Differentials
Eczema
SIGNS / SYMPTOMS
Dry, scaly, red skin sometimes with excoriations visible (scratch marks). Exacerbations may be associated with skin infection causing weeping or oozing skin. The border of eczema is usually less well-defined than a plaque of psoriasis.
INVESTIGATIONS
Skin biopsy shows changes consistent with atopic dermatitis.
Pityriasis rosea
SIGNS / SYMPTOMS
More common in children.
Lesions may show features of guttate psoriasis but are in a characteristic Christmas tree-shaped distribution.
Usually subsides within 8 weeks.
INVESTIGATIONS
Clinical diagnosis is usually sufficient.
Seborrhoeic dermatitis
SIGNS / SYMPTOMS
Scaly eruptions usually limited to scalp, eyebrows, paranasal region, ears, and chest, but can be widespread.
Scales are fine, not lamellar.
INVESTIGATIONS
Skin biopsy shows changes consistent with seborrhoeic dermatitis.
Mycosis fungoides
SIGNS / SYMPTOMS
Usually presents with patches and plaques on the lower half of the body but can be widespread.
Does not involve joints.
INVESTIGATIONS
Skin biopsy shows atypical lymphocytes and Pautrier abscess.
Tinea corporis
SIGNS / SYMPTOMS
Annular scaly patches.
INVESTIGATIONS
Skin scraping or biopsy confirms diagnosis.
Nappy dermatitis
SIGNS / SYMPTOMS
Oozy, weepy.
Only in nappy region.
INVESTIGATIONS
Clinical diagnosis is usually sufficient.
Onychomycosis
SIGNS / SYMPTOMS
Only involves nails.
INVESTIGATIONS
Culture of nail shows fungus.
Squamous cell cancer/actinic keratosis
SIGNS / SYMPTOMS
Actinic keratosis or actinic field change often affect the forehead and dorsal aspect of hands, which are less common sites for psoriasis.
Usually presents at an older age.
INVESTIGATIONS
Skin biopsy shows proliferating atypical squamous cells.
Lichen planus
SIGNS / SYMPTOMS
Violaceous papules. Oral mucosa is more likely to be involved than in psoriasis.
INVESTIGATIONS
Skin biopsy shows lichenoid lymphocyte infiltrates under epidermis.
Lichen simplex chronicus
SIGNS / SYMPTOMS
Usually limited to a few areas easily reached by hands.
Lesions are thick and mostly without scaly or desquamated appearance
INVESTIGATIONS
Skin biopsy shows chronic dermatitis with epidermal acanthosis.
Subcorneal pustular dermatosis
SIGNS / SYMPTOMS
A differential for pustular psoriasis.
Pustular lesions are subcorneal and in annular/serpiginous forms, present on the abdomen, axillae, and groin.
INVESTIGATIONS
Culture of pustules shows no bacteria. Skin biopsy shows predominantly neutrophilic perivascular infiltrate; minimal spongiosis.
Keratoderma blennorrhagicum (reactive arthritis)
SIGNS / SYMPTOMS
Lesions are circular, scaly, scalloped-edged hyperkeratotic psoriasiform papules and plaques, which are sometimes painful and pustular (at the centre of lesions); appear on soles and toes, and, less commonly, legs, palms, scalp, and penis.
INVESTIGATIONS
Skin biopsy may be done, but may show overlapping features with psoriasis.
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