What is the most likely cause of reddish‑purple discoloration of the knuckles and how should it be evaluated and managed?

Reddish-Purple Knuckles: Dermatomyositis Until Proven Otherwise

Reddish-purple discoloration over the knuckles is Gottron's papules or Gottron's sign—pathognomonic cutaneous manifestations of dermatomyositis—and requires immediate evaluation for inflammatory myopathy and associated systemic complications including interstitial lung disease and malignancy. 1

Diagnostic Approach

Distinguish Gottron's Papules from Gottron's Sign

• Gottron's papules are erythematous to violaceous papules over the extensor surfaces of joints (finger joints, elbows, knees, malleoli, toes) that are palpable and sometimes scaly, scoring 2.1-2.7 points in the EULAR/ACR classification criteria 1

• Gottron's sign presents as erythematous to violaceous macules (flat, non-palpable) over the same extensor surfaces, scoring 3.3-3.7 points—higher than papules because it is more specific 1

• Both lesions result from the Koebner phenomenon, vasculopathy, or photosensitivity characteristic of dermatomyositis 2

Examine for Additional Pathognomonic Features

• Heliotrope rash: Purple, lilac-colored or erythematous patches over the eyelids or periorbital distribution, often with periorbital edema (scores 3.1-3.2 points) 1

• Periungual changes: Look for periungual telangiectasias and cuticular overgrowth at the nail folds 3, 4

• Photodistributed eruptions: V-sign (anterior chest/neck), shawl sign (posterior neck/shoulders), and poikiloderma 3, 5

• Mechanic's hands: Hyperkeratotic, cracked skin on the palmar and lateral aspects of fingers 3, 2

Assess for Muscle Involvement

• Proximal muscle weakness: Objective symmetric weakness of proximal upper extremities (0.7 points) and proximal lower extremities (0.5-0.8 points), usually progressive 1

• Neck flexor weakness: Neck flexors relatively weaker than extensors (1.6-1.9 points) 1

• Muscle enzymes: Elevated creatine kinase (CK), lactate dehydrogenase (LDH), AST, or ALT (1.3-1.4 points) 1

• Note: Clinically amyopathic dermatomyositis presents with classic cutaneous manifestations for >6 months without muscle weakness or enzyme elevation 3

Essential Laboratory Workup

• Myositis-specific autoantibodies: Anti-Jo-1 (anti-histidyl-tRNA synthetase) scores 3.8-3.9 points when present 1

• Anti-Mi-2 antibody: Associated with classic DM features including heliotrope rash, Gottron's papules, V-neck sign, shawl sign, and photosensitivity 5

• Anti-TIF1γ antibody: Associated with diffuse photoerythema and "dusky red face"; strongly linked to malignancy risk 5

• Anti-MDA5 antibody: Associated with skin ulcerations, palmar papules (inverse Gottron), diffuse hair loss, panniculitis, and oral ulcers; high risk for rapidly progressive interstitial lung disease 5, 6

• Muscle enzymes: CK, LDH, AST, ALT to quantify muscle inflammation 1

Imaging and Tissue Diagnosis

• Electromyogram (EMG): Demonstrates myopathic changes in inflammatory myopathy 3

• Magnetic resonance imaging (MRI): Identifies muscle inflammation and guides biopsy site selection 3

• Muscle biopsy: Look for endomysial infiltration (1.7 points), perimysial/perivascular infiltration (1.2 points), perifascicular atrophy (1.9 points), or rimmed vacuoles (3.1 points) 1

• Skin biopsy: Shows interface dermatitis with liquefaction degeneration, vacuolar degeneration, edema, and mucin deposition 3, 2

Screen for Life-Threatening Complications

Interstitial Lung Disease (35-40% of patients)

• Obtain baseline chest CT and pulmonary function tests, especially if antisynthetase antibodies are present 3

• Anti-MDA5 antibody carries particularly high risk for rapidly progressive interstitial lung disease 5, 6

Malignancy Screening (3-8× increased risk)

• All patients with dermatomyositis require comprehensive malignancy evaluation at diagnosis 3, 4

• Cancer-associated disease is more common in older patients and confers poor prognosis 4

• Ovarian cancer is particularly linked to dermatomyositis 4

• Anti-TIF1γ antibody is strongly associated with malignancy 5

Other Systemic Manifestations

• Dysphagia or esophageal dysmotility (0.6-0.7 points): Assess swallowing function 1, 3

• Cardiopulmonary dysfunction: Monitor for cardiac involvement 4

• Arthritis: Nonerosive inflammatory polyarthritis may occur 3

Critical Pitfalls to Avoid

• Do not dismiss skin findings without muscle weakness: Clinically amyopathic dermatomyositis has identical malignancy and interstitial lung disease risks 3

• Do not delay malignancy screening: The 3-8× increased cancer risk requires immediate comprehensive evaluation 3, 4

• Do not overlook interstitial lung disease: This complication affects 35-40% of patients and can be rapidly progressive, especially with anti-MDA5 antibodies 3, 5

• Do not confuse with other conditions: Adult Still's disease, contact dermatitis, and sarcoidosis can mimic dermatomyositis cutaneous manifestations; skin biopsy aids differentiation 2

• Do not ignore autoantibody profiles: Specific antibodies predict distinct clinical phenotypes, systemic involvement patterns, and prognosis 5, 6

Classification Criteria Application

• The 2017 EULAR/ACR classification criteria use a point-based system where ≥5.5 points (without biopsy) or ≥6.7 points (with biopsy) classifies definite inflammatory myopathy 1

• Gottron's sign alone (3.3-3.7 points) plus heliotrope rash (3.1-3.2 points) nearly meets criteria without any muscle findings 1

• Age ≥40 years at symptom onset adds 2.1-2.2 points 1