Clinical Distinction Between Prurigo Pigmentosa and Confluent and Reticulated Papillomatosis
The key clinical distinction is that prurigo pigmentosa presents with pruritic erythematous papules and vesicles that evolve through inflammatory stages before leaving reticular hyperpigmentation, while confluent and reticulated papillomatosis (CARP) presents as asymptomatic scaly hyperpigmented macules and papules from onset without a preceding inflammatory phase. 1, 2
Distribution Pattern
Prurigo pigmentosa:
- Predominantly affects the back, chest, neck, and nape in a symmetrical distribution 1, 3
- The armpit is frequently involved in Chinese populations 3
- Lesions appear in a reticular (net-like) pattern but only after the inflammatory phase resolves 1
CARP:
- Classic presentation involves the trunk (chest most commonly affected) in a confluent and reticular pattern 2, 4
- A generalized form can occur affecting face, neck, extremities, and trunk 2
- The intermammary and interscapular areas are typical sites 4
Clinical Presentation and Symptoms
Prurigo pigmentosa:
- Intensely pruritic erythematous papules, papulovesicles, and vesicles are the hallmark 1, 3
- Lesions evolve through distinct stages: inflammatory papules → vesicles → crusting → reticular hyperpigmentation 1, 3
- Different stages commonly co-exist in the same patient 3
- Bullous forms can rarely occur 5
- Predominantly affects young females (mean age 27.4 years in Chinese cohort) 3
CARP:
- Asymptomatic in 80% of cases (8 of 10 patients in one series) 4
- When symptomatic, presents with itching rather than intense pruritus 2
- Scaly hyperpigmented macules, patches, and papules present from onset 2, 4
- No vesicular or inflammatory papular stage 2, 4
- Affects young people (mean age 19 years) with equal gender distribution 4
Morphology of Individual Lesions
Prurigo pigmentosa:
- Early lesions are erythematous inflammatory papules and papulovesicles 1, 3
- Vesicles and bullae may be present 5
- Crusting occurs as lesions evolve 3
- Final stage shows reticulated hyperpigmentation without active inflammation 1
CARP:
- Scaly brownish patches, thin papules, and plaques from onset 2
- Reticulated (net-like) and confluent pattern of hyperpigmented macules 2, 4
- No vesiculation or crusting 2, 4
- Wood's lamp examination is negative 2
Histopathological Differences
Prurigo pigmentosa:
- Early stage: Superficial perivascular infiltrate of neutrophils (pathognomonic finding) 1, 3
- Later stages show spongiosis and necrotic keratinocytes 1, 3
- Lymphocytic infiltration in later stages 3
- Follicular abscesses may occasionally occur 3
- Different histological stages can co-exist in the same biopsy 3
CARP:
- Hyperkeratosis, papillomatosis, and acanthosis 2, 4
- Hypergranulosis 2
- Follicular plugging in 90% of cases (9 of 10 patients) 4
- Anastomosis of rete ridges in 30% 4
- Perivascular inflammatory infiltrate (non-neutrophilic) 2
- PAS stain may reveal Malassezia furfur yeast forms in 60% of cases 4
Response to Treatment
Prurigo pigmentosa:
- Responds rapidly to oral minocycline (mean 8.62 days) 3
- Doxycycline, macrolides, or dapsone are alternatives 1
- Lower recurrence rate (28.3%) after minocycline treatment 3
CARP:
- Responds to oral antibiotics (minocycline is first-line) 2
- May require adjunctive topical tacrolimus and selenium sulfide 2
- Treatment duration typically longer (2 months) 2
Common Diagnostic Pitfalls
- Do not rely solely on reticular hyperpigmentation to distinguish these conditions, as both can present with this pattern 1, 2
- The presence or absence of pruritus is critical: intense pruritus strongly suggests prurigo pigmentosa, while asymptomatic lesions favor CARP 1, 2, 4
- Timing of biopsy matters for prurigo pigmentosa: early-stage biopsies showing neutrophils are diagnostic, while late-stage biopsies may be non-specific 1, 3
- Look for vesicles or crusting: their presence essentially rules out CARP and confirms prurigo pigmentosa 1, 5
- Follicular plugging on histology strongly suggests CARP rather than prurigo pigmentosa 4