Dermatomyositis

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Dermatomyositis

 

An inheritable inflammatory disease of the skin, muscles, and vasculature that develops in young collies, Shetland sheepdogs, and their crossbreeds.

 

PATHOPHYSIOLOGY

  • The exact pathogenesis of dermatomyositis is unknown.
  • A familial predisposition has been reported in collies and Shetland sheepdogs; however, possible triggers for the disease include infectious agents (especially viral), vaccines, drugs, malignancy, toxins, infection—as seen with ischemic dermatopathy in other breeds.
  • Based on the clinical and histopathologic evidence, an immune-mediated or auto-immune process may be involved.

 

SYSTEMS AFFECTED

Skin/Exocrine

Musculoskeletal

 

GENETICS

Autosomal dominant inheritance, with variable expression in collies and Shetland sheepdogs.

 

SIGNALMENT

Species

Dogs

 

Breed Predilections

  • Inheritable disease of collies, Shetland sheepdogs, and their crossbreeds, Beauceron shepherds, Belgian Tervurens, and Portuguese water dogs.
  • Similar symptoms are reported in the mongrel, Welsh corgi, Lakeland terrier, chow chow, German shepherd dog, schipperke, and Kuvasz.
  • Some animals in other breeds with similar signs are now classified as ischemic dermatopathy (dermatomyositis-like), and not dermatomyositis as previously reported.

 

Mean Age and Range

Cutaneous lesions typically develop before 6 months, and may develop as early as 7 weeks of age.

The full extent of lesions is usually present by 1 year of age, and may lessen thereafter.

Adult-onset dermatomyositis can occur, but is rare, and is usually less severe.

 

Predominant Sex

None reported

 

SIGNS

General Comments

  • The clinical signs vary from subtle skin lesions and subclinical myositis to severe skin lesions with generalized muscle atrophy, abnormal gait and megaesophagus.
  • Several littermates may be affected, but the severity of the disease often varies significantly among affected dogs.

 

Physical Examination Findings

  • Waxing and waning lesions around the eyes, lips, face, inner ear pinnae, tip of the tail, and bony prominences—usually seen in affected dogs before they are 6 months old.
  • Scarring—often a sequela to the initial skin lesions.
  • Atrophy of the masseter and temporal muscles.
  • Severe cases may have difficulty eating, drinking, and swallowing.
  • Stiff or high-stepping gait.
  • Skin lesions—characterized by variable degrees of crusted erosions, ulcers, and alopecia, with erythema, scaling, and scarring on the face, around the lips and eyes, in the inner ear pinnae, on the tip of the tail; the entire face may be involved.
  • Pressure points over bony prominences, especially the carpal and tarsal regions.
  • Foot pad and oral ulcers, as well as nail abnormality or loss may occur.
  • Myositis—signs may be absent or vary from subtle decrease in the mass of the temporalis muscles to generalized symmetric muscle atrophy and stiff high-stepping gait.
  • Dogs with megaesophagus may present with aspiration pneumonia.

 

CAUSES

  • Hereditary in collies, Shetland sheepdogs, and their crosses.
  • Infectious agents, toxins, malignancy, vaccines or drugs may be a triggering event.
  • Immune-mediated disease in other breeds.

 

RISK FACTORS

Mechanical pressure and trauma, and ultraviolet light exposure may worsen cutaneous lesions.

 

DIAGNOSIS

DIFFERENTIAL DIAGNOSIS

  • Demodicosis
  • Dermatophytosis
  • Bacterial folliculitis
  • Juvenile cellulitis
  • Discoid lupus erythematosus
  • Systemic lupus erythematosus
  • Polymyositis
  • Ischemic dermatopathy
  • Epidermolysis bullosa simplex

 

CBC/BIOCHEMISTRY/URINALYSIS

Serum creatine kinase may be elevated due to muscle damage.

 

OTHER LABORATORY TESTS

  • Antinuclear antibody titers—rule out systemic lupus erythematosus.
  • Elevated levels of immunoglobulin G and circulating immune complex correlated with disease severity.

 

DIAGNOSTIC PROCEDURES

  • Skin biopsy—may be diagnostic for dermatomyositis, although this disease can be difficult to definitively diagnose; avoid infected and scarred lesions.
  • Muscle biopsy—proper muscle selection can be difficult because pathologic changes may be mild consisting of muscle necrosis and atrophy.
  • EMG—ideally, is used to select affected muscles for biopsy; if EMG is not available, atrophied muscles should be biopsied.

