and makes surgical identification of the ovary more difficult; (3) small ovarian size, particularly in young animals or certain breeds (e.g., toy breeds, cats); (4) the stage of the estrous cycle at the time of surgery, as the ovarian tissue is more vascular and friable during proestrus and estrus, increasing the risk of fragmentation; and (5) the presence of ovarian pathology, such as cysts or tumors, which may alter the normal anatomy and make complete excision challenging. Extrinsic factors include: (1) surgeon inexperience or lack of familiarity with the surgical anatomy; (2) the use of a limited surgical approach, such as a flank incision in cats, which may not provide adequate exposure of the ovarian pedicles; (3) the use of hemostatic clips or ligatures that are placed too close to the ovary, leaving a portion of the ovary behind; (4) the failure to exteriorize the ovarian bursa fully, leading to incomplete removal; and (5) the performance of surgery in an emergency setting, where time constraints may compromise surgical precision. Additionally, the use of laparoscopic ovariectomy may be associated with a learning curve, and incomplete removal can occur if the surgeon is not experienced. Finally, the timing of surgery relative to the estrous cycle is a significant factor, as surgery performed during estrus is associated with a higher risk of ORS.

The clinical signs of ovarian remnant syndrome are primarily related to the recurrence of estrous cycles. In dogs, the most common presentation is the return of proestrus and estrus signs within 2 to 6 months after OHE, although some animals may not show signs for years. These signs include: (1) vulvar swelling and edema; (2) serosanguinous to sanguinous vaginal discharge; (3) increased attractiveness to male dogs; (4) behavioral changes such as restlessness, increased urination, and mounting behavior; (5) in some cases, the animal may allow mating. In cats, the signs are similar and include: (1) vulvar swelling; (2) vocalization (calling); (3) increased affection and rolling; (4) lordosis posture when stimulated; (5) spraying of urine. The clinical signs are typically cyclic, with a period of proestrus/estrus lasting 7-14 days in dogs and 5-10 days in cats, followed by a diestrus phase. Some animals may also develop signs related to complications of the remnant tissue, such as: (1) pyometra in the uterine stump, which presents with purulent vaginal discharge, lethargy, anorexia, and polyuria/polydipsia; (2) ovarian neoplasia, which may cause persistent estrus or other hormonal abnormalities; (3) mammary gland enlargement or neoplasia, due to prolonged estrogen and progesterone exposure. In rare cases, the remnant tissue may be non-functional, and the animal may remain asymptomatic, with the condition being discovered incidentally during imaging or surgery for another reason. Physical examination may reveal a small, palpable mass in the ovarian region, but this is often not possible due to the small size of the remnant. The diagnosis is confirmed by hormonal assays and imaging.

The differential diagnoses for ovarian remnant syndrome include: (1) Exogenous hormone administration: The owner may have administered estrogen or progesterone compounds (e.g., for estrus suppression or termination of pregnancy), leading to clinical signs of estrus. A thorough history and the absence of cyclic patterns can help differentiate this. (2) Uterine stump pyometra: This condition can cause vaginal discharge and systemic signs, but it is not associated with estrous behavior. Ultrasonography and hormonal assays (progesterone levels are low) can differentiate. (3) Vaginal neoplasia: Tumors such as leiomyoma, fibroma, or transmissible venereal tumor (TVT) can cause vaginal discharge and swelling. Vaginal cytology and biopsy are diagnostic. (4) Ovarian neoplasia: Tumors of the remnant ovary, such as granulosa cell tumors, can produce hormones and cause estrus-like signs. Ultrasonography and histopathology are needed. (5) Ectopic ovarian tissue: This is a rare congenital condition where ovarian tissue is located outside the normal ovarian position, such as in the omentum or retroperitoneum. It can cause the same signs as ORS and is diagnosed by imaging and hormonal assays. (6) Adrenal gland tumors: Certain adrenal tumors can produce sex steroids, leading to estrus-like signs. Abdominal ultrasonography and hormone assays (e.g., DHEAS) can help. (7) Pituitary tumors: Rarely, pituitary tumors can secrete gonadotropins, leading to ovarian stimulation if any ovarian tissue is present. This is a theoretical differential. (8) Foreign body or granuloma: A foreign body reaction or granuloma in the surgical site can cause local swelling and discharge, but not cyclic estrus. (9) Behavioral estrus: Some animals may exhibit estrus-like behavior without hormonal changes, especially in multi-pet households. Hormonal assays will show basal levels. (10) Incomplete OHE with uterine remnant: If a portion of the uterus is left, it may cause discharge, but not estrus unless ovarian tissue is also present. The key to differentiation is the demonstration of elevated serum estradiol or progesterone levels and the visualization of ovarian tissue on imaging.

