DERMOID SINUS ... What is it ?

by Michele Steed, BVSc

The Dermoid Sinus is a sinus, which connects the skin with the dura mater (outer covering of the spinal cord) in the vertebral canal or other structures in the area. It is a congenital (present at birth) and hereditary (transmitted from parent to offspring) abnormality in Rhodesian Ridgeback dogs. The word "dermoid" means skin like and the word "sinus" means channel, recess or cavity.

Dermoid Sinus is classified into 4 types depending on the depth to which it penetrates, with Type I penetrating only through the skin and Type IV reaching the spinal cord (this is rare). Dermoid sinus occurs only rarely in dogs that aren't Ridgebacks or Ridgeback crosses.

It is characterized by the presence of a tuft of hair protruding from each sinus and is sometimes complicated by infection. Other common names used to describe a dermoid sinus include dermoid cyst, epidermal inclusion cyst, hair cyst and African cyst.

Where is it found on the dog?

The most common places for this defect to occur are on the midline at the back of the neck and on the midline of the rump - so in front of and behind the ridge. One or more sinuses may be present on the same animal.

How does it become a problem?

The narrow tube of skin, which descends below the skin surface, is lined with the usual dermal structures - hair, and sweat and oil glands which continue to grow and produce excretions. This channel with time, becomes filled with hair, skin cells and oil and can become infected, resulting in abscess formation and a chronic discharging wound. This infection may spread to the spinal cord if the sinus is Type IV. Treatment for an infected dermoid sinus involves surgery and supportive medical procedures and is difficult, and expensive with some Type IV cases proving incurable. Type I-III sinuses are curable if the surgery is done by an experienced surgeon.

How can it be detected?

It can be palpated (felt) in newborn pups. The skin from the back of the head to the tail is examined carefully by an experienced breeder or Veterinarian and any pups found with a sinus should be humanely put to sleep (if not euthanased, they should be desexed). If there is any doubt as to whether a sinus is present or not shaving the hair in the suspected area can reveal the external opening in the skin.

What can we do to prevent it?

The mode of inheritance of Dermoid Sinus is complex and not well understood, with different theories on heritability touted by different geneticists. By only breeding from stock that does not have DS there should be a reduction in numbers of affected pups being born. However, it would be unlikely for the elimination of DS to occur entirely. Recent research by Dr John G. Roberts and Felicity A. Nicholls-Grzemski indicated that high folate levels in the diet of the breeding RR bitch led to a significant and important reduction in the incidence of DS in their progeny. This study was prompted by a similar study on the effect of Vitamin supplementation in pregnant women and its effect on the incidence of neural tube defects (conditions in humans similar to DS in Ridgebacks, eg. Spina bifida).

When injecting Ridgebacks it is a good idea to avoid the midline between the back of the head and the ridge, as occasionally reactions to certain products can cause an inflammation that resembles Dermoid Sinus.

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