Abstract
Meningoencephalitis of unknown origin (MUO) is an immune-mediated central nervous system disorder in dogs in which magnetic resonance imaging (MRI) plays a key role in diagnosis and monitoring. This case report describes a five-year-old spayed female Chihuahua (3.9 kg) presented with acute right-sided weakness, anorexia, and vomiting. The initial MRI demonstrated multifocal T2/FLAIR hyperintense lesions in the left frontal lobe with suspected disruption of the blood–brain barrier, consistent with MUO. Immunosuppressive therapy with prednisolone (0.5 mg/kg twice daily, tapered to 0.15 mg/kg once daily) was administered in combination with imatinib (10 mg/kg once daily). Follow-up MRI after seven weeks revealed partial remission of lesions, restoration of blood–brain barrier integrity, and mild ventricular enlargement. At eleven months, chronic gliosis, progressive cerebral atrophy, and further ventricular dilation (39% vs. 29% at 7 weeks) were evident without new lesions. Clinical signs resolved within 25 days, but long-term therapy caused polyphagia, polydipsia, persistent alkaline phosphatase elevation, transient alanine aminotransferase increase, and episodes of hypertriglyceridemia. This case highlights the value of sequential MRI for documenting the transition from acute inflammation to partial remission and chronic structural change in MUO. Prednisolone-based immunosuppression combined with imatinib achieved sustained clinical improvement, while long-term monitoring was essential to detect progressive atrophy and treatment of related adverse effects.