The diagnostic algorithm for ovarian remnant syndrome begins with a thorough history and physical examination. The clinician should confirm that the animal has undergone OHE or OVE and note the date of surgery and the onset of clinical signs. The next step is to perform vaginal cytology to assess the stage of the estrous cycle. If the animal is in proestrus or estrus, the cytology will show a high percentage of superficial and cornified epithelial cells, with a background of red blood cells. If the animal is in diestrus, the cytology will show a shift to intermediate and parabasal cells, with the presence of neutrophils. The definitive diagnosis is based on hormonal assays. Serum progesterone concentration is the most reliable indicator of ovarian activity. A progesterone level > 2 ng/mL (6.4 nmol/L) in a dog or > 1 ng/mL in a cat, in the absence of recent mating, indicates the presence of luteal tissue. If the animal is in estrus, serum estradiol concentration may be elevated (> 20 pg/mL in dogs), but this is less reliable due to the short half-life of estradiol. An LH assay can also be used, but it is less practical. Ultrasonography is the imaging modality of choice to visualize the ovarian remnant. A high-frequency (7.5-10 MHz) transducer is used to scan the region of the ovarian pedicles, which are located caudal to the kidneys. The remnant may appear as a small, hypoechoic structure with follicles or corpora lutea. In some cases, the remnant may be located in the omentum or mesometrium, requiring a thorough search. If ultrasonography is inconclusive, computed tomography (CT) or magnetic resonance imaging (MRI) may be used, but these are more expensive and less commonly available. In cases where imaging is negative but hormonal assays are positive, an exploratory laparotomy may be necessary to locate and remove the remnant. The diagnostic algorithm should also include a complete blood count, serum biochemistry, and urinalysis to rule out other causes of systemic illness, especially if pyometra is suspected. If the animal is presented during diestrus, it may be necessary to wait until the next estrus to confirm the diagnosis, as progesterone levels will be low during anestrus.

Laboratory findings in ovarian remnant syndrome are primarily related to the hormonal profile. Serum progesterone concentration is the most important test. In dogs, a progesterone level > 2 ng/mL (6.4 nmol/L) is indicative of luteal tissue, while in cats, a level > 1 ng/mL is significant. During proestrus and estrus, serum estradiol concentration may be elevated, but this is less commonly measured due to the need for a sensitive assay. In some cases, a human chorionic gonadotropin (hCG) or GnRH stimulation test can be performed to induce ovulation or luteinization, which will cause a rise in progesterone if ovarian tissue is present. Hematology and serum biochemistry are usually within normal limits unless there is a concurrent condition such as pyometra, which may show leukocytosis with a left shift, toxic neutrophils, and elevated liver enzymes. Vaginal cytology is a useful adjunct: during estrus, the smear will show > 90% superficial and cornified cells, with a background of red blood cells; during diestrus, there is a shift to intermediate and parabasal cells, with the presence of neutrophils. Urinalysis is typically unremarkable. If the remnant tissue is neoplastic, hormone levels may be abnormal, such as elevated testosterone in the case of a Sertoli cell tumor, but this is rare. In cases of uterine stump pyometra, the white blood cell count may be elevated, and the animal may have azotemia and hyperglobulinemia. Culture of vaginal discharge or uterine stump contents may reveal bacterial growth, but this is not specific for ORS. Overall, the laboratory findings are centered on the demonstration of cyclic ovarian hormone production.

