Dermatomyositis

Question 1

Familial Canine Dermatomyositis Background

Answer 1

Hereditary idiopathic inflammatory condition of the skin and muscles of young dogs

Question 2

Dermatomyositis predisposed breeds

Answer 2

collies, shetland sheepdogs, beauceron shepherds, belgian tervurens, and Portuguese water dogs; also reported in mixed, welsh corgi, lakeland terrier, chow chow, GSD, and kuvasz

Question 3

Dermatomyositis cause and pathogenesis

Answer 3

Cause unknown
genetically determined immune-mediated pathogenesis is suspected
could be induced by drugs, vaccines, infections (especially viral), toxins, or internal malignancies

Question 4

Mode of inheritance of dermatomyositis

Answer 4

familial history common in collies and shetland sheepdogs, studies suggest autosomal dominant mode with variable expressivity in collies and shetland sheepdogs; study suggests locus affecting the dermatophytosis phenotype is near marker FH3570 near chromosome 35 but not confirmed by additional study

Question 5

Clinical features of dermatomyositis

Answer 5

no coat color or length associated, either sex
lesions appear early in life, typically before 6m of age (as early as 7-11 weeks)
mildly affected dogs can have few lesions that heal rapidly without scarring
most dogs have new lesions after the first are recognized but variable rate of progression
extent of skin lesions known by 1 yr of age, decrease in severity and number from that point on

Question 6

Areas affected by dermatomyositis

Answer 6

areas of mechanical trauma
common on the face (especially around the eyes) and muzzle
tips of the ears
carpal and tarsal regions
digits
tip of the tail
some dogs have onychorrhexis, onychoschizia, or onychomadesis
oral and footpad lesions are common but especially in Beauceron

Question 7

Myositis in Dermatomyositis

Answer 7

months after skin lesions and correlates with skin lesion severity
mildly affected dogs have no clinical muscle disease (supports that no evidence may be found on EMG testing or muscle biopsy): epidermolysis bulls simplex, dermatomyositis with focal but undetected myositis, or dermatomyositis without the myositis
rare dog that is related to dogs with classic dermatomyositis has EMG changes of myositis but no skin lesions (unknown significance)

Question 8

Clinical signs of myositis in dermatomyositis

Answer 8

dirty water bowl containing food particles, do not have trouble chewing their food but do not swallow it completely (residual pieces washed from mouth during drinking)
severely affected dogs drink, chew, and swallow with difficulty, have megaesophagus, and often have secondary aspiration pneumonia
some have peculiar high-stepping stiff gait
most common sign is asymptomatic atrophy, especially muscles of mastication and distal limbs
rarely dogs have skin lesions only in adulthood

Question 9

Differentials for Dermatomyositis

Answer 9

Demodicosis, Staph folliculitis, dermatophytosis, DLE, and epidermolysis bullosa simplex (EBS particularly if no muscle signs or lesions and if vesicles are present

Question 10

Diagnosis of Dermatomyositis

Answer 10

history, PE, biopsy of affected skin and muscle, EMG, and lab tests to r/o other conditions

Question 11

Histopath of dermatomyositis

Answer 11

scattered hydronic degeneration of the surface and follicular basal cells
apoptotic basal cells (Civatte bodies) occasionally visible
with confluent hydronic change, intranasal or sub epidermal cleating may be present
dermal inflammation can be absent
most cases have mild perivascular to interstitial dermatitis with predominantly lymphocytes, plasma cells, and histiocytes
mild pigmentary incontinence in superficial dermis
follicular atrophy and perifollicular fibrosis are common and may be the only findings in chronic skin lesions
vasculitis occasionally present in skin

Question 12

Lab abnormalities in Dermatomyositis

Answer 12

CBC and chem usually unremarkable, but CK levels can be increased
Elevated concentrations of IgG and circulating immune complexes may be found in active disease, magnitude correlates with severity of skin disease

Question 13

Management of Dermatomyositis

Answer 13

skin lesions worsened by trauma and prolonged solar exposure, avoid these secondary insults
mildly affected dogs require no additional treatment b/c skin lesions heal spontaneously (some dogs may have permanently alopecia, hyper pigmented, and scarred areas where the most severe active lesions were)
severely affected dogs difficult: widespread skin lesions and generalized myopathy, often resulting in lameness and difficulty in drinking and eating, often have aspiration pneumonia

Question 14

Management of mildly to moderately affected dogs

Answer 14

some skin lesions remain and muscle atrophy is apparent
oral doses of Vitamin E (200-800 IU/day) and OFA can benefit skin but not muscle
periodic flares and inflammatory lesions common, short course of pred +/- pentoxyfylline (25mg/kg BID with food)

Question 15

Combination therapy for Dermatomyositis

Answer 15

Pentoxyfylline +/- Prednisone
or
doxycycline and niacin amide

Topical tacrolimus BID can reduce skin lesion size alone or in combination

Question 16

Treatment of muscle disease in Dermatomyositis

Answer 16

muscle disease progresses and old dogs have profound atrophy of the muscles of the head, distal limbs, and sometimes body
with severe atrophy, ability to eat/drink can be compromised and dietary manipulations become necessary
limb and body atrophy can cause abnormal gait
amyloidosis in some chronically affected dogs