HISTORY

A six-month-old male Bull terrier was presented with a five-month history of spinal pain. Physical examination disclosed no abnormalities. On neurological examination the dog showed mild paraparesis, proprioceptive deficits in the pelvic limbs, and cranial thoracic pain on spinal palpation. Main differential diagnoses included: congenital anomalies (arachnoid cyst, pilonidal cyst, vertebral malformation), inflammatory / infectious disorders and neoplasia.

Routine haematology, biochemistry, urinanalysis and thoracic radiographs showed no abnormalities. A lumbar myelogram was performed. Myelography revealed dorsal displacement of the spinal cord at T3-T4, caused by an intradural-extramedullary lesion in the dorsal contrast column. An MRI of the cranial thoracic spine was performed. Transverse sections showed compression and displacement of the spinal cord to the right by an abnormal mass, which was connected with a sinus tract running towards the skin surface. A diagnosis of a dermoid sinus was made.

The sinus was surgically excised. A left hemilaminectomy was performed, and a durotomy was needed to completely remove the sinus as its lining was continuous with the dura mater. One week after surgery, the dog's neurological status was normal. No relapse of clinical signs was observed three months later. Histopathology revealed features consistent with a dermoid sinus. The sinus tract was lined by stratified squamous epithelium and hair follicles, sweat and sebaceous glands were observed in the sinus walls.

DISCUSSION

Dermoid sinuses (pilonidal sinuses) are congenital abnormalities caused by an incomplete separation of the ectoderm and the neural tube during embryonic development. Sinuses can be observed as fistulous tracts on the skin surface dorsal to the spine, extending below the skin to variable depths. In some cases, the sinus can extend as far as the dura mater and it may communicate with the subarachnoid space.

Purebred and crossbred Rhodesian ridgeback dogs are most commonly affected, and the condition in this breed is thought to be inherited as an autosomal recessive trait. Individual reports of dermoid sinuses have been reported in other breeds. Diagnosis is frequently based on clinical signs. An opening in the skin on the dorsal midline is usually found. In the present case, the opening in the skin was not seen until the hair over the sinus was clipped prior to surgery. However a fistulous tract could be palpated subcutaneously under a hair tuft in the cranial thoracic region. A dermoid sinus was suspected based on clinical and myelographic signs. Information provided by the MRI scan allowed to establish a more precise diagnosis of a dermoid sinus.

Complete surgical excision is the treatment of choice. If the sinus is attached to the dura mater, a laminectomy is required to fully remove all abnormal tissue. In the present case, a complete excision was performed and no relapse of clinical signs was observed. To the authors' knowledge, this is the first reported case of a dermoid sinus in a Bull terrier, and the first report of MRI diagnosis of this condition.