Nephrosplenic Entrapment and Right Dorsal Displacement of the Large Colon

Nephrosplenic entrapment (NSE) and right dorsal displacement (RDD) are two distinct forms of large colon displacement in horses, representing significant causes of acute abdominal pain (colic). NSE, also known as left dorsal displacement of the large colon, occurs when the left colon (pelvic flexure and left ventral colon) becomes entrapped within the nephrosplenic space, a potential space bounded by the spleen medially, the left kidney dorsally, the left body wall laterally, and the nephrosplenic ligament ventrally. This entrapment can lead to partial or complete luminal obstruction and venous congestion, but strangulation is rare. RDD involves the displacement of the large colon to the right side of the cecum, often with the colon folding between the cecum and the right body wall, resulting in a non-strangulating obstruction. Both conditions are common in adult horses, particularly those used for pleasure riding, showing, and racing, and require prompt diagnosis and management to prevent complications such as colonic ischemia, rupture, and death.

The exact etiology of NSE and RDD is multifactorial, involving anatomical, dietary, and management factors. NSE is often associated with sudden changes in diet, high-grain feeding, lack of access to pasture, and recent exercise or rolling. The displacement may occur when the colon migrates dorsally and becomes trapped in the nephrosplenic space, possibly due to excessive gas accumulation in the colon, which increases its buoyancy and mobility. RDD is similarly linked to colonic gas distention and abnormal motility, often precipitated by abrupt dietary changes, inadequate water intake, or stress. In both conditions, the colon's mobility within the abdominal cavity is influenced by its mesenteric attachments, and any factor that increases colonic gas or alters motility can predispose to displacement. Additionally, conformational factors such as a deep, narrow body shape may increase the risk of NSE. There is no evidence of infectious or parasitic causes directly, but conditions that cause colic, such as ileal impaction or enterolithiasis, may predispose to secondary displacements.

NSE and RDD are most commonly diagnosed in adult horses, with a median age of 8-12 years. There is no strong breed predisposition, but larger breeds such as Warmbloods, Thoroughbreds, and Quarter Horses may be overrepresented due to their body size and use in disciplines that involve intermittent high-intensity exercise. Geldings and mares appear equally affected. The conditions are more frequent in horses that are stalled for prolonged periods and fed high-concentrate diets, as opposed to those on pasture. Seasonal variation has been reported, with a higher incidence in the spring and fall, possibly related to changes in forage quality and management. Morbidity rates are significant, with NSE accounting for approximately 5-10% of all colic cases in referral hospitals, while RDD is less common. Mortality is generally low for uncomplicated cases, but can increase if surgical intervention is delayed or if concurrent conditions such as colitis or peritonitis develop. Athletic career impact is variable; most horses that recover without complications can return to their previous level of performance, but those requiring surgery may have a longer recovery period.

The pathophysiology of NSE involves the migration of the left colon into the nephrosplenic space, leading to partial or complete obstruction of the colonic lumen. The entrapment typically involves the pelvic flexure and adjacent colon, which become compressed between the spleen and left kidney. This compression impedes the passage of gas and ingesta, causing progressive distension of the colon proximal to the obstruction. Venous congestion develops due to impaired venous return, leading to edema and thickening of the colonic wall. Arterial blood supply is usually maintained, so ischemia is not a primary feature unless the entrapment is severe and prolonged. In RDD, the colon becomes displaced to the right side, often folding between the cecum and the right body wall. This results in a non-strangulating obstruction, but the colon may become distended and the cecum may also be affected. The displacement can cause kinking of the colon, leading to luminal obstruction and similar sequelae of gas accumulation and venous congestion. In both conditions, if left untreated, the increasing intraluminal pressure can compromise mucosal integrity, leading to bacterial translocation, endotoxemia, and systemic inflammatory response syndrome (SIRS).

