Itch

Itch
A man scratching his back
Classification and external resources
Specialty	Dermatology
ICD-10	L29
ICD-9-CM	698
DiseasesDB	25363
MedlinePlus	003217
eMedicine	derm/946
Patient UK	Itch
MeSH	D011537

Itch (also known as pruritus) is a sensation that causes the desire or reflex to scratch.^[1] Itch has resisted many attempts to classify it as any one type of sensory experience. Modern science has shown that itch has many similarities to pain, and while both are unpleasant sensory experiences, their behavioral response patterns are different. Pain creates a withdrawal reflex, whereas itch leads to a scratch reflex.^[2]

Unmyelinated nerve fibers for itch and pain both originate in the skin; however, information for them is conveyed centrally in two distinct systems that both use the same nerve bundle and spinothalamic tract.^[3]

Signs and symptoms

Pain and itch have very different behavioral response patterns. Pain evokes 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 hedonic aspects of scratching, as one would find noxious scratching highly pleasurable.^[2] This can be problematic with chronic itch patients, such as ones with atopic dermatitis, who may scratch affected spots until they no longer produce a pleasant or painful sensation, instead of when the itch sensation disappears.^[4] It has been hypothesized that motivational aspects of scratching include the frontal brain areas of reward and decision making. These aspects might therefore contribute to the compulsive nature of itch and scratching.^[2]

Contagious itch

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 we scratch. 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 mirror neuron system exists in which we imitate certain motor actions when we view others performing the same action. A similar phenomenon in which mirror neurons are used to explain the cause is contagious yawning.^[2]

Pain inhibition by itching

The sensation of itch can be reduced by many painful sensations. Studies done in the last decade have shown that itch can be inhibited by many other forms of painful stimuli, such as noxious heat,^[5] physical rubbing/scratching, noxious chemicals, and electric shock.^[6]

Causes

Scabies is one cause of itching.

Swimmer's itch

Athlete's foot (showing the toes from below plus the front part of the sole)

Infections

• 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.^[7][8] Certain allergens may be diagnosed in a patch test.^[9][10]
• Body louse, found in substandard living conditions
• Cutaneous larva migrans, a skin disease
• Head lice, if limited to the neck and scalp
• Herpes, a viral disease
• Insect bites, such as those from mosquitos or chiggers
• Photodermatitis – sunlight reacts with chemicals in the skin, leading to the formation of irritant metabolites
• Pubic lice, if limited to the 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

Environmental and allergic

• Foreign objects on the skin are the most common cause of non-pathological itching.
• Urticaria (also called hives) usually causes itching

Skin disorders

• 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
• Skin conditions (such as psoriasis, eczema, 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, the most common cause, 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

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)^[11]
• Iron deficiency anemia, a common anemia (low red blood cell or hemoglobin levels)
• Jaundice and cholestasis – bilirubin is a skin irritant at high concentrations
• Malignancy or internal cancer, such as lymphoma or Hodgkin's disease^[12]
• 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

Medication

• Drugs (such as opioids) that activate histamine (H1) receptors or trigger histamine release
• Chloroquine, a drug

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

Mechanism

Itch can originate in the peripheral nervous system (dermal or neuropathic) or in the central nervous system (neuropathic, neurogenic, or psychogenic).^[13][14]

Dermal/pruritoceptive

A squirrel scratching itself.

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. The primary afferent neurons responsible for histamine-induced itch are unmyelinated C-fibres.^[1]

Two major classes of human C-fibre nociceptors exist: mechano-responsive nociceptors and mechano-insensitive nociceptors. Mechano-responsive nociceptors have been shown in studies to respond to mostly pain, and mechano-insensitive receptors respond mostly to itch induced by histamine. However, it does not explain mechanically induced itch or when itch is produced without a flare reaction which involves no histamine.^[1] Therefore, it is possible that pruritoceptive nerve fibres have different classes of fibres, which is unclear in current research.^[2]

Studies have been done to show that itch receptors are found only on the top two skin layers, the epidermis and the epidermal/dermal transition layers. Shelley and Arthur had verified the depth by injecting individual itch powder spicules (Mucuna pruriens), and found that maximal sensitivity was found at the basal cell layer or the innermost layer of the epidermis. Surgical removal of those skin layers removed the ability for a patient to perceive itch. Itch is never felt in muscle or joints, which strongly suggests that deep tissue probably does not contain itch signaling apparatuses.

