History
A 3.5-year-old spayed female Chihuahua weighing 5.6 kg was presented for progressive lethargy and imbalance. Six days before, the patient was seen by the primary veterinarian for lethargy. A urinary tract infection was suspected based on urinalysis. Carprofen, maropitant, cobalamin, and cefovecin were administered. The lethargy was cyclical for 3 days, and the patient developed stiffened extremities, outstretched neck, and incoordination progressing to inability to ambulate. The patient then presented to an emergency facility where the patient experienced transient hypertension, bradycardia, and respiratory arrest. The patient was intubated and manually ventilated and subsequently treated for increased intracranial pressure with a 5-mL/kg bolus of 7.2% hypertonic saline. Additionally, an immunosuppressive regimen of corticosteroids was begun for presumptive meningoencephalitis of unknown origin. The patient regained the ability to ventilate, neurologically improved, and was referred for further workup.
Upon examination, approximately 48 hours after the episode of respiratory arrest, the patient was ambulatory with mild proprioceptive and cerebellar ataxia in all 4 limbs. The patient tended to circle widely to the right and would fall to the left. The patient had a mild right-sided head tilt and mild positional horizontal nystagmus with fast phase to the right. Pupillary light reflexes were slightly slow but responsive, and the patient had a mild right ventral strabismus.
Assessment
Anatomic diagnosis
The proprioceptive ataxia affecting all 4 limbs was consistent with a lesion affecting the ascending white matter tracts bilaterally, which are present within the cervical spinal cord and brainstem, or the cuneatus and gracile nuclei in the caudal medulla oblongata. The cerebellar ataxia and vestibular deficits (which varied in laterality) necessitated an intracranial or multifocal neurolocalization. A single large lesion within the caudal fossa could have explained the proprioceptive ataxia, cerebellar ataxia, and vestibular deficits and also caused the bilaterally decreased pupillary light reflexes and right ventral strabismus. Although less likely, breed variation could also have explained the unilateral strabismus.
Likely location of the lesion
Bilateral brainstem and cerebellum localization (caudal fossa) was most likely, with a potential for a component of paradoxical vestibular syndrome. A multifocal localization was also possible.
Etiologic diagnosis
Based on signalment and the response to immunosuppressive corticosteroids, the primary differential diagnosis was meningoencephalitis of unknown origin. Infectious meningoencephalitis and a noninflammatory space-occupying mass (cyst, abscess, neoplasia) were considered less likely, largely due to these conditions being uncommon and the young age of the patient. Vascular accident, toxin exposure, and metabolic disease were considered unlikely due to the cyclical and progressive nature of the neurologic signs, slightly prolonged clinical course, lack of corresponding history, and lack of severe blood work abnormalities.
Initial diagnostics included a CBC, serum chemistry, and thoracic radiographs. Magnetic resonance imaging of the brain was pursued to assess for evidence of inflammatory or structural intracranial disease. Cerebrospinal fluid analysis was performed to assess for evidence of inflammation.
Diagnostic Test Findings
The CBC revealed a mild leukocytosis (15.67 X 103 cells/µL; reference interval [RI], 5 X 103 to 14.2 X 103 cells/µL) characterized by mature neutrophilia (13.8 X 103 cells/µL; RI, 3.1 X 103 to 11.8 X 103 cells/µL) and mild lymphopenia (783 cells/µL; RI, 1,100 to 4,800 cells/µL). The chemistry profile revealed a mildly increased ALT (161 U/L; RI, 10 to 90 U/L) and bilirubin (0.8 mg/dL; RI, 0.2 to 0.6 mg/dL) and mild decrease in total calcium (8.3 mg/dL; RI, 8.8 to 11.2 mg/dL). Due to the mild nature, the chemistry changes were considered clinically insignificant.
The patient was anesthetized, and MRI of the brain was performed with a 3.0-T scanner (Ingenia Evolution; Koninklijke Philips NV). Multiplanar (sagittal, transverse, and dorsal) images were obtained by use of the following sequences: T1-weighted with fat saturation (T1W-FS), T2-weighted (T2W), T1W FLAIR, T2W FLAIR, T2*W gradient-recalled echo (T2*W-GRE), diffusion-weighted apparent diffusion coefficient map, and postcontrast T1W-FS. Centered within the fourth ventricle, there was a large, heterogeneously T2W–iso- and hyperintense, T1W-hypointense, ovoid, asymmetrically peripherally contrast-enhancing septated mass measuring approximately 1.2 X 1.2 X 2.1 cm. The rostral margin of the mass was indistinct with heterogeneous, amorphous T1W-FS– and T2W–hyperintense (to CSF) material extending into the dilated rostral portion of the fourth ventricle. Within the caudodorsal aspect of this mass, there was a thin, semilunar, T1W-FS/T1W FLAIR–hyperintense, T2W- and T2*W-hypointense partial rim (Figure 1).
Sagittal T1-weighted FLAIR (A), T2-weighted fast spin echo (B), and T1-weighted postcontrast (C) images of the brain in a 3.5-year-old spayed female Chihuahua presenting for progressive brainstem and cerebellar signs. A peripherally contrast-enhancing intraventricular mass (white arrow) with an indistinct rostral margin (white bracket in panel C) and secondary mass effect of the cerebellum and brainstem was revealed. Note the caudodorsal hypointense rim on T2-weighted imaging (B), suspected to be early subacute hemorrhage. Transverse T2-weighted FLAIR (D) at the level of the mass and medulla oblongata shows lack of attenuation of the fluid signal, which can help differentiate between epidermoid cysts and arachnoid cysts.
