Psoriasis: histological picture and forms
At the microscopic level (with hematoxylin and eosin staining), psoriasis is characterized by pronounced epidermal changes. Parakeratosis is observed (which sometimes sloughs off during specimen preparation), along with a complete absence of the granular layer and acanthosis with characteristic elongation and broadening of the interpapillary pegs at their tips.
The inflammatory infiltrate consists predominantly of lymphocytes mixed with histiocytes, eosinophils, and occasional neutrophils.
When localized to the palms and soles, the disease occurs in two forms:
- Pustular psoriasis.
- Psoriatic keratoderma. Histologically, it is characterized by alternating hyperkeratosis and parakeratosis, pronounced acanthosis, preservation of fragments of the thinned granular layer, and increased exudative-inflammatory phenomena.
Lichen planus
Lichen planus (Lichen ruber planus, Wilson's lichen) is a chronic dermatosis of unknown etiology characterized by pruritic papular eruptions on the skin and mucous membranes. The disease accounts for up to 2.4% of all dermatoses and more frequently affects women across all age groups.
The pathogenesis is frequently based on delayed-type hypersensitivity (DTH). The pathology may act as a trigger for or be associated with the following conditions:
- Neoplastic processes
- Primary biliary cholangitis and chronic hepatitis
- Ulcerative colitis
- Urological infections
Several clinical forms are distinguished: classical, hypertrophic, atrophic, bullous, follicular, pigmented, and erythematous.
Vesiculobullous lesions: herpetiform dermatitis
This is a long-standing, cyclic dermatosis with polymorphic eruptions. The etiology is complex: the dermatitis may be a manifestation of gluten-sensitive celiac disease or a paraneoplastic process. In familial forms, women are more frequently affected.
Clinical evolution: Urticarial, papular, and vesicular elements appear on an erythematous background. They tend to group. Vesicles progress to pustules, rupture with the formation of erosions and crusts, leaving behind persistent pigmentation.
Pathological anatomy (histology):
- Beginning: Neutrophils and eosinophils accumulate in the papillary dermis, forming microabscesses with fibrin and necrosis. Subepidermal clefts arise.
- Blister: Initially, it is multilocular (due to the preserved connection between the epidermis and dermis), then it becomes unilocular. The roof consists of the full-thickness epidermis, and the content consists of fibrin and eosinophils.
- Direct immunofluorescence (DIF) reveals IgA deposits.
- Eosinophilia is frequently detected in the patients' blood.
Skin manifestations in metabolic disorders and COVID-19
Metabolic dermatoses In hereditary metabolic diseases (thesaurismoses), the skin can be affected secondarily. For example, in phenylketonuria (phenylalanine hydroxylase deficiency), the conversion of phenylalanine to tyrosine is blocked. This leads to hypopigmentation (fair hair, pale skin), increased photosensitivity, and a tendency to eczema and dermatitis.
Skin lesions in COVID-19
- Miliaria (eccrine prickly heat): Arises due to low-grade fever and prolonged sweating. Unlike common prickly heat, it is extensive in nature.
- Toxicoderma: Drug allergy associated with treatment, not directly related to the virus.
- Urticaria: Can be infection-associated (a precursor to COVID-19, often with an acral distribution of wheals) or drug-induced.
- Artificial lesions: Trophic changes in facial tissues due to prolonged prone position ventilation.
Class and exam questions
Where is the blister localized in seborrheic pemphigus?
Histologically, the blister forms in the upper part of the epidermis, at the level of the granular layer.
What is the difference between the initial blister in herpetiform dermatitis and a late-stage blister?
At an early stage, the blister is multilocular because the connections between the interpapillary epidermal pegs and the dermis are preserved. Later, these connections rupture, and it becomes unilocular.
Why do patients with phenylketonuria have a tendency toward dermatitis and hypopigmentation?
Due to the enzymatic block, the synthesis of tyrosine from phenylalanine is impaired, leading to a deficiency of the melanin pigment, skin pallor, photosensitivity, and a tendency toward eczema.
What else to study on this topic
- Pathogenesis and classification of thesaurismoses (storage diseases)
- Phenylketonuria: neurological symptoms and diagnostics
- Differential diagnosis of psoriasis types (pustular vs keratoderma)
- Drug-induced toxicodermas: mechanisms of development
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