Sensitivity to pruritic stimuli is evenly distributed across the skin, and has a clear spot distribution with similar density to that of pain. The different substances that elicit itch upon intracutaneous injection (injection within the skin) elicit only pain when injected subcutaneously (beneath the skin).

Itch is readily abolished in skin areas treated with nociceptor excitotoxin capsaicin, but remains unchanged in skin areas which were rendered touch-insensitive by pretreatment with saponins, an anti-inflammatory agent. 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.^[15]

Neuropathic

Neuropathic itch can originate at any point along the afferent pathway as a result of damage of the nervous system. They could include diseases or disorders in the central nervous system or peripheral nervous system.^[14] Examples of neuropathic itch in origin are notalgia paresthetica, brachioradial pruritus, brain tumors, multiple sclerosis, peripheral neuropathy, and nerve irritation.^[16]

Neurogenic

Neurogenic itch, which is itch induced centrally but with no neural damage, is mostly associated with increased accumulation of exogenous opioids and possibly synthetic opioids.^[14]

Psychogenic

Itch is also associated with some symptoms of psychiatric disorders such as tactile hallucinations, delusions of parasitosis, or obsessive-compulsive disorders (as in OCD-related neurotic scratching).^[14]

Peripheral sensitization

Inflammatory mediators—such as bradykinin, serotonin (5-HT) and prostaglandins—released during a painful or pruritic inflammatory condition not only activate pruriceptors but also cause acute sensitization of the nociceptors. In addition, expression of neuro growth factors (NGF) can cause structural changes in nociceptors, such as sprouting. NGF is high in injured or inflamed tissue. Increased NGF is also found in atopic dermatitis, a hereditary and non-contagious skin disease with chronic inflammation.^[17] NGF is known to up-regulate neuropeptides, especially substance P. Substance P has been found to have an important role in inducing pain; however, there is no confirmation that substance P directly causes acute sensitization. Instead, substance P may contribute to itch by increasing neuronal sensitization and may affect release of mast cells, which contain many granules rich in histamine, during long-term interaction.^[2]

Central sensitization

Noxious input to the spinal cord is known to produce central sensitization, which consists of allodynia, exaggeration of pain, and punctuate hyperalgesia, extreme sensitivity to pain. Two types of mechanical hyperalgesia can occur: 1) touch that is normally painless in the uninjured surroundings of a cut or tear can trigger painful sensations (touch-evoked hyperalgesia), and 2) a slightly painful pin prick stimulation is perceived as more painful around a focused area of inflammation (punctuate hyperalgesia). Touch-evoked hyperalgesia requires continuous firing of primary afferent nociceptors, and punctuate hyperalgesia does not require continuous firing which means it can persist for hours after a trauma and can be stronger than normally experienced. In addition, it was found that patients with neuropathic pain, histamine ionophoresis resulted in a sensation of burning pain rather than itch, which would be induced in normal healthy patients. This shows that there is spinal hypersensitivity to C-fiber input in chronic pain.^[2]

Treatment

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.

Topical antipruritics in the form of creams and sprays are often available over-the-counter. Oral anti-itch drugs also exist and are usually prescription drugs. The active ingredients usually belong to the following classes:

• Antihistamines, such as diphenhydramine (Benadryl)
• Corticosteroids, such as hydrocortisone topical cream; see topical steroid
• Counterirritants, such as mint oil, menthol, or camphor^[18]
• 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)

Phototherapy is helpful for severe itching, especially if caused by renal failure. The common type of light used is UVB.^[12]

Sometimes scratching relieves isolated itches, hence the existence of devices such as the back scratcher. Often, however, scratching can intensify itching and even cause further damage to the skin, dubbed the "itch-scratch-itch cycle."

The mainstay of therapy for dry skin is maintaining adequate skin moisture and topical emollients.

Epidemiology

Approximately 280 million people globally, 4% of the population, have difficulty with itchiness.^[19] This is comparable to the 2–3% of the population suffering from psoriasis.

See also

• 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

References

Further reading

• Han, Liang; Dong, Xinzhong (6 May 2014). "Itch Mechanisms and Circuits". Annual Review of Biophysics. 43 (1): 331–355. doi:10.1146/annurev-biophys-051013-022826. 
• Andrew D, Craig AD (2001). "Spinothalamic lamina I neurons selectively sensitive to histamine: a central neural pathway for itch". Nat. Neurosci. 4 (1): 72–7. PMID 11135647. doi:10.1038/82924. 
• National Cancer Institute (2003) "Pruritus" Retrieved Aug. 22, 2005.