Psoriasis
Psoriasis is a long-lasting, noncontagious autoimmune disease characterized by patches of abnormal skin. These areas are red, pink, or purple, dry, itchy, and scaly. Psoriasis varies in severity from small localized patches to complete body coverage. Injury to the skin can trigger psoriatic skin changes at that spot, which is known as the Koebner phenomenon.
The five main types of psoriasis are plaque, guttate, inverse, pustular, and erythrodermic. Plaque psoriasis, also known as psoriasis vulgaris, makes up about 90% of cases. It typically presents as red patches with white scales on top. Areas of the body most commonly affected are the back of the forearms, shins, navel area, and scalp. Guttate psoriasis has drop-shaped lesions. Pustular psoriasis presents as small, noninfectious, pus-filled blisters. Inverse psoriasis forms red patches in skin folds. Erythrodermic psoriasis occurs when the rash becomes very widespread and can develop from any of the other types. Fingernails and toenails are affected in most people with psoriasis at some point in time. This may include pits in the nails or changes in nail color.
Psoriasis is generally thought to be a genetic disease that is triggered by environmental factors. If one twin has psoriasis, the other twin is three times more likely to be affected if the twins are identical than if they are nonidentical. This suggests that genetic factors predispose to psoriasis. Symptoms often worsen during winter and with certain medications, such as beta blockers or NSAIDs. Infections and psychological stress can also play a role. The underlying mechanism involves the immune system reacting to skin cells. Diagnosis is typically based on the signs and symptoms.
There is no known cure for psoriasis, but various treatments can help control the symptoms. These treatments include steroid creams, vitamin D3 cream, ultraviolet light, immunosuppressive drugs, such as methotrexate, and biologic therapies targeting specific immunologic pathways. About 75% of skin involvement improves with creams alone. The disease affects 2–4% of the population. Men and women are affected with equal frequency. The disease may begin at any age, but typically starts in adulthood. Psoriasis is associated with an increased risk of psoriatic arthritis, lymphomas, cardiovascular disease, Crohn's disease, and depression. Psoriatic arthritis affects up to 30% of individuals with psoriasis.
The word "psoriasis" is from Greek ψωρίασις meaning or, from wikt:psora|, and wikt:-iasis#Suffix|.
Signs and symptoms
Plaque psoriasis
Psoriasis vulgaris is the most common form and affects 85–90% of people with psoriasis. Plaque psoriasis typically appears as raised areas of inflamed skin covered with silvery-white, scaly skin. These areas are called plaques and are most commonly found on the elbows, knees, scalp, and back.Other forms
Additional types of psoriasis comprise about 10% of cases. They include pustular, inverse, napkin, guttate, oral, and seborrheic-like forms.Pustular psoriasis
appears as raised bumps filled with noninfectious pus. The skin under and surrounding the pustules is red and tender. Pustular psoriasis can either be localized or more widespread throughout the body. Two types of localized pustular psoriasis include psoriasis pustulosa palmoplantaris and acrodermatitis continua of Hallopeau; both forms are localized to the hands and feet.Inverse psoriasis
appears as smooth, inflamed patches of skin. The patches frequently affect skin folds, particularly around the genitals, the armpits, in the skin folds of an overweight abdomen, between the buttocks in the intergluteal cleft, and under the breasts in the inframammary fold. Heat, trauma, and infection are thought to play a role in the development of this atypical form of psoriasis.Napkin psoriasis
is a subtype of psoriasis common in infants under the age of two and is characterized by red papules with silver scales in the diaper area that may extend to the torso or limbs. Napkin psoriasis is often misdiagnosed as napkin dermatitis. It typically improves as children age and may later present in more common forms as plaque psoriasis or inverse psoriasis.Guttate psoriasis
is an inflammatory condition characterized by numerous small, scaly, red or pink, droplet-like lesions. These numerous papules appear over large areas of the body, primarily the trunk, limbs, and scalp, but typically spare the palms and soles. Guttate psoriasis is often triggered by a streptococcal infection and typically occurs 1–3 weeks post-infection. Guttate psoriasis is most commonly seen in children and young adults, and diagnosis is typically made based on history and clinical exam findings. Skin biopsy can also be performed which typically shows a psoriasiform reaction pattern characterized by epidermal hyperplasia with elongation of the rete ridges.There is no firm evidence regarding the best management for guttate psoriasis; however, first-line therapy for mild guttate psoriasis typically includes topical corticosteroids. Phototherapy can be used for moderate or severe guttate psoriasis. Biologic treatments have not been well studied in the treatment of guttate psoriasis.
Guttate psoriasis has a better prognosis than plaque psoriasis and typically resolves within 1–3 weeks; however, up to 40% of patients with guttate psoriasis eventually convert to plaque psoriasis.
