Psoriasis is a common chronic inflammatory skin disorder affecting individuals with an underlying genetic predisposition. The disease manifests following exposure to various triggers (e.g., infection, medication). The typical lesions are sharply demarcated, erythematous, scaly, pruritic plaques, which occur most often on the extensor surfaces of the knees and elbows but may also affect the scalp and back. Other common clinical findings include involvement of the nails (e.g., pitting or discoloration) or joints, which generally manifests with arthritis of the fingers and lower spine. As psoriasis presents with several subtypes, the size, location, and severity of the lesions vary. The diagnosis is based primarily on clinical findings but may also be confirmed with tests (e.g., Auspitz sign) or biopsy. Mild psoriasis is treated with topical agents such as steroids, whereas moderate to severe disease requires systemic therapy (e.g., PUVA, biologics).
- Prevalence: ∼ 2% of the white population 
- Age of onset: 20–40 years 
Epidemiological data refers to the US, unless otherwise specified.
|Classification of psoriasis |
|Characteristics||Psoriasis type I||Psoriasis type II|
|Onset|| || |
|Prevalence|| || |
|Genetic predisposition|| || |
|Correlation with HLA|| |
|Clinical presentation|| || |
The mechanism causing the immune response is not yet well understood.
- Increased proliferation of keratinocytes
- T cells secrete cytokines, which mediate an inflammatory response.
- Course: relapsing, with symptom-free intervals
Lesions: Initially, a few single lesions typically appear, which then often become confluent.
- Well-demarcated, erythematous lesions, silvery-white scaling plaques, and papules
- Mainly on scalp, back, elbows, and knees (extensor surfaces) but any other site may be involved 
- Pruritus in ∼ 80% of cases (typically mild, but may also be severe) 
- Lesions characteristically show the (see “Diagnostics” below for more information).
- Involvement of nails (in ∼ 50% of cases) 
Subtypes and variants
- Definition: inflammation of joints (primarily on hands, feet, spine) that may occur with psoriasis
- Epidemiology: 5–30% of psoriasis patients affected 
- Psoriasis and psoriatic arthritis may occur independently or together.
- There are several types of psoriatic arthritis:
- Other rheumatological features 
- Enthesitis: inflammation of the enthesis (the connective tissue where tendons and ligaments insert into the bone)
- Dactylitis: inflammation and swelling of fingers or toes (“sausage digit”)
- Arthritis mutilans: destruction of the IP joints and resorption of the phalanges with further collapse of the soft tissue of the fingers (“telescoping fingers” or “opera glass hand”) 
- Classification criteria for psoriatic arthritis (CASPAR): ≥ 3 points are required 
- Plaque psoriasis: most common variant characterized by symmetrically distributed, thick, scaly, erythematous lesions 
- Guttate psoriasis
- Erythrodermic psoriasis
- Inverse psoriasis: : mainly affects skin folds and flexural creases of large joints (flexural psoriasis)
- Pustular psoriasis
- Koebner phenomenon: Physical stimuli or skin injury (e.g., trauma, scratching, irritating clothing) lead to skin lesions typical of the underlying condition appearing on previously healthy skin ("isomorphic response").
- Auspitz sign
- Skin biopsy: rarely needed, but may be performed to rule out other diseases
- Laboratory tests
|Differential diagnosis of scaling|
|Atopic dermatitis|| |
|Pityriasis rubra pilaris|| |
The differential diagnoses listed here are not exhaustive.
Treatment choice depends on disease severity, with consideration for patient preference and response to treatment.
|Overview of psoriasis treatment|
|Mild psoriasis (< 3% body surface area involvement)|| |
| || |
|Moderate psoriasis (3–10% body surface area involvement)|| || |
|Severe psoriasis (> 10% body surface area involvement)|| || |
Psoriatic arthritis always requires systemic treatment.
Ultraviolet light is effective in treating dermatological conditions due to its antiproliferative (e.g., slowing of keratinization) and antiinflammatory effects (e.g., inducing apoptosis of pathogenic T cells).
- UVA phototherapy
- UVB therapy: irradiation with narrowband UVB
- PUVA therapy: (psoralen PLUS UVA): topical or oral psoralen → ↑ photosensitivity of skin → ↑ antiproliferative and antiinflammatory effects of UVA light
- Lifelong disease, usually benign
- Patients may experience remissions of varying lengths; acute episodes of exacerbation possible.
- Psoriasis is associated with depression and a decreased quality of life.