Canine Dermatomyositis

Also known as: Canine familial dermatomyositis

Introduction

Canine familial dermatomyositis is a hereditary inflammatory disease of skin and muscle characterised by symmetrical scarring alopecia of the face and limbs and atrophy of the muscles of mastication.

It affects Shetland Sheepdogs and Collies from as early as 7 weeks of age and has an autosomal dominant mode of inheritance with variable expressivity. It has also been known to occur in other breeds such as the Chow Chow and Corgi.

The aetiopathogenesis of the condition is unknown. It is thought that immunological damage occurs to blood vessels resulting in ischaemic damage to the skin and muscles.

Clinical Signs

Lesions have usually developped by 6 months of age, but they can also occur in adults.

The rate of development of lesions is very variable, and the disease may wax and wane and may undergo spontaneous regression.

Typical lesion distribution is the face (bridge of nose, tips of ears and around the eyes), carpal and tarsal areas, digits and tip of the tail.

There is scarring alopecia, erythema, scaling and crusting.

Myositis is a feature of the disease and its severity correlates with the severity of the skin lesions. There is commonly temporal and masseter muscle atrophy.

Severely affected dogs may present with dysphagia, may have growth retardation, megaoesophagus, lameness and widespread muscle atrophy.

The condition is not usually pruritic nor painful.

Diagnosis

A clinical diagnosis can be made based upon the presentation, history and clinical signs.

Other common causes of follicular dermatitis in young dogs, such as dermatophytosis, demodecosis and pyoderma, should be ruled out with skin scrapes, cytology and culture.

Skin biopsies can provide supportive evidence, and there is hydropic degeneration of basal cells, intrabasalar or subepidermal clefting, pigmentary incontinence, follicular atrophy and possibly vasculitis.

Muscle biopsies reveal inflammatory exudates, muscle fibre necrosis and muscle atrophy. The best place for a biopsy is the anterior and most superficial temporalis muscle.

EMG can be performed and will reveal abnormalities consisting of positive sharp waves, fibrillation potentials and bizarre high-frequency discharges of affected muscles.

Treatment

It is difficult to determine the effectiveness of a treatment as lesions can wax and wane on their own.

Mild lesions may not need treatment and may spontaneously resolve.

Pentoxifylline is the treatment of choice and it improves circulation and decreases levels of inflammatory mediators. 2-3 months may be needed before any benefits are seen.

Vitamin E can lead to an improvement in the skin lesions, but not in muscle.

Prednisolone can be given during acute flare-ups but prolonged use is discouraged as it can contribute to muscle atrophy.

As the condition is heritable, affected animals should not be used for breeding.

Prognosis

Complete resolution is rarely possible and treatment only minimises the development of new lesions and lessens the severity of those present. Scarring and muscle atrophy may never resolve. Animals may suffer from secondary infections and aspiration pneumonia from megaoesophagus. This means prognosis can be difficult to predict.

Mild cases generally have a good prognosis and may resolve spontaneously.

Canine Dermatomyositis Learning Resources
Flashcards Test your knowledge using flashcard type questions	Small Animal Orthopaedics Q&A 05

References

Moriello, K. (2005) Self-assessment colour review of small animal dermatology Manson Publishing

Harvey, R. (2009) A colour handbook of skin diseases of the dog and cat Manson Publishing