An itchy rash can pose a risk of infection if you constantly scratch it. A dark purple rash can also appear unsightly, which can make you self-conscious. Since diagnosing skin rashes requires the trained eye of a physician, schedule an appointment with your doctor if you experience any type of unexplained itchy purple rash or skin lesion.
Although a number of conditions can cause itchy rashes -- or non-itchy purple spots -- the Family Doctor website indicates that the most probable cause of an itchy, purple rash is lichen planus. Lichen planus occurs most frequently in middle-aged adults, and less commonly in children. The purple or reddish-purple bumps of lichen planus commonly affect the wrists, ankles and forearms, but can also appear on the scalp, genitals, anus and the inside of the mouth.
Your doctor can often diagnose lichen planus by conducting a physical examination of the skin rash. A lichen planus rash involves uneven bumps with flat tops, often sprinkled with white scales or flakes. The bumps are usually 2 to 4 cm in diameter, and shiny or scaly in appearance. If the doctor cannot diagnose your rash by its appearance, he might conduct a biopsy, in which he removes part of the bump and examines it under a microscope.
Lichen planus lacks both a concrete cause and a cure, but Medline Plus notes that it probably occurs as a result of an allergic or immune reaction. Exposure to dyes, medications and other chemical substances, including antibiotics, gold, arsenic and diuretics can place you at a higher risk of developing the rash. Hepatitis C, which can cause liver disease, can also cause lichen planus, so your doctor might order a blood test to check for this condition.
Treatments such as antihistamines and topical corticosteroids can often soothe itching and speed healing of the rash. Although these medications will often improve the appearance of the rash, they are not a cure. Lichen planus can last weeks or months, and might come and go for years, according to Medline Plus.
If you recently returned from a scuba diving trip, barotrauma -- an injury caused by changes in pressure -- might be the cause of your itchy purple skin. The Drugs.com database indicates that "dive suit squeeze," caused by a too-tight diving suit, can cause an itchy rash and patches of purple skin on the body. The symptoms might appear right after your trip or occur over time. Since some cases of barotrauma require surgery or medical treatment, schedule an appointment with your primary care provider if you suspect barotrauma.