Endolymphatic sac tumor in an 8-month-old cat.

Background: The endolymphatic sac is an organ devoid of sensory receptors. It is connected with the endolymphatic compartment and contains endolymph. Endolymphatic sac tumor (ELST) is a rare neoplasm involving the middle and inner ear described in humans and dogs that does not show cellular characteristics of malignancy, but can be locally invasive and involve destruction of the temporal bone and adjacent structures.

Case Description: An 8-month-old female cat was referred because of sudden onset of vestibular signs starting 3 days prior to referral. On clinical examination, the patient showed depression, right head tilt, left-sided facial paralysis, and horizontal nystagmus with fast phase to the left. The magnetic resonance images showed a voluminous extra-axial lesion, with irregular morphology and well-defined margins, with intracranial extension in the region of the pons, rostral medulla oblongata, cerebellar vermis, floccule, and left cerebellar hemisphere. Due to progressive clinical deterioration, the cat was euthanized 2 weeks later. A necropsy was then performed and histological samples were taken. The necropsy revealed the presence of a voluminous dark red irregular mass extending from the tympanic bulla to the posterior cranial fossa following the left glossopharyngeal nerve. The histopathological exam of the extra-axial lesion featured a nonencapsulated, moderately cellular, rather loose, proliferation of cuboidal to columnar epithelium breaching through chunks of an otherwise normal appearing dura mater and invading some cranial nerves. Sections of the cerebellum and brainstem revealed moderate, focal, impingement of the parenchyma with a very mild extension of the proliferating cells into the ventral left side of the medulla oblongata. Based on these histological characteristics, the lesion was defined as ELST, a rare neoplasm described in human beings and with two reports in dogs.

Conclusion: To our knowledge, this is the first report describing an ELST in a cat.