CAUSE AND PATHOGENESIS
The cause of dermatomyositis in humans
or dogs is unknown. An immune-mediated pathogenesis is suspected
because of detectable immunologic abnormalities, but it is unclear whether
this immunologic reaction causes all of the changes seen or is in response
to some pre-existing muscle or skin damage. Although dermatomyositis
could be induced by drugs, infections (especially viral ones), toxins,
or internal malignancies, their causal relationship in dermatomyositis
remains unproved.
A familial history is rare in humans,
but common in collies and Shetland sheepdogs. Breeding studies in
collies support an autosomal dominant mode of inheritance with variable
expressivity. Limited studies in the Shetland sheepdog conducted
by one author (WHM) suggest a similar mode of inheritance in this breed.
CLINICAL FEATURES
In collies and Shetland sheepdogs, there
is no sex, coat colour, or coat length association with dermatomyositis.
This may or may not be true for other breeds. Because of the familial
predisposition, lesions occur early in life, typically before 6 months
of age. Signs in some dogs appear as early as 7 to 11 weeks of age.
The progression of lesions is variable. Some mildly affected dogs
have few lesions, which heal rapidly without scarring. Most dogs
have new lesions after the first ones are recognized, but the rate of progression
is variable. The extent of the skin lesions is known by one year
of age. Unless management changes occur, lesions usually decrease
in number and severity from that point on.
Skin lesions occur in areas of mechanical
trauma and are commonly seen on the face (see Fig. 11:22C and D), especially
around the eyes (Fig. 11:22E); on the tips of the ears; on caral and tarsal
regions (Fig. 11:22F); on the digits; and on the tip of the tail (Fig.
11:22G). Oral and footpad lesions can be seen but are rare.
Although intack vesicles can be seen in some dogs, primary lesions are
usually absent. Typical skin lesions are characterized by alopecia,
erythema, scaling, and mild crusting. Ulceration can be seen in severely
affected dogs, large areas of normal skin remain, whereas severely affected
dogs have involvement of the entire face, distal limbs, and tail. Some
dogs have long, soft claws.
The myositits typically occurs after
the skin lesions are recognized and correlated with the severity of the
skin lesions. Mildly affected dogs have no clinical muscle disease
and convincing evidence of this may not be found on EMG testing or muscle
biopsy. These cases may have epidermolyisis bullosa, dermatomyositis
with focal but undetected myotitis, or dermatomyositis without the myositis.
In humans, the diagnosis of amyopathic dermatomyositis can be made only
when no muscle changes are detected for 4 years or longer after the skin
lesions have occurred. To the authors’ knowledge, this type of follow-up
testing has not been done in dogs. The rare dog related to dogs with
classic dermatomyositis has EMG changes of myositis with no skin lesions.
The significance of these findings is unknown.
Clinical signs of myositis are variable.
A common finding is a dirty water bowl that contains food particles.
These dogs do not have trouble chewing their food but do not swallow it
completely, so residual pieces are washed from the mouth during drinking.
Some dogs have a peculiar high-stepping gait. Severely affected dogs
drink, chew, and swallow with difficulty; have a stiff gait; have megaoesophagus;
and often have secondary aspiration pneumonia. The most common sign of
the mypositis is asymptomatic atrophy, especially of the muscles of mastication
and distal limbs.
The rare dog has skin lesions only in
adulthood. The lesions can be the classic superficial lesions of
dermatomyositis or be more deeply ulcerated (see Idiopathic Ulcerative
Dermatomyositis in Shetland Sheepdogs and Collies in this chapter).
These cases could represent an adult-onset variant of the diseases or be
dogs who had mild, unrecognized disease as puppies.
DIAGNOSIS
The differential diagnostic considerations
should include demodicosis, staphylococcal folliculitis, dermatophytosis,
discoid lupus erythematosus, and epidermolyisis bullosa. The latter
might be considered if there are no muscle signs or lesions and if vesicles
are present.
Diagnosis is made by history, physical
examination findings, biopsy of affected skin and muscle, EMG, and laboratory
tests to rule out other conditions. Biopsy of affected skin shows
scattered vacuolar change of the surface and follicular basal cells (Fig.
11:23). Occasional apoptotic basal cells (Civatt’s bodies) may be
seen. With confluent hydropic change, intrabasal or subepidermal
clefting may be seen (Fig. 11:24). Dermal inflammation can be absent.
Most cases show a mild perivascular to interstitial dermatitis in which
lymphocytes, plasma cells and histiocytes predominate. Mild pigmentary
incontinence may be present in the superficial dermis. Follicular
atrophy and fibrosis are common findings (Figs 11:25 and 11:26).
Vasculitis may occasionally be seen in the skin. Muscle biopsy may show
mixed inflammatory exudates, accompanied by muscle fibre necrosis and atrophy.
In some cases, a vasculitis may be found. Needle EMG abnormalities
include positive sharp waves and fibrillation potentials in muscles of
the head and of distal extremities.
Hemograms and serum chemistry profiles
are usually unremarkable. Creatine kinase levels are normal or slightly
increased. Neurologic examination and nerve conduction studies are
usually normal. Elevated concentrations of immunoglobulin G and circulating
immune complexes may be found in active disease. The magnitude of
the elevations in immunoglobulin G and circulating immune complex levels
correlates with the severity of the skin disease.
CLINICAL MANAGEMENT
The skin lesions of dermatomyositis are
worsened by trauma and prolonged solar exposure. Management changes
to avoid these secondary insults should be instituted. Mildly affected
dogs usually require no additional treatment, as their skin lesions heal
spontaneously. Severely affected dogs are difficult to manage.
These dogs have widespread skin lesions and a generalized myopathy, which
results in lameness and difficulty in drinking and eating. These
dogs often have aspiration pneumonia. Although large doses of Prednisolone
(1 mg/kg q24h) improves the skin lesions, the disease is progressive and
euthanasia should be encouraged.
Mildly to moderately affected dogs can
usually be maintained as acceptable pets for extended periods. Some
skin lesions remain and muscle atrophy, especially of the muscles of mastication,
is noted. Oral does of vitamin E (200 to 800 IU/day) or marine lip
supplements (e.g. DVM Derm Caps) appear to be beneficial for the skin but
not the muscle lesions.. Occasional use of Prednisolone (1 mg/kg
q24h) is necessary in some dogs for traumatic flares. Continued use of
glucocorticoids should be discouraged because the muscle atrophy may be
aggravated. Treatment with pentoxifylline (Trental [Hoecsht-Roussel])
has been recommended. This drug increases tissue oxygenation by increasing
microvascular blood flow. It is a gastric irritant and must be given
with food. Dosages of 400 mg every 24 to 48 hours have been suggested.
Response is slow, and 2 to 3 months of treatment are necessary before efficacy
can be determined. No data are available on the efficacy of this
treatment.
The above treatments usually minimize
the development of new skin lesions, and those that do occur tend to be
milder. Muscle disease progresses and old dogs have profound atrophy
of the muscles of the head, the distal limbs, and sometimes the body.
With severe atrophy, the animal’s ability to eat and drink can be compromised
and dietary manipulations are necessary. The limb and body atrophy
can cause an abnormal gait, but locomotion is till possible. Amyloidosis
can occur in some chronically affected dogs.