A 24-year-old 732-kg (1,610-lb) Belgian mare was admitted to the veterinary teaching hospital for evaluation of intermittent colic of 3 days' duration. The mare was reported to be 320 days' pregnant, with prior gestations lasting between 340 and 360 days. The mare had undergone surgery at 3 years of age to relieve sand impaction of the large colon. Prior to admission, the mare reportedly had an increase in gastrointestinal tract sounds, and distention of the large colon was detected during palpation per rectum. Treatment by the referring veterinarian included administration of flunixin meglumine (1.1 mg/kg [0.5 mg/lb], IV) and nasogastric intubation (no reflux was obtained), with administration of water containing 12 oz of disodium succinate. Over the next 24 hours, the mare appeared comfortable and was passing manure. The following morning, however, the mare again appeared uncomfortable and was therefore referred to the veterinary teaching hospital for further diagnostic testing.
On initial examination at the veterinary teaching hospital, heart rate (60 beats/min) and respiratory rate (36 breaths/min) were high, and capillary refill time (3 seconds) was prolonged. Reduced borborygmi were ausculted in the right dorsal and ventral quadrants. Palpation per rectum revealed distention of the large intestine in the upper right quadrant of the abdomen and a gravid uterus. Abdominal ultrasonography revealed normal motility in visualized bowel segments as well as a viable fetus with a heart rate of 93 beats/min and normal-appearing uterine and placental walls. Results of a CBC, serum biochemical profile, and venous blood gas analyses were all within reference limits. The mare was sedated with xylazine hydrochloride (0.2 mg/kg [0.09 mg/lb], IV) during physical examination.
A tentative diagnosis of right dorsal displacement of the large colon was made. The mare was treated with lactated Ringer's solution (2 L/h, IV) and taken for frequent walks. Colic signs were mild at this time. Approximately 7 hours after admission, the mare became acutely painful with noticeable abdominal distention. A nasogastric tube was passed at this time, and 14 L of enterogastric reflux was obtained. In addition, palpation per rectum now revealed gas distention of the small and large intestines. Therefore, a decision was made to take the mare immediately to surgery. Potassium penicillin (22,000 U/kg [10,000 U/lb], IV), gentamicin (6.6 mg/kg [3 mg/lb], IV), and flunixin meglumine (1.1 mg/kg, IV) were administered prior to surgery.
The mare was sedated with xylazine (0.2 mg/kg, IV) and butorphanol (0.014 mg/kg [0.006 mg/lb], IV) and anesthetized with guaifenesin (48 mg/kg [22 mg/lb], IV) and ketamine (2.2 mg/kg [1 mg/lb], IV). The mare was hoisted via hobbles attached to all 4 limbs and placed in dorsal recumbency on 6-inch–thick padding. Anesthesia was maintained with isoflurane in oxygen administered with a large animal circle rebreathing circuit, and a balanced electrolyte solution was administered (27 mL/kg [12.3 mL/lb], IV) over a period of 3 hours. Mean arterial blood pressure was maintained at ≥ 70 mm Hg by adjustment of anesthetic depth and intermittent administration of dobutamine (5 μg/kg/min [2.3 μg/lb/min], IV). Blood gas analysis 20 minutes after anesthetic induction revealed hypoxemia with a PaO2 of 41 mm Hg. Adjustment of ventilatory parameters and administration of aerosolized albuterol (990 Mg divided into 3 doses over 60 minutes) increased the PaO2 to 77 mm Hg.
At surgery, extreme distention of the small intestine, cecum, and large colon was found. The cause of the gas distention was believed to be external compression of the ileum by the fetus. No displacements or torsions were found. The intestines were decompressed multiple times and placed back in the abdomen, and the incision was routinely closed. Total anesthesia time, including the time necessary for positioning on the surgery table, sterile preparation, and draping of the abdomen, was 3.25 hours. Surgery time was approximately 2.5 hours. Following discontinuation of anesthesia, the mare was hoisted again and placed in lateral recumbency in a padded recovery stall. Head and tail ropes were placed on the mare to assist recovery. The mare stood approximately 45 minutes after placement in the recovery stall. The recovery was complicated by an inability to bear weight on the left hind limb and flexion of the proximal interphalangeal, tarsal, and stifle joints in that limb. These signs were attributed to femoral nerve paresis. Palpation of the muscles of the hindquarters did not reveal any signs of pain, and the muscles were soft during palpation, suggesting that there was no concurrent myositis. Following surgery, potassium penicillin (22,000 U/kg, IV, q 6 h), gentamicin (6.6 mg/kg, IV, q 24 h), and flunixin meglumine (1.1 mg/kg, IV, q 12 h) were administered, along with lactated Ringer's solution (2 L/h, IV) to which calcium gluconate had been added. Butorphanol was also added to the lactated Ringer's solution at a quantity sufficient to provide a dosage of 0.015 mg/kg/h (0.007 mg/lb/h). A 5% solution of dimethyl sulfoxide (1 g/kg [0.45 g/lb], IV) was administered once after surgery.
