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Veterinarian Technician November 2006 (Vol 27, No 11) Focus: Equine Medicine

Case Report: "Intracranial Surgery in a Foal"

by Charity Johnson

    This case was originally presented in Janicek JC, Kramer J, Coates JR, et al: Intracranial abscess caused by Rhodococcus equi infection in a foal. JAVMA 228(2):251-253, 2006.

    Rhodococcus equi is found in the soil and is relatively widespread in equine environments throughout the world. Severe bronchopneumonia is the most common result of R. equi infection in foals, and lung abscesses may be seen on radiographs. Occasionally, abscesses may form elsewhere in the body. In this case, unusual neurologic signs in a foal with R. equi infection led veterinarians to discover and treat an intracranial abscess.

    A 3-month-old Missouri Fox Trotter foal was referred to the University of Missouri Veterinary Medical Teaching Hospital on December 10, 2004, for examination of respiratory distress.

    Initial Presentation and Therapy

    On presentation, the foal was de­pressed and febrile and had increased bronchovesicular sounds and a slight head tilt. Initially, a complete blood cell count (CBC), serum blood chemistry, thoracic radiography, and a transtracheal wash were conducted. The CBC revealed an elevated white blood cell count (15.9 x 103/µl [normal: 5.4 to 14.3 x 103/µl]) and fibrinogen level (0.9 g/dl [normal: 0.1 to 0.4 g/dl]). Thoracic radiography revealed a moderate to severe unstructured interstitial pattern suggestive of bacterial pneumonia in the caudodorsal lung parenchyma. Bacteria were not isolated from the trans­tracheal wash fluid; however, medical therapy for R. equi bronchopneumonia was initiated based on signalment and clinical signs.

    Erythromycin ethylsuccinate and rifampin were administered; these antibiotics are the most commonly used agents for the treatment of R. equi and are typically used together because of their synergistic qualities. Resistance to rifampin can develop rapidly when this agent is used alone. Flunixin meglumine was started to alleviate the fever, and because stress predisposes horses to gastric ulcers, omeprazole was given to prevent ulcer development. Intravenous fluids were given for 24 hours to ensure good hydration, and blood glucose levels were monitored twice a day. While the foal was hospitalized, head and neck radiographs were taken to help determine the cause of the head tilt; radiographic findings were within normal limits.

    After 7 days of hospitalization, the foal's white blood cell count returned to normal, its fibrinogen level had decreased to 0.6 g/dl, and its attitude had improved greatly. Subsequently, the foal was discharged from the hospital with the recommendation to continue the antimicrobial regimen for 6 weeks and to monitor the head tilt.

    Computed Tomography Findings

    After 5 weeks of antimicrobial therapy, the foal was readmitted for computed tomography (CT) of the head. The bronchopneumonia had resolved, but a mild head tilt remained. While resting, the foal showed very mild neurologic signs; however, a left-sided head tilt, ataxia, nystagmus, and strabismus were evident when the animal became excited. The left side of the poll also appeared to be enlarged. CT showed osteolysis of the left parietal and occipital bones and an associated soft tissue mass. The mass compressed the cerebellum and displaced the occipital lobe of the cerebrum rostrally, while the occipital bone was displaced caudally. The veterinary team decided that the best course of action was to decompress the affected area and obtain tissue samples for culture and sensitivity to determine appropriate antibiotic therapy.

    Surgical Treatment

    On January 20, 2005, a craniectomy of the parietal and occipital bones was performed. The foal was placed in sternal recumbency with all four legs adequately supported, and its head was stabilized in an air cushion and positioned with a head-holding device. Following induction with xylazine hydrochloride and ketamine hydrochloride, anesthesia was maintained with isoflurane mixed with oxygen. Direct blood pressure was monitored using an arterial line placed in the lateral metatarsal artery. Intravenous fluids were initiated and continued throughout the surgery. While the patient's head and neck were being aseptically prepared for surgery, 25% mannitol solution (0.25 g/kg) was given as an IV bolus over a 20-minute period to decrease cranial pressure.