 

PATHOLOGIC FINDINGS

Skin Biopsy

  • Scattered apoptosis or vacuolation of individual and follicular basal cells;may lead to intrabasal or subepidermal clefting.
  • Mild pigmentary incontinence.
  • Superficial, mild, diffuse dermal and perivascular cellular infiltrates—composed of lymphocytes, plasma cells, and histiocytes.
  • Follicular atrophy and perifollicular fibrosis in chronic cases.
  • Secondary epidermal ulceration and dermal scarring—may be present.
  • Histopathologic features may be subtle and consist mostly of atrophic changes; however, the combination of epidermal and follicular basal cell degeneration, perivascular inflammation, and follicular atrophy with fibrosis is highly suggestive of dermatomyositis.

Muscle Biopsy

  • Variable multifocal accumulations of inflammatory cells, including lymphocytes, plasma cells, macrophages, and neutrophils.
  • Myofibril degeneration—characterized by fragmentation, vacuolation, atrophy, fibrosis, and regeneration.

Electromyography

  • EMG abnormalities are present especially in the muscles of the head and distal limbs; findings include fibrillation potentials (rapid, irregular, and unsynchronized contraction of muscle fibers), and positive sharp waves.

 

TREATMENT

APPROPRIATE HEALTH CARE

  • Most dogs can be treated as outpatients.
  • Dogs with severe myositis and megaesophagus may need to be hospitalized for supportive care.
  • Severe cases may warrant euthanasia.
  • Assist to eat if muscles of mastication are affected; feed at an elevated position if megaesophagus develops.
  • Nonspecific supportive therapy includes gentle bathing and soaking to remove crusts, and treatment of secondary bacterial folliculitis (if present).

 

ACTIVITY

Avoid activities that may traumatize the skin.

Keep indoors during the day to avoid solar radiation.

 

CLIENT EDUCATION

  • Discuss the hereditary nature of the disease.
  • Note that affected dogs should not be bred.
  • Inform the owner that the disease is not curable, although spontaneous resolution or waxing and waning of symptoms may occur.
  • Discuss prognosis and possible complications, especially in severely affected dogs.
  • Therapeutic efficacy of medical treatment can be difficult to assess because the disease tends to be cyclic in nature and is often self-limiting.

 

CONTRAINDICATIONS

Pentoxifylline should not be used in dogs that are sensitive to methylxanthine derivatives.

 

PRECAUTIONS

  • Pentoxifylline—rarely causes gastric irritation; can affect clotting times (PT/PTT prolongation and thrombocytopenia) and dogs receiving anticoagulant therapy should be monitored carefully when treated with this drug; possible rare seizure or reduction of seizure threshold in epileptics.
  • Glucocorticoids—discuss possible side effects with the owner.
  • Tacrolimus can cause local irritation.

 

POSSIBLE INTERACTIONS

Glucocorticoids and nonsteroidal anti-inflammatory medications can cause GI bleeding if used concurrently.

 

PREVENTION/AVOIDANCE

  • Do not breed affected animals.
  • Neuter intact animals to reduce hormonal influence on symptoms.
  • Minimize trauma and exposure to sunlight.

 

POSSIBLE COMPLICATIONS

  • Secondary bacterial folliculitis.
  • Mildly to moderately affected dogs may have residual scarring.
  • Severely affected dogs may have trouble chewing, drinking, and swallowing due to scarring of the masticatory and esophageal muscles.
  • Megaesophagus may develop, predisposing the dog to aspiration pneumonia.
  • Dogs may be lame due to damage of the muscles of the extremities.

EXPECTED COURSE AND PROGNOSIS

  • Long-term prognosis—variable, depending on severity of disease.
  • Minimal disease—prognosis good; tends to spontaneously resolve with no evidence of scarring.
  • Mild-to-moderate disease—tends to resolve spontaneously, but residual scarring is common.
  • Severe disease—prognosis for long-term survival is poor as damage to the skin and muscle may be life-long.

 

MISCELLANEOUS

 

ASSOCIATED CONDITIONS

None

 

AGE-RELATED FACTORS

 

Initial clinical signs usually occur in dogs younger than 6 months.

Adult-onset—rare; more commonly seen in dogs that had subtle lesions as puppies.

 

ZOONOTIC POTENTIAL

None

 

PREGNANCY/FERTILITY/BREEDING

  • Do not breed affected dogs.
  • Pregnancy may exacerbate clinical symptoms.
  • Oestrus may exacerbate clinical symptoms.

 

ABBREVIATIONS

EMG = electromyography, electromyographic

GI = gastrointestinal

PT = prothrombin time

PTT = partial thromboplastin time

 

WORD COUNT: 1287

 

WORD FREQUENCY: 

28 (5%)may 

16 (3%)dogs 

14 (2%)disease 

12 (2%)affected 

10 (2%)skin 

10 (2%)lesions 

9 (1%)muscles

 

META DESCRIPTION:

 

Dermatomyositis is an inheritable inflammatory disease of the skin, muscles, and vasculature that develops in young collies, Shetland sheepdogs, and their crossbreeds.

 

Visit your veterinarian as early recognition, diagnosis, and treatment are essential.

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