Unilateral facial paresis secondary to a suspected brainstem arachnoid cyst in a Maltese dog

An 8-year-old, intact female Maltese dog was presented with decreased tear production and unilateral loss of eye blinking. Neuro-ophthalmic examination and brain magnetic resonance imaging were performed to determine the origin of facial paresis. A cystic lesion in the left pontomedullary region which displayed equal intensity to cerebrospinal fluid was revealed. Hyposignality was noted on fluid attenuated inversion recovery sequences, and the lesion was suggestive of an arachnoid cyst. This report described unilateral facial nerve dysfunction that resulted from a suspected brainstem arachnoid cyst in an unusual anatomic location.