Abstract: Background – Equine Protozoal Myeloencephalitis (EPM) is an infectious, debilitating disease affecting the central nervous system of horses. Definitive diagnosis requires histopathology. Magnetic resonance imaging (MRI) of the neurologic horse is uncommon, and brain MRI findings in horses with EPM have not been well described. Hypothesis/Objectives – To report MRI abnormalities in the brains of horses with EPM. Methods – This is a multicenter, retrospective, secondary analysis, cross-sectional study. Client-owned horses that underwent anesthesia for a brain MRI and received a definitive diagnosis of EPM were included. All cases had cerebrospinal fluid (CSF) testing for Sarcocystis neurona, followed by necropsy with histopathologic evaluation. A blinded, consensus reading of the MRI images was performed by a board-certified veterinary radiologist and equine surgeon. Animals –Eight horses diagnosed with EPM. Results – Seven horses had ≥2 lesions. The most common locations were the myelencephalon (n=4) and pons (n=3). All lesions were intra-axial and hyperintense to grey matter on T2W sequences, and six contacted the 4th ventricle. One lesion had mild contrast enhancement. No T2* signal void was appreciated. Cranial nerve enlargement and ventricular dilation were not observed. Three horses had unilateral masticatory muscle atrophy. Conclusions and Clinical Importance – Horses with EPM tend to have multiple, T2 hyperintense, non-contrast enhancing lesions, most often located in the pons or myelencephalon. These characteristic findings support MRI as a useful tool in the clinical investigation of neurologic horses.