Citation: Journal of the American Veterinary Medical Association 262, 12; 10.2460/javma.24.06.0413
The caudal aspect of the fourth ventricle was completely occupied by the mass. Additionally, there was severe caudodorsal displacement and rostroventral flattening of the cerebellum and ventral displacement and flattening of the brainstem. The cerebellar nodulus and rostroventral aspect of the cerebellum were reduced to absent in volume. The cerebellar vermis was compressed and displaced caudally into the foramen magnum. The ventricular system rostral to this mass was moderately dilated. There was mild periventricular hyperintensity on T2W FLAIR (Figure 1).
Cerebrospinal fluid analysis was performed. Grossly, the sample appeared red tinged and cloudy. The nucleated cell count was 10 cells/µL (RI, 0 to 5 cells/µL), the RBC count was 2,395 cells/µL (RI, 0 cells/µL), and the total protein was 44.0 mg/dL (cisternal RI, 25 to 30 mg/dL). During cytological evaluation, the nucleated cell count appeared higher than reported by the technologist. The nucleated cell count differential consisted of 75% large mononuclear cells (macrophages, ependymal lining cells, meningothelial lining cells, choroid plexus cells), 23% small lymphocytes, and 2% nondegenerate neutrophils. Erythrophagocytosis was present. The cytological interpretation was mild mononuclear pleocytosis with minimal evidence of acute hemorrhage.
Given the imaging and CSF findings, the primary differential diagnosis was an epidermoid/dermoid cyst with secondary obstructive hydrocephalus. Less likely differential diagnoses included an infected subarachnoid cyst, granuloma, or neoplasia as with ependymoma or atypical medulloblastoma. The lack of intense contrast enhancement made a choroid plexus tumor unlikely, though still possible. The T1W-FS–hyperintense, T2W- and T2*W-hypointense caudodorsal region was thought to be early subacute hemorrhage. The indistinct rostral margin raised concern for rupture of the mass with secondary sterile meningoencephalitis.
Surgical debulking was elected, and a craniotomy was performed. Internal contents and suspected portions of the capsule of the mass were removed and submitted for analysis. Intraoperative impression smears (stained with Wright-Giemsa staining) consisted of peripheral blood constituents, distorted cells associated with capillaries and embedded in an amphophilic substance (suspected normal neuropil). Moderate to marked amounts of light-blue staining keratin that often occurred in large, dense aggregates were also seen (Figure 2). The cytological findings were most compatible with a keratin-containing cystic lesion. Histopathology of surgical biopsies revealed scattered aggregates of keratin and rare naked hair shafts embedded within neuropil and hemorrhage. The anatomic location and the presence of keratin and hair shafts supported the suspected diagnosis of an epidermoid/dermoid cyst, with dermoid cyst being favored given the presence of adnexal structures (hair) within the lesion. No cyst epithelium was present in the samples, precluding a definitive diagnosis.
Photomicrographs of an impression smear from an intracranial mass. A—Normal neuropil (arrow) and a moderate amount of light-blue staining aggregates of keratin (rectangle; X10 objective; bar 100 µm) Wright-Giemsa stain. Inset—A marked amount of variably staining keratin aggregates (X4 objective; bar 200 µm). B—Histologic section of the same mass. Tissues are largely composed of fragments of gray matter and abundant hemorrhage. Embedded within the tissues are occasional keratin flakes (black arrow) and rare naked hair shafts (white arrow). H&E stain (X20 objective).
Citation: Journal of the American Veterinary Medical Association 262, 12; 10.2460/javma.24.06.0413
Comments
Intracranial epidermoid/dermoid cysts develop as a result of abnormal inclusion of embryologic ectoderm within neuroectoderm tissue.1 These are uncommon in dogs but are commonly on midline within the fourth ventricle or within the cerebellopontine angle.1,2–5 Clinical signs usually occur within young adulthood and are attributed to mass effect on surrounding structures within the cerebellum and medulla oblongata.1,2,4,5
Magnetic resonance imaging characteristics of epidermoid/dermoid cysts generally include a heterogeneous T1W-hypointense and T2W-hyperintense mass with or without rim contrast enhancement.1,3,4 They may be differentiated from intracranial arachnoid cysts by remaining hyperintense on T2W FLAIR imaging (whereas arachnoid cysts are hypointense on FLAIR). Epidermoid cysts may be differentiated from dermoid cysts by T1 hypointensity (whereas dermoid cysts tend to be T1 hyperintense due to the inclusion of large amounts of lipid).1 Histologically, these lesions are keratin-filled cysts lined by stratified squamous epithelium.1,2 Dermoid cysts additionally contain adnexal components (such as hair or glands), which are absent within epidermoid cysts.4
Prognosis for intracranial epidermoid cysts is poor, and surgical resection is a potential treatment.3,4 In 1 previously reported case3 of surgical treatment of an intracranial epidermoid cyst, neurologic signs worsened immediately postoperatively, gradually improved (relative to preoperative clinical signs) for a period of 4 months, then gradually worsened again for an additional few months until humane euthanasia was elected.
In the current case, the indistinct rostral margin of the mass raised concern for rupture and released cystic contents into the CSF. This likely resulted in aseptic meningoencephalitis and may have contributed to the progression of clinical signs later in disease. Aseptic meningoencephalitis secondary to rupture of an epidermoid cyst has been reported previously.5
Neurologic signs worsened immediately postoperatively, and on day 3 of recovery, the patient experienced respiratory arrest after a large volume of dark fluid was expelled from the mouth. Necropsy was not performed to definitively identify the cause of death, though the most likely cause was a severe aspiration event, with brainstem dysfunction likely serving as a contributing factor.
Acknowledgments
None reported.
Disclosures
The authors have nothing to disclose. No AI-assisted technologies were used in the generation of this manuscript.
Funding
The authors have nothing to disclose.
ORCID
M. Talavera https://orcid.org/0009-0001-3817-6824
References
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