What is your diagnosis? Cerebrospinal fluid from a Labrador Retriever

A 3-year-old male intact Labrador Retriever was presented to a veterinary emergency service for cluster seizures 1 day after being hit by a car. The dog had a history of six seizures 1 year prior to presentation, with the last known seizure approximately 8 months ago. The dog had not been medically treated for the seizures. On physical examination, vital parameters were within normal limits. The dog was alert but dull. A painful response was elicited on palpation of the lower jaw and masticatory muscles. On neurologic examination, gait and postural reactions were normal. Spinal reflexes were intact, but the thoracic limbs showed increased spasticity when the dog was recumbent. Based on the seizures and abnormal mentation, neuroanatomic localization was intracranial and forebrain. Differentials for seizures include idiopathic epilepsy or an acquired disease process. A complete blood count and biochemistry panel were performed with relevant findings indicating mild inflammation with a mild nonregenerative anemia of 39.6% (Hct reference interval: 40.4%–56.9%) and leukocytosis of 19.77 × 103 cells/μL (reference interval: 4.8–13.40 × 103cells/μL) with a mature neutrophilia 16.41 × 103 cells/μL (reference interval: 2.49–9.28 × 103cells/μL); the remainder of CBC was normal. The mature neutrophilia could alternatively be due to a corticosteroid or epinephrine-mediated response. After brain MRI, cerebrospinal fluid was collected from the cerebellomedullary cistern (Table 1). During sample collection, the gravity flow of CSF was slow, and the cloudy fluid congealed in the collection tube. After immediate transport to the laboratory, a red gelatinous clot was noted within the fluid, and impression smears were made of it (Figures 1 and 2). Cytologic interpretation: Septic neutrophilic inflammation with evidence of hemorrhage. The intralesional anucleate squamous epithelial cells were presumed to originate from a ruptured tympanokeratoma. Impression smears of the clot noted within the CSF contained many nucleated cells and erythrocytes. A 200 nucleated cell differential count on a cytocentrifuged slide of the CSF included 98% often degenerates neutrophils and 2% macrophages. Occasional eosinophils and small lymphocytes were observed. Rarely, neutrophils contained phagocytized bacterial cocci and macrophages displayed erythrophagia on both the impression smears and cytocentrifuged slides. Moderate numbers of anucleate squamous epithelial cells were seen individually and in small clusters that were often surrounded by neutrophils. A brain MRI was performed on a 3.0 Tesla scanner (Canon, Vantage Titan 3 T) (Figure 3). A large, round, T2/FLAIR hyperintense structure measuring 2.0 cm diameter lateral to the left tympanic bulla extended distally within the proximal ear canal and axially into the tympanic bulla. There was loss of structural detail in the left inner ear. On T2*, there was an ovoid susceptibility artifact in the structure. After intravenous gadolinium administration, there was increased meningeal enhancement surrounding the midbrain, including the meninges of the cranial cervical spinal cord. These findings suggested an otogenic meningitis and encephalitis. The left middle ear lesion was aspirated, and slides were submitted for review (Figure 4). The middle ear mass had abundant anucleate squamous epithelial cells present in large aggregates. There were abundant bacterial cocci, low numbers of cholesterol crystals, and degenerate neutrophils (not pictured). The combination of imaging and CSF findings led to a final diagnosis of a ruptured tympanokeratoma with septic, suppurative meningoencephalitis. Bacterial culture of the middle ear aspirate yielded a heavy isolate of Staphylococcus sp. MALDI-TOF mass spectrometry could not differentiate between S. pseudintermedius and S. delphini. Aural tympanokeratoma, previously referred to as cholesteatoma, is a non-neoplastic, typically unilateral, proliferative lesion composed of keratinizing squamous epithelial cells surrounding keratin debris and cholesterol.1, 2 Associated clinical signs include pain upon opening the mouth, otic discharge, and neurologic signs.3 Cytologically, these closely resemble the aspiration of a follicular cyst. This is an uncommon and likely underdiagnosed lesion reported in various species, including dogs, people, cats, and gerbils.1, 4-7 Tympanokeratomas in dogs are often acquired secondary to chronic otitis media.2 Several theories exist on how keratinized squamous epithelium forms a tympanokeratoma at this location. Dysregulated pressure equalization of the middle ear with chronic otitis media can lead to “retraction pockets” of the tympanic membrane, which gives rise to a tympanokeratoma.8 Migration of external ear canal epithelium after tympanic membrane rupture has been proposed.2, 8 Metaplasia of the middle ear mucosal epithelium into keratinizing squamous epithelium is another theory.8 However, studies on cytokeratin expression of tympanokeratomas are similar to epithelial cells in otitis externa, suggesting migration is more likely epidermal in origin.8 Congenital tympanokeratomas have been reported in people and gerbils but not dogs and result from failure of epidermoid remnant involution (similar to an epidermoid cyst).4, 8 In this case, the traumatic event of being hit by a car likely led to the rupture of a tympanokeratoma into the inner ear with extension into the meninges, resulting in identifiable anucleate squamous epithelial cells and bacterial cocci within the CSF. To the author's knowledge, there are currently no reports of anucleate squamous epithelial cells in the CSF due to a ruptured tympanokeratoma in animals. The presence of anucleate squamous epithelial cells and bacteria within the intrathecal space led to dramatic inflammation with corresponding increases in the total nucleated cell count and total protein concentration. While pathologic hemorrhage was present, the marked increase in protein concentration likely contributed to the xanthochromia in this case as well.9 Other potential sources of anucleate squamous epithelial cells within CSF samples include superficial skin contamination, ruptured epidermoid cyst, or neoplasia (squamous cell carcinoma, craniopharyngioma, or germ cell tumor). Surgical removal is the treatment of choice for aural tympanokeratomas; however, recurrence rates of approximately 50% have been reported in dogs after surgical removal.2, 3 A total ear canal ablation and bulla osteotomy with removal of the tympanokeratoma were recommended, but the owner declined surgical treatment. The dog was administered oral amoxicillin and clavulanate potassium tablets at 20 mg/kg q 12 h and enrofloxacin at 15 mg/kg q 24 h for 8 weeks. Prednisone 0.6 mg/kg per os q 24 h was administered for 2 weeks with instructions to taper. Based on the initial phone follow-up after 1 week, the dog improved with medical management. The case was later lost to follow-up. The authors have indicated that they have no affiliations or financial involvement with any organization or entity with a financial interest in, or in financial competition with, the subject matter or materials discussed in this article.