Dermatology MCQ - Inflammatory Dermatoses - Dermatomyositis diagnosis

A 60-year-old man presents with a 4-month history of a violaceous rash on his eyelids and over his knuckles, along with significant difficulty rising from a chair and climbing stairs. Which of the following findings is both necessary and sufficient, when combined with the classic skin rash, to establish a definitive diagnosis of dermatomyositis according to widely accepted diagnostic criteria?

A. Presence of anti-Jo-1 antibodies
B. An MRI showing diffuse muscle edema in the thigh muscles
C. An electromyogram (EMG) demonstrating irritative myopathy
D. A muscle biopsy revealing perfascicular atrophy
E. An elevated serum creatine kinase (CK) level

Correct Answer: D. A muscle biopsy revealing perfascicular atrophy

Explanation

The diagnosis of dermatomyositis (DM) rests on the combination of characteristic skin disease and objective evidence of myositis. The most widely accepted criteria, such as those from Bohan and Peter or the more recent EULAR/ACR criteria, provide multiple pathways to confirm muscle involvement.

• Definitive Histopathologic Evidence: Among the options, the finding of perifascicular atrophy on muscle biopsy is considered a pathognomonic feature for dermatomyositis. When this specific histologic finding is present in a patient with the classic cutaneous manifestations of DM (e.g., heliotrope rash, Gottron's papules), it is considered definitive for the diagnosis. No other test carries this level of specificity.

Why other options are incorrect

• A. Presence of anti-Jo-1 antibodies: Anti-Jo-1 is a myositis-specific antibody associated with the antisynthetase syndrome, which includes myositis, interstitial lung disease, fever, Raynaud's, and mechanic's hands. While it supports a diagnosis of an inflammatory myopathy, it is not specific to DM (it's more associated with polymyositis) and is not sufficient for a definitive diagnosis on its own.

• B. An MRI showing diffuse muscle edema in the thigh muscles: MRI is a highly sensitive tool for detecting muscle inflammation and is very useful for guiding a biopsy site. However, it is not specific for DM, as muscle edema can be seen in other inflammatory myopathies, infections, or even severe muscle injury.

• C. An electromyogram (EMG) demonstrating irritative myopathy: An EMG showing short-duration, low-amplitude motor unit potentials with fibrillations and positive sharp waves is indicative of an inflammatory myopathy. However, these findings are not specific to DM and can be seen in polymyositis and inclusion body myositis.

• E. An elevated serum creatine kinase (CK) level: An elevated CK is a sensitive marker of muscle damage but is entirely non-specific. It can be elevated in many other conditions, including muscle trauma, strenuous exercise, drug effects, and other myopathies. Furthermore, in some cases of DM (especially amyopathic DM), the CK can be normal.

Key Associations for Dermatomyositis Diagnosis

• Diagnostic Criteria (Bohan & Peter Framework): A definitive diagnosis typically requires the classic rash plus at least three of the following four criteria:

  1. Progressive symmetrical proximal muscle weakness.

  2. Elevated serum muscle enzymes (CK, aldolase, LDH, AST, ALT).

  3. An abnormal EMG findings of myopathy.

  4. A diagnostic muscle biopsy showing perifascicular atrophy, inflammation, and necrosis.

• Pathognomonic Features:

  • Skin: Gottron's papules, Gottron's sign, and heliotrope rash are considered pathognomonic.

  • Muscle Biopsy: Perifascicular atrophy is the hallmark histologic finding, reflecting the underlying microangiopathy and endofascicular hypoxia.

• Amyopathic Dermatomyositis: This is a defined subset where patients have the classic skin manifestations of DM for 6 months or longer without clinical or laboratory evidence of muscle disease. Diagnosis requires two skin biopsies showing the characteristic interface dermatitis and no objective muscle weakness.

• Role of Myositis-Specific Autoantibodies (MSAs): While not part of the classic diagnostic criteria, testing for MSAs is now crucial as they define clinical phenotypes, prognosis, and associated risks (e.g., anti-TIF1γ and anti-NXP2 with malignancy; anti-MDA5 with rapidly progressive interstitial lung disease).

• Differential Diagnosis: Includes systemic lupus erythematosus (can have similar rash and myositis), polymyositis (lacks the classic DM rash), scleroderma, and drug-induced myopathies (e.g., from statins).