Introduction

Intraosseous keratinized cysts are rare lesions formed by the proliferation of epidermal cells within osseous tissues. They are characterized histologically by the accumulation of laminated keratin with an external layer of squamous epithelium. The etiology of intraosseous keratinized cysts is largely unknown, but several theories have been described.^1–3 Possible etiologies include trauma to the epidermis, embryonal misplacement of epithelial cells, or erosion of soft epidermal tissue into bone.^1–6 The most common location of intraosseous keratinized cysts in humans is in the phalanges, although other sites have been reported, including in a sacral vertebra.⁷ The location of the cysts in the few reports in dogs are in the distal phalanges, although there is one report of an intraosseous cyst in the vertebral body in a dog.^8–10 A dog described therein had progressive paresis and cervical pain that was diagnosed postmortem after osteolysis of the T10 vertebral body was noted on radiographs. This is the first case report that describes MRI findings and successful postsurgical outcome of an intraosseous keratinized cyst in the vertebral body in the dog.

Case Report

An 8 yr old (30 kg) female spayed golden retriever was presented for evaluation of a 3 wk history of progressive pelvic limb ataxia. There were no prior health concerns. Prednisone (0.5 mg/kg q 12 hr), methocarbamol (33 mg/kg q 8 hr) and gabapentin (3–6 mg/kg q 8–12 hr) were prescribed by the referring veterinarian prior to referral for evaluation by the authors. This treatment resulted in transient improvement, but the ataxia recurred after the prednisone dose was decreased. Immediately prior to presentation, a marked worsening of the pelvic limb ataxia occurred, as did fecal incontinence and stranguria.

No abnormalities were noted on general physical examination. Neurologic examination revealed moderate pelvic limb proprioceptive ataxia with incoordination of the pelvic limbs, a narrowed pelvic limb stance, kyphotic posture, and absent conscious proprioception in the right pelvic limb. Conscious proprioception in the left pelvic limb was normal. Muscle tone and reflexes in all four limbs were normal. There was no pain elicited on spinal palpation. Neurologic localization was consistent with a T3–L3 myelopathy and neoplastic, infectious/inflammatory conditions (such as bacterial/fungal/protozoal myelitis, chronic discospondylitis), and chronic intervertebral disc disease were considered the top differential diagnoses. Complete blood count showed no abnormalities. Serum biochemical profile abnormalities included elevations in alanine aminotransferase of 257 U/L (16–75 U/L), alkaline phosphatase of 843 U/L (8–70 U/L), and gamma-glutamyl transferase of 14 U/L (1–5U/L), which were thought to be a consequence of prednisone administration. The only abnormalities on abdominal ultrasound were a hyperechoic hepatic parenchyma and suspected hepatomegaly thought to be consistent with steroid hepatopathy. Thoracic radiographs showed no abnormalities. Spinal radiographs revealed an expansile lytic lesion in the pedicle of L1 with remodeling leading to a diminished T13–L1 inter-vertebral formation. The dorsal cranial border of the lesion adjacent to the cranial articular process was sclerotic. Incidentally, there was also chronic spondylosis deformans ventrally between T13 and L1 (Figure 1).

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MRI of the dog’s thoracolumbar spine was performed using a 3 Tesla magnet^a. A large, well-circumscribed, T2-weighted hyperintense, T1-weighted iso/hypointense, extradural mass that enhanced poorly after gadolinium^b administration that appeared to originate from the L1 vertebral body was noted on MRI (Figure 2). The height, length, and width of the mass was 1.88 cm, 2.35 cm, and 1.8 cm, respectively. Extension into the vertebral canal resulted in marked spinal cord compression. Intraosseous expansion of the mass resulted in deviation of the L1 dorsal lamina and right pedicle. There were several degenerate discs in the thoracolumbar spine and a mild L7–S1 disc protrusion, but no significant spinal cord or nerve root compression was present. Differential diagnoses for the L1 extradural mass included neoplasia (sarcomas, plasma cell tumors, lymphoma, hemangiosarcoma), abscess, hematoma, myelolipoma, granuloma, and a cyst.

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A right-sided hemilaminectomy was performed on the T13–L1 vertebrae. The lamina of L1 appeared irregular and soft. A large, encapsulated mass was identified in the spinal canal. The center of the mass was soft, mucoid, green, and friable. The mass was easily removed from the canal with minimal adhesion to the spinal cord. Samples were obtained for culture and histopathology. The dog recovered uneventfully from surgery. An indwelling urinary catheter was placed immediately postoperatively, and urinalysis was unremarkable with no growth on aerobic bacterial culture of the urine. Outcome was favorable immediately after surgery with normal urination and improved ataxia. Within a week, the owners reported complete resolution of the ataxia. Neurological exams performed at 2 and 4 wk postoperatively were normal.

