Свербіж

Histamine is the well-known mediator. It is synthesized and stored in mast cells in the skin and is released in response to various stimuli. Other mediators (eg, neuropeptides) can either cause the release of histamine or act as pruritogens themselves, thus explaining why antihistamines ameliorate some cases of itching and not others. Opioids have a central pruritic action as well as stimulating the peripherally mediated histamine itch.

There are 3 mechanisms of itch:

• Dermatologic: This mechanism is typically caused by inflammatory or pathologic processes (eg, urticaria, eczema).

• Systemic: This mechanism is related to diseases of organs other than skin (eg, cholestasis).

• Neurogenic: This mechanism is related to disorders of the central or peripheral nervous system (eg, multiple sclerosis).

Intense itching stimulates vigorous scratching, which in turn can cause secondary skin conditions (eg, inflammation, excoriation, infection), which can lead to more itching through disruption of the skin barrier. Although scratching can temporarily reduce the sensation of itch by activating inhibitory neuronal circuits, it also leads to amplification of itching at the level of the brain, exacerbating the itch–scratch cycle.

Many skin disorders cause itching. The most common include

• Dry skin

• Atopic dermatitis (eczema)

• Contact dermatitis

• Fungal skin infections

In systemic disorders, itching may occur with or without skin lesions. However, when itching is prominent without any identifiable skin lesions, systemic disorders, medications, and drugs should be considered more strongly. Systemic disorders are less often a cause of itching than skin disorders, but some of the more common causes include

• Allergic reaction (eg, to foods and medications)

• Cholestasis

• Chronic kidney disease

Less common systemic causes of itching include hyperthyroidism, hypothyroidism, diabetes, iron deficiency, dermatitis herpetiformis, and polycythemia vera.

Medications and other substances can cause itching as an allergic reaction or by directly triggering histamine release (most commonly morphine, some IV contrast agents).

History of present illness should determine onset of itching, initial location, course, duration, patterns of itching (eg, nocturnal or diurnal, intermittent or persistent, seasonal variation), and whether any rash is present. A careful medication history should be obtained including both prescription and over-the-counter medications with particular attention paid to recently started medications. The patient's use of moisturizers and other topicals (eg, hydrocortisone, diphenhydramine) should be reviewed. History should include any factors that make the itching better or worse.

Review of systems should seek symptoms of causative disorders, including

• Irritability, sweating, weight loss, and palpitations (hyperthyroidism)

• Depression, dry skin, and weight gain (hypothyroidism)

• Headache, pica, hair thinning, and exercise intolerance (iron deficiency anemia)

• Constitutional symptoms of weight loss, fatigue, and night sweats (cancer)

• Intermittent weakness, numbness, tingling, and visual disturbances or loss (multiple sclerosis)

• Steatorrhea, jaundice, and right upper quadrant pain (cholestasis)

• Urinary frequency, excessive thirst, and weight loss (diabetes)

Past medical history should identify known causative disorders (eg, renal disease, cholestatic disorder, cancer being treated with chemotherapy) and the patient’s emotional state. Social history should focus on family members with similar itching and skin symptoms (eg, scabies, pediculosis); relationship of itching to occupation or exposures to plants, animals, or chemicals; and history of recent travel.

Physical examination begins with a review of clinical appearance for signs of jaundice, weight loss or gain, and fatigue. Close examination of the skin should be done, taking note of presence, morphology, extent, and distribution of lesions. Cutaneous examination also should make note of signs of secondary infection (eg, erythema, swelling, warmth, yellow or honey-colored crusting).

The examination should make note of significant adenopathy suggestive of cancer. Abdominal examination should focus on organomegaly, masses, and tenderness (cholestatic disorder or cancer). Neurologic examination should focus on weakness, spasticity, or numbness (multiple sclerosis).

The following findings are of particular concern:

• Constitutional symptoms of weight loss, fatigue, and night sweats

• Extremity weakness, numbness, or tingling

• Abdominal pain and jaundice

• Urinary frequency, excessive thirst, and weight loss

Generalized itching that begins shortly after use of a medication or exposure to a substance (eg, IV contrast) is likely caused by that agent. Localized itching (often with rash) that occurs in the area of contact with a substance is likely caused by that substance. However, many systemic allergies can be difficult to identify because patients typically have consumed multiple different foods and have been in contact with many substances before developing itching. Similarly, identifying a drug cause in a patient taking several medications may be difficult. Sometimes the patient has been taking the offending medication for months or even years before developing a reaction.

If an etiology is not immediately obvious, the appearance and location of skin lesions can suggest a diagnosis (see table Some Causes of Itching).

In the minority of patients in whom no skin lesions are evident, a systemic disorder should be considered. Some disorders that cause itching are readily apparent on evaluation (eg, chronic kidney disease, cholestatic jaundice). Other systemic disorders that cause itching are suggested by findings (see table Some Causes of Itching). Rarely, itching is the first manifestation of significant systemic disorders (eg, polycythemia vera, certain cancers, hyperthyroidism).

Many dermatologic disorders are diagnosed clinically. However, when itching is accompanied by discrete skin lesions of uncertain etiology, biopsy can be appropriate.

When an allergic reaction is suspected but the substance is unknown, skin testing (either prick or patch testing depending on suspected etiology) is often done.

When a systemic disorder is suspected, testing is directed by the suspected cause and usually involves complete blood count; liver, renal, and thyroid function measurements; and appropriate evaluation for underlying cancer.

Itching due to any cause benefits from use of cool or lukewarm (instead of hot) water when bathing, mild or moisturizing soap, limited bathing duration and frequency, frequent lubrication, humidification of dry air, and avoidance of irritating clothing. Avoidance of contact irritants (eg, wool clothing) also may be helpful.

Topical medications may help localized itching. Options include lotions or creams that contain camphor and/or menthol, pramoxine, capsaicin, or corticosteroids. Corticosteroids are effective in relieving itch caused by inflammation but should be avoided for conditions that have no evidence of inflammation.

Topical benzocaine, diphenhydramine, and doxepin should be avoided because they may sensitize the skin.

Systemic medications are indicated for generalized itching or local itching resistant to topical agents. Antihistamines, most notably hydroxyzine, are effective, especially for nocturnal itch, and are most commonly used. Sedating antihistamines must be used cautiously in older adults during the day because they can lead to falls; newer nonsedating antihistamines such as loratadine, fexofenadine, and cetirizine can be useful for daytime itch. Other medications include doxepin (typically taken at night due to high level of sedation), cholestyramine (for renal failure, cholestasis, and polycythemia vera), opioid antagonists such as naltrexone (for biliary pruritus), and possibly gabapentin (for uremic pruritus).

Physical agents that may be effective for itching include ultraviolet phototherapy.