We all know the sensation of itchy skin and the response to this stimulus is obvious and almost universal: scratch where it itches. Itching is not considered a disease since it is a process that occurs in healthy individuals, but in some cases it can be a big problem if it becomes a chronic or symptomatic stimulus.
What is itching?
The medical term pruritus or itching was defined for the first time in 1660 by the German physician Samuel Hafenreffer as an “unpleasant sensation that provokes the desire or reflex to scratch” . Although it is obvious that itching was known long before, when it was included in the first dermatological treatises, its mechanism began to be studied as a symptom of some diseases.
To understand the evolutionary meaning or function of this natural mechanism, we must first explain what happens in our skin that leads to this sensation. In a simple way, itching, also known as pruritus, itching or stinging, is an uncomfortable stimulus or irritation that causes the desire to scratch the area where the stimulus appears.
This reflex or unconscious act is conserved among mammals because it is an evolutionary advantage. The main objective of itchy skin is that, when scratching that point, the harmful stimulus is eliminated , since in most cases it will be a dangerous agent such as parasites or toxic substances.
This sensation can be localized to one point or generalized throughout almost the entire body. Although decades ago itching was considered to be a mild form of pain, it is now known that it is a different stimulus.
This stimulus can originate in the peripheral or central nervous system, and this difference can also indicate the cause of its appearance. The stimulus is transmitted through neurons and sometimes shares the same messaging system as pain stimuli, but the message it sends (neurotransmitter) is different.
The mechanism by which neurons send the itch message can be activated through many different molecules. However, there is a molecule known as the “itch regulator” or pruritic cytokine: interleukin 31 (IL-31). This molecule is capable of activating the sensory neurons of itching.
However, although patients suffering from pruritic (itchy) and allergic diseases have high levels of IL-31 in the skin and in the blood, this molecule also has other functions. Among them, IL-31 can create a proinflammatory environment, with immunomodulatory effects on skin remodeling .
What causes itching?
A study published in 2012 in the prestigious scientific journal The Lancet analyzed the prevalence of itching as a debilitating symptom in relation to another disease. This study determined that around 4% of the global population have difficulties in their routine caused by itching .
The agents that can cause itching are many and varied. We can find external agents, such as bites or stings, irritating substances whether from plants or synthetic compounds. and environmental factors such as the sun itself.
However, itching caused by internal factors can also be experienced as occurs in various allergic, infectious, autoimmune, parasitic, psychogenic or unclear (idiopathic) diseases.
In general, itching can also be classified if it appears acutely or chronically. Acute itching is punctual and localized, such as in the case of a mosquito bite. Chronic itching occurs repeatedly, usually caused by an underlying disease, although the mechanism is only partially understood.
What is good when itchy skin?
Although all the causes of itching in each of the diseases are not known, some can be treated using drugs that attack the IL-31 molecule. Studies conducted with an IL-31 blocker in patients with atopic dermatitis and nodular prurigo have shown it to be a highly effective treatment for eliminating this symptom.
In the case of itching associated with allergic rhinitis or conjunctivitis, the symptoms can be controlled with the same oral or local antihistamine (eye drops or nasal sprays). Nasal corticosteroids are also very fast and effective, although they can cause side effects.
However, in other cases where the recurring chronic itching has an unknown origin, it is necessary to use other types of treatment. A 2017 study showed that an alternative medication could relieve symptoms in patients who did not improve with standard treatment . Although the study included few patients, it helps to understand the mechanisms that can cause itching.
There are still many unknowns to resolve about the causes and mechanisms of itching, but the search for new treatments and basic research on molecular processes are gradually providing the necessary evidence.
References:
Serra-Baldrich et al. 2021. Nemolizumab: an innovative biological treatment for the control of key interleukin 31 (IL-31) in atopic dermatitis and nodular pruritus. Dermo-Syphilographic MINUTES. doi: 10.1016 / j.ad.2021.12.014.
Vos et al. 2012. Years lived with disability (YLDs) for 1160 sequelae of 289 diseases and injuries 1990–2010: a systematic analysis for the Global Burden of Disease Study 2010. Lancet. doi: 10.1016/S0140-6736(12)61729-2.
