Histologic assessment of the excised tissues revealed a dermoid sinus cyst with rupture and extensive chronic active dermatitis
and panniculitis. Within the inflamed subcutaneous tissues, a cystic mass structure was noted, lined by well-differentiated
basal and squamous epithelium with moderate hyperkeratosis. Subjacent to the cystic lining were numerous adnexal structures
including follicles and apocrine glands.
Dermoid sinus cysts, also described as pilonidal sinuses or pilonidal cysts, are congenital defects stemming from incomplete neuroectodermal separation during embryogenesis.1-7 The condition is commonly seen in Rhodesian ridgebacks and is hereditary in the breed.8 Several reports have also described the condition in other breeds.2-3,9-13 These cysts are commonly reported to occur over the spine and less commonly in the nasal aspect of the dorsal midline.7,12 Other reported locations include the tongue, the parieto-occipital region, and inside the skull.14-16
Dermoid sinuses and cysts contain normal skin structures such as epidermis and adnexa.17 These factors differentiate the cyst in this report from infundibular or epidermal inclusion cysts, as the latter conditions
do not have hair or adnexal structures. This patient's lesion was diagnosed as a dermoid sinus cyst partly because of its
gross surgical appearance and the confirmatory findings of adnexal structures on histopathology.
Five types of dermoid sinuses have been described18 :
- Type I extends ventrally as a cylindrical sac.
- Type II has a saclike portion more superficial than type I.
- Type III has a superficial sac with no deep attachments.
- Type IV extends into and communicates with dura mater.
- Type V is a true cyst with a epithelial-lined sac.
The cyst in this report appears to be a type V dermoid sinus since it was a true cyst consisting of a closed, epithelial-lined
sac. To our knowledge, the anatomic location of this unusual lesion has not been reported in the veterinary literature. In
people, similar dermoid sinus cysts have been described in neonates and children.19 Some researchers have hypothesized that parasagittal dermoid sinuses on the head may occur along embryological fusion lines.16 In the context of this hypothesis, the lesion's location in this report may have originated from temporofrontal suture lines.
Dermoid sinus cysts are most effectively treated by complete surgical excision of involved tissues. Partial excision, as was
initially performed in this patient, is likely to lead to recurrence of the clinical signs associated with the cystic lesion.
Advanced imaging such as computed tomography or MRI are highly useful in assessing and delineating the lesion and any potential
associated deep tracts.
Alon Kramer, DVM, DACVS
Department of Small Animal Surgery
Cornell University Veterinary Specialists
880 Canal St.
Stamford, CT 06902
Kei Hayashi, DVM, PhD, DACVS
Department of Surgical and Radiological Sciences
School of Veterinary Medicine
University of California
Davis, CA 95616