Predisposing factors for NSE and RDD include intrinsic factors such as age (adult horses), breed (larger breeds), and individual variations in colonic anatomy and mesenteric attachments. Extrinsic factors are more significant and include sudden changes in diet, particularly an increase in concentrate feeding or a decrease in forage quality, which can alter colonic fermentation and gas production. Inadequate water intake, especially in winter, can lead to impaction and increased colonic motility. Management practices such as prolonged stall confinement, lack of regular exercise, and transportation stress can also predispose to displacement. Additionally, horses that are prone to colic due to previous episodes of large colon displacement or impaction may be at higher risk. High-performance horses undergoing intense training may experience altered gastrointestinal motility due to stress and corticosteroid release. Finally, conditions that cause abdominal pain, such as gastric ulcers or enteroliths, may lead to rolling behavior that can precipitate a displacement.

Clinical signs of NSE and RDD are typical of moderate to severe colic. Horses may exhibit signs of abdominal pain including pawing, flank watching, rolling, lying down, and getting up frequently. The severity of pain can vary; some horses may show only mild discomfort, while others may be severely painful and unresponsive to analgesics. Heart rate is often elevated (60-100 bpm) and may be accompanied by tachypnea. Mucous membranes may be injected or toxic, with prolonged capillary refill time (>2 seconds). Gastrointestinal sounds may be reduced or absent, particularly in the left dorsal quadrant in NSE. Nasogastric reflux is often present, with volumes exceeding 2 liters, and may be foul-smelling and brownish if the obstruction is prolonged. Rectal palpation is a key diagnostic step: in NSE, the spleen is medially displaced, and the nephrosplenic ligament can be palpated, with the colon felt dorsal to it; in RDD, the cecum may be distended and the colon may be palpable on the right side. In severe cases, signs of shock, such as cold extremities and weak pulse, may develop.

Differential diagnoses for NSE and RDD include other causes of large colon obstruction and strangulation: 1) Large colon volvulus (LCV) - presents with acute severe pain, rapid deterioration, and often a distended colon on rectal palpation; peritoneal fluid is often serosanguineous with elevated protein and lactate. 2) Large colon impaction - typically causes mild to moderate pain, with a firm mass palpable per rectum; nasogastric reflux is less common. 3) Cecal impaction - similar to large colon impaction but located in the cecum; may cause chronic mild colic. 4) Enterolithiasis - may cause acute obstruction, often with a history of feeding alfalfa hay; radiography may reveal the enterolith. 5) Sand impaction - history of grazing on sandy soil; sand may be palpable per rectum or visible on radiographs. 6) Strangulating lipoma - more common in older horses, causes acute severe pain and often a palpable mass on rectal exam. 7) Ileal impaction - causes nasogastric reflux and small intestinal distension on ultrasound. 8) Gastric dilatation - may cause reflux and pain, but is often secondary to other obstructions. 9) Peritonitis - may cause fever, depression, and diffuse abdominal pain; peritoneal fluid analysis is diagnostic. 10) Colitis - causes diarrhea and endotoxemia, but may also cause colic. Definitive differentiation relies on rectal palpation, ultrasonography, and response to medical therapy.

The diagnostic algorithm for suspected NSE or RDD begins with a thorough physical examination, including assessment of pain severity, heart rate, mucous membranes, and abdominal auscultation. Nasogastric intubation is performed to check for reflux and to decompress the stomach. Rectal palpation is the next critical step; in NSE, the spleen is medially displaced, and the nephrosplenic ligament is palpable, with the colon entrapped dorsally. In RDD, the cecum may be distended, and the colon may be palpable on the right side. If rectal palpation is inconclusive, abdominal ultrasonography is performed to visualize the nephrosplenic space and assess colonic wall thickness and motility. In NSE, the colon may be seen entrapped between the spleen and kidney, and the spleen may be displaced medially. In RDD, the colon may be seen on the right side, and the cecum may be distended. Abdominocentesis is performed to evaluate peritoneal fluid for evidence of ischemia or peritonitis; normal peritoneal fluid has a protein <2.0 g/dL and WBC <5000/µL. If the diagnosis remains unclear, or if the horse does not respond to medical management, exploratory laparotomy is indicated. In cases where surgery is not immediately available, a trial of medical therapy with analgesics and fluid therapy may be attempted, but if pain persists or worsens, surgery is necessary.