Erythrodermic psoriasis
involves widespread inflammation and exfoliation of the skin over most of the body surface, often involving greater than 90% of the body surface area. It may be accompanied by severe dryness, itching, swelling, and pain. It can develop from any type of psoriasis. It is often the result of an exacerbation of unstable plaque psoriasis, particularly following the abrupt withdrawal of systemic glucocorticoids. This form of psoriasis can be fatal as the extreme inflammation and exfoliation disrupt the body's ability to regulate temperature and perform barrier functions.Mouth
Psoriasis in the mouth is very rare, in contrast to lichen planus, another common papulosquamous disorder that commonly involves both the skin and mouth. When psoriasis involves the oral mucosa, it may be asymptomatic, but it may appear as white or grey-yellow plaques. Fissured tongue is the most common finding in those with oral psoriasis and has been reported to occur in 6.5–20% of people with psoriasis affecting the skin. The microscopic appearance of oral mucosa affected by geographic tongue is very similar to the appearance of psoriasis. A recent study found an association between the two conditions, and it suggests that geographic tongue might be a predictor for psoriasis.Seborrheic-like psoriasis
is a common form of psoriasis with clinical aspects of psoriasis and seborrheic dermatitis, and it may be difficult to distinguish from the latter. This form of psoriasis typically manifests as red plaques with greasy scales in areas of higher sebum production such as the scalp, forehead, skin folds next to the nose, the skin surrounding the mouth, skin on the chest above the sternum, and in skin folds.Psoriatic arthritis
is a form of chronic inflammatory arthritis that has a highly variable clinical presentation and frequently occurs in association with skin and nail psoriasis. It typically involves painful inflammation of the joints and surrounding connective tissue and can occur in any joint, but most commonly affects the joints of the fingers and toes. This can result in a sausage-shaped swelling of the fingers and toes known as dactylitis. Psoriatic arthritis can also affect the hips, knees, spine, and sacroiliac joint. About 30% of individuals with psoriasis will develop psoriatic arthritis. Skin manifestations of psoriasis tend to occur before arthritic manifestations in about 75% of cases.Nail changes
and produces a variety of changes in the appearance of fingers and toenails. Nail psoriasis occurs in 40–45% of people with psoriasis affecting the skin, and has a lifetime incidence of 80–90% in those with psoriatic arthritis. These changes include pitting of the nails, whitening of the nail, small areas of bleeding from capillaries under the nail, yellow-reddish discoloration of the nails known as the oil drop or salmon spots, dryness, thickening of the skin under the nail, loosening and separation of the nail, and crumbling of the nail.Medical signs
In addition to the appearance and distribution of the rash, specific medical signs may be used by medical practitioners to assist with diagnosis. These may include Auspitz's sign, Koebner phenomenon, and itching and pain localized to papules and plaques.Causes
The cause of psoriasis is not fully understood. Genetics, seasonal changes, skin damage, climate, immunocompromised state, specific infections, and the use of some medications have been connected with different types of psoriasis.Genetics
Around one-third of people with psoriasis report a family history of the disease, and researchers have identified genetic loci associated with the condition. Identical twin studies suggest a 70% chance of a twin developing psoriasis if the other twin has the disorder. The risk is around 20% for fraternal twins. These findings suggest both a genetic susceptibility and an environmental response in developing psoriasis.Psoriasis has a strong hereditary component, and many genes are associated with it, but how those genes work together is unclear. Most of the identified genes relate to the immune system, particularly the major histocompatibility complex and T cells. Genetic studies are valuable due to their ability to identify molecular mechanisms and pathways for further study and potential medication targets.
Classic genome-wide linkage analysis has identified nine loci on different chromosomes associated with psoriasis. They are called psoriasis susceptibility 1 through 9. Within those loci are genes on pathways that lead to inflammation. Certain variations of those genes are commonly found in psoriasis. Genome-wide association scans have identified other genes that are altered to characteristic variants in psoriasis. Some of these genes express inflammatory signal proteins, which affect cells in the immune system that are also involved in psoriasis. Some of these genes are also involved in other autoimmune diseases.
The major determinant is PSORS1, which probably accounts for 35–50% of psoriasis heritability. It controls genes that affect the immune system or encode skin proteins that are overabundant in psoriasis. PSORS1 is located on chromosome 6 in the MHC, which controls important immune functions. Three genes in the PSORS1 locus have a strong association with psoriasis vulgaris: HLA-C variant HLA-Cw6, which encodes an MHC class I protein; CCHCR1, variant WWC, which encodes a coiled coil protein overexpressed in psoriatic epidermis; and CDSN, variant allele 5, which encodes corneodesmosin, a protein expressed in the granular and cornified layers of the epidermis and upregulated in psoriasis.
Two major immune system genes under investigation are interleukin-12 subunit beta on chromosome 5q, which expresses interleukin-12B; and IL23R on chromosome 1p, which expresses the interleukin-23 receptor and is involved in T cell differentiation. Interleukin-23 receptor and IL12B have both been strongly linked with psoriasis. T cells are involved in the inflammatory process that leads to psoriasis. These genes are on the pathway that upregulates tumor necrosis factor-α and nuclear factor κB, two genes involved in inflammation. The first gene directly linked to psoriasis was identified as the CARD14 gene located in the PSORS2 locus. A rare mutation in the gene encoding for the CARD14-regulated protein, plus an environmental trigger, was enough to cause plaque psoriasis.