Psoriasis is a chronic, noncontagious skin disease affecting up to 1 percent of the human population. Its etiology is still unknown, but it is clear that it has genetic and environmental components. Therefore, if you have an immediate family member with psoriasis, you will most likely develop this disease. Psoriasis is typically found on the skin of the elbows, knees, scalp, lower back area, intergluteal cleft (the cleft between the two cheeks of your buttocks) and penis. It has its own clinical appearance and is variably itchy. It may also be associated with joint pain and disease, muscle disease, gut disease or the acquired immunodeficiency syndrome.
Doctors diagnose psoriasis clinically, which means that diagnosis is based on the patient's story and on what the skin lesions look like. Laboratory tests may not be done. However, in difficult cases such as when the appearance or distribution of skin lesions is atypical, a skin biopsy can be done wherein a sample of skin affected with the lesion is taken, processed and viewed under a light microscope. A skin sample is obtained by using a small scalpel blade (shave biopsy) or a small round knife (punch biopsy), or by making a cut through the dermis and subcutaneous fat (incisional biopsy). If your doctor believes that it is necessary for you to have a skin biopsy, then the area to be biopsied is injected with a local anesthetic such as lidocaine. However, the biopsy procedure is rarely necessary for doctors to diagnose psoriasis, unless they want to rule out rare infectious, immunological or malignant conditions.
Size, Color and Margins of Lesions
The skin lesions of psoriasis can range from being a papule to a plaque. A papule is an elevated dome-shaped or flat-topped skin lesion 5 mm or less across, while a plaque is an elevated flat-topped skin lesion more than 5 mm across. Whether it is a papule or a plaque, the lesion is always elevated. The color of this skin lesion varies from pink to salmon. Some types of psoriasis have a very distinctive color. Psoriasis on the legs may sometimes have a bluish or violet tint. The lesions are well defined and sharply demarcated. Occasionally, they appear to be immediately encircled by a paler peripheral zone.
The psoriatic papules and/or plaques are covered by loosely adherent scales, which are white-silver in color. The amount and thickness of the scales vary. When a scale is removed from a plaque, it reveals a smooth, red, glossy membrane with needlelike, punctured, multiple bleeding points. Medical practitioners call this the Auspitz sign.
Shape and Distribution
Psoriatic papules and/or plaques have variations in their configurations. Some lesions take the linear, annular (ringlike), gyrate or serpiginous (serpent-like) configuration. Its distribution all over the body tends to be symmetrical. This means that the lesions on the two sides of the body are generally uniform with few differences in appearance. If you have psoriasis on your right elbow and right leg, then you would also have psoriasis on your left elbow and left leg.
Nail psoriasis causes distinct changes in nail appearance. These consist of yellow-brown discoloration, pitting, thickening, crumbling, dimpling and separation.