Blog

What is pruritus?

Pruritus is the medical term we use to refer to “itching“. It is described as the unpleasant sensation that causes the desire to scratch. It can be mild or so annoying as to be unbearable. It is not a disease in itself, but a symptom: it is the form in which many skin diseases manifest themselves, but it also accompanies some systemic diseases (those affecting different organs or systems). For all these reasons, it is very complex to fully address the causes and treatment of pruritus. It may originate in the skin (“proprioceptive”) or in the central nervous system (“neurogenic”) or be a combination of both. Stimulation of the central nervous system may be involved not only in itching associated with systemic diseases, but also in dermatological diseases. Itching originating in the skin is transmitted through nerve fibers whose endings are located in the dermoepidermal junction or epidermis. The substances capable of activating these fibers are called pharmacological mediators and include histamine, neuropeptide P, serotonin, bradykinin, various proteases such as tryptase and endothelin. These impulses are transmitted to the central nervous system (first to the spinal cord and then to the thalamus, a neural structure located in the brain). These mediators are also known to be important when pruritus originates in the central nervous system although it is not well known in what form or to what extent. Although this explanation may seem very technical, knowledge of these substances and how they work opens up new avenues of treatment.

Causes

Most skin diseases cause pruritus. Some representative pruritic diseases are atopic dermatitis (AD), urticaria and scabies. In AD, pruritus is not clearly related to the intensity of the disease, causes a decrease in the number of hours of sleep and alteration in its quality, causes scratching that further alters the barrier function of the skin and favors bacterial infection and aggravation of dermatitis. In the picture I show you a typical image of eczema with the consequences of scratching (thickening and wrinkling of the skin as well as erosions and scabs). If you want to know more about atomic dermatitis click here. In urticaria, itching is mainly due to the release of histamine, a pharmacological mediator, by cells called mast cells. It is characterized by “wheals” or “welts”, as you can see in the image, which disappear in minutes or hours. It is a disorder preferably treated with non-sedating antihistamine drugs. I will not emphasize it any further since it can be developed in depth later on.

Scabies is caused by a mite and produces a very intense itching, especially at night. The type and location of lesions, often shared by other people in the environment, help in the diagnosis. In addition to dermatological diseases, generalized pruritus can be found in renal diseases, cholestasis (obstruction of bile flow to the intestine), hematological diseases (iron deficiency, polycythemia vera), endocrine diseases (hyper or hypothyroidism, hyperparathyroidism, diabetes mellitus…) and neoplastic diseases (for example in lymphomas).

Diagnosis

In a patient with pruritus, it is essential to rule out a dermatologic disease before considering another cause. In addition to the physical examination, it is essential to take a clinical history to detail the time of onset, location, intensity, provoking factors or associated symptoms. It is important to inquire about drugs, alcohol intake or emotional stress. If dermatological disease is ruled out and a systemic disorder is suspected, a blood test, imaging tests (X-ray, ultrasound…) and even a skin biopsy may be necessary. It is important to emphasize that tests are not always necessary, and sometimes it is necessary for some time to pass in order to assess the patient’s evolution and possible response to simple treatment. It should not be forgotten that sometimes the tests may cause discomfort or discomfort to the patient, so the doctor will weigh the need to perform them, what type and at what time.

Treatment

It depends on the originating disease. Although new drugs are appearing thanks to advances in the knowledge of the diseases, it is often frustrating, especially if the underlying disease is not controlled. In addition to skin care with hydration and oral antihistamines, doxepin, mirtazapine, gabapentin and aprepitant have been useful. It is important to emphasize that these drugs must be prescribed by a physician after a complete assessment of the patient. There are also other more specific treatment options depending on the underlying process, which are not detailed here as they are far from the general idea I wish to convey. Just one more note about antihistamines (AH). I have previously mentioned that histamine is a molecule that is involved in itching; it exerts its action when it binds to certain cells. An “antihistamine” drug prevents such binding. There are sedative and non-sedative HAs: this refers to their ability to reach the brain and produce symptoms such as sedation or drowsiness. Although HAs have similar pharmacological properties to each other, their effects depend on both the patient and the type of HA administered. Dr. Beatriz Fleta