Abnormal Uterine Involution

Abnormal uterine involution refers to the delayed, incomplete, or pathological regression of the uterus to its non-pregnant state following parturition. In the bitch and queen, normal uterine involution is a rapid process involving myometrial contraction, endometrial sloughing and regeneration, and reduction in uterine size, typically completed within 4 to 6 weeks postpartum. Abnormal involution encompasses a spectrum of conditions including subinvolution of placental sites (SIPS), retained fetal membranes, postpartum metritis, and uterine torsion or prolapse. These conditions are characterized by persistent uterine enlargement, abnormal vaginal discharge, systemic signs of illness, and potential impairment of future fertility. The condition is clinically significant due to its association with maternal morbidity, mortality, and compromised subsequent reproductive performance.

The etiology of abnormal uterine involution is multifactorial. Primary causes include: 1) Retained fetal membranes (RFM), which occur when the placental zonary attachment fails to separate normally, often due to uterine inertia, dystocia, or premature placental separation. 2) Subinvolution of placental sites (SIPS), a condition of unknown etiology but possibly related to incomplete phagocytosis of the placental bed by macrophages, leading to persistent hemorrhage. 3) Postpartum metritis, typically caused by bacterial infection (Escherichia coli, Streptococcus spp., Staphylococcus spp., and anaerobic organisms) ascending from the vagina or introduced during obstetrical manipulation. 4) Uterine torsion or prolapse, which may result from excessive straining or uterine relaxation. 5) Hormonal imbalances, such as persistent luteal function or inadequate prostaglandin F2alpha release, leading to delayed myometrial contraction. 6) Iatrogenic factors, including improper use of oxytocin or ergot alkaloids, or trauma during manual delivery. 7) Congenital or acquired uterine abnormalities, such as uterine hypoplasia or fibrosis, which impair contractility. 8) Systemic diseases, including hypocalcemia, which reduces myometrial contractility.

Abnormal uterine involution is reported in both dogs and cats, with a higher incidence in bitches than queens. In dogs, the condition is more common in middle-aged, multiparous females, particularly those with a history of dystocia or cesarean section. Certain breeds, such as Boxers, Golden Retrievers, and German Shepherds, may be predisposed to uterine inertia and subsequent RFM. The incidence of SIPS is estimated at 3-5% of postpartum bitches, while postpartum metritis occurs in up to 5% of bitches. In cats, abnormal involution is less frequently reported but can occur following dystocia or cesarean section. Risk factors include prolonged parturition, retained fetuses, poor hygiene during delivery, and concurrent systemic illness. The condition is more prevalent in animals housed in kennels or catteries with high infectious pressure.

Normal uterine involution involves a series of coordinated events: myometrial contractions to expel lochia and reduce uterine size, endometrial degeneration and sloughing, and regeneration of the endometrium. Prostaglandin F2alpha and oxytocin are key mediators of myometrial contraction. In abnormal involution, these processes are disrupted. In RFM, the placental zonary attachment persists due to inadequate myometrial contraction or failure of the placental separation mechanism, leading to retention of fetal membranes and providing a nidus for bacterial infection. In SIPS, the placental sites fail to undergo normal involution, with persistent hemorrhage from incompletely thrombosed uterine vessels. Histologically, there is an accumulation of hyalinized material and trophoblastic remnants within the endometrium. In postpartum metritis, bacterial colonization of the uterine lumen occurs, leading to an inflammatory response characterized by neutrophilic infiltration, tissue necrosis, and systemic signs of toxemia. The release of bacterial toxins and inflammatory mediators can cause endotoxemia and sepsis. Uterine torsion or prolapse results in vascular compromise and tissue ischemia, leading to necrosis and potential rupture.

Intrinsic predisposing factors include: 1) Age: older females have reduced myometrial contractility. 2) Breed: brachycephalic breeds and those with a history of uterine inertia. 3) Parity: multiparous animals are at higher risk due to uterine muscle fatigue. 4) Hormonal imbalances: luteal insufficiency or persistent corpus luteum. 5) Genetic anomalies: uterine malformations. 6) Obesity: excessive fat deposition impairs uterine contraction. Extrinsic factors include: 1) Exogenous steroid administration during pregnancy, which can delay parturition and involution. 2) Improper breeding timing leading to prolonged gestation. 3) Poor hygiene during delivery, increasing infection risk. 4) Stressful kenneling conditions. 5) Inappropriate obstetrical intervention, such as excessive manual traction or use of oxytocin without proper indication.

