Psoriasis is a chronic skin disease marked by hyperproliferation of epidermal cells and inflammation that results in thickened, scaly, erythematous plaques. The cause of psoriasis is incompletely understood and appears to be multifactorial, with genetic and environmental components.

Psoriasis is fairly common in childhood but may also occur in infancy (see infantile psoriasis). It occurs worldwide and is estimated to affect 1%-2% of the population, with 20% of cases presenting before 20 years of age.

Several clinical patterns exist, and multiple forms may be observed in a single patient. Typical plaque-type psoriasis (psoriasis vulgaris) is discussed here; other forms include guttate psoriasis (which often follows streptococcal pharyngitis), palmar-plantar psoriasis, erythrodermic psoriasis, and pustular psoriasis. Inverse psoriasis occurs in intertriginous or thin-skinned areas of the body, such as the axillae, groin, genitals, inframammary areas, face, and eyelids.
 
Pediatric psoriasis often presents as red, scaly plaques on the face. Psoriasis can also be limited to the nails or to body areas such as the genitals, scalp, feet, or even a solitary fingertip. Fingernails are more often involved compared to toenails in children. A review of pediatric psoriasis found that scalp psoriasis was more common in girls, while nail involvement was more common in boys, suggesting a possible role of chronic incidental trauma in these locations.

Approximately 8% of patients develop psoriatic arthritis. Those with nail involvement seem to be at increased risk for developing this erosive seronegative arthritis, which can be a source of considerable morbidity. Warning signs of psoriatic arthritis include tender, swollen joints that may be warm to the touch or painful swelling of the fingers.

Comorbidities may include obesity, hyperlipidemia, diabetes mellitus, Crohn disease, ulcerative colitis, rheumatologic disease, juvenile idiopathic arthropathy, depression, anxiety, bipolar disorder, substance abuse, and eating disorders.

The National Psoriasis Foundation is an excellent resource for patients: www.psoriasis.org.