Itch

Itch (also known as pruritus) is a

Most commonly, an itch is felt in one place. If it is felt all over the body, then it is called generalized itch or generalized pruritus.[4] Generalized itch is infrequently a symptom of a serious underlying condition, such as cholestatic liver disease.

If the sensation of itching persists for six weeks or longer, then it is called chronic itch or chronic pruritus.^[4][5] Chronic idiopathic pruritus or Chronic Pruritus of Unknown Origin is a form of itch that persists for longer than six weeks, and for which no clear cause can be identified.^[6][7]

Signs and symptoms

Pain and itch have very different behavioral response patterns. Pain elicits a withdrawal reflex, which leads to retraction and therefore a reaction trying to protect an endangered part of the body. Itch in contrast creates a scratch reflex, which draws one to the affected skin site. Itch generates stimulus of a foreign object underneath or upon the skin and also the urge to remove it. For example, responding to a local itch sensation is an effective way to remove insects from one's skin.

Scratching has traditionally been regarded as a way to relieve oneself by reducing the annoying itch sensation. However, there are

Events of "contagious itch" are very common occurrences. Even a discussion on the topic of itch can give one the desire to scratch. Itch is likely to be more than a localized phenomenon in the place one scratches. Results from a study showed that itching and scratching were induced purely by visual stimuli in a public lecture on itching. The sensation of pain can also be induced in a similar fashion, often by listening to a description of an injury, or viewing an injury itself.

There is little detailed data on central activation for contagious itching, but it is hypothesized that a human

Infections

• Body louse, found in substandard living conditions
• Cutaneous larva migrans, a skin disease caused by hookworm infection
• Head lice
  , if limited to the neck and scalp
• Herpes, a viral disease
• chiggers
• genital area
• Scabies, especially when several other persons in close contact also itch
• Shaving, which may irritate the skin
• Swimmer's itch, a short-term immune reaction
• Varicella – i.e. chickenpox, prevalent among young children and highly contagious
• Tungiasis, ectoparasite of skin

Environmental and allergic

• Allergic reaction to contact with specific chemicals, such as urushiol, derived from poison ivy or poison oak, or Balsam of Peru, found in many foods and fragrances.^[11][12] Certain allergens may be diagnosed in a patch test.^[13][14]
• Foreign objects on the skin are the most common cause of non-pathological itching.
• Photodermatitis – sunlight reacts with chemicals in the skin, leading to the formation of irritant metabolites.
• Urticaria (also called hives
  ) usually causes itching.

• Dandruff, an unusually large amount of flaking is associated with this sensation.
• Punctate palmoplantar keratoderma
  , a group of disorders characterized by abnormal thickening of the palms and soles.
• eczema, seborrhoeic dermatitis, sunburn, athlete's foot, and hidradenitis suppurativa
  ). Most are of an inflammatory nature.
• Scab healing, scar growth, and the development or emergence of moles, pimples, and ingrown hairs from below the epidermis.
• Xerosis
  , dry skin, frequently seen in the winter and also associated with older age, frequent bathing in hot showers or baths, and high-temperature and low-humidity environments.

Other medical disorders

• Diabetes mellitus
  , a group of metabolic diseases in which a person has high blood sugar
• Hyperparathyroidism, overactivity of the parathyroid glands resulting in excess production of parathyroid hormone (PTH)^[15]
• Iron deficiency anemia
  , a common anemia (low red blood cell or hemoglobin levels)
• Cholestasis, where bile acids leaking into the serum activate peripheral opioid receptors, resulting in the characteristic generalized, severe itching
• Hodgkin's disease^[16]
• Polycythemia, which can cause generalized itching due to increased histamines
• Psychiatric disease ("psychogenic itch", as may be seen in delusional parasitosis
  )
• Thyroid illness
• Uraemia – the itching sensation this causes is known as uremic pruritus

• Drugs (such as
  opioids
  ) that activate histamine (H1) receptors or trigger histamine release
• Chloroquine, a drug used in the treatment and prevention of malaria
• Bile acid congeners such as obeticholic acid

Related to pregnancy

• Gestational pemphigoid, a dermatosis of pregnancy
• Intrahepatic cholestasis of pregnancy, a medical condition in which cholestasis occurs
• Pruritic urticarial papules and plaques of pregnancy (PUPPP), a chronic hives-like rash

Other

• Menopause, or changes in hormonal balances associated with aging
• Terminal illness^[17]

Mechanism

Itch can originate in the

A chipmunk scratching itself.