The mare's condition was stable overnight, and signs of femoral neuropathy remained unchanged. The following morning, a CBC revealed a decrease in WBC count (3.82 × 103 cells/μL; reference range, 5.5 to 10.49 cells/μL), and serum biochemical analyses revealed mild increases in serum creatine kinase (2,865 U/L; reference range, 147 to 635 U/L) and aspartate transaminase (639 U/L; reference range, 216 to 365 U/L) activities. Results were otherwise unremarkable. A milk calcium test was performed, and results (calcium, 14.9 mg/dL; reference range, < 20 mg/dL) were inconsistent with impending parturition.
Beginning approximately 33 hours after recovery from surgery, the mare began to show signs of worsening pain in the hind limbs, including muscle tremors, stomping of the hind feet, and agitated circling in the stall. The muscle tremors and foot stomping were worse in the left hind limb than in the right hind limb. The mare also had generalized hind limb weakness and was having difficulty standing. Administration of butorphanol was discontinued in case the agitation was a reaction to the opioid. Xylazine (0.25 to 0.3 mg/kg [0.11 to 0.14 mg/lb], IV) was administered on 3 occasions within an hour to help control pain, but the mare became progressively more anxious over the next hour. Detomidine hydrochloride (0.01 mg/kg [0.0045 mg/lb], IV) was then administered to allow for more extensive examination. Abdominal ultrasonography was performed and did not reveal any abnormalities of the fetus or placenta. Subjectively, motility of the small and large intestines was also normal. Palpation per rectum did not reveal any abnormalities, although the examination was incomplete because the mare showed signs of extreme agitation when forced to stand still. The mare began to sweat profusely, have diffuse muscle tremors, and circle compulsively to the left in its stall. If forced to stand still, the mare would obsessively stomp its left hind limb and, occasionally, its right hind limb and then begin circling again. When forced to turn around, the mare would begin circling to the right, but when taken out of the stall, would walk in a straight line. There had been no signs of CNS disease prior to the onset of circling.
Given the clinical signs, the history of femoral nerve damage, and the lack of response to conventional pain medications, a diagnosis of neuropathic pain was made. A constant rate infusion of lidocaine was initiated at a dosage of 0.05 mg/kg/min (0.023 mg/lb/min), IV, for additional analgesia. However, the mare remained extremely agitated and uncomfortable. Therefore, the constant rate infusion of lidocaine was discontinued, and a constant rate infusion of detomidine (4 μg/kg/h [1.8 μg/lb/h], IV) was begun. At this dosage, the mare became quite sedate and lay down in the stall. When the dosage was decreased to 3 μg/kg/h (1.4 μg/lb/h), the mare would again become agitated, despite being heavily sedated. Acepromazine maleate was administered at a dosage of 0.03 mg/kg (0.014 mg/lb), IV, with no apparent improvement in clinical signs. At this time, epidural administration of morphine was attempted; however, the procedure could not be performed because of excessive motion on the part of the mare.
Given the tentative diagnosis of neuropathic pain, a decision was made to begin treatment with gabapentin (2.5 mg/kg [1.1 mg/lb], PO, q 8 h), with the dosage extrapolated from the dosage used in small animals. The constant rate infusion of detomidine was continued at a dosage of 3 μg/kg/h, and 2 additional doses of acepromazine were administered 6 hours apart. Within 2 hours after administration of the first dose of gabapentin, the mare appeared less agitated. The compulsive circling in the stall decreased in frequency and eventually stopped within 36 hours after initiation of gabapentin administration, and although the mare continued to stomp its left hind limb, the frequency of stomping was greatly decreased.
The morning after administration of gabapentin was begun, treatment with detomidine and acepromazine was discontinued. After 24 hours of treatment with gabapentin, the dosage was decreased to 2.5 mg/kg, PO, every 12 hours, for an additional 3 days. During this time, the mare's condition continued to improve, with only mild muscle tremors and weakness in the left hind limb after walking and grazing. Intravenous administration of antimicrobials was discontinued 5 days after surgery, and the mare was weaned from the fluid therapy and flunixin treatment. Abdominal ultrasonography performed at that time did not reveal any abnormalities of the fetus or placenta; fetal heart rate was 85 beats/min. The mare had been started back on feed 2 days after surgery and was gradually reintroduced to a normal ration over 5 days.
After 3 days of administration at a dosage of 2.5 mg/kg every 12 hours, the gabapentin dosage was again decreased to 2.5 mg/kg every 24 hours for an additional 2 days, and administration was then discontinued. The mare was discharged 7 days after surgery. Results of serum biochemical analyses performed at the time of discharge were unremarkable, with the exception of slightly high serum aspartate transaminase activity (776 U/L). The mare was taken to a local veterinary practice to foal. Foaling was observed, but unassisted, and proceeded without complications. No abnormalities were detected in the foal. Shortly after parturition, the mare developed a uterine infection that was treated with uterine lavage and intrauterine administration of antimicrobials. The uterine infection resolved without complications, and the mare was reportedly doing well 6 weeks after surgery, with only mild signs of femoral nerve damage persisting in the left hind limb.
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