Opthalmoplegia Caused by Thyroid Adenocarcinoma Invasion of the Cavernous Sinuses in the Dog

External ophthalmoplegia (paralysis of extraocular muscles) and internal ophthalmoplegia (paralysis of intraocular muscles) may occur as isolated or concomitant lesions of one or both eyes.10’13’24’34 Total unilateral ophthalmoplegia has previously been described in two dogs with tumor invasion of the cavernous sinuses by an adenocarcinoma of unknown origin and by a chromophobe cell adenoma of the pitui- tary gland.13 The two cases presented here were initially referred forevaluation of a seemingly painful ocular disorder. Neurophthalmological exami- nations disclosed deficits of parasympathetic, sympathetic, sensory, and ocular motor innervation to affected eyes. Destruction of cranial nerves III, IV VT, and the ophthalmic division of V by thyroid adeno- carcinoma invasion of the cavernous sinuses was confirmed in one case at necropsy.