How can leprosy be distinguished from psoriasis?

Differentiating Leprosy from Psoriasis

The key to distinguishing leprosy from psoriasis is testing for sensory loss within skin lesions—leprosy characteristically presents with definite sensory deficit in patches, while psoriasis lesions retain normal sensation and are typically pruritic.

Critical Distinguishing Features

Sensory Examination (Most Important)

• Leprosy: Definite sensory deficit (loss of light touch, pain, or temperature sensation) within hypopigmented or erythematous patches is a cardinal clinical sign 1
• Psoriasis: Lesions are often severely pruritic with intact sensation 2

Morphological Characteristics

Leprosy 1:

• Hypopigmented or erythematous macules, papules, plaques with loss of sensation
• Lesions have ill-defined or well-defined borders depending on type
• May show central healing
• Typically asymmetric distribution
• Paucibacillary: ≤5 lesions
• Multibacillary: ≥6 lesions

Psoriasis 3, 4:

• Well-demarcated, erythematous plaques with silvery scale
• Commonly symmetric distribution
• Classic locations: scalp, elbows, knees, presacral region
• Lesions are pruritic or painful, not anesthetic
• No sensory deficit

Nerve Involvement

• Leprosy: Thickened peripheral nerves (ulnar, radial cutaneous, common peroneal, posterior tibial) with tenderness and dysfunction—this is the second clinical cardinal sign 1
• Psoriasis: No nerve thickening or peripheral neuropathy

Diagnostic Algorithm

1. Examine all skin lesions for sensory deficit using light touch, pinprick, and temperature testing

   • If sensory loss present → strongly suggests leprosy

2. Palpate peripheral nerves (ulnar at elbow, radial cutaneous at wrist, common peroneal at fibular head, great auricular)

   • If thickened/tender → diagnostic of leprosy

3. Perform slit-skin smear from ear lobes and lesion edges

   • Demonstration of acid-fast bacilli confirms leprosy 1
   • Should be done when clinical suspicion exists but only one cardinal sign present

4. Assess scale characteristics

   • Silvery, micaceous scale → psoriasis
   • Minimal or absent scale with sensory loss → leprosy

5. Evaluate distribution pattern

   • Symmetric, extensor surfaces, well-demarcated → psoriasis
   • Asymmetric with sensory deficit → leprosy

Critical Pitfalls to Avoid

Leprosy is frequently misdiagnosed as psoriasis 5, leading to delayed treatment and potential nerve damage. The most common error is failing to test for sensory loss in skin lesions.

Common misdiagnosis scenarios:

• Hypopigmented patches without obvious sensory testing may be dismissed as early psoriasis
• Erythematous plaques in leprosy can mimic psoriatic plaques if sensation is not formally assessed
• Inverse leprosy lesions in skin folds may resemble inverse psoriasis 2

Additional Distinguishing Points

Nail Involvement

• Psoriasis: Pitting, onycholysis, subungual hyperkeratosis, oil-drop sign in 50% of patients 2
• Leprosy: Nail changes are secondary to nerve damage, not primary disease manifestation

Associated Features

• Psoriasis: May have psoriatic arthritis, metabolic syndrome, cardiovascular disease 3
• Leprosy: Nerve dysfunction, deformities in untreated cases, occurs in <2% with proper treatment 1

Rare Coexistence

The two conditions rarely coexist in the same patient 6, 7, with prevalence of psoriasis in leprosy patients as low as 0.014%. This mutual exclusivity has led to hypotheses about protective mechanisms, though both can occasionally occur together 6.

When in doubt, always perform sensory testing and consider slit-skin smear examination—missing leprosy has far greater consequences for morbidity than delaying psoriasis treatment.