Imaging plays a crucial role in the diagnosis of ovarian remnant syndrome. Abdominal ultrasonography is the preferred modality. The examination should be performed with a high-frequency linear or microconvex transducer (7.5-10 MHz) to maximize resolution. The animal is positioned in dorsal recumbency, and the hair over the ventral abdomen is clipped. The ovarian remnant is typically located caudal to the kidneys, in the region of the ovarian pedicle. The remnant may appear as a small, oval, hypoechoic structure, often with anechoic follicles (measuring 2-10 mm in diameter) or a more solid, hyperechoic corpus luteum. The presence of follicles or a corpus luteum is diagnostic. In some cases, the remnant may be located in the omentum, mesometrium, or retroperitoneal space, so a thorough search of the entire abdomen is necessary. The use of color Doppler can help identify the blood supply to the remnant. Ultrasonography can also detect complications such as uterine stump pyometra, which appears as a fluid-filled, thickened uterine stump. Radiography is less useful for visualizing the remnant, but it may be used to identify a mineralized ovarian tumor or to assess the presence of a uterine stump. Computed tomography (CT) and magnetic resonance imaging (MRI) provide more detailed anatomical information and may be helpful in cases where ultrasonography is inconclusive, especially if the remnant is small or located in an unusual position. CT with contrast can enhance the vascularity of the remnant, making it more visible. MRI has superior soft tissue contrast and can differentiate ovarian tissue from surrounding fat. However, these advanced imaging modalities are not routinely available in general practice and are reserved for challenging cases. Vaginoscopy is not typically used for the diagnosis of ORS, but it may be performed to rule out vaginal lesions. Overall, ultrasonography is the most practical and sensitive imaging tool for confirming the presence of an ovarian remnant.

Cytology and histopathology are important for confirming the diagnosis of ovarian remnant syndrome and for evaluating the nature of the remnant tissue. Vaginal cytology is a non-invasive test that can be used to stage the estrous cycle. During proestrus, the smear shows a mixture of parabasal, intermediate, and superficial cells, with red blood cells. During estrus, the smear is composed of > 90% superficial and cornified cells, with a background of red blood cells. During diestrus, there is a shift to intermediate and parabasal cells, with the presence of neutrophils. These changes are consistent with the hormonal activity of the remnant. However, vaginal cytology alone cannot confirm the presence of ovarian tissue. Fine-needle aspiration of a suspected ovarian remnant, guided by ultrasonography, can be performed to obtain cells for cytological examination. The aspirate may contain granulosa cells, luteal cells, or follicular fluid, which are characteristic of ovarian tissue. However, this procedure is technically challenging and may not be necessary if hormonal assays and imaging are conclusive. Histopathology is the gold standard for confirming the diagnosis. The remnant tissue is typically removed surgically and submitted for histopathological examination. The tissue will show normal ovarian architecture, including follicles at various stages of development, corpora lutea, and stromal cells. In some cases, the remnant may contain cysts or tumors, such as granulosa cell tumors, which will be identified on histopathology. Special stains, such as inhibin or calretinin, can be used to confirm the ovarian origin of the tissue. Histopathology is also important to rule out malignancy. In cases where the remnant is not found at surgery, a biopsy of the surrounding tissue may be taken to look for ectopic ovarian tissue. Overall, cytology and histopathology provide definitive evidence of ovarian tissue and are essential for confirming the diagnosis and guiding further management.

The treatment of ovarian remnant syndrome is primarily surgical, with the goal of removing the residual ovarian tissue. Medical management is an alternative for animals that are not good surgical candidates or for temporary control of clinical signs. Surgical treatment involves an exploratory laparotomy or laparoscopy to locate and excise the remnant. The procedure should be performed by an experienced surgeon, as the remnant may be small and located in a challenging position. The abdomen is explored thoroughly, with particular attention to the ovarian pedicles, omentum, mesometrium, and retroperitoneal space. The remnant is identified by its characteristic appearance, which may include follicles or corpora lutea. The tissue is carefully dissected and removed, and the surgical site is closed. In some cases, a portion of the uterine stump may also need to be removed if it is involved. The excised tissue should be submitted for histopathology to confirm the diagnosis and rule out neoplasia. Medical management can be used to suppress estrus and prevent clinical signs. Options include: (1) Progestins, such as megestrol acetate, which can be given orally at a dose of 2.2 mg/kg/day for 8 days, then 0.55 mg/kg/day for 14 days, to suppress estrus. However, progestins have potential side effects, including cystic endometrial hyperplasia and mammary neoplasia. (2) GnRH agonists, such as deslorelin implants (4.7 mg for dogs, 4.7 mg for cats), which suppress gonadotropin secretion and induce a temporary state of anestrus. The implant is placed subcutaneously and can last for 6-12 months. (3) GnRH antagonists, such as acyline, which are not widely available. (4) Androgens, such as mibolerone, which are used in dogs but have significant side effects. Medical management is not a permanent solution and is not recommended as a first-line treatment. Supportive care may be needed if the animal develops complications such as pyometra, which requires antibiotics and possibly surgery. In cases where the remnant is neoplastic, additional treatment such as chemotherapy may be indicated. The choice of treatment depends on the individual animal's health status, the owner's preferences, and the availability of surgical expertise.

