Intramedullary Epidermoid Cyst in the Thoracic Spine of a Dog

Introduction

Epidermoid cysts are congenital lesions within the neuraxis. They are thought to form when the neural tube closes and adjacent ectodermal cells become entrapped during embryologic development.^1,2 Epidermoid cysts may be extradural, intradural and extramedullary, or intramedullary in location.³ Histopathologically, epidermoid cysts have been described as lined by one to three layers of stratified squamous epithelium containing intraluminal desquamated keratin and occasional inflammatory cells.^4–7 They may be an incidental finding at autopsy or become large enough to cause neurologic dysfunction.

Epidermoid cysts are a subgroup of congenital tumor-like lesions in humans, occurring more frequently in children and at cranial sites. In adult humans, the incidence of true epidermoid cysts within the spinal canal represents <1% of all intraspinal tumors.⁸

The majority of reported epidermoid cysts in dogs are intracranial, with a possible anatomic predilection for the caudal fossa.^4–6 Epidermoid cysts within the spinal canal of dogs are rare. Three previous case reports described these cysts in the vertebral canal of dogs. Two of these reports described intramedullary epidermoid spinal cord cysts visualized by myelography in a 2 yr old rottweiler and in a 1.5 yr old flat-coated retriever, and one report described a vertebral body associated epidermoid cyst in a 5 yr old boxer visualized by computed tomography.^7,9,10

To the author's knowledge, this is the first report of the magnetic resonance imaging (MRI) features of a histopathologically confirmed intramedullary epidermoid cyst in the spinal cord of a dog.

Case Report

A 5 yr old female spayed mastiff was referred to the University of Wisconsin-Madison Veterinary Medical Teaching Hospital for a 3–4 mo history of progressive paraparesis, intermittently decreased appetite, mild weight loss, and episodes of loose stool. There was a 3 day history of worsening paraparesis, pain, and urinary incontinence that occurred acutely after play. Physical examination revealed a Grade I/VI left apical systolic heart murmur and mild atrophy of the proximal limb musculature of both pelvic limbs consistent with disuse atrophy; there were no abnormalities on orthopedic examination. Neurologic examination revealed ambulatory paraparesis, proprioceptive deficits in both pelvic limbs, decreased gastrocnemius and withdrawal reflexes in the right pelvic limb, and pain on dorsal tail extension. Muscle tone was normal in the left pelvic limb and mildly reduced in the right pelvic limb; there was no evidence of neurogenic atrophy. An L6–S3 myelopathy or cauda equina syndrome was suspected.

A complete blood cell count and a serum chemistry panel were within normal limits. Urinalysis and urine culture revealed a urinary tract infection caused by Escherichia coli. Thoracic radiographs were within normal limits. An abdominal ultrasound was unremarkable except for hyperechoic urine consistent with the urinary tract infection.

MRI of the spine was performed under general anesthesia with a 1.0-T magnet^a. Spin echo T1-weighted images (T1-W; time to repeat [TR], 550, time to echo [TE], 15, slice thickness, 3 mm) and T2-weighted images (T2-W; TR, 4000, TE, 98, slice thickness, 3 mm) were acquired from the L1–S3 spinal cord segments in sagittal, dorsal, and transverse planes. Gradient echo images (GE; TR, 500, TE, 18, slice thickness, 3 mm) were acquired in the sagittal plane using a 20° flip angle. T1-W transverse and sagittal sequences were repeated after intravenous administration of 0.1 mmol/kg gadodiamide contrast medium^b.

MRI of the L1–S3 spinal cord segments and cauda equina showed lesions interpreted as synovial cysts at the L4–5, L5–6, and L6–7 articular facet joints. No compression of the spinal cord or cauda equina was seen (Figure 1). MRI of the T3–L3 spine was elected using the same protocol as previously described. A round, 1 cm diameter well-demarcated spinal cord lesion was visualized at the level of the ninth thoracic vertebra. The lesion was hyperintense on T2-W images compared with normal spinal cord tissue and hypointense surrounded by a hyperintense rim on T1-W postcontrast images (Figures 2, 3). On transverse T2-W images, hyperintensity was present within the spinal cord parenchyma (Figure 4). Precontrast T1-W images were unavailable because the examination was performed after completion of the L1–S3 spinal cord segments. Cranial to the spinal cord lesion, there was diffuse spinal cord hyperintensity on T2-W imaging, consistent with spinal cord edema. Mild syringohydromyelia was observed cranial to the lesion on T2-W sequences; however, a fluid-attenuated inversion recovery sequence would have been necessary to confirm that this fluid was consistent with syringoydromyelia, because spinal cord edema was also hyperintense on T2-W sequences. The lesion remained hyperintense on GE images, making hemorrhage unlikely, as the majority of hemorrhagic lesions produce a hypointense void. Diagnostic interpretation was a spinal cord lesion at T9 with spinal cord edema and mild syringohydromyelia cranial to the lesion. Differential diagnoses included neoplasia (most likely a glioma) or a fungal granuloma.

