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.
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.
Hall air drill, Zimmer-Smith & Associates, Creve Couer, Mo.
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