History
A 13-year-old Haflinger mare was evaluated for a firm swelling and draining tract at the base of the right ear associated with minimal signs of pain. The swelling had been present since the horse was obtained at auction 1.5 years earlier with an unknown history. During current ownership, the wound was debrided and flushed several times and several bony fragments were recovered during the procedure. Additionally, the horse was treated with several courses of antimicrobials, but with no improvement in clinical signs. Radiographs of the skull were obtained (Figure 1).
Right lateral (A) and left dorsal–right ventral oblique (B) radiographic views of the skull of a 13-year-old Haflinger mare with a firm swelling and draining tract ventral to the right pinna. The radiographs are centered on the right temporal region.
Citation: Journal of the American Veterinary Medical Association 247, 10; 10.2460/javma.247.10.1097
Determine whether additional imaging studies are required, or make your diagnosis from Figure 1—then turn the page →
Diagnostic Imaging Findings and Interpretation
The radiographic images depict a medium-sized, irregularly marginated, heterogeneous mass of mineral opacity in the right temporal region. The mass is ventral to the right pinna and dorsal to the right external ear canal (horizontal portion). A gas-filled tract is associated with the lateral margin of the mass (Figure 2). The history, physical examination findings, and radiographic findings were attributed to a dentigerous cyst with a draining tract.
Same radiographic images as in Figure 1. In the right temporal region, there is a medium-sized, heterogeneous, somewhat rectangular mass of mineral opacity (circle) located ventral to the base of the right pinna (a). In the oblique view, a gas-filled draining tract (asterisk) is associated with the lateral margin of the mass.
Citation: Journal of the American Veterinary Medical Association 247, 10; 10.2460/javma.247.10.1097
General anesthesia was induced, and surgical debridement was performed. However, the dentigerous cyst could not be removed; thus, CT of the skull was performed to identify any potential underlying cause for difficult removal and to plan for a second surgery. Computed tomography was performed with a 16-slice helical CT scanner (contiguous 2-mm-thick slices). A transverse image (Figure 3) was acquired at the level of the tympanic bullae with a bone display algorithm (window width, 3,700 Hounsfield units [HU]; window center, 380 HU).
Transverse CT image of the skull of the same horse as in Figure 1. There is a large, rectangular, heterogeneously mineralized mass (c) with rounded margins that is located dorsal to the external ear canal (a), right of the skull, and dorsolateral to the right tympanic bulla (b). Adjacent to the mass is a gas-filled draining tract (asterisk) extending laterally toward the skin.
Citation: Journal of the American Veterinary Medical Association 247, 10; 10.2460/javma.247.10.1097
On CT, the mass is of medium size (2.7 × 4.0 × 5.4 cm) and has smooth margins. There is contact between the mass and the caudomedial aspect of the right zygomatic arch. The mass has a rectangular shape with well-defined internal structures characterized by layering typical of a tooth. The mass displaces the right external ear canal ventrally with moderate compression of the lumen. The wall of the external ear canal is mildly mineralized with normal thickness. Lateral to the mass and dorsal to the external ear canal, there is a gas-filled tract that extends to the skin.
Treatment and Outcome
A second surgery was performed, and the dentigerous cyst was completely excised with a good cosmetic result. The horse did well after surgery, with no recurrence of clinical signs reported.
Comments
Differential diagnoses for a draining tract and soft tissue swelling in the temporal region include bone sequestrum, foreign body, abscess, neoplasia, and dentigerous cyst. The diagnosis of dentigerous cyst often is made with sufficient certainty simply on the basis of signalment, history, physical examination, and, in some cases, results of radiography. A dentigerous cyst (also known as an ear tooth or temporal teratoma) is a congenital defect in the closing of the first branchial cleft resulting in ectopic dental tissue, which may or may not be well organized.1 Because of the typical migration of embryological tissue, dentigerous cysts in horses are usually unilateral and identified at the base of 1 ear but can occasionally occur bilaterally or in other locations such as the paranasal sinuses and cranium.1 Dentigerous cysts are characteristically lined by a stratified mucous membrane that secretes a mucoid discharge, leading to a perpetual draining tract.2 Surgical removal can be performed for a purely cosmetic result and is warranted in cases where the cyst causes a functional interference.
Because of the location of most dentigerous cysts, surgical removal can be associated with complications that range in severity. Minor complications include aesthetically unpleasing results such as damage to the auricular muscles or scutiform cartilage. Major complications include temporomandibular joint sepsis or cranial nerve trauma.3 Additionally, if the entire cystic lining is not removed, dehiscence of the incision and reformation of a fistulous tract may occur.1 Despite potential surgical complications for horses with a dentigerous cyst, these congenital lesions are benign and the prognosis is good to excellent.
In most horses with dentigerous cysts, cross-sectional diagnostic imaging is unnecessary. However, in the horse of the present report, the CT scan was valuable for characterizing the extent and attachments of the dentigerous cyst to better evaluate why the first surgery was unsuccessful and to plan accordingly for the second surgery. The use of contrast material, albeit not used in the horse of the present report, can be helpful in outlining the draining tract and, if administered IV, can help define the extent of inflamed or infected tissue. Particularly for complicated cases, the use of cross-sectional diagnostic imaging will allow for a more careful surgical approach and better informed advice to the owner in regard to risks of the procedure.
1. Auer JA, Stick JA. Equine surgery. 4th ed. St Louis: Elsevier, 2012.
2. Jubb KVF, Kennedy PC, Palmer N. Pathology of domestic animals. 4th ed. San Diego: Academic Press, 1992.
3. Barakzai SZ, Dixon PM. Dentigerous cysts. Equine Vet Educ 2012;24:579–581.