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Cerebrospinal fluid (CSF) was collected from the L5–6 interarcuate space. Analysis showed elevated total protein concentration of 87.3 mg/dL (reference range 0–35 mg/dL), normal total nucleated cell count of 3 cell/μL (reference range 0–5 cells/μL), and 190 red blood cells/μL. The differential cell count was 86% lymphocytes and 14% macrophages.

Due to the poor prognosis associated with an extensive intramedullary spinal lesion, the owners elected humane euthanasia. A postmortem examination limited to the affected area of the spinal cord was performed; necropsy examination of the remainder of the nervous system was not permitted.

At gross necropsy, there was a 1.5 cm firm segment of spinal cord at the level of the ninth thoracic vertebra with loss of vascular filling in the leptomeninges. Within the spinal cord parenchyma, there was a 6 mm diameter, 9 mm long, yellow gelatinous mass. Histologic examination showed a well-demarcated cystic mass entirely within the parenchyma of the spinal cord (Figure 5). The mass was encircled by a thick dense fibrous capsule with lymphoplasmacytic infiltrate; this capsule was focally confluent with the leptomeninges in one area where it may have arisen from entrapped ectodermal cells. The cyst was surrounded by spinal cord parenchyma. The inner surface of the cyst was lined by one to several layers of squamous epithelial cells. The cyst contained many sloughed necrotic epithelial cells, few foamy macrophages, and small to moderate amounts of cellular debris. Sloughed epithelial cells were occasionally anucleate or partially keratinized, and there were few admixed keratin flakes. Abundant cyst contents had focally escaped the cyst and an inflammatory response was present in the surrounding spinal cord tissue (Figure 6). The compressed spinal cord parenchyma contained few round vacuoles and rare spheroids. Cranial and caudal to the mass, the meninges were thickened by dense fibrous connective tissue. There was no evidence of necrosis; however, locally extensive rarefaction was present in the surrounding spinal cord parenchyma surrounding the cyst consistent with edema. There was no histologic evidence of hydromyelia; however, the central canal might have been artifactually collapsed on transverse sectioning of the spinal cord cranial to the lesion. Histopathologic diagnosis was an intramedullary epidermoid spinal cord cyst.

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Discussion

This is the first description of the MRI appearance of a histopathologically confirmed epidermoid cyst in the spinal cord of a dog. The cyst was hyperintense on T2-W images, and it was surrounded by a hyperintense ring on postcontrast T1-W images. Similar MRI features have been described in dogs with intracranial epidermoid cysts and in humans with spinal epidermoid cysts.^3,6,11 The hyperintensity in the spinal cord cranial to the cyst on T2-W imaging was consistent with edema on histopathological examination. T1-W precontrast images were not available because the lesion was imaged after administration of intravenous contrast for the original L1–S3 examination. Fat, proteinaceous fluid, melanin, and hemorrhage (in the subacute and acute phases) were hyperintense on T-1W imaging before contrast administration.¹² However, a ring of one of these around the lesion would be an unlikely cause of peripheral hyperintensity in this case. Furthermore, T1-W hypointensity and peripheral ring enhancement was reported as an MRI feature of intracranial epidermoid cysts in dogs and spinal and intracranial epidermoid cysts in humans.^3,6,11

The progressive neurologic signs were most likely due to slow growth of a congenital lesion that formed as a result of inclusion of ectodermal tissue during closure of the neural tube. Histopathologically, the entirety of the cyst was within the spinal cord parenchyma, consistent with an intramedullary lesion. Cyst contents and inflammation in the surrounding tissue might have indicated recent rupture, which corresponded to the acute worsening of clinical signs. However, the inflammatory response in the spinal cord was minimal compared with the large amount of extracystic material; some cyst contents might have been expressed on postmortem manipulation.

The neuroanatomical localization of L6–S3 or cauda equina was based on reduced pelvic limb reflexes. The compressive lesion at T9 might have caused reduced pelvic limb reflexes due to loss of reflex facilitation from the cranial spinal cord segment. Upper motor neuron injuries could be associated with lower motor neuron signs in both acute spinal cord injury such as spinal shock and with chronic lesions such as cranial cervical disc disease.^13,14 The authors have seen this at their institution in several cases with chronic compressive T3–L3 myelopathies; reflexes normalized after surgical removal of the compression. Alternatively, an incidental lower motor neuron lesion in the cauda equina or right sciatic nerve causing hyporeflexia in the right pelvic limb might have been missed because this area was not examined postmortem.

CSF analysis showed albuminocytologic dissocation, consistent with spinal cord compression secondary to the presence of the cyst. Elevated CSF protein concentration might also occur with an intraspinal tumor, which was the primary differential diagnosis.¹⁵ Although CSF analysis is typically helpful in differentiating infectious or inflammatory lesions from cystic lesions, it did not aid in distinguishing between a neoplasm and a cyst.

Epidermoid cysts are rare intraspinal congenital lesions in dogs. However, an epidermoid cyst should be considered as a differential diagnosis if a discrete, T2-W hyperintense, peripherally contrast enhancing lesion is visualized on MRI in the spinal cord of young to middle-aged dogs.