A3-month-old Missouri Fox Trotter colt that weighed 105 kg (231 lb) was examined for respiratory distress and a mild left-sided head tilt at the University of Missouri Veterinary Medical Teaching Hospital. Thoracic radiography revealed bronchointerstitial pneumonia; radiographs of the skull and cervical portion of the vertebral column yielded inconclusive results. Fluid obtained from a transtracheal lavage did not yield bacteria; however, medical treatment for bronchopneumonia caused by Rhodococcus equi was administered with erythromycin ethylsuccinate (25 mg/kg [11.4 mg/lb], PO, q 8 h), rifampin (5 mg/kg [2.3 mg/lb], PO, q 12 h), flunixin meglumine (1.1 mg/kg [0.5 mg/lb], IV, q 12 h), and omeprazole (4 mg/kg [1.8 mg/lb], PO, q 24 h) on the basis of signalment and clinical signs. After 1 week of hospitalization, the foal was discharged with the same antimicrobial regimen. Computed tomography of the head was recommended if the head tilt persisted.
Upon readmission 5 weeks later, the foal was bright and alert and all physical examination variables were within reference ranges. Clinical signs of respiratory tract disease were not identified, and auscultation of the lung fields yielded normal results. At rest, neurologic deficits were minimal; however, when excited, a leftsided head tilt and ataxia became more apparent. Cranial nerve examination revealed ventrolateral strabismus of the left eye and a right fast-phase nystagmus in both eyes. The soft tissues overlying the external occipital protuberance (ie, the poll) appeared enlarged on the left side. Complete blood count and serum biochemical results were within reference ranges. Erythromycin and rifampin administration was continued.
Computed tomography revealed an osteolytic process in the left parietal and occipital bones resulting in caudal displacement of the occipital bone (Figure 1). A soft tissue mass associated with the osteolytic bone compressed the cerebellum and rostrally displaced the occipital lobe of the cerebrum. The mass extended into the soft tissues dorsal to the left ear canal and medially occupied the left half of the caudal fossa and dorsal one third of the foramen magnum. The mass did not appear to invade the atlanto-occipital joint.
Transverse contrast computed tomography images of the caudal fossa of the skull of a foal. Contiguous images are shown rostral (top of figure) to caudal (bottom of figure). Soft tissue window (A, C, and E) and corresponding hard tissue window (B, D, and F) images were obtained. Soft tissue slices were acquired in 8-mm sections and hard tissue slices were acquired in 5-mm sections. An osteolytic process resulting in fragmentation (white arrow) and caudal displacement of the occipital bone is evident. An ossified fragment (black arrow) associated with the temporal and occipital bones was resected from within the cranium. A soft tissue mass (asterisk [A]) associated with the osteolytic region extends intracranially, resulting in compression of the cerebellum and rostral displacement of the occipital lobe of the cerebrum. The extent of compression and displacement is difficult to discern because of volume-averaging artifact.
Citation: Journal of the American Veterinary Medical Association 228, 2; 10.2460/javma.228.2.251
Seven days later, a rostrotentorial-suboccipital approach for a craniectomy of the parietal and occipital bones was performed during general anesthesia. The foal was positioned in sternal recumbency with all 4 limbs supported on respective sides of the table. The head was positioned in slight ventroflexion and stabilized with an air cushion and a head-holding device. A 25% mannitol solution (0.25 g/kg [0.114 g/lb], IV) was administered as an IV bolus over 20 minutes while a dorsal midline skin incision was centered over the external occipital protuberance. Periosteal attachments of the left temporalis and interscutilaris muscles were elevated from the parietal and occipital bones. A 1.5 × 1.0-cm section of proliferative tissue that was adhered to the ventral portion of the temporalis muscle was isolated and determined to have penetrated through the occipital and temporal bones. The tendon of the obliquus capitis cranialis muscle was transected from the external occipital protuberance and nuchal line to gain exposure. A 2.0 × 4.0-cm craniectomy was performed through the caudoventral aspect of the parietal bone, the temporal bone, and the rostral aspect of the occipital bone with a high-speed pneumatic drilla and ronguers. A 1.0 × 1.0-cm ossified fragment associated with the temporal and occipital bones was removed from within the cranium. Proliferative tissue was further isolated and debulked for histologic examination and bacterial culture. The lateral aspect of the osseous tentorium cerebelli and left transverse sinus were obliterated by the mass. The entire mass was not removed because further manipulation of the proliferative tissue might have resulted in iatrogenic brain injury. The transected tendon of the obliquus capitis dorsalis muscle was apposed, followed by routine closure.
