Pathological Features of Pityriasis Lichenoides Chronica on Skin Biopsy
Pityriasis lichenoides chronica (PLC) demonstrates a characteristic "T-shaped" deep dermal lymphocytic infiltrate with periadnexal arrangement, vacuolar changes or necrotic keratinocytes, and superficial perivascular red blood cell extravasation—features that distinguish it from other inflammatory dermatoses.
Core Histopathological Features
The essential microscopic findings in PLC include:
- Vacuolar changes or necrotic keratinocytes are present in 100% of cases, representing a pathognomonic feature 1
- Both superficial and deep lymphocytic infiltrates occur in 99% of cases, creating a distinctive pattern 1
- Dermal wedge-shaped lymphocytic infiltrate with variable necrosis of keratinocytes is characteristic 2
- Epidermal spongiosis and parakeratosis are consistently observed 2
Distinctive Diagnostic Features
Adnexal Involvement
- Infiltration of lymphocytes into the adnexal epithelium occurs in 97% of cases 1
- Deep dermal lymphocytic infiltrate with "T-shaped" periadnexal arrangement extends along the full length of follicular and sudoral epithelia—this feature is particularly valuable for differentiating PLC from other diseases 1
Vascular Changes
- Superficial perivascular and/or intraepidermal red blood cells are observed in 83% of cases 1
- Slight vascular deposits of IgM and C3 are present in most lesions on immunofluorescence 3
- Perivascular and interstitial deposits of fibrin may be detected, though less extensive than in acute variants 3
Cellular Characteristics
Lymphocyte Phenotype
- Small- to medium-sized lymphocytes comprise the inflammatory infiltrate without eosinophils 1
- T lymphocytes predominate, with cytotoxic/suppressor cells (CD8+/T8+) generally outnumbering helper/inducer cells (CD4+/T4+) 3
- Decreased epidermal Langerhans cells (T6-positive) in the lower stratum spinosum compared to upper layers 3
Histological Spectrum Considerations
PLC demonstrates more subtle histological features compared to the acute variant (PLEVA), which shows more exaggerated changes 2. The overlapping clinical and histopathologic features across the pityriasis lichenoides spectrum make classification challenging, though PLC maintains its characteristic pattern of chronic inflammation 2.
Critical Differential Diagnosis Pitfalls
Distinguishing from Lichen Planus
- PLC lacks the band-like lichenoid infiltrate directly hugging the dermoepidermal junction that characterizes lichen planus 4
- The deep periadnexal "T-shaped" pattern in PLC differs from the superficial band-like pattern of lichen planus 1
Distinguishing from Mycosis Fungoides
- Absence of atypical lymphocytes is key—PL-like mycosis fungoides would show atypical lymphocytic infiltrate with epidermotropism 5
- Biopsy is mandatory when PL-like eruptions occur, particularly in adults, to exclude mycosis fungoides 5
- The CD4/CD8 ratio and presence of atypical cells help differentiate these conditions 5
Distinguishing from Psoriasis
- PLC shows irregular acanthosis rather than the regular, uniform elongated rete ridges of psoriasis 4
- Psoriasis typically has more confluent parakeratosis with neutrophils (Munro microabscesses), which are absent in PLC 4
Clinical Correlation
The histopathological diagnosis should always be correlated with clinical presentation: PLC manifests as gradual onset of very small red-to-brown flat maculopapules with mica-like scale following a relapsing course with long remission periods 2. The more subtle histology of PLC reflects its chronic, less aggressive clinical course compared to acute variants 2.