Itching is a symptom that can cause significant discomfort and is one of the most common reasons for consultation with a dermatologist. Itching leads to scratching, which can cause inflammation, skin degradation, and possible secondary infection. The skin can become lichenified, scaly, and excoriated.
(See also Evaluation of the Dermatologic Patient.)
Pathophysiology of Itching
Itch can be prompted by diverse stimuli, including light touch, vibration, and wool fibers. There are a number of chemical mediators as well as different mechanisms by which the sensation of itch occurs. Specific peripheral sensory neurons mediate the itch sensation. These neurons are distinct from those that respond to light touch or pain; they contain a receptor, MrgA3, the stimulation of which causes the sensation of itching.
Mediators
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.
Mechanisms
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.
Etiology of Itching
Itching can be a symptom of a primary skin disease or a systemic disease. Also, drugs and medications can cause itching (see table Some Causes of Itching).
Skin disorders
Many skin disorders cause itching. The most common include
Systemic disorders
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
Less common systemic causes of itching include hyperthyroidism, hypothyroidism, diabetes, iron deficiency, dermatitis herpetiformis, and polycythemia vera.
Medications
Evaluation of Itching
History
History of present illness
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
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).
Red flags
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
Interpretation of findings
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).
Testing
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.
Treatment of Itching
Any underlying disorder is treated. Supportive treatment involves the following (see also table Some Therapeutic Approaches to Itching):
Local skin care
Topical treatment
Systemic treatment
Skin care
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 treatment
Systemic treatment
Physical agents that may be effective for itching include ultraviolet phototherapy.
Geriatrics Essentials: Itching
Age-related changes in the immune system and in nerve fibers may contribute to the high prevalence of itch in older adults.
Xerotic eczema is very common among older adults. It is especially likely if itching is primarily on the lower extremities.
Severe, diffuse itching in older adults should raise concern for cancer, especially if another etiology is not immediately apparent.
When treating older adults, sedation can be a significant problem with antihistamines, so dose reduction may be appropriate. Use of nonsedating antihistamines during the day and sedating antihistamines at night, liberal use of topical ointments and corticosteroids (when appropriate), and consideration of ultraviolet phototherapy can help avoid the complications of sedation.
Key Points
Itching is usually a symptom of a skin disorder or systemic allergic reaction but can result from a systemic disorder.
If skin lesions are not evident, systemic causes should be investigated.
Skin care (eg, limiting bathing, avoiding irritants, moisturizing regularly, humidifying environment) should be observed.
Symptoms can be relieved by topical or systemic medications.