A wolf scratching itself.

A lioness scratching herself.

Itch originating in the skin is known as pruritoceptive, and can be induced by a variety of stimuli, including mechanical, chemical, thermal, and electrical stimulation, or infection. The primary

Itch is readily abolished in skin areas treated with nociceptor excitotoxin capsaicin but remains unchanged in skin areas rendered touch insensitive by pretreatment with anti-inflammatory saponins. Although experimentally induced itch can still be perceived under a complete A-fiber conduction block, it is significantly diminished. Overall, itch sensation is mediated by A-delta and C nociceptors located in the uppermost layer of the skin.^[21]

Gene expression. Using single-cell mRNA sequencing, clusters of genes expressed in itch-related tissues were identified, e.g. NP1-3, transmitting itch information; where NP3 expresses neuropeptides Nppb and Sst as well as genes involved in inflammatory itch (Il31ra, Osmr and Crystrl2). The histamine receptor gene Hrh1 was found in NP2 and NP3, suggesting that histaminergic itch is transmitted by both these pruriceptive sub clusters.^[22]

Infection. Staphylococcus aureus, a bacterial pathogen associated with itchy skin diseases, directly activates pruriceptor sensory neurons to drive itch. Skin exposure to S. aureus causes robust itch and scratch-induced damage. This reaction is mediated by S. aureus serine protease V8 which cleaves proteinase-activated receptor 1 (PAR1) on mouse and human sensory neurons. Targeting PAR1 through genetic deficiency, small interfering RNA (siRNA) knockdown, or pharmacological blockade decreases itch and skin damage caused by V8 and S. aureus exposure.^[23]

The spinal itch pathway

After the pruriceptive primary afferent has been activated, the signal is transmitted from the skin into the spinal dorsal horn. In this area, a number of interneurons will either be inhibited or activated to promote activation of projection neurons, mediating the pruriceptive signal to the brain. The GRP-GRPR interneuron system has been found to be important for mediating both histaminergic and non-histaminergic itch, where the GRP neurons activate GRPR neurons to promote itch ^[24][25]

Neuropathic

Neuropathic itch can originate at any point along the afferent pathway as a result of damage of the

A variety of over-the-counter and prescription anti-itch drugs are available. Some plant products have been found to be effective anti-pruritics, others not. Non-chemical remedies include cooling, warming, soft stimulation.

• Antihistamines, such as diphenhydramine (Benadryl)^[28]
• Corticosteroids, such as hydrocortisone topical cream; see topical steroid
• Counterirritants, such as mint oil, menthol, or camphor^[29]
• Crotamiton (trade name Eurax) is an antipruritic agent available as a cream or lotion, often used to treat scabies. Its mechanism of action remains unknown.
• Local anesthetics, such as benzocaine topical cream (Lanacane)

No studies have been conducted to investigate the effectiveness of emollient creams, cooling lotions, topical corticosteroids, topical antidepressants, systemic antihistamines, systemic antidepressants, systemic anticonvulsants, and phototherapy on chronic pruritus of unknown origin.[28] However, there are clinical trials currently underway with dupilumab which is thought to alleviate itch by acting on the IL-4 receptor on sensory neurons.^[31][32] The effectiveness of therapeutic options for people who are terminally ill with malignant cancer is not known.^[17]

History

In 1660, German physician Samuel Hafenreffer introduced the definition of pruritus (itch).^[33]

Epidemiology

Approximately 280 million people globally, 4% of the population, have difficulty with itchiness.^[34] This is comparable to the 2–3% of the population who have psoriasis.

See also

• Feeling, a perceptual state of conscious experience.
• Formication, a sensation that resembles that of small insects crawling on or under the skin
• Pruritus ani (also known as anusitis), irritation of skin at the exit of the rectum (anus), causing the desire to scratch
• Referred itch, a phenomenon in which a stimulus applied in one region of the body is felt as an itch or irritation in a different part of the body
• Itching powder, a powder or powder-like substance that induces itching when applied onto human skin.

References