Itching
Itching (pruritus) is a sensation that instinctively demands scratching.
Itching may be caused by a skin disorder or by a disease that affects the whole body (systemic disease). Skin disorders that cause severe itching include infestations with parasites (scabies, mites, or lice), insect bites, hives, atopic dermatitis, and allergic and contact dermatitis. These disorders usually also produce a rash. Systemic diseases that can cause itching include liver disease, kidney failure, lymphomas, leukemias and other blood disorders, and, occasionally, thyroid disease, diabetes, and cancer. However, itching from these diseases usually does not result in a rash.
Many drugs can cause itching, including barbiturates and aspirin as well as any drug to which a person has an allergy.
Itching is also common during the later months of pregnancy. Usually, pregnancy-related itching does not indicate any abnormality, but it can result from mild liver problems.
Often, contact with wool clothing or irritants, such as solvents or cosmetics, causes itching. Dry skin (xerosis), which is especially common in older people, can cause severe, widespread itching. Dry skin also can result from cold weather or prolonged exposure to water. Hot baths typically worsen itching.
The act of scratching can itself irritate the skin and lead to more itching, creating an itching-scratching-itching cycle. Vigorous scratching may cause redness and deep scrapes in the skin. In some people, even gentle scratching causes raised, red streaks that can itch intensely. Prolonged scratching and rubbing can thicken and scar the skin.
 See the sidebar When the Skin Is Dry.
Diagnosis
Doctors try to determine the cause of itching to eliminate it. Often, the cause is obvious, such as an insect bite or poison ivy. Itching that lasts longer than a few days or that comes and goes frequently without an obvious cause usually requires testing. If an allergy is suspected, skin tests may be performed (see Section 16, Chapter 185 and Section 18, Chapter 202). If a systemic disease is suspected, blood tests are usually performed to check liver function, kidney function, and blood sugar levels. The number of eosinophils, a type of white blood cell, may be checked as well, because a high number may indicate an allergic reaction. Sometimes, the doctor may have a person discontinue one or more drugs to see if the itching is relieved. A biopsy or skin scraping (see Section 18, Chapter 202) may help identify the cause, including an infectious one.
Treatment
For itching of any cause, bathing should be kept brief and preferably in cool or lukewarm water with very little or no soap. The skin should be patted dry gently rather than rubbed vigorously. Many people with itching benefit from an over-the-counter moisturizing cream applied right after bathing. The moisturizer should be odorless and colorless, because additives that provide color or scent may irritate the skin and even cause itching. Fingernails, especially children's, should be kept short to minimize abrasions from scratching. Coating the affected area with soothing compounds, such as menthol, camphor, chamomile, eucalyptus, or calamine, also can help.
Taking antihistamines by mouth may decrease itching. Some antihistamines, such as hydroxyzine and diphenhydramine, usually cause sleepiness and dry mouth and are mainly used at bedtime. Other antihistamines, such as loratadine and cetirizine, usually do not cause sleepiness. Generally, creams containing antihistamines (such as diphenhydramine) should not be used, because they can themselves cause an allergic reaction.
Corticosteroid creams decrease inflammation and control itching and may be used when itching is limited to a small area. Itching from some conditions, such as poison ivy, may require high-strength corticosteroid creams. However, only mild corticosteroids, such as 1% hydrocortisone, should be applied to the face and genitals, because stronger corticosteroids may thin the sensitive skin in these areas. Also, powerful corticosteroid creams applied over large areas or for a long time can cause serious medical problems (see Section 5, Chapter 67), especially in infants, because these drugs are absorbed into the bloodstream. Oral corticosteroids are sometimes used when large areas of the body are involved.
Specific treatments may be needed. For example, when fungal, parasitic, or bacterial infections cause itching, topical or systemic drugs may be required. Topical drugs are applied directly to the affected area of the skin. Systemic drugs are taken by mouth or injected and are distributed throughout the body.
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