Laboratory findings in NSE and RDD are often nonspecific but can reflect the severity of the obstruction and the presence of systemic inflammation. Complete blood count may show hemoconcentration (elevated PCV) due to dehydration, and leukopenia or leukocytosis depending on the stage. Plasma lactate is a valuable biomarker; a normal value is <2 mmol/L, but in cases of colonic distension and venous congestion, it may be elevated, though not as high as in strangulating lesions. Peritoneal fluid analysis is crucial: in uncomplicated NSE or RDD, the fluid is typically normal or mildly modified, with protein <2.5 g/dL and WBC <5000/µL. If the fluid becomes serosanguineous or has a protein >3.0 g/dL and lactate >5 mmol/L, it suggests ischemia or strangulation, which is uncommon in these displacements. Serum amyloid A (SAA) may be elevated in cases with inflammation. Blood gas analysis may reveal metabolic acidosis with elevated lactate and decreased bicarbonate. Electrolyte imbalances, particularly hypokalemia and hypocalcemia, may be present due to decreased feed intake and gastrointestinal losses. In horses with concurrent endotoxemia, fibrinogen may be elevated.

Abdominal ultrasonography is the primary imaging modality for NSE and RDD. In NSE, the nephrosplenic space is visualized on the left side, and the entrapped colon appears as a gas-filled structure with a thickened wall (>4 mm) and reduced motility. The spleen may be displaced medially, and the left kidney may be difficult to visualize. In RDD, the colon is seen on the right side, often with a distended cecum. Ultrasonography can also assess the degree of colonic distension and wall thickness, which helps guide treatment. Radiography is less commonly used but may be helpful in diagnosing enteroliths or sand impactions. In NSE, a lateral abdominal radiograph may show a gas-filled colon in the nephrosplenic space. Endoscopy is not directly useful for these conditions but may be performed to rule out gastric ulcers. Advanced imaging such as CT or MRI is rarely indicated but may be used in research settings. Scintigraphy is not used clinically for colic.

Cytological examination of peritoneal fluid is essential in the diagnostic workup. Normal peritoneal fluid is clear to pale yellow, with a total protein <2.0 g/dL and a nucleated cell count <5000/µL, predominantly macrophages and lymphocytes. In NSE or RDD, the fluid may be normal or show mild increases in protein and cell count due to inflammation. If the fluid becomes serosanguineous or has a protein >3.0 g/dL and lactate >5 mmol/L, it indicates intestinal ischemia or necrosis, which is rare in these displacements. Histopathology is not typically performed on biopsy samples from live horses, but if surgery is performed, resected colon segments may show edema, congestion, and mucosal sloughing in severe cases. In chronic cases, fibrosis may be present.

Treatment for NSE and RDD depends on the severity and duration of the displacement. Initial medical management is often attempted for NSE, as many cases can be corrected non-surgically. This includes withholding feed, providing intravenous fluid therapy (e.g., lactated Ringer's solution at 5-10 mL/kg/h) to correct dehydration and electrolyte imbalances, and administering analgesics such as flunixin meglumine (1.1 mg/kg IV q12h) or detomidine (0.01-0.02 mg/kg IV) for pain control. Phenylephrine (3-6 µg/kg IV over 15 minutes) is often used to induce splenic contraction, which may help release the entrapped colon. After administration, the horse is exercised or lunged to encourage the colon to fall out of the nephrosplenic space. If medical management fails or if the horse is in severe pain, surgical intervention is required. For RDD, medical management is less likely to be successful, and surgery is often necessary. Surgical treatment involves exploratory celiotomy, during which the colon is manually repositioned. In NSE, the colon is gently pushed out of the nephrosplenic space; in RDD, the colon is repositioned to its normal anatomical position. In cases of colonic ischemia or necrosis, resection and anastomosis may be required. Postoperative care includes continued fluid therapy, analgesics, and monitoring for complications such as ileus, peritonitis, and laminitis.

The prognosis for NSE and RDD is generally good, especially if treated early. For NSE, the success rate of medical management is high, with up to 90% of cases resolving without surgery. Surgical correction also has a good prognosis, with survival rates exceeding 80%. For RDD, surgical treatment is often required, and the prognosis is also good, with survival rates around 75-85%. Negative prognostic indicators include delayed treatment, the presence of colonic ischemia or necrosis, and the development of complications such as peritonitis or laminitis. Horses that recover without complications can usually return to their previous level of athletic performance, though a period of rest and gradual return to work is recommended. Recurrence rates are low, but horses that have had one episode may be at slightly higher risk for future displacements.

