What is prurigo pigmentosa, a skin disorder characterized by pruritic pigmented papules and plaques, commonly seen in Asian women?

What is Prurigo Pigmentosa?

Prurigo pigmentosa is a rare inflammatory dermatosis characterized by sudden-onset pruritic erythematous papules, papulovesicles, and plaques that coalesce into a distinctive reticulated (net-like) pattern, healing within weeks to leave characteristic reticulated brownish hyperpigmentation. 1, 2

Clinical Presentation

Primary Features

• Pruritic erythematous papules and papulovesicles that appear suddenly and coalesce to form a reticulated or net-like pattern on the skin 1, 2
• Predilection for the trunk (particularly the back and chest), neck, and proximal upper extremities 1, 3
• Reticulated brownish hyperpigmentation that persists after the inflammatory lesions resolve, typically within weeks 2, 4
• Some patients develop annular lesions arranged en cocarde or pustular variants 1

Demographics

• Mean age of presentation is 25.6 years (range 13-72 years), with a strong female predominance (72.1% of cases) 2
• Originally described in Japan by Masaji Nagashima in 1971, but now recognized worldwide across all ethnic backgrounds 2, 5
• Likely underdiagnosed outside Asia due to lack of clinician awareness 1, 5

Pathogenesis and Associated Factors

Ketoacidotic States (Most Important Association)

• Ketoacidosis is strongly implicated in the pathogenesis of prurigo pigmentosa 1, 4
• Dietary changes are the most common trigger (25.5% of cases), with 40.4% of these patients having started a ketogenic diet 2
• Fasting is a significant trigger, particularly during religious practices like Ramadan in Middle Eastern populations 1, 4
• Other documented associations include diabetes mellitus, anorexia nervosa, and strict dieting 4, 3

Other Aggravating Factors

• Friction (8.4% of cases) and sweat (7.6% of cases) can trigger or worsen lesions 2
• Ketonuria was documented in 5.1% of patients 2

Histopathological Features

Stage-Dependent Findings

• Early stage: Superficial perivascular infiltrate of neutrophils with neutrophilic exocytosis, which can mimic psoriasis or impetiginized spongiotic dermatitis 1, 3
• Later stages: Spongiosis, necrotic keratinocytes, ballooning and reticular degeneration (can mimic viral exanthema) 1, 3
• Chronic stage: Features resembling chronic spongiotic dermatitis 1
• Psoriasiform hyperplasia may be present, further complicating diagnosis 1

Treatment Approach

First-Line Therapy

• Minocycline monotherapy is the most frequently prescribed and effective treatment (20.9% of all cases), achieving complete resolution in 48.1% of patients 2
• Oral tetracyclines are the treatment of choice for managing prurigo pigmentosa effectively 2

Alternative Options

• Doxycycline can be used as an alternative tetracycline 3
• Macrolide antibiotics may be indicated for patients who cannot tolerate tetracyclines 3
• Dapsone (diaminodiphenyl sulfone) is another therapeutic option for refractory cases 3

Critical Diagnostic Pitfalls

High Index of Suspicion Required

• Many cases are misdiagnosed due to nonspecific clinical and histological features that can mimic psoriasis, viral exanthema, or chronic eczema 1, 4
• Increased awareness is essential, particularly in regions where fasting is common religious practice (Middle East, Muslim populations during Ramadan) 1, 4
• The disease is significantly underrecognized outside Japan, with likely many undiagnosed cases worldwide 5

Key Distinguishing Features

• Look for the characteristic reticulated pattern of both active lesions and residual hyperpigmentation 2, 4
• Obtain a detailed dietary history, specifically asking about ketogenic diets, fasting, or recent significant dietary changes 2, 4
• Consider testing for ketonuria when clinical suspicion is high 2
• The rapid resolution with residual net-like pigmentation is highly characteristic and distinguishes prurigo pigmentosa from other inflammatory dermatoses 4, 3