Clinical signs vary depending on the underlying cause. In RFM, there may be a visible portion of fetal membranes protruding from the vulva, often accompanied by a foul-smelling, dark green or brown vaginal discharge. SIPS is characterized by persistent serosanguineous to hemorrhagic vaginal discharge lasting beyond 3 weeks postpartum, without systemic signs. Postpartum metritis presents with systemic signs including fever (often >103.5°F), lethargy, anorexia, depression, and a purulent, malodorous vaginal discharge. Abdominal palpation may reveal an enlarged, doughy uterus, and the animal may show signs of abdominal pain. In severe cases, septicemia can lead to shock, disseminated intravascular coagulation, and death. Uterine torsion or prolapse presents with acute abdominal pain, straining, and a visible mass protruding from the vulva (prolapse) or signs of acute abdomen (torsion).

Differential diagnoses include: 1) Normal postpartum involution: physiological discharge and uterine size reduction, but no systemic signs. 2) Retained fetus: abdominal palpation and imaging reveal fetal remnants. 3) Pyometra: occurs in intact females, typically 1-2 months post-estrus, with closed or open cervix, and characteristic ultrasonographic findings. 4) Vaginal trauma or laceration: history of difficult delivery, visible vaginal tears on vaginoscopy. 5) Urinary tract infection: dysuria, hematuria, and bacteriuria. 6) Coagulopathy: bleeding disorders leading to persistent hemorrhage. 7) Uterine neoplasia: rare, but can cause abnormal discharge and uterine enlargement. 8) Peritonitis: secondary to uterine rupture, with acute abdomen and effusion. 9) Mastitis: mammary gland inflammation, but may coexist with uterine disease. 10) Metritis secondary to Brucella canis infection: zoonotic, with abortion and infertility.

The diagnostic approach should be systematic: 1) Obtain a thorough history including parturition details, duration, and any interventions. 2) Perform a complete physical examination, including temperature, heart rate, respiratory rate, and abdominal palpation. 3) Evaluate vaginal discharge: note color, consistency, odor, and quantity. 4) Perform vaginal cytology to assess for inflammatory cells and bacteria. 5) Obtain blood samples for complete blood count, serum biochemistry, and serum progesterone measurement. 6) Perform abdominal ultrasonography to assess uterine size, wall thickness, luminal contents, and ovarian structures. 7) If RFM is suspected, perform a digital vaginal examination or vaginoscopy to identify retained membranes. 8) Obtain uterine culture and sensitivity via guarded swab if metritis is suspected. 9) In cases of suspected torsion or prolapse, radiography or advanced imaging (CT/MRI) may be indicated. 10) Consider histopathology of uterine biopsy if chronic SIPS is suspected.

Hematology may reveal leukocytosis with a left shift, toxic neutrophils, and anemia in cases of metritis or SIPS. Biochemistry may show azotemia, elevated liver enzymes, and hypocalcemia. Serum progesterone levels are typically low (<1 ng/mL) in normal postpartum involution, but may be elevated in cases of retained luteal tissue. Vaginal cytology in metritis shows numerous neutrophils, bacteria, and degenerate epithelial cells. In SIPS, cytology may show red blood cells and macrophages. Uterine culture may yield E. coli, Streptococcus, Staphylococcus, or other pathogens. Histopathology of uterine biopsy in SIPS reveals hyalinized placental site remnants and trophoblastic cells.

Abdominal ultrasonography is the primary imaging modality. In normal involution, the uterus should decrease in size and return to a tubular structure within 4 weeks. In abnormal involution, the uterus may appear enlarged with thickened walls and echogenic luminal contents. In metritis, the uterine lumen may contain hypoechoic to hyperechoic fluid with gas foci. In SIPS, the placental sites may appear as hyperechoic areas with shadowing. Radiography can be used to detect retained fetal skeletons or uterine torsion (loss of abdominal detail). CT and MRI are rarely needed but can provide detailed assessment of uterine pathology. Vaginoscopy can visualize the vaginal vault and cervix, identifying retained membranes or discharge.

Vaginal cytology is a rapid diagnostic tool. In metritis, there is a predominance of neutrophils, often with intracellular bacteria. In SIPS, cytology shows red blood cells, macrophages, and occasional trophoblastic cells. Histopathology of uterine biopsy is the gold standard for diagnosing SIPS, revealing hyalinized placental site remnants, trophoblastic cells, and fibrosis. In metritis, histopathology shows diffuse neutrophilic infiltration, necrosis, and bacterial colonies. Special stains such as Gram stain can identify bacterial types.