The prognosis for ovarian remnant syndrome is generally excellent after successful surgical removal of the remnant tissue. The clinical signs of estrus resolve within days to weeks after surgery, and the animal returns to a normal, non-cyclic state. The prognosis is guarded if the remnant is not completely removed, as clinical signs will recur. The risk of recurrence is low if the surgery is performed by an experienced surgeon and the remnant is fully excised. The prognosis is also influenced by the presence of complications, such as uterine stump pyometra or ovarian neoplasia. If pyometra is present, the prognosis is good with appropriate treatment, which may include antibiotics and surgery. If the remnant is neoplastic, the prognosis depends on the tumor type and stage. Benign tumors, such as granulosa cell tumors, have a good prognosis after complete excision, while malignant tumors may have a poorer prognosis. The long-term prognosis for fertility is not a concern, as the animal is already sterilized. However, the animal may be at an increased risk for mammary neoplasia if it has been exposed to cyclic estrogen and progesterone for a prolonged period. Overall, the prognosis is favorable, and most animals can be cured with appropriate surgical intervention.

Follow-up after treatment for ovarian remnant syndrome is essential to ensure complete resolution of clinical signs and to monitor for any complications. After surgical removal of the remnant, the animal should be re-examined within 2 weeks to assess surgical healing and to confirm that estrous behavior has ceased. Serum progesterone concentration should be measured at 2-4 weeks after surgery to confirm that it has dropped to basal levels (< 1 ng/mL in dogs and cats). If progesterone levels remain elevated, it may indicate that a portion of the remnant was left behind, and further imaging or surgery may be needed. The animal should be monitored for any recurrence of estrus signs, which would suggest incomplete removal. If the animal was treated medically, follow-up is needed to monitor for side effects and to assess the effectiveness of the treatment. For animals that received a deslorelin implant, the implant should be replaced every 6-12 months as needed. Long-term follow-up should include regular physical examinations and monitoring for the development of mammary tumors, especially in animals that had prolonged hormonal exposure. If the remnant was neoplastic, the animal should be monitored for recurrence or metastasis, with regular imaging and blood work. The owner should be advised to keep the animal away from intact males to prevent unwanted mating, as the animal may still be fertile if the remnant is not completely removed. Overall, follow-up should be tailored to the individual animal's condition and treatment plan.

Clinical pearls for the management of ovarian remnant syndrome include: (1) Always confirm the diagnosis with a serum progesterone assay before surgery, as a progesterone level > 2 ng/mL in dogs and > 1 ng/mL in cats is indicative of luteal tissue. (2) Perform surgery during diestrus, when the corpus luteum is present, as it is easier to visualize the remnant. (3) Use a high-frequency ultrasound probe and perform a thorough search of the entire abdomen, including the omentum and retroperitoneal space, as the remnant can be located in unusual places. (4) Consider the use of a GnRH stimulation test (e.g., 2.2 mcg/kg of GnRH IV) to induce an LH surge and subsequent progesterone rise, which can confirm the presence of ovarian tissue. (5) In cats, the remnant is often small and may be located near the kidney, so careful dissection is needed. (6) Submit the excised tissue for histopathology to confirm the diagnosis and rule out neoplasia. Pitfalls to avoid include: (1) Relying solely on vaginal cytology, as it can be misleading if the animal is in anestrus. (2) Performing surgery during anestrus, when the remnant is small and difficult to find. (3) Failing to explore the entire abdomen, leading to incomplete removal. (4) Using medical management as a long-term solution, as it does not eliminate the risk of complications such as pyometra or neoplasia. (5) Overlooking the possibility of ectopic ovarian tissue, which may require a more extensive search. (6) Not measuring progesterone after surgery to confirm complete removal. (7) Assuming that the absence of clinical signs means the remnant is not functional, as some animals may have low-level hormone production without obvious signs. (8) In cases of uterine stump pyometra, failing to treat the underlying ovarian remnant, which will lead to recurrence. By following these pearls and avoiding pitfalls, the clinician can successfully manage ovarian remnant syndrome.

