Learning Objective
By the end of this lesson, the learner should be able to describe the clinical features, causes, associations, and histopathology of major erythematous skin diseases and differentiate them based on presentation.
Polymorphous Light Eruption
Most common idiopathic photodermatosis: Triggered by sunlight exposure.
Clinical Features: Pruritic erythematous macules, papules, plaques, or vesicles.
Microscopy: Dermal edema and inflammation.
Pityriasis Rosea
Self-limiting dermatitis.
Begins with a herald patch, followed weeks later by multiple patches and papular eruption on the trunk.
Often affects children and young adults.
May be a viral exanthem, not contagious.
Symptoms: Pruritus is common.
Erythema Nodosum
Painful, erythematous nodules involving subcutaneous fat.
Typically on the anterior shins.
Associations:
- Streptococcal infections
- Granulomatous diseases (e.g., sarcoidosis, Crohn’s disease)
- Drugs, infections
Represents panniculitis (inflammation of subcutaneous fat).
Psoriasis
Chronic autoimmune disorder with genetic predisposition.
Due to increased proliferation and turnover of keratinocytes.
Affects ~1% of the population.
Common sites: Knees, elbows, scalp.
Classic lesion:
- Well-demarcated erythematous plaque with silvery scale.
- Removal of scale → Auspitz sign (pinpoint bleeding).
- Nail changes: Pitting, discoloration.
Associations:
- Arthritis (psoriatic arthritis)
- Enteropathy
- Myopathy
Microscopic Findings
- Epidermal hyperplasia (acanthosis)
- Patchy hyperkeratinization with parakeratosis
- Uniform elongation/thickening of rete ridges
- Thinning of the epidermis over the dermal papillae
- Munro microabscesses (neutrophils in stratum corneum)
Treatment
- Topical steroids
- Ultraviolet therapy
- Severe disease: Methotrexate