Treatment depends on the underlying cause. For RFM, manual removal may be attempted if membranes are easily accessible, but care must be taken to avoid uterine trauma. Oxytocin (0.5-2 IU/kg IM or SC) can be administered to stimulate uterine contraction, but only after ensuring the cervix is open. For metritis, treatment includes: 1) Fluid therapy with crystalloids to correct dehydration and electrolyte imbalances. 2) Broad-spectrum antibiotics (e.g., amoxicillin-clavulanate 12.5-25 mg/kg PO q8h, or enrofloxacin 5-10 mg/kg PO/IM q24h) based on culture and sensitivity. 3) Prostaglandin F2alpha (dinoprost 0.1-0.25 mg/kg SC q8h, or cloprostenol 1-5 mcg/kg SC q24h) to promote uterine evacuation. 4) Oxytocin (0.5-2 IU/kg IM q2-4h) may be used if the cervix is open. 5) In severe cases, ovariohysterectomy may be necessary to remove the infected uterus. For SIPS, treatment is primarily supportive, with antibiotics if secondary infection is present, and surgical intervention (ovariohysterectomy) if hemorrhage is severe or persistent. For uterine torsion or prolapse, immediate surgical correction is required.

The prognosis for abnormal uterine involution is generally good with prompt and appropriate treatment. For RFM, the prognosis is excellent if treated early. For metritis, the prognosis is good to fair, with a mortality rate of up to 10% in severe cases. For SIPS, the prognosis is good, with most cases resolving spontaneously within 4-6 weeks; however, severe hemorrhage may require surgery. Future fertility may be impaired in cases of severe metritis or SIPS, with a risk of recurrent disease. Negative prognostic indicators include systemic signs, prolonged duration of illness, and development of sepsis or peritonitis.

Post-treatment monitoring should include: 1) Serial ultrasonography every 3-5 days to assess uterine involution. 2) Serum progesterone levels weekly until <1 ng/mL. 3) Vaginal cytology every 3-5 days to monitor resolution of inflammation. 4) Complete blood count and biochemistry every 3-5 days until normalized. 5) In cases of metritis, repeat uterine culture 2 weeks after antibiotic therapy. 6) For breeding animals, a breeding soundness examination should be performed at the next estrus, including vaginal cytology, progesterone, and ultrasonography to confirm normal uterine health.

Pearls: 1) Normal postpartum discharge (lochia) is dark green to brown and may persist for up to 3 weeks; persistent serosanguineous discharge beyond 3 weeks suggests SIPS. 2) In metritis, systemic signs are often severe; early aggressive fluid therapy and antibiotics are crucial. 3) Prostaglandin F2alpha should be used with caution in animals with closed cervix to avoid uterine rupture. 4) Oxytocin is contraindicated in cases of uterine torsion or obstruction. Pitfalls: 1) Failure to recognize retained fetal membranes, leading to metritis. 2) Overuse of oxytocin without ensuring cervical patency. 3) Delaying surgical intervention in cases of uterine torsion or prolapse. 4) Using antibiotics without culture and sensitivity, leading to resistance. 5) Neglecting to monitor for DIC in septic cases.

Based on Plumb's Veterinary Drug Handbook and theriogenology guidelines: 1) Oxytocin: 0.5-2 IU/kg IM or SC, q2-4h, for uterine contraction. 2) Prostaglandin F2alpha (dinoprost tromethamine): 0.1-0.25 mg/kg SC, q8h, for metritis. 3) Cloprostenol: 1-5 mcg/kg SC, q24h, for metritis. 4) Cabergoline: 5 mcg/kg PO, q24h, for 5-7 days, to reduce prolactin and luteal function. 5) Amoxicillin-clavulanate: 12.5-25 mg/kg PO, q8h, for 7-14 days. 6) Enrofloxacin: 5-10 mg/kg PO or IM, q24h, for 7-14 days (use with caution in young animals). 7) Metronidazole: 10-15 mg/kg PO, q12h, for anaerobic coverage. 8) Calcium gluconate: 10% solution, 0.5-1.5 mL/kg IV slowly, for hypocalcemia. 9) Fluid therapy: Lactated Ringer's solution at 60-90 mL/kg/day IV, adjusted based on hydration status.

Key studies include: 1) Johnston, Kustritz, and Olson (2001) in 'Canine and Feline Theriogenology' provide comprehensive guidelines on postpartum uterine involution and disorders. 2) Noakes, Parkinson, and England (2019) in 'Veterinary Reproduction and Obstetrics' discuss the pathophysiology of retained fetal membranes and metritis. 3) England and von Heimendahl (2010) in 'BSAVA Manual of Small Animal Reproduction' outline diagnostic and therapeutic protocols. 4) A study by Fieni et al. (2014) evaluated the use of aglepristone in the management of postpartum metritis in bitches, showing improved outcomes. 5) A retrospective study by Smith (2005) reported that early ovariohysterectomy in severe metritis reduces mortality. 6) Consensus guidelines from the European Society for Small Animal Reproduction (EVSSAR) recommend the use of prostaglandins and antibiotics for metritis, with surgery reserved for refractory cases.