A 10-year old, spayed female, Irish water spaniel was presented with a 2-week history of anisocoria characterized by mydriasis of the right eye compared to the left eye in ambient light. Ophthalmic and neurological examinations, combined with pharmacological testing, identified a disease process affecting the right parasympathetic nucleus of cranial nerve 3(CN III) and/or the parasympathetic component of CN III. Magnetic resonance imaging (MRI) identified a mass involving the right midbrain and extending caudally to the rostral border of the medulla oblongata. The dog became comatose within 12 h following an MRI and was euthanized. Histopathology identified the intracranial mass as a meningioma.
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