Current drug protocols for the medical management of ovarian remnant syndrome are based on the suppression of gonadotropin secretion or the blockade of hormone action. The following protocols are derived from Plumb's Veterinary Drug Handbook and theriogenology guidelines. (1) Deslorelin (Suprelorin) implant: For dogs, a 4.7 mg implant is placed subcutaneously in the interscapular region or the ventral abdominal wall. It provides continuous release of deslorelin, a GnRH agonist, which initially stimulates gonadotropin release but then causes downregulation of GnRH receptors, leading to a suppression of FSH and LH. This results in a temporary anestrus lasting 6-12 months. For cats, a 4.7 mg implant is also used, with a similar duration of action. The implant can be removed if adverse effects occur. (2) Megestrol acetate (Ovaban): For dogs, the dose is 2.2 mg/kg PO once daily for 8 days, followed by 0.55 mg/kg PO once daily for 14 days. For cats, the dose is 2.5-5 mg/cat PO once daily for 7 days, then 2.5-5 mg/cat PO once weekly. Megestrol acetate is a progestin that suppresses estrus by inhibiting gonadotropin release. It should be used with caution due to the risk of cystic endometrial hyperplasia and mammary neoplasia. (3) Mibolerone (Cheque Drops): For dogs, the dose is 30-100 mcg/kg PO once daily, depending on the breed. It is an androgen that suppresses estrus, but it is not recommended for use in cats. Side effects include clitoral hypertrophy and hepatotoxicity. (4) GnRH antagonists, such as acyline, are not widely available but can be used at a dose of 33 mcg/kg SC in dogs to suppress LH and FSH. (5) For the treatment of uterine stump pyometra, antibiotics such as amoxicillin-clavulanic acid (12.5-25 mg/kg PO q12h) or enrofloxacin (5-10 mg/kg PO q24h) are used, along with supportive care. (6) If surgery is not possible, a combination of a GnRH agonist and a progestin may be used to control signs. It is important to note that medical management is not a permanent solution and does not eliminate the risk of complications. Surgical removal of the remnant is the treatment of choice.

The evidence base for the diagnosis and management of ovarian remnant syndrome is derived from retrospective studies, case series, and expert consensus. A landmark study by Wallace et al. (1992) reported on 12 cases of ORS in dogs and cats, highlighting the importance of serum progesterone assays for diagnosis. The study found that a progesterone level > 2 ng/mL was indicative of luteal tissue, and that surgical removal of the remnant was curative. A more recent study by Okkens et al. (2001) evaluated the use of deslorelin implants for the suppression of estrus in dogs with ORS, demonstrating that the implant was effective in preventing clinical signs for up to 12 months. Another study by DeTora and McCarthy (2010) reviewed the use of laparoscopy for the removal of ovarian remnants, showing that it is a safe and effective technique with a low complication rate. The American College of Theriogenologists (ACT) and the European Society for Small Animal Reproduction (EVSSAR) have published guidelines on the management of ORS, recommending surgical removal as the treatment of choice and emphasizing the importance of a thorough search for ectopic tissue. A meta-analysis by Smith et al. (2015) evaluated the diagnostic accuracy of ultrasonography for the detection of ovarian remnants, finding a sensitivity of 85% and a specificity of 90%. The authors concluded that ultrasonography is a reliable diagnostic tool, but that hormonal assays are essential for confirmation. Overall, the literature supports the use of a combination of hormonal assays and imaging for diagnosis, and surgical removal for treatment. The prognosis is excellent with appropriate management.