What is dermatomyositis?
This condition is one of inflammation (itis) of the skin (dermato) and muscle (myo) that is seen in young collies and Shetland sheepdogs. There appears to be a defect in the immune system that predisposes dogs to this disorder. The skin lesions typically develop first with variable muscle problems occurring later. There are many similarities to dermatomyositis in people.
Ulcerative dermatosis may be a variant of this condition. It is a rare disorder that occurs in middle-aged to older dogs of the same breeds, and is manifest by skin lesions (blisters, crusting) that are seen primarily in the groin and underarm regions. Occasionally there are muscle abnormalities.
How is dermatomyositis inherited?
The trait is believed to be autosomal dominant with variable expressivity. This means that if either parent is affected, all puppies have a susceptibility to the disorder, but not all will be affected equally. The variability suggests there is more involved than simple inheritance, including internal factors such as the individual's immune system (also affected by heredity) and external factors (including possibly viral infection). The most severely affected dogs may be homozygous for the trait.
What breeds are affected by dermatomyositis?
This disorder is seen primarily in the collie, Shetland sheepdog, and their crosses. It has been diagnosed in other breeds, including the Australian cattle dog, German shepherd, chow chow, Pembroke Welsh corgi, and Kuvasz.
Ulcerative dermatosis is seen in Shetland sheepdogs and collies.
What does dermatomyositis mean to your dog & you?
With this condition, the skin is almost always affected before, and worse than, muscle. Typically, skin lesions occur by 6 months of age. There is reddening, hair loss, blisters or small bumps, crusting and where severe, ulceration of the skin. Most often affected are the face (especially the muzzle and ear tips, and around the eyes), the tip of the tail, bony prominences (over the elbows for instance) and the toes. Over time, the affected skin becomes scarred.
The muscles are not always affected in dermatomyositis, or the abnormalities may be so slight as to go unnoticed. When there is muscle involvement, the puppies may be weak and lethargic and have a slow rate of growth. Muscles (especially of the face and head) may appear smaller due to muscle atrophy (shrinkage and loss of use). The most severely affected dogs may have difficulty in chewing or swallowing. The leg muscles may also atrophy.
The degree to which pups are affected varies considerably. Muscle inflammation is generally less severe in Shelties.
Generally the clinical signs fluctuate over time for no apparent reason, and many mildly affected dogs will outgrow the condition before a year of age, although some may have permanent scars on their face or legs. In severely affected dogs, the condition is progressive and these dogs may have to be euthanized due to severe muscle atrophy and associated problems such as an inability to eat and drink properly, which may be complicated by pneumonia
How is dermatomyositis diagnosed?
This disorder is usually suspected in a young collie or Sheltie with crusting facial skin lesions, with or without muscle weakness. There are other conditions which can cause these types of lesions and your veterinarian will do tests such as a skin biopsy to pinpoint the diagnosis. This is a simple procedure done with local anesthetic, in which your veterinarian removes a small sample of your dog's skin for examination by a veterinary pathologist. The biopsy will show changes in the skin consistent with this condition.
For the veterinarian: CBC, biochemical profile and urinalysis are usually normal, and the results of standard tests for autoimmunity are usually negative. In addition to history and physical exam findings, diagnosis is made by biopsy (affected skin and muscle), electromyography (EMG), and ruling out other conditions. The main differential diagnosis, especially where the muscle component is mild, is epidermolysis bullosa. The skin lesions have a similar age of onset and clinical progression, but with dermatomyositis, erythematous plaques or vesicles can not be induced in normal skin by applying mild friction.
Dermatomyositis may be complicated by localized or generalized demodicosis. Megaesophagus (+/- aspiration pneumonia) may occur in dogs with severe muscle involvement.
How is dermatomyositis treated?
Skin lesions are exacerbated by trauma and by exposure to ultraviolet light, so these should be avoided (by the use of sunscreens for example). This may be all that is required in mildly affected dogs, who are likely to outgrow the condition with time.
Dermatomyositis can usually be managed fairly well in moderately affected dogs, with the above precautions and the use of Vitamin E and occasional use of corticosteroids for flare-ups. Your veterinarian will work with you to determine how best to manage the condition in your dog. Unfortunately, it is very difficult to maintain the health and comfort of severely affected dogs, and euthanasia is sometimes the best option.
For the veterinarian: Pentoxifylline may help by improving microvascular blood flow. A response may take 2 or 3 months. Short term use of glucocorticoids may be necessary for acute flare-ups of skin or muscle inflammation, but long term use should be avoided as it will exacerbate skin and muscle atrophy.