Itch - The major symptom of skin disease and yet still enigmatic.

Abstract

There is no doubt that itch – also called pruritus – is the major symptom of skin diseases; some even use it to define dermatology as the specialty dealing with diseases of all surfaces which are able to itch, from skin to the invisible borders at the level of mucous membranes. In some diseases, itch has even been called the ‘primary lesion’, before visible skin changes appear like in some cases of atopic eczema. Although every human being knows the sensation of itch, the understanding of the etiopathophysiological mechanisms involved in the wide variety of different itch sensations is still incomplete. Until today, we do not have a better definition of itch than the old one given by Samuel Hafenreffer 350 years ago as ‘the untoward sensation inducing the urge to scratch’.1 Shakespeare described itch as the disease of the kings because of the orgiastic feeling of relief during scratching. Everybody knows that there are different kinds of itch; yet, the classification until today remains a matter of debate and is continuously developed by task forces and study groups.2 In the classification of itch, there is agreement to distinguish between itch occurring together with visible skin lesions versus itch without visible skin lesions – formerly called ‘pruritus sine materia’, some also use the term ‘pruritus of unknown origin’ (PUO). Here, systemic diseases of various organs like kidney, liver, nerves and brain but also cancer or systemic drugs may be causal. Generally, when people talk about itchy dermatoses they think of inflammatory skin diseases like atopic eczema, urticaria, lichen planus and bullous pemphigoid, but also psoriasis can itch, and this has been underestimated. Of course, contagious skin conditions like scabies are highly pruritic dermatoses, but also genetically determined skin diseases can go along with severe pruritus as is discussed in this issue in an article focussing on quite different genodermatoses such as epidermolysis bullosa, several forms of ichthyosis, Darier’s disease and Hailey–Hailey disease, but also immunodeficiency syndromes.3 The difficulty in classification is evident in conditions such as so-called pruritus senilis, itch in the elderly, where the xerosis could be regarded as altered skin, yet it remains to be defined what is physiological and or pathophysiological. The major problem in itch research is the subjective nature of the sensation which makes it difficult to be measured objectively. A variety of questionnaires and visual analogue scales, as well as imaging techniques studying activation of certain brain areas triggered by itch-inducing stimuli, have been used. As difficult as it is to measure itch quantitatively, it becomes even more difficult to distinguish between different qualities of itch. Here, we depend upon the description by the individuals experiencing the sensation and their language used; obviously, there are differences in language when they describe subjective/passive sensations and active words for the inducer, for example tickle. In some languages or dialects, the situation is described in an active sense like ‘I am itching myself’ (instead of correctly stating ‘it itches, therefore, I scratch myself’). On the skin, itch itself is not visible, but only the result of mechanical manoeuvers, such as scratching but also pinching, rubbing, pressing and others, is observed. Since itch is a very personal sensation accompanied by intense emotional feelings, the degree and experience of itch can often be better described in dialect than in the plain official language.4 In spite of big gaps in understanding the complex pathophysiology of itch, there is tremendous progress in therapeutic options reflected in guidelines and position statements including topical, systemic, anti-inflammatory, immunosuppressive, psycho-neurologic, phototherapeutic as well as psychologic and educational strategies.2, 5, 6