Immediately after surgery, the foal began nursing and was closely monitored for worsening of neurologic signs and seizures. Diazepam (0.05 mg/kg [0.023 mg/lb], IV, q 4 h) was administered for 24 hours in an attempt to decrease the foal's physical activity and lessen the chance of seizures. All neurologic deficits except for a mild head tilt diminished within 3 days after surgery. The head tilt was only present during head elevation or when the foal became excited.
Histopathologic diagnosis of the proliferative tissue was granulomatous inflammation. Bacterial culture yielded R equi from the proliferative tissue and bone fragments. Antimicrobial susceptibility testing indicated a strong susceptibility to erythromycin and an intermediate susceptibility to rifampin. Six days after surgery, clarithromycin (7.5 mg/kg [3.4 mg/lb], PO, q 12 h) was administered in place of erythromycin.
The foal was discharged 9 days after surgery with recommendations for 2 weeks of strict stall confinement followed by 3 weeks of turnout in a round pen. At that time, if no obvious neurologic signs were present, access to a small paddock for 4 weeks was allowed, followed by gradual return to a normal turnout schedule. Antimicrobial treatment was discontinued 3 weeks after surgery.
Discussion
Rhodococcus equi is one of the most common pathogens in foals from 1 to 6 months of age.1 Severe suppurative bronchopneumonia is the predominant clinical manifestation.2 Extrapulmonary disease is less common and includes ulcerative enterocolitis, lymphadenitis, nonseptic polysynovitis, septic arthritis, septic physitis, and vertebral body and pelvic symphysis osteomyelitis.2–4 To our knowledge, intracranial abscess formation and occipital osteomyelitis resulting from R equi infection and craniectomy and surgical debulking for treatment for R equi infections have not been described. Intracranial abscess formation and bacterial meningitis secondary to Streptococcus equi infection have been reported in horses, with the outcome being poor.5–11 Successful intracranial decompression through surgical drainage, debulking, or both has been reported in 2 horses8; however, complications after surgery have limited the success of other craniectomy procedures in horses.7
Acquiring computed tomography or magnetic resonance images before attempting intracranial surgery is recommended to characterize and determine the extent of intracranial disease. Intracranial surgery in veterinary medicine has been primarily limited to dogs and cats.12 However, with increasing utilization of noninvasive diagnostic imaging and advancements in critical care, extrapolation of intracranial surgical techniques from humans and small animals should improve treatment of intracranial diseases in horses.
Appropriate head positioning helps minimize increased intracranial pressure. Specifically, head elevation to 30° above cardiac level decreases intracranial pressure primarily by facilitating venous drainage.13 This foal's head was positioned in a head-holding device to maintain this position and to prevent jugular vein occlusion. In addition, a 25% mannitol solution was administered at an estimated time point at which the maximal osmotic effect would coincide with the craniectomy. A single dose of 20% to 25% mannitol solution (0.15 to 2.5 g/kg [0.07 to 1.14 g/lb]), administered IV has clinically and experimentally resulted in a decrease in intracranial pressure within 5 minutes.14
Specimens obtained during surgery for bacteriologic culture guided further medical therapy. Combined treatment with erythromycin and rifampin has greatly improved survival rates of foals infected with R equi.15 Azithromycin and clarithromycin are alternatives to erythromycin for treatment of foals with R equi infections. Compared with erythromycin, these drugs have greater bioavailability after oral administration and achieve higher concentrations in phagocytic cells and tissues.16 Foals with R equi infection and treated with clarithromycin-rifampin have substantially better radiographic improvement and increased short-term and long-term treatment success, compared with erythromycin-rifampin or azithromycin-rifampin treatments.15 Because of the known complications with erythromycin and potential benefits of clarithromycin, clarithromycin was substituted for erythromycin after culture results confirmed susceptibility to erythromycin.
The primary goals of intracranial surgery should be to acquire tissue samples that aid in diagnosis and guide treatment after surgery and to perform sufficient intracranial mass debulking or resection and decompression of neural tissue while limiting manipulation of normal brain tissue. Surgical success in this horse was attributable to the limited severity of presurgical neurologic deficits, advanced imaging techniques, surgical planning, and identification of R equi. Because of the limited number of appropriate surgical candidates, rapid advancements in brain surgery for horses will likely not occur. However, increasing access to advanced imaging and appropriate selection of surgical candidates should result in improved outcome for treatment of horses with intracranial diseases.
Hall air drill, Zimmer-Smith & Associates, Creve Couer, Mo.
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