Definition and Etiology
Psoriasis is a common; typically chronic papulosquamous skin disease that may be associated with a seronegative spondyloarthropathy. The etiology of psoriasis is unknown.
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Prevalence and Risk Factors
Psoriasis affects 2% of the U.S. population, and about 11% of these patients have psoriatic arthritis (PsA). Psoriasis may begin at any age however generally there are two peaks of onset, the first at 20-30 years and the second at 50-60 years. Men and women are equally affected.
U.S. primary care physicians initially see 58% of the estimated 150,000 new cases of psoriasis per year, however dermatologists manage 80% of the 3 million office and hospital visits for psoriasis each year.
The type and clinical manifestations of psoriasis in a patient depend on a combination of genetic influences, environmental factors (i.e. trauma and climate) and associated diseases (particularly bacterial infections). Additionally, certain medications, notably lithium, antimalarials, beta blockers, interferon, and ethanol (if abused) have been reported to induce psoriasis or exacerbate preexisting disease in some patients. Emotional stress may also lead to psoriasis flares.
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Associations
Psoriasis is associated with the metabolic syndrome and cardiovascular (CV) disease. Psoriasis patients are not only more likely to have CV risk factors but severe psoriasis may serve as an independent risk factor for CV mortality.
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Pathophysiology
Psoriatic skin lesions are the result of inflammation in the dermis and hyperproliferation with abnormal differentiation of the epidermis. The primary pathologic process is most likely dysregulation of activated T cell interactions with antigen-presenting cells and overproduction of pro-inflammatory cytokines such as interferon-α and tumor necrosis factor-α (TNF-α ). Evidence for this theory derives from the dramatic improvement of severe psoriasis in patients treated with immunosuppressive therapies such as cyclosporine (a potent T cell inhibitor used to prevent transplant rejection) or with TNF-α inhibitors (used in other inflammatory diseases such as inflammatory bowel disease, rheumatoid arthritis and ankylosing spondylitis).
Recently, additional cytokine mediators, IL-12 and IL-23, have been linked to psoriasis as they promote differentiation of naïve CD4+ lymphocytes into Th1 and Th17 cells respectively. The U.S Food and Drug Administration (FDA) has recently approved a novel therapy for psoriasis targeting Il-12 and IL-23, which will be discussed in the therapy section.
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Signs and Symptoms
Although considered a single disease, psoriasis has several morphologic expressions and a full range of severity.
Plaque-type psoriasis, or psoriasis vulgaris, is the most common form, occurring in about 80% of all psoriasis patients. A typical lesion is a well-demarcated, red-violet plaque with adherent white silvery scales (Fig. 1).
Plaque psoriasis.
Figure 1: Click to Enlarge
Guttate psoriasis.
Figure 2: Click to Enlarge
Pustular psoriasis.
Figure 3A: Click to Enlarge
Pustular psoriasis.
Figure 3B: Click to Enlarge
Nail Psoriasis.
Figure 4: Click to Enlarge
Lesions are typically symmetrical and the face is usually spared. The most commonly involved areas are the elbows and knees, scalp, sacrum, umbilicus, intergluteal cleft, and genitalia. In addition to physical trauma (Koebner phenomenon), other causes of cutaneous injury such as viral exanthems or sunburn may elicit the formation of any type of psoriatic lesion. About 70% of patients complain of pruritus, skin pain, or burning, especially when the scalp is involved. A characteristic finding, coined Auspitz sign, is pinpoint bleeding when psoriatic scale is lifted and correlates with histologic elongation of dermal papillae vessels in combination with suprapapillary epidermal thinning.
Guttate psoriasis (Fig. 2), named for its small droplet-shaped lesions, accounts for about 18% of all cases. This type is more common among children and young adults and is more likely to involve the face. Patients frequently have a history of a preceding upper respiratory tract infection or pharyngitis, particularly Group A Streptococcus. Some cases of acute guttate flares following streptococcal infection are precipitated by its superantigen exotoxin.
Pustular psoriasis (Fig. 3 and B) accounts for approximately 1.7% of cases. It is characterized by sterile pustules, which may be generalized or localized to the palms and soles. There is a female predominance in localized pustular psoriasis, however the incidence is equal in men and women in the generalized type. The average age at onset for pustular psoriasis is 50 years. Pregnancy and rapid tapering of systemic corticosteroids are known triggers. Generalized pustular psoriasis in pregnancy is also known as impetigo herpetiformis. Impetigo herpetiformis and generalized pustular psoriasis must be treated more aggressively because untreated, may lead to serious complications such as sepsis and bacterial superinfection.
Inverse psoriasis involves intertriginous areas (i.e skin folds of axilla, inguinal, intergluteal and inframammary regions). Plaques are typically pink to red and minimally scaly. Lesions may mimic cutaneous candidiasis however satellite lesions (if present) distinguish candidiasis from inverse psoriasis. Consider inverse psoriasis if candidiasis is recalcitrant to appropriate therapies.
The least common form of psoriasis is exfoliative dermatitis or psoriatic erythroderma, which accounts for 1% to 2% of all cases. Erythroderma is defined as a scaling pruritic, erythematous inflammatory skin eruption that involves over 90% of the body surface. Erythrodermic psoriasis may develop gradually or acutely during the course of chronic plaque-type psoriasis, but it may be the first manifestation of psoriasis, even in children. Psoriasis is the most common cause of erythroderma in adults and the second (following drug eruptions) in children. The mean age at onset is approximately 50 years. Men with the condition outnumber women, and concomitant psoriatic arthropathy is common. The most common precipitating factor is the withdrawal of potent topical, oral, and intramuscular corticosteroids. Although psoriasis patients are typically thought to be at decreased risk of cutaneous infection, those with erythrodermic psoriasis may be at risk for Staphylococcus aureus septicemia as a result of their compromised skin barrier therefore it is important for emergent evaluation by a dermatologist. Additionally, erythroderma may result in temperature dysregulation, hypoalbuminemia, and high output cardiac failure.
The nails (Fig. 4) are involved in up to 50% of psoriasis patients; in patients with psoriatic arthritis (PsA), the prevalence exceeds 80%. Pitting of the nail plate is the most common manifestation and is the result of damage to the proximal nail matrix. The pits tend to be large, deep, and randomly dispersed on the nail plate. Distal onycholysis, or lifting of the nail plate, is a common finding in psoriatic nail disease. Yellow-brown dyschromia (oil droplet sign) of the nail bed corresponds to psoriasis in that location and is the result of abnormal keratinization of the nail bed.
PsA affects up to one third of patients with psoriasis and is a destructive arthropathy and enthesopathy. Although PsA may share clinical features with rheumatoid arthritis (involving small and medium sized joints), it most commonly presents as inflammation of the proximal and distal interphalangeal joints in the hands and feet. Arthritis occurs after the onset of skin involvement in two thirds of cases however in 10-15% of patients, it occurs prior to the development of skin lesions. The severity of skin and nail involvement does not correlate with the severity of joint disease in patients with PsA. Early recognition and intervention is important as PsA may lead to loss of function. For this reason, patients with joint involvement are typically treated with more aggressive therapies such as a TNF inhibitor.