Itch

Itching or pruritus (from Latin prurire, meaning to itch) is an unpleasant sensation of the skin that provokes scratching, i.e. rubbing, scrubbing, etc. of the skin with the ends of the fingernails or other suitable materials or mechanical aids (e.g. back scratcher). Itching may be a symptom of disease. The skin lesions (scratching artifacts) caused by scratching present as line or area redness, bleeding areas (with subsequent crusting), hyperpigmentation, lichenification, and pyoderma. According to a study, about 70 % of the pruritus patients additionally suffer from psychosomatic or psychiatric diseases.

The messenger substances (mediators) that trigger itching, e.g. histamine, which is released by mast cells among others, can also be released by drugs (e.g. hydroxyethyl starch), food, allergens, plant or insect toxins and the like. Chronic forms of pruritus are frequently resistant to therapy.

In the past, it was often assumed that itch stimuli as subliminal pain stimuli on the skin are perceived by the same receptors as pain and are transmitted via the same signalling pathways. More recent research on histamine-mediated itch blames nerve fibers in the skin that cannot trigger pain. In addition, there appear to be separate signaling pathways for pain and itch. Important transmitters for itch seem to be the gastrin releasing peptide (GRP) and the natriuretic polypeptide b (Nppb).

Itching - similar to pain - is linked to memory. This means that people with chronic pruritus already perceive itching from a lower level. In addition, itching has a contagious effect via mirror neurons - an effect that also occurs when yawning.

Forms and triggers

Pruritus cum materia is itching accompanying skin diseases such as atopic eczema, dermatomycoses, psoriasis or urticaria.

Pruritus sine materia is pruritus without primary visible skin changes which may indicate disease of internal organs (e.g. cholestasis syndrome and primary biliary cholangitis due to an increase in bile acids in the blood plasma, renal insufficiency, uraemia, diabetes mellitus, leukaemia, lymphomas, malignant tumours) or, in approximately 50 % of cases, is without detectable triggering factors (idiopathic), for example in the form of aquagenic pruritus.

Pruritus senilis or itching of old age is due to excessively dry skin caused by age-related degenerative skin changes.

Pruritus ani is itching in the area of the anus.

Neuropathic pruritus is the term used to describe itching that results from compression or degeneration of nerve fibers. This is or may be the case in the following conditions, among others: Notalgia paraesthetica (on the back), Cheiralgia paraesthetica (on the hand), Meralgia paraesthetica (on the thigh).

Uremic pruritus is very common in renal failure and under hemodialysis. The cause is not clear, but chronic polyneuropathy, uremic skin changes and chronic inflammation play a role.

Itching may also occur as an adverse drug reaction. The above mentioned hydroxyethyl starch (HES), which causes pruritus in almost 100 % of the treatments, plays practically no role therapeutically today. Drugs that cause pruritus at least frequently (≥ 1 %) are ACE inhibitors, calcium antagonists, sulfonylureas, penicillins and cephalosporins, the combination of trimethoprim with sulfamethoxazole, tetracyclines, quinolones, metronidazole, chloroquine, opiates, cytokines and the direct anticoagulants: edoxaban and rivaroxaban.

Treatment

Besides the treatment of a possible underlying disease and basic measures like skin care, symptomatic therapy may be performed with H1-antihistamines among others. Opioid antagonists (naltrexone) could achieve success according to a study. Cooling helps with local itching. In part, warm water also helps, which however may lead to dry skin and thus to itching again. A therapy with UVB rays is also used.