Itch, pruritus

Author: Hon A/Prof혻Amanda Oakley, Dermatologist, Hamilton, New Zealand, 2003.

The medical term for itch is pruritus. Itch is an unpleasant sensation on the skin that provokes the desire to rub or scratch the area to obtain relief. Itch can cause discomfort and frustration; in severe cases it can lead to disturbed sleep, anxiety and depression. Constant scratching to obtain relief can damage the skin (excoriation, lichenification) and reduce its effectiveness as a major protective barrier.

What causes pruritus?

There are numerous causes of pruritus. They can be classified under 5 main headings as follows:

Localised pruritus

Localised pruritus is pruritus that is confined to a certain part of the body. It can occur in association with a primary rash (e.g. dermatitis) or may occur because of hypersensitive nerves in the skin (neuropathic pruritus). Neuropathic pruritus is due to compression or degeneration of nerves in the skin, on route to the spine or in the spine itself. Neuropathic itch is sometimes associated with reduced or absent sweating in the affected area of skin.

Typical causes of localised itchy rashes
Scalp Seborrhoeic dermatitis
Head lice
Back Grover disease
Hands Pompholyx
Hand dermatitis
Genitals Vulvovaginal candida
Vulvar and vaginal problems in prepubertal females
Lichen sclerosus
Legs Gravitational eczema
Feet Tinea pedis
Neuropathic causes of localised pruritus without primary rash
Face Trigeminal trophic syndrome
Arm Brachioradial pruritus
Back Notalgia paraesthetica
Vulva Pruritus vulvae
Anus Pruritus ani
Any dermatome Herpes zoster (shingles) during recovery phase
Anywhere Brain lesion: tumour, abscess, infarct

Scratching a localised itch may lead to lichen simplex, prurigo or prurigo nodularis.

Systemic causes of pruritus

Sytemic diseases may cause generalised pruritus. This is sometimes called metabolic itch. There is nothing wrong with the skin itself, at least until it's been scratched.

Some systemic disorders that cause pruritus
Kidney disease Chronic kidney disease
Liver disease Intrahepatic and extrahepatic biliary obstruction (cholestatic jaundice), associated with autotaxin (the enzyme that forms lysophosphatidic acid)
Endocrine/metabolic Diabetes
Blood Iron deficiency anaemia
Lymphatic leukaemia
Hodgkin lymphoma
Infection Human immunodeficiency virus (HIV)
Nervous system Generalised neuropathic pruritus, e.g. following stroke, multiple sclerosis, small fibre neuropathy
Neurotic excoriations related to psychosis, anxiety, obsessive compulsive disorder, depression
Skin picking

Pruritic skin diseases

Pruritus is often a symptom of many skin diseases. Some of these are included in the following list.

Exposure-related pruritus

Pruritus may arise as a result of exposure to certain external factors.

Hormonal reasons for pruritus

About 2% of pregnant women have pruritus without any obvious dermatological cause. In some cases the itch is due to cholestasis (pooling of bile in the gall bladder and liver). It usually occurs in the 3rd trimester and is relieved after giving birth.

Generalised itch is also a common symptom of menopause.

Explain how itch occurs

Itch, like pain, can originate anywhere along the neural itch pathway, from the central nervous system (brain and spinal cord) to the peripheral nervous system and the skin.

Researchers have found two itch-sensitive neural pathways:

Imaging studies have revealed diffuse itch areas that overlap with pain within the brain.

The way scratching stops itching has been explained by an interaction with pain pathways within the dorsal horn of the spinal cord.

What treatment is available for itch?

The management of pruritus relies heavily on establishing the cause and then either removing or treating the cause to prevent further itching. In many cases, tests are necessary to determine the cause; while these are in progress, treatment to provide symptomatic relief of pruritus may be given.

In addition to specific therapy for any underlying skin or internal disease, topical treatment may include:

Other measures that can be useful in preventing pruritus include avoiding precipitating factors such as rough clothing or fabrics, overheating, and vasodilators if they provoke itching (e.g. caffeine, alcohol, spices). Fingernails should be kept short and clean. If the urge to scratch is irresistible then rub the area with your palm.

Topical antihistamines should not be used for chronic itch, as they may sensitise the skin and result in allergic contact dermatitis.

If pruritus is severe and sleep is disturbed, then treatment with oral medication may be necessary. Some drugs may help to relieve the itch whilst others are given solely for their sedative effects.

Patients with generalised itch may benefit from phototherapy (medically supervised exposure to ultraviolet radiation). This is particularly useful if there is an underlying skin condition, in patients with chronic renal failure on dialysis, and in patients with eosinophilic infiltrations.

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