A3-year-old castrated male Golden Retriever was evaluated because of progressive vestibular ataxia and signs of cervical pain of 2 weeks’ duration following running into a tree. On initial neurologic assessment, the dog had moderate vestibular ataxia with leaning and listing to the left side, a mild left-sided head tilt, and resistance to ventral flexion of the neck. Results of a CBC and serum biochemical profile were unremarkable other than a mild hypercholesterolemia (412 mg/dL; reference range, 120 to 310 mg/dL). Further endocrine testing to rule out other causes of hypercholesterolemia was not performed.
Etiologic diagnosis—Considering the dog's clinical signs, differential diagnoses included congenital malformation, hemorrhage or hematoma secondary to trauma, meningoencephalitis, and neoplasia. The diagnostic plan included MRI of the brain and bullae (to assess for structural neurologic disease) and, if necessary, collection of a CSF sample for analysis (to evaluate the cellular response to neurologic disease).
Diagnostic test findings—The MR images of the brain (sagittal T2-weighted and T1-weighted after contrast medium administration; transverse T2-weighted, T1-weighted, T1-weighted after contrast medium administration, fluid attenuated inversion recovery (FLAIR), and T2*-weighted; and dorsal T1-weighted after contrast medium administration) were obtained with a 1.0-T MRI unit. The images revealed a large (1.8 × 1.2 × 1.2-cm) ring-enhancing, cyst-like mass within the fourth ventricle with smaller masses within its wall; the mass was hyperintense (relative to brain and CSF) on T2-weighted and FLAIR images and hypointense on T1-weighted images (Figure 1). Additionally, there was moderate edema within the adjacent medulla, obstructive hydrocephalus with transependymal edema, and mild cervical syringohydromyelia with patchy intramedullary edema. Analysis of a CSF sample was not performed as it was not thought to be of aid in further determining differential diagnoses. On the basis of the MRI findings, differential diagnoses included choroid plexus cyst, choroid plexus tumor with cystic dissemination, epidermoid cyst, and respiratory epithelial (neuroenteric) cyst.
Magnetic resonance images of the brain of a dog that was evaluated because of progressive vestibular ataxia and signs of cervical pain of 2 weeks’ duration following running into a tree. A—Mid-sagittal T2-weighted image. Notice the hyperintense (relative to brain and CSF) cystic mass within the fourth ventricle (white arrow). Obstructive hydrocephalus with transependymal edema (long black arrow) as a result of ventricular hypertension, vasogenic edema in the adjacent medulla (short black arrow), and a rostrocerebellar fluid collection (asterisk) are also evident. B—Transverse fluid-attenuated inversion recovery image. The cystic mass is very hyperintense (white arrow). Also noted is a smaller mass within the right dorsal wall (short black arrow). C—Transverse TI-weighted image obtained after contrast medium administration. The cystic mass has intense ring enhancement (white arrow). Small ring-enhancing masses are present within its wall (short black arrows).
Citation: Journal of the American Veterinary Medical Association 249, 8; 10.2460/javma.249.8.895
Comments
Surgery was recommended with the goals of tumor removal to decompress the brainstem, alleviation of obstructive hydrocephalus, and collection of tissue samples for histologic diagnosis. Foramen magnum decompression was performed, and a green-yellow mass was removed in 2 large pieces. This mass was well encapsulated and contained thick, yellow material. Histologic examination of removed tissues revealed a large cystic structure lined by nonkeratinized stratified squamous epithelium, most consistent with an atypical epidermoid cyst given that the epithelial lining was not keratinized and the lumen of the cyst was not filled with keratin as with typical epidermoid cysts.1 The fibrous connective tissue wall of the cyst had multiple cholesterol granulomas and occasional hemorrhage.
The dog of the present report developed worsened vestibular signs after surgery, likely secondary to tissue manipulation during the procedure; however, these signs improved considerably with time and the dog appeared normal 73 days after surgery. One year after surgery, the owner reported that the dog had no return of clinical signs and its condition was normal.
Intracranial epidermoid cysts are rare congenital, slow-growing masses that develop as a result of defective separation of neuroectoderm during formation of the neural tube.2 In humans, atypical epidermoid cysts comprise 5.6% of all epidermoid cysts3 and may differ from typical epidermoid cysts with regard to diagnostic imaging characteristics, gross findings, and histopathologic features.
Atypical epidermoid cysts account for a small percentage of total epidermoid cysts in people.3 Atypical epidermoid cysts are generally larger than classic epidermoid cysts and often are associated with pre- and postoperative hemorrhage.3 Histologically, they often contain granulation tissue and cholesterol crystals, both of which may be related to hemorrhage. The granulation tissue is highly vascular, which predisposes to bleeding; cholesterol crystals may then develop secondary to hemorrhage.3–6 The presence of hemorrhage, cholesterol crystals, and granulation tissue may account for the atypical gross and MRI findings in atypical epidermoid cysts.3
Epidermoid cysts have an associated capsule, and complete excision of this capsule appears to be necessary to prevent regrowth.3,7 The capsule often adheres to surrounding neurovascular structures, making excision difficult.3,7 Epidermoid cysts are not responsive to chemotherapy or radiation therapy8; therefore, aggressive surgical removal, while sparing as much normal tissue as possible, is important. Although follow-up images are lacking for the dog of this report and it was not possible to assess recurrence, the surgeon's impression and post-operative MRI findings together with the absence of neurologic signs 1 year after surgery supported complete excision of the mass and associated capsule.
Acknowledgments
Presented as an oral presentation at the Southeastern Veterinary Neurology meeting, Starkville, Miss, September 2014.
References
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