Psoriasis

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 ^[1][2]
• Age of onset: 20–40 years ^[3]

Epidemiological data refers to the US, unless otherwise specified.

• Genetic predisposition: most likely determined via polygenic inheritance ^[4]
• Trigger factors ^[5]
  • Infectious
    • Infections of the upper respiratory tract caused by β-hemolytic streptococci
    • Staphylococcal infections
    • HIV
  • Mechanical irritation
  • Drugs (e.g., beta-blockers, chloroquine, lithium, interferon)

The mechanism causing the immune response is not yet well understood.

• Increased proliferation of keratinocytes
  • Acanthosis: thickening of the epidermis
  • Parakeratosis: retention of nucleated keratinocytes in the stratum corneum
• T cells secrete cytokines, which mediate an inflammatory response.

References:^[8]

• Course: relapsing, with symptom-free intervals
• Lesions: Initially, a few single lesions typically appear, which then often become confluent. ^[5]
  • 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 ^[2]
  • Pruritus in ∼ 80% of cases (typically mild, but may also be severe) ^[9]
  • Lesions characteristically show the Auspitz sign (see “Diagnostics” below for more information).
• Involvement of nails (in ∼ 50% of cases) ^[10]
  • Nail pitting: small, round depressions in the nail
  • Brittle nails: nail dystrophy with crumbling of the nail
  • Onycholysis: partial and mostly distal separation of the nail plate
  • Oil drop sign (or salmon spot): well-circumscribed, yellow-reddish discoloration of the nail

Psoriatic arthritis

• Definition: inflammation of joints (primarily on hands, feet, spine) that may occur with psoriasis
• Epidemiology: 5–30% of psoriasis patients affected ^[11]
• Clinical features
  • Psoriasis and psoriatic arthritis may occur independently or together.
  • There are several types of psoriatic arthritis:
    • Oligoarthritis; (most common, accounting for 70% of cases): typically with asymmetric involvement of both the distal and proximal interphalangeal joints (DIP and PIP) ^[1][11]
    • Spinal involvement (up to 40% of cases)
  • Other rheumatological features ^[12]
    • Enthesitis: inflammation of the enthesis (the connective tissue where tendons and ligaments insert into the bone)
    • Tenosynovitis
    • 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”) ^[13]
• Diagnosis ^[5]
  • There is no specific test for diagnosing psoriatic arthritis
  • Imaging studies: joint destruction, ankylosis
    • Fingers: pencil-in-cup deformity of DIP joints on x-ray
    • Spine: syndesmophytes, and in particular asymmetric paravertebral ossification
• Classification criteria for psoriatic arthritis (CASPAR) are helpful for diagnosing psoriatic arthritis; ≥ 3 points in the following 5 categories are required: ^[14]
  • Evidence of psoriasis (2 points)
    • Current disease manifestation
    • Personal or family history of the disease
  • Personal or family history of the disease (1 point)
  • Psoriatic nail dystrophy (1 point)
  • Negative rheumatoid factor (1 point)
  • Dactylitis (1 point)
  • Radiologic signs (1 point)
• Treatment
  • Mild disease: NSAIDs
  • Moderate to severe disease: DMARDs
  • Physical therapy

If first-degree relatives of patients with psoriasis have joint problems, psoriatic arthritis should be considered.

Cutaneous variants

• Plaque psoriasis: most common variant characterized by symmetrically distributed, thick, scaly, erythematous lesions ^[11]
• Guttate psoriasis: lesions the size of drops of water: ; may develop into psoriasis; occurs mainly in children and adolescents after streptococcal infection ^[15]
• Erythrodermic psoriasis: generalized erythematous lesion with diffuse scaling: ; may lead to severe illness with fever and dehydration ^[16]
• Inverse psoriasis: : mainly affects skin folds and flexural creases of large joints (flexural psoriasis)
• Pustular psoriasis
  • Rare; high correlation with HLA-B27
  • Generalized pustular psoriasis (most common subtype)
    • Generalized erythroderma; confluent, white pustules over the entire body; involvement of oral mucosa and tongue ^[1]
    • Relapsing course with pronounced malaise (fever, weakness, chills); possibly fatal

• 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
  • Small pinpoint bleeding when scales are scraped off
  • Removal of the scales exposes the dermal papillae → bleeding
• Skin biopsy: rarely needed, but may be performed to rule out other diseases
  • Acanthosis and parakeratosis
  • Thickening of stratum spinosum, thinning of stratum granulosum
  • Munro microabscesses: accumulation of neutrophils in the stratum corneum surrounded by parakeratosis
• Laboratory tests
  • In case of psoriatic arthritis: ↑ ESR and CRP
  • Rheumatoid factor (RF) negative
  • Anti-CCP antibodies present in 10–15% of patients ^[17]

References:^[5]

Differential diagnosis of scaling [5]
Disorder	Lesion	Distribution
Psoriasis	Clearly demarcated, erythematous plaques with silvery scaling, pruritus	Scalp Extensor surfaces of joints (knees, elbows) Back
Atopic dermatitis	Poorly demarcated, eczema, white scales, severe xerosis, and pruritus	Extensor surfaces of extremities (e.g., shins) Flexural creases (antecubital, popliteal)
Seborrheic dermatitis	Clearly demarcated, erythematous plaques, greasy-looking yellow scales	“Cradle cap” in infants Facial involvement (hairline, periocular, nasolabial folds) Trunk
Pityriasis rubra pilaris	Disseminated, plaques with orange-pink scales, hyperkeratosis, erythroderma	Typically palms and soles Islands of unaffected skin (sparing) Follicular keratosis
Erythroderma [18]	Erythema, scaling 2–6 days following onset, lesions of underlying disease, pruritus	Generalized erythema Scaling initially in flexural creases

The differential diagnoses listed here are not exhaustive.

Medical therapy

Mild to moderate psoriasis	Moderate to severe psoriasis	Severe psoriasis
First-line: moisturizers and topical medications Steroids (triamcinolone, fluocinonide, clobetasol) Vitamin D derivatives (calcipotriene) Tar preparations (anthralin) Retinoids/vitamin A derivatives	Topical medication as combination therapy Add systemic agents as needed (methotrexate, retinoid, cyclosporine) Phototherapy	Topical + systemic treatment Phototherapy Biologicals (etanercept, adalimumab, infliximab)

Systemic treatment is always required for psoriatic arthritis (immunosuppressants and NSAIDs)!

Phototherapy

Ultraviolet light is effective in treating dermatological conditions, as it has antiproliferative effects (slowing keratinization) and anti-inflammatory effects (inducing apoptosis of pathogenic T cells) on the skin.

• UVB therapy
  • Indications: psoriasis, vitiligo, chronic pruritus, and atopic dermatitis
  • Irradiation with narrowband UVB
  • Accelerates photodamage (e.g., sunburn) and increases cancer risk
• PUVA therapy (psoralen + UVA)
  • Indications: psoriasis, lichen planus, scleroderma, vitiligo, mycosis fungoides
  • Topical (bath, lotions) or oral administration of psoralen → increases photosensitivity of skin → UVA radiation
  • Increases risk of squamous cell carcinoma

References:^{[1][2][11][19][20]}