Follow-up care after treatment for NSE or RDD is crucial to ensure full recovery and prevent complications. Immediately after surgery, horses should be monitored closely for signs of ileus, such as nasogastric reflux, and for signs of peritonitis or laminitis. Serial nasogastric reflux measurements should be performed every 4-6 hours, and abdominal ultrasonography may be repeated to assess colonic motility. Horses should be gradually reintroduced to feed, starting with small amounts of high-quality forage and then slowly increasing to their normal diet over several days. Pain management should be tapered gradually. After discharge, horses should have a period of stall rest for 2-4 weeks, followed by hand walking and then gradual return to exercise over 4-6 weeks. Regular farriery is important to maintain hoof health, especially if the horse was at risk for laminitis. Recheck examinations, including physical examination and possibly abdominal ultrasound, should be performed at 2 weeks, 6 weeks, and 3 months post-treatment to ensure complete resolution.

Clinical pearls: 1) Always perform nasogastric intubation in any colic case to check for reflux and decompress the stomach. 2) In NSE, the spleen is medially displaced on rectal palpation, and the nephrosplenic ligament is palpable. 3) Phenylephrine is a useful adjunct to medical therapy for NSE, but it should be used with caution in horses with cardiac disease. 4) If a horse with suspected NSE does not respond to medical management within 24 hours, surgery is indicated. 5) Monitor for laminitis in any horse with severe colic, especially if there is endotoxemia. Pitfalls: 1) Failing to perform a rectal examination can lead to a missed diagnosis. 2) Administering phenylephrine without adequate fluid therapy can cause hypertension and worsen colic. 3) Delaying surgery in a horse with persistent pain can lead to colonic rupture or ischemia. 4) Overlooking the possibility of a concurrent lesion, such as a lipoma or enterolith, during surgery. 5) Not monitoring for postoperative ileus, which can be fatal.

Current drug protocols for NSE and RDD are based on Plumb's Veterinary Drug Handbook and ACVIM guidelines. For analgesia, flunixin meglumine is administered at 1.1 mg/kg IV q12h for up to 3 days; phenylbutazone at 4.4 mg/kg IV or PO q24h may be used for longer-term pain management. For sedation and pain control, detomidine (0.01-0.02 mg/kg IV) or xylazine (0.3-0.5 mg/kg IV) can be used as needed. For splenic contraction in NSE, phenylephrine is given at 3-6 µg/kg IV over 15 minutes, once. For fluid therapy, lactated Ringer's solution is administered at 5-10 mL/kg/h IV, with adjustments based on hydration status and electrolyte levels. In cases of endotoxemia, polymyxin B (1000-6000 IU/kg IV q12h) may be used. Antibiotics are indicated if surgery is performed; a common protocol is penicillin G (22,000 IU/kg IV q6h) and gentamicin (6.6 mg/kg IV q24h). For gastric ulcer prophylaxis, omeprazole (4 mg/kg PO q24h) is recommended. In the postoperative period, lidocaine CRI (1.3 mg/kg IV bolus followed by 0.05 mg/kg/min) may be used to promote gastrointestinal motility. For laminitis prevention, cryotherapy and nonsteroidal anti-inflammatory drugs are used.

Evidence-based literature on NSE and RDD includes several landmark studies. A retrospective study by Hardy et al. (2000) reported that medical management with phenylephrine and exercise was successful in 85% of NSE cases, with a low recurrence rate. A study by Abutarbush et al. (2005) compared medical versus surgical treatment for NSE and found that surgical correction had a higher success rate in horses with severe pain or failure of medical therapy. For RDD, a study by Pankowski et al. (1987) described the surgical technique and reported a survival rate of 80%. More recent studies have focused on the use of ultrasonography for diagnosis and the role of inflammatory markers such as SAA in predicting outcome. Consensus guidelines from the American College of Veterinary Surgeons (ACVS) recommend early surgical intervention for horses with persistent pain or evidence of ischemia. The American Association of Equine Practitioners (AAEP) has published guidelines on colic management, emphasizing the importance of prompt referral for surgical candidates. Overall, the evidence supports a good prognosis for both conditions with appropriate treatment.