Itch overview

# Overview
An itch (Latin: pruritus) is a sensation felt on an area of skin that evokes the desire or reflex to scratch that area. Itching can be related to anything from dry skin to cancer.  Itch has many similarities to pain and both are unpleasant sensory experiences but their behavioral response patterns are different. Pain creates a reflex withdrawal while itch leads to a scratch reflex. Unmyelinated nerve fibers for itch and pain both originate in the skin, however, information for them are conveyed centrally in two distinct systems that both use the same peripheral nerve bundle and spinothalamic tract.

# Historical perspective
Until recent times, sensations of itch were considered to be largely the same as those of pain. Currently, pruritus is considered to be an autonomous, pain-independent sensation.
Itch, also known as medically pruritus, The first documented definition of itch was given by the German physician Samuel Hafenreffer as an unpleasant sensation that elicits the desire or reflex to scratch about 350 years ago. In recent years, pruritus has been determined to be an autonomous, pain-independent sensation.
pruritus is a frequently misspelled word, often misspelled as "pruritis".
Historically, the sensations of itch and pain have not been considered to be independent of each other until recently where it was found that itch has several features in common with pain but exhibits notable differences. Pruritic stimuli mostly create the same reactions as noxious stimuli in experimental animals. As a result, it is difficult to study the physiologic mechanisms of itch in animal models as they lack the ability to discern the differences between both stimuli. Therefore human studies have provided most of the information regarding the processing of pruritic stimuli. It was initially thought histamine was the major chemical involved in mediating pruritus through binding to H1R and activating phospholipase C3 (PLC3) and phospholipase A2 (PLA2). Bell et al. demonstrated that histamine promotes the influx of calcium at the axon terminals of the spinal cord neurons via a transient receptor vanilloid 1 (TRPV1) receptor activation and then, through the promotion of a series of intracellular signal activation, eventually lead to the generation of itching sensation. However, the current consensus is that there are other chemical mediators that are highly involved in the promotion of pruritus.

# Pathophysiology
The pathogenesis of itch is characterized by  dermal/pruritoceptive which originates in the skin, neuropathic which is due to damage along the afferent pathway of a nerve, neurogenic itch which originates centrally without the involvement of the peripheral nerves, a psychogenic itch which is associated with psychiatric disorders and phobias, and a mixed picture which is due to two or more causes of itching

# Causes
There are many causes of itching. Examples include skin diseases, systemic diseases, allergies, medications, insect stings among many others

# Differentiating Itch from other Disorders
There is a very broad differential diagnosis for itching due to an extensive list of possible causes. Itching may be due to dermatologic disease, systemic disease, reactions to chemicals and medications, weather, or psychiatric disease. Symptoms, laboratory findings, and other diagnostic tests can help narrow down the list.

# Epidemiology and Demographics
About 280 million people globally, roughly 4% of the world's population, have difficulty with itchiness.
pruritusmay be associated with virtually any malignancy. Certain neoplasms, especially hematologic malignancies, have pruritus as a common symptom. For example, in patients with polycythemia vera, 48% to 70% have aquagenic pruritus, which is itching that is associated with water coming in contact with the skin. Also, about 30% of people with Hodgkin's disease suffer from pruritus. The incidence of pruritus in other types of hematologic malignancies is currently not known, however, investigators have reported its presence in approximately 3% of patients with nonHodgkin's lymphoma. Solid tumors may be associated with paraneoplasticpruritus. In fact, pruritus may one of the early symptoms, starting months or years before a patient is diagnosed. pruritus is also a frequent finding in cutaneous lymphomas. Additionally, it is frequently seen in cancers involving the biliary tract. Retrospective studies have revealed that 2% to 11% of chronic itch cases are attributable to malignancies.
pruritus has been reported to be an early symptom in some non-malignant diseases such as liver disease, renal insufficiency, thyroid disease, Sjogren's syndrome, diabetes mellitus, iron deficiency, paraproteinemia, and other conditions. In internal diseases, itch has been best studied in cholestatic pruritus and uremic pruritus.
About a third of uremic patients treated without dialysis complain of uremic pruritus, and on maintenance hemodialysis, the incidence of uremic itching rises to 70%-80%.
Cholestatic pruritus]] affects nearly if not all patients with biliary cirrhosis and is the first symptom in nearly 50% of the patients with this disease. Furthermore, the prevalence of pruritus in patients with endstage HIV is over 20%.

# Risk Factors
Common risk factors in the development of itch are allergic reactions, cholestasis , diabetes mellitus, and drug side effect.

# Natural History, Complications and Prognosis
The scientific term for itching is pruritus. The word pruritusis derived from the Latin word Prurire which means to itch.
In 1660, the German physician Samuel Hafenreffer defined pruritus as "an unpleasant sensation associated with the desire to scratch".
In modern medicine, pruritus is used in reference to a pathological condition in which the sensations of itch are intense and often generalized and stimulate repeated scratching in an attempt to alleviate the discomfort. pruritus is not a disease, but rather a common and still poorly understood symptom of both localized and systemic disorders that may accompany many conditions.
pruritus is important in many medical specialties, and notably to palliative care. Although pruritus is not the most commonly seen symptom seen in advanced terminal disease, it is among the most puzzling symptoms. It can cause considerable discomfort in patients receiving cancer therapy or in other types of terminal diseases. The itchscratchitch cycle adversely affects skin integrity, resistance to infections, and quality of life as much as pain can affect a patient. It can also lead to feelings of embarrassment which can impact a social patient's life.

# Diagnosis
A proposed approach to the evaluation of itching can be summarized on Itch diagnosis page

# Treatment

## Medical Therapy
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.
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.

## Primary Prevention
Effective measures for the primary prevention of  include avoidance of causes, and emollients.

## Secondary Prevention
Effective measures for the secondary prevention of itch include anti-pruritics, scratching, cooling, warming, soft stimulation.

## Cost-Effectiveness of Therapy
There has been no discussion on the cost-effectiveness of therapy foritch

## Future or Investigational Therapies
There has been no discussion on future or investigational therapies for itch.