Dermatomyositis (DM)

A 58 year old female presents with progressive difficulty climbing stairs, rising from a chair, and lifting objects overhead for 3 months. She also reports fatigue, weight loss, and mild exertional dyspnea. Physical examination reveals symmetric proximal muscle weakness, violaceous discoloration around the eyelids with edema, erythematous papules over the MCP and PIP joints, and a photosensitive rash over the upper chest. Fine bibasilar crackles are heard on lung examination. Laboratory investigations demonstrate markedly elevated creatine kinase levels. MRI shows proximal muscle edema, and muscle biopsy reveals perifascicular atrophy with perivascular inflammation. Diagnosis?

Diagnosis is dermatomyositis.

1. Definition

Dermatomyositis is an autoimmune inflammatory myopathy characterized by:

1. Symmetric proximal muscle weakness
2. Characteristic cutaneous manifestations
3. Immune-mediated muscle inflammation and microangiopathy

It is distinct from polymyositis in:

1. Pathophysiology
2. Histology
3. Autoantibody profile
4. Malignancy association

2. Epidemiology

1. Female predominance
2. Juvenile form: 5–15 years
3. Adult form: 40–65 years
4. Malignancy risk highest in adults >50 years
5. Cancer risk is greatest within first 2–3 years after diagnosis

3. Pathophysiology

Dermatomyositis is primarily a humoral immune-mediated microangiopathy.

Key Mechanisms

1. Complement-mediated capillary injury with C5b-9 deposition
2. Endothelial damage causing muscle ischemia
3. Perifascicular atrophy develops due to ischemic injury
4. Perivascular and perimysial inflammation occur
5. CD4+ T cells, B cells, and macrophages predominate

Important Contrast

Dermatomyositis

• Perivascular and perimysial inflammation
• CD4+ predominant infiltrates
• Perifascicular atrophy

Polymyositis / Inclusion Body Myositis

• Endomysial CD8+ T-cell mediated injury

4. Clinical Features

4.1 Muscular Manifestations

1. Progressive painless symmetric proximal muscle weakness
2. Hip flexors are often affected earlier and more severely than shoulder abductors
3. Difficulty:
• Rising from a chair
• Climbing stairs
• Lifting objects overhead
• Combing hair
4. Neck flexor weakness is common
5. Myalgias may occur
6. Distal weakness is uncommon and suggests inclusion body myositis

4.2 Bulbar and Respiratory Involvement

1. Dysphagia due to oropharyngeal muscle weakness
2. Dysphonia due to laryngeal involvement
3. Aspiration risk
4. Respiratory muscle weakness may lead to respiratory failure

5. Cutaneous Manifestations

Heliotrope Rash

1. Violaceous rash around eyelids
2. Often associated with periorbital edema
3. Highly characteristic of dermatomyositis

Gottron Papules

1. Violaceous papules over MCP and PIP joints
2. Pathognomonic for dermatomyositis

Gottron Sign

1. Flat erythema over extensor surfaces

Shawl Sign

1. Photosensitive erythematous rash over shoulders and upper back

V Sign

1. Photosensitive rash over anterior chest

Nailfold Changes

1. Dilated capillaries
2. Capillary dropout
3. Periungual erythema

Mechanic’s Hands

1. Hyperkeratotic rough cracked skin over lateral and palmar fingers
2. Often associated with antisynthetase syndrome

Skin findings may precede muscle weakness

 

6. Extramuscular Manifestations

Pulmonary

1. Interstitial lung disease (ILD)
2. Major cause of morbidity and mortality
3. Anti-MDA5-associated ILD may progress rapidly despite mild muscle disease

Cardiac

1. Arrhythmias
2. Conduction abnormalities
3. Rare myocarditis
4. Congestive heart failure may occur
5. Cardiac involvement contributes to mortality

Musculoskeletal

1. Nonerosive inflammatory arthritis
2. Arthralgias and myalgias

Systemic Features

1. Fever
2. Fatigue
3. Weight loss

7. Dermatomyositis Subtypes

Classic Dermatomyositis

1. Rash with proximal muscle weakness

Amyopathic Dermatomyositis

1. Typical rash without clinical muscle weakness
2. CK may be normal
3. Still associated with ILD and malignancy risk

Paraneoplastic Dermatomyositis

1. Older age at presentation
2. Severe skin disease
3. Strong malignancy association

8. Antisynthetase Syndrome

Characterized by:

1. Inflammatory myopathy
2. Interstitial lung disease
3. Fever
4. Raynaud phenomenon
5. Mechanic’s hands
6. Nonerosive arthritis

Most commonly associated with:

• Anti-Jo-1 antibodies (histidyl-tRNA synthetase)

Important:

• Anti-Jo-1 is associated with antisynthetase syndrome rather than classic dermatomyositis

