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 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|
Vulvar and vaginal problems in prepubertal females
|Neuropathic causes of localised pruritus without primary rash|
|Face||Trigeminal trophic syndrome|
|Any dermatome||Herpes zoster (shingles) during recovery phase|
|Anywhere||Brain lesion: tumour, abscess, infarct|
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)|
|Blood||Iron deficiency anaemia
|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
Pruritic skin diseases
Pruritus is often a symptom of many skin diseases. Some of these are included in the following list.
- Allergic contact dermatitis
- Dry skin
- Atopic dermatitis
- Dermatitis herpetiformis
- Lichen simplex
- Lichen planus
- Bullous pemphigoid
- Pityriasis rosea
- Mycosis fungoides
Pruritus may arise as a result of exposure to certain external factors.
- Allergens or irritants
- Physical urticaria, e.g. dermographism
- Aquagenic pruritus
- Insects and infestations
- Medications (topical or systemic) e.g. opioids, aspirin
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:
- Histamine-stimulated pathway via mechanically insensitive C-fibres
- Cowhage-stimulated pathway via polymodal C-fibres.
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:
- Wet dressings or tepid shower to cool the skin
- Calamine lotion (contains phenol, which cools the skin): avoid on dry skin and limit use to a few days
- Menthol/camphor lotion: gives a chilling sensation
- Regular use of emollients, especially if skin is dry
- Mild topical corticosteroids for short periods.
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.
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.
- Antihistamines are most useful in urticaria. However, the non-sedating antihistamine cetirizine has antipruritic action. Sedating antihistamines may be used for their sedative effects.
- Doxepin and amitriptyline: tricyclic antidepressants have antipruritic action and act on the central and peripheral nervous systems. Tetracyclic antidepressants such as mirtazepine and selective serotonin reuptake inhibitors (paroxetine, sertraline, fluoxetine) may also help some patients with severe itch including when it is caused by a metabolic disease, malignancy or a neuropathic condition.
- Aspirin is sometimes effective if pruritus is mediated by kinins or prostaglandins. Note: aspirin may cause or aggravate itch in some patients.
- Anti-epileptic drugs such as sodium valproate, gabapentin and pregabalin may also be of benefit to some patients, e.g., those with itch associated with renal failure or neuropathic itch. The mechanism of action is uncertain.
- Thalidomide has been very successful in treating nodular prurigo and chronic pruritus of various kinds but is rarely used because of serious adverse effects and expense.
- Opioid antagonists such as butorphanol intranasal spray, naltrexone tablets and nalfurafine have been effective in patients suffering from intractable pruritus in association with liver disease.
- Rifampicin is effective for patients with pruritus associated with cholestasis (some forms of liver disease)
- Isolated case reports in severe itch associated with malignancy have reported success with the NKR1 antagonist aprepitant (normally used short-term for postoperative or chemotherapy-induced nausea). This is under investigation for neuropathic itch and nodular prurigo.
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.