History

An 11-month-old 44-kg (97-lb) sexually intact male Mastiff was evaluated because of a fluctuant subcutaneous mass on the dorsal aspect of the cervical region. The mass did not cause signs of pain and was first noticed approximately 2 months prior to hospital admission. Historically, the size of the mass decreased while the dog received antimicrobial treatment (ie, cephalexin; 10 mg/kg [4.5 mg/lb], PO, q 8 h) and increased in size once antimicrobial treatment was discontinued.

Physical examination revealed a soft subcutaneous mass along the dorsal aspect of the second cervical vertebra. A firm tubular band was palpable extending from the subcutaneous mass into the deeper cervical musculature. No abnormalities were detected on the remainder of the physical examination or on a neurologic examination. Hemoconcentration was evident; the dog had a high PCV (57%; reference range, 37% to 55%) with a plasma total protein concentration (6.8 g/dL) within reference range (6 to 8 g/dL). During anesthesia, MRI of the cervical vertebral column was performed (Figure 1).

Precontrast sagittal short-tau inversion recovery (A) and transverse T2* gradient (B) MRI images of the cranial portion of the cervical region of an 11-month-old sexually intact male Mastiff evaluated because of a globoid mass in the dorsal aspect of the cervical region.

Citation: Journal of the American Veterinary Medical Association 249, 2; 10.2460/javma.249.2.149

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Precontrast sagittal short-tau inversion recovery (A) and transverse T2* gradient (B) MRI images of the cranial portion of the cervical region of an 11-month-old sexually intact male Mastiff evaluated because of a globoid mass in the dorsal aspect of the cervical region.

Citation: Journal of the American Veterinary Medical Association 249, 2; 10.2460/javma.249.2.149

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Precontrast sagittal short-tau inversion recovery (A) and transverse T2* gradient (B) MRI images of the cranial portion of the cervical region of an 11-month-old sexually intact male Mastiff evaluated because of a globoid mass in the dorsal aspect of the cervical region.

Citation: Journal of the American Veterinary Medical Association 249, 2; 10.2460/javma.249.2.149

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Determine whether additional imaging studies are required, or make your diagnosis from Figure 1—then turn the page →

Diagnostic Imaging Findings and Interpretation

Magnetic resonance imaging was performed with a 0.3-T permanent magnet and a body coil.^a Postcontrast sequences were obtained following administration of gadopentate dimeglumine^b (101 mg/kg [46 mg/lb], IV, once).

Two well-defined subcutaneous nodules are observed dorsal to C2 (Figure 2). These nodules are hyperintense, compared with the surrounding musculature, on the T2-weighted and short-tau inversion recovery (STIR) sequences and isointense to hyperintense on T1-weighted and proton density (PD) sequences.

Same MRI images as in Figure 1. A—Two nodules are evident within the subcutaneous space at the level of C2 (asterisks). A hyperintense tract traverses the spinous process of C2 (long arrow). A second hyperintense tract courses cranial to the C2 spinous process (arrowheads). B—A single subcutaneous nodule (asterisk) is seen with an associated linear tract. The tract has a mixed hypointense and hyperintense signal and it connects with the subarachnoid space (arrows).

Citation: Journal of the American Veterinary Medical Association 249, 2; 10.2460/javma.249.2.149

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Same MRI images as in Figure 1. A—Two nodules are evident within the subcutaneous space at the level of C2 (asterisks). A hyperintense tract traverses the spinous process of C2 (long arrow). A second hyperintense tract courses cranial to the C2 spinous process (arrowheads). B—A single subcutaneous nodule (asterisk) is seen with an associated linear tract. The tract has a mixed hypointense and hyperintense signal and it connects with the subarachnoid space (arrows).

Citation: Journal of the American Veterinary Medical Association 249, 2; 10.2460/javma.249.2.149

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Same MRI images as in Figure 1. A—Two nodules are evident within the subcutaneous space at the level of C2 (asterisks). A hyperintense tract traverses the spinous process of C2 (long arrow). A second hyperintense tract courses cranial to the C2 spinous process (arrowheads). B—A single subcutaneous nodule (asterisk) is seen with an associated linear tract. The tract has a mixed hypointense and hyperintense signal and it connects with the subarachnoid space (arrows).