Histopathology of the excised tissue showed a large amount of keratinized material with diffusely dispersed fragments of bone that were largely necrotic and/or undergoing resorption. The wall of the cystic lesion was characterized by areas of fibrosis with infiltrates of lymphocytes and numerous macrophages containing hemosiderin and hematoidin pigment. Keratinized debris was entrapped within the capsule wall as well. Aerobic bacterial culture of the mass revealed no growth. The clinical and histopathologic diagnosis was an intraosseous keratinized cyst.

Discussion

In humans, intraosseous keratinized cysts are extremely rare and are reported most commonly in the skull and phalanges. The etiology remains unclear, but it is thought that trauma to the epidermis, embryonal misplacement of epithelial cells, or erosion of soft epidermal tissue into bone may play a role. Specifically, phalangeal intraosseous keratinized cysts are considered to be traumatic in origin, due to the implantation of the epidermis into the bone.^1–6 Epidermoid cysts seen in the skulls of humans are not considered to be associated with trauma but may be related to abnormalities in lines of closure in fetal development.¹¹ A similar pathogenesis of formation of these cysts may be applicable to animals. There are case reports of intraosseous epidermoid cysts in the distal phalanges in dogs and horses.^8,12–14 All digital intraosseous epidermoid cysts were suspected to be traumatic in origin.^8,14 In the case reported here and the previous case of intraosseous keratinized cyst in a thoracic vertebra, there was no known trauma, making the etiology of cyst formation in the vertebral body unclear.⁸ It is possible that this represents ectopic embryological tissue that became cystic.

Intraosseous keratinized cysts of the vertebral canal cause slowly progressive clinical signs in humans that can mimic other spinal diseases. Radiographs may reveal an osteolytic focus around the affected area, but MRI is indicated to define the cyst and assess spinal cord compression and parenchymal change.¹⁵ Although the specificity and sensitivity of MRI features of neoplastic, inflammatory, and vascular causes of spinal cord diseases have been evaluated in a previous study in dogs, MRI of intraosseous keratinized cysts has not been previously described.¹⁶ In addition, there are limited data on the MRI characteristics of extradural, compressive lesions in dogs. Several studies have described MRI findings in dogs with intervertebral disk herniation, but not for other extradural conditions.^17,18

On MRI, cysts appear as sharply and well-defined, homogeneous fluid-filled lesions, surrounded by an outer low-signal margin due to fibrous tissue that are typically characterized as hyperintense on T2-weighted images and hypo-isointense on T1-weighted images, as was noted in the present case. Other MRI features include wall thickening or internal complexity (heterogeneous signal or septation) and the lack of internal contrast enhancement. In addition to characterizing the lesion on MRI, the differential diagnosis may be further narrowed by considering the anatomic location of the lesion and the associated affected structures. Other more common extradural cysts such as synovial, ganglion, subarachnoid, and discal cysts are described in veterinary medicine, and differentiation on MRI is based on location and other associated pathologies.¹⁹ Intraosseous extradural cysts are infrequently described in the human literature, and differentiating them from neoplasia can be challenging.¹⁵ Intraosseous keratinized cysts in the phalanges and skulls of humans have characteristics typical of cysts but also have an intimate association with the surrounding bone and variable degree of osteolysis and/or secondary bone remodeling.^6,11,20 Extradural neoplastic masses, which include primary and secondary mesenchymal tumors (osteosarcoma, fibrosarcoma, and chondrosarcoma), hemangiosarcoma, plasma cell tumors, liposarcoma, and lymphosarcoma, will typically have contrast enhancement, but this can be variable, so histopathology is often needed for differentiation.^21,22 Lastly, granulomas typically also have contrast enhancement and usually cause multifocal or diffuse MRI changes as well as abnormal cerebrospinal fluid due to the significant inflammation.²³

Conclusion

Vertebral/spinal/neural cysts are rare in dogs and typically arise from the synovium, meninges, arachnoid, or nerves/ganglia. To the authors’ knowledge, this is the first report of an intraosseous keratinized cyst arising from the vertebrae that underwent advanced imaging and successful surgical management. The characteristics and the anatomic location of the lesion on MRI may help differentiate intraosseous keratinized cysts from other causes of spinal disease, which alters the prognosis. This is a case in which characteristics of the lesion on MRI led to surgical intervention and a successful outcome that may have been labeled terminal based on radiographs alone. Intraosseous cysts are benign lesions, and removal of these lesions can be curative. Although intraosseous keratinized cysts arising from the vertebral bodies are extremely rare in dogs, they should be considered a differential in dogs with progressive myelopathy, as outcome can be very successful.