    A 20-cm midline incision was made through the skin and subcutaneous tissues, the left temporalis muscle was elevated, and the obliquus capitis cranialis muscle was transected. A granulomatous mass of proliferative tissue (abscess capsule remnant) approximately 1.5 x 1.0 cm was found adhered to the temporalis muscle and invading the parietal bone. Once the mass had been identified, a craniectomy was performed and an ossified fragment associated with the temporal and occipital bones was removed from within the cranium. Approximately 50% of the mass was removed for histopathology and culture; the remaining portion was left in place to avoid the risk of iatrogenic trauma to the brain. Surgical closure was performed in three layers. Anesthetic recovery from surgery was uneventful; the foal stood and nursed well following surgery with only a slight head tilt obvious at rest.

    Diazepam (6.5 mg IV q4h) was started for 24 hours after surgery to reduce both physical activity and any chance of seizures. The excised proliferative tissue was submitted for histopathologic diagnosis, which was determined to be granulomatous inflammation. R. equi was cultured from the proliferative tissue and bone fragments and was found to have a strong susceptibility to erythromycin and an intermediate susceptibility to rifampin. Antibiotic therapy was continued for 3 weeks following surgery. After 6 days, clarithromycin (7.5 mg/kg [3.75 mg/lb] PO q12h) was substituted for erythromycin because of the potential complications associated with erythromycin, which are rare but can include diarrhea or dyspnea.1

    The foal's recovery from surgery was unremarkable. No seizures were ob­served, and the diazepam was discontinued after 24 hours. No overt neurologic signs were noted 48 hours after surgery, and the head tilt only appeared when the foal was very active. Although head shy and sensitive around the incision, the foal was otherwise easy to handle. On January 30, 2005, the foal was discharged. The owners were instructed to continue administering antibiotic therapy for 3 more weeks, maintain strict stall rest for 2 weeks, and avoid handling the foal's head or introducing a halter. Gradual access to a small paddock was allowed if no signs of head tilt were present. At last follow-up, the foal was doing well with no neurologic signs.

    Discussion

    Intracranial surgery is generally limited to companion animals, mostly because of the danger of recovering a large animal with neurologic deficits, limited ability to locate the source of the problem, and expense involved. However, advances in veterinary medicine and the increased availability of CT and magnetic resonance imaging have resolved many of these drawbacks.

    Reducing intracranial pressure during surgery is vital to a successful outcome. In this case, it was obtained through head positioning and appropriate drug therapy. Keeping the head elevated 30° above the level of the heart decreases intracranial pressure by facilitating venous drainage. The head-holding device used to retain this position in this case also prevented jugular occlusion.2 Furthermore, the administration of the 25% mannitol solution was timed to give the best possible osmotic effect during surgery. A single IV dose of 20% to 25% mannitol solution (0.15 to 2.5 g/kg) can decrease cranial pressure within 5 minutes.3

    Conclusion

    The unique aspect of this case is the conclusive diagnosis of R. equi infection. There have been no other reported cases of R. equi infection resulting in intracranial abscess. The success of this case de­pended in large part on the advanced imaging available, the foal's mild neurologic signs, pre- and postsurgical planning, and positive identification of R. equi. Because of the complications mentioned above, brain surgery in horses will likely remain the exception rather than the rule. However, with continued advances in ­veterinary medicine, treatment of intracranial disease in horses should continue to improve.

    Acknowledgment

    The author would like to thank John C. Janicek, DVM, Joanne Kramer, DVM, DACVS, Joan R. Coates, DVM, MS, DACVIM, Jimmy C. Lattimer, DVM, DAVCR, Alison M. LaCarrubba, DVM, and Nat T. Messner, DVM, DABVP, for their assistance in writing this paper.

    1. Beech J: Inflammatory, infectious, and immune diseases, in Equine Medicine and Surgery, vol 1, ed 5. St. Louis, Mosby, 1999, pp 530-535.

    2. Mavrocordatos P, Bissonnette B, Ravussin P: Effects of neck position and head elevation on intracranial pressure in anesthetized neurosurgical patients: Preliminary results. J Neurosurg Anesthesiol 12:10-14, 2000.

    3. McCraw CP, Alexander E, Howard G: Effect of dose and dose schedule on the response of intracranial pressure to mannitol. Surg Neurol 10:127-130, 1978.

    References »

    NEXT: Digestion in the Horse

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