9. Autoantibodies

Myositis-Specific Antibodies

Anti-Mi-2

1. Classic dermatomyositis
2. Good prognosis

 

Anti-MDA5

1. Amyopathic dermatomyositis
2. Rapidly progressive ILD

 

Anti-TIF1-γ

1. Strong malignancy association

 

Anti-NXP2

1. Malignancy association
2. Severe muscle weakness

 

Anti-SAE

1. Skin-predominant dermatomyositis
2. Muscle involvement may develop later

10. Diagnosis

10.1 Laboratory Findings

Muscle Enzymes

1. Elevated CK
2. Elevated aldolase
3. Elevated LDH
4. Elevated AST and ALT due to muscle injury

A normal CK does not exclude dermatomyositis

Inflammatory Markers

1. ESR and CRP may be elevated
2. ESR and CRP may also be normal

Other Tests

1. ANA positive in many patients
2. TSH should be checked to exclude endocrine myopathy

10.2 MRI

1. Muscle edema and inflammation
2. Helps identify biopsy site

10.3 HRCT Chest

1. Evaluates interstitial lung disease

10.4 Pulmonary Function Tests

1. Baseline assessment for ILD

10.5 Electromyography (EMG)

1. Increased insertional activity
2. Fibrillation potentials
3. Short-duration polyphasic motor units

10.6 Muscle Biopsy (Gold Standard)

Classic findings:

1. Perifascicular atrophy
2. Perimysial inflammation
3. Perivascular inflammation
4. Capillary dropout
5. CD4+ predominant infiltrates

Perifascicular atrophy is highly characteristic of dermatomyositis

11. Malignancy Association

Adult dermatomyositis is strongly associated with malignancy

Common associated cancers:

1. Ovarian cancer
2. Lung cancer
3. Breast cancer
4. Gastrointestinal malignancies
5. Hematologic malignancies

Age-appropriate malignancy screening is mandatory

Enhanced screening is particularly important in:

1. Anti-TIF1-γ positivity
2. Anti-NXP2 positivity
3. Older age
4. Severe dysphagia
5. Weight loss

12. Differential Diagnosis

1. Polymyositis
2. Inclusion body myositis
3. Immune-mediated necrotizing myopathy
4. Statin-induced myopathy
5. Hypothyroid myopathy
6. Myasthenia gravis
7. Amyotrophic lateral sclerosis

13. Treatment

13.1 Corticosteroids

1. High-dose glucocorticoids are first-line therapy
2. Prednisone approximately 1 mg/kg/day
3. IV methylprednisolone may be used in severe disease

13.2 Steroid-Sparing Immunosuppressive Therapy

1. Methotrexate
2. Azathioprine
3. Mycophenolate mofetil:
• Preferred in ILD

Early initiation is recommended to reduce steroid toxicity

13.3 Rituximab

1. Used in refractory disease
2. May benefit severe ILD or resistant myositis

13.4 Intravenous Immunoglobulin (IVIG)

Useful in:

1. Severe weakness
2. Dysphagia
3. Refractory disease
4. Rapidly progressive ILD

13.5 Skin-Directed Therapy

1. Hydroxychloroquine
2. Topical corticosteroids
3. Strict sun protection

13.6 Supportive Care

1. Physical therapy and rehabilitation
2. Swallow evaluation if dysphagia
3. Vaccination before immunosuppression when possible
4. Monitoring with pulmonary function tests and imaging

14. Prognosis

1. Major causes of death:
• ILD
• Malignancy
2. Early treatment improves outcomes

Favorable Prognostic Factors

1. Anti-Mi-2 positivity
2. Younger age
3. Early treatment

Poor Prognostic Factors

1. Rapidly progressive ILD
2. Anti-MDA5 positivity
3. Older age
4. Malignancy
5. Severe dysphagia

15. Complications

1. Interstitial lung disease
2. Respiratory failure
3. Aspiration pneumonia
4. Cardiac arrhythmias
5. Malignancy-associated mortality
6. Chronic disability

16. Key Clinical Insight

Patient with symmetric proximal muscle weakness and characteristic cutaneous findings such as heliotrope rash and Gottron papules strongly suggests dermatomyositis

17. Key Exam Points

1. Dermatomyositis is a complement-mediated humoral microangiopathy
2. Gottron papules are pathognomonic for dermatomyositis
3. Perifascicular atrophy is highly characteristic on biopsy
4. Anti-Mi-2 is associated with classic dermatomyositis and favorable prognosis
5. Anti-MDA5 is associated with rapidly progressive ILD
6. Anti-TIF1-γ and anti-NXP2 are associated with malignancy
7. Anti-Jo-1 suggests antisynthetase syndrome rather than classic dermatomyositis
8. Normal CK does not exclude dermatomyositis
9. Adult patients require malignancy screening
10. High-dose corticosteroids with early steroid-sparing therapy are standard treatment