Itch
An itch is a sensation that causes a strong desire or reflex to scratch. Itches have many similarities to pain, and while both are unpleasant sensory experiences, their behavioral response patterns are different. Pain creates a withdrawal reflex, whereas itches lead to a scratch reflex.
Unmyelinated nerve fibers for itches and pain both originate in the skin. Information for them is conveyed centrally in two distinct systems that both use the same nerve bundle and spinothalamic tract.
Classification
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. 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. 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.
Signs and symptoms
Pain and itch have different mechanisms of onset and different behavioral responses. Pain elicits a withdrawal reflex, which leads to retraction, and therefore a reaction trying to protect an endangered part of the body. By contrast, an itch 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 to scratching, as one would find noxious scratching highly pleasurable. 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. These aspects might contribute to the compulsive nature of itch and scratching.
Contagious itch
Events of "contagious itch" are common occurrences and may be more than a localized phenomenon in the place one scratches.Itches due to specific stimuli
In polycythemia vera itching is particularly caused by skin contact with warm water, such as in bath or shower.Itch inhibition due to pain
Studies done in the last decade have shown that itch can be inhibited by many other forms of painful stimuli, such as noxious heat, physical rubbing or scratching, noxious chemicals, and electric shock.Causes
Infectious
- 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
- Insect bites, such as those from mosquitos or chiggers
- 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
- 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. Certain allergens may be diagnosed in a patch test.
- 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 usually causes itching.
Skin
- 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. 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 diseases
- 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
- Iron deficiency anemia, a common anemia
- Cholestasis, where bile acids leaking into the serum activate peripheral opioid receptors, resulting in the characteristic generalized, severe itching
- Malignancy or internal cancer, such as lymphoma or Hodgkin's disease
- Polycythemia, which can cause generalized itching due to increased histamines. In polycythemia vera itching is particularly caused by skin contact with warm water, such as in bath or shower.
- Psychiatric disease
- Thyroid illness
- Uraemia – the itching sensation this causes is known as uremic pruritus
- mast cell activation syndrome.
- fibromyalgia
Medication
- Drugs that activate histamine 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, a chronic hives-like rash
Other
- Menopause, or changes in hormonal balances associated with aging
- Terminal illness
Mechanism
Pruritoceptive
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 afferent neurons responsible for histamine-induced itch are unmyelinated C-fibres.Nociceptors. 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 itch produced without a flare reaction that involves no histamine. Therefore, it is possible that pruritoceptive nerve fibres have different classes of fibres, which is unclear in current research.
Histology and skin layers. 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 elicit only pain when injected subcutaneously.
Molecular basis
Itch is often classified as that which is histamine mediated and nonhistaminergic.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.
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. The histamine receptor gene Hrh1 was found in NP2 and NP3, suggesting that histaminergic itch is transmitted by both these pruriceptive sub clusters.
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.