What is the differential diagnosis for a non-diabetic patient with dark patch-like lesions, skin cracking, itching, and pigmentation on knuckles, affecting all fingers on one hand with progression to the other?

Differential Diagnosis for Dark Patch-Like Lesions on Fingers with Cracking, Itching, and Knuckle Pigmentation

The most likely diagnosis is chronic hand eczema (hand dermatitis), particularly given the unilateral onset with progression to the other hand, cracking, itching, and pigmentation—all hallmark features of this condition. 1, 2

Primary Differential Diagnoses

1. Chronic Hand Eczema (Most Likely)

• Hand eczema presents with dark patches, cracking, itching, and post-inflammatory hyperpigmentation on knuckles, matching this clinical presentation exactly 1, 2
• The unilateral onset with slow progression to the contralateral hand is characteristic of hand dermatitis 1
• Cracking and fissuring occur due to barrier dysfunction and chronic inflammation 2
• Post-inflammatory hyperpigmentation on knuckles is common in chronic hand eczema, particularly in darker skin types 2
• Risk factors include wet work, atopic dermatitis history, and contact allergen exposure 2, 3

Key diagnostic features to assess:

• Occupational and recreational exposure history (wet work, irritants, allergens) 3
• Personal or family history of atopic dermatitis 2, 3
• Distribution pattern: palmar, dorsal, fingers, or combination 1
• Duration: if persisting >3 months, patch testing is mandatory to identify contact allergens 3

2. Allergic Contact Dermatitis

• Contact allergen exposure causes hand dermatitis in approximately 50% of cases 3
• Presents with similar dark patches, cracking, and itching 3
• Unilateral onset may suggest asymmetric exposure to allergen 3
• Patch testing is essential for all patients with hand symptoms persisting >3 months 3

3. Irritant Contact Dermatitis

• Chronic irritant exposure (soaps, detergents, chemicals) causes cumulative damage 2
• Presents with dryness, cracking, hyperpigmentation 2
• Female gender is a risk factor 3

4. Pigmented Purpuric Dermatosis (Less Likely but Consider)

• Rare presentation on hands, typically unilateral on dorsum 4
• Presents with red, brown, or golden macules with "cayenne pepper-like spots" 4
• Histopathology shows perivascular lymphocytic infiltrate with erythrocyte extravasation and hemosiderin deposition 4
• Biopsy required for diagnosis due to unusual presentation 4

5. Actinic Keratoses (Consider if Sun-Exposed)

• Occurs on chronically sun-exposed dorsal hands 5
• Presents as rough, scaly patches that may be pigmented 5
• Can itch occasionally 5
• However, typically does not cause cracking or involve all fingers bilaterally 5

6. Post-Inflammatory Hyperpigmentation from Other Causes

• Secondary to previous eczema, psoriasis, trauma, or infection 6
• History of preceding inflammatory lesion is key 6
• Hyperpigmentation is partial and may improve over time 6

Critical Diagnostic Workup

Essential history elements:

• Occupational exposure: wet work, chemicals, irritants, allergens 3
• Personal/family history of atopic dermatitis 2, 3
• Timeline: onset, progression pattern, duration 1
• Aggravating factors: work activities, hobbies, exposures 3

Physical examination specifics:

• Distribution: palmar vs. dorsal, finger involvement pattern 1
• Morphology: vesicles, scaling, fissuring, lichenification 1
• Extent and severity assessment 1

Mandatory investigations:

• Patch testing if symptoms persist >3 months 3
• Consider skin biopsy if presentation is atypical or diagnosis uncertain 4
• Wood's light examination can help assess pigmentation patterns 7, 8

Common Pitfalls to Avoid

• Failing to perform patch testing in chronic hand dermatitis (>3 months duration)—this misses contact allergens in 50% of cases 3
• Not obtaining detailed occupational exposure history, which is critical for identifying triggers 3
• Overlooking atopic dermatitis as a major risk factor for hand eczema 2, 3
• Assuming pigmentation rules out eczema—post-inflammatory hyperpigmentation is common in chronic hand dermatitis 2
• Not considering early notification of occupational accident insurer if work-related triggers are suspected 3
• Dismissing rare diagnoses like pigmented purpuric dermatosis without biopsy when presentation is atypical 4

Management Implications

Once diagnosis is established:

• Identify and eliminate individual triggers (allergens, irritants) 3
• Optimize preventive measures and barrier protection 3
• Graded therapy based on clinical severity 3
• Early intervention is critical—early onset (before age 20), extent, and atopic/allergic etiology predict persistence 3