Citation: Journal of the American Veterinary Medical Association 249, 2; 10.2460/javma.249.2.149

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A thin linear tract, which is hyperintense on T2-weighted and STIR sequences and isointense on T1-weighted and PD sequences (relative to epaxial muscle), extends ventrally from the level of the nodules through the spinous process of C2 and terminates at the level of the dorsal aspect of the subarachnoid space (Figure 2). The tract is well-defined as it courses through the spinous process of C2 and is hyperintense on T1-weighted and T2-weighted sequences, relative to the spinous process.

A second hyperintense tract was identified on T2-weighted and STIR sequences, coursing parallel to the craniodorsal margin of the C2 spinous process. Absence of fusion of the cranial aspect of the C2 hemiarches was also observed. There was no evidence of meningeal enhancement in the postcontrast sequences. On the basis of MRI findings, diagnoses of a dermoid cyst and C2 spina bifida were made.

Treatment and Outcome

Excision of the cyst and associated linear tracts was completed via a dorsal laminectomy. During resection, a multilobulated subcutaneous cystic structure connected to a diverging fibrous linear tract was identified. One tract coursed through the midline of the spinous process of C2, and the second tract coursed through the interarcuate ligament and foramina of C1–2. Within the vertebral canal, the fibrous tract was found to connect to the dura mater. The dog recovered uneventfully from surgery and was discharged from the hospital 48 hours later.

The cyst and fibrous linear tracts were submitted for histologic evaluation. The larger portion of the cyst was confirmed to be a dermoid cyst with pyogranulomatous furunculosis and fibrosis. The second lobulated portion was consistent with an apocrine gland cyst. The incision healed uneventfully, and the dog was doing well 6 months after surgery without evidence of recurrent swelling in the cervical region.

Comments

Dermoid sinus, also termed pilonidal sinus, pilonidal cyst, or dermoid cyst, is a focal tubular structure lined by squamous epithelium and hair follicles.¹ Dermoid sinus is a congenital defect that results from incomplete separation of the ectoderm and neural tube of the spinal cord and skin during embryonic development. The condition manifests as a sac containing sebum, keratin, debris, and hair, with variable penetration of the skin, subcutaneous tissues, and dura mater of the spinal cord.^2,3 A true dermoid sinus is a blind sac that does not have any direct contact with the surface of the skin.^2–4 The condition is most commonly reported in Rhodesian Ridgebacks, but has been described less frequently in many other species and breeds.^2,5 Clinical signs may vary depending on the extent and location of the sinus, with neurologic deficits present in dogs that have a direct connection between the cyst and dura mater.^4,5 Clinical differential diagnoses to consider include follicular infundibular cyst, folliculosebaceous hamartoma, trichofolliculoma, menigocele, and meningomyocele.

Diagnostic tests historically performed to define the extent of the cyst and associated tracts include radiography, ultrasonography, fistulography, myelography, and CT.² Recently, MRI has been used in an effort to delineate sinus structures and fibrous tracts.^2,4,6 Magnetic resonance imaging is considered an ideal method to visualize soft tissue disease, as it provides better contrast resolution than other imaging modalities. In previous reports, MRI was useful for evaluating the cyst-like structures but not for identifying the exact path or termination of the sinus. For these reasons, MRI was considered the most ideal imaging modality, compared with CT, for the dog in the present report. In contrast to previous reports, the termination and path of both tracts were readily visualized on the MRI images and were instrumental in the surgical planning process.

Treatment may not be required if the sinus is quiescent and lacks a direct connection to the dura mater. Because the cyst in the dog of the present report was progressively increasing in size over a 2-month period and because of the palpable extent of the fibrous tract, surgical resection was performed. A direct communication between the fibrous tract and subarachnoid space was considered unlikely in the absence of epidural or focal meningeal enhancement, despite the fact that subtraction imaging was not performed. Because of the absence of neurologic signs and lack of a draining tract or clinical evidence of an infection, the prognosis for the dog of the present report was considered to be favorable following complete resection. Prognosis may be considered more guarded in dogs with neurologic signs related to meningitis or myelitis.