International Academy of Cosmetic Dermatology
Itching (pruritus) is a sensation that produces the desire to scratch; it is exclusive to the skin and the cornea. Teleologically, itching, with resultant scratching, is of possible biological value in combating parasitic infestation.
Pruritus probably originates in the specialized fine afferent unmyelinated C fibers located close to the dermal-epidermal junction. This sensation is transmitted via the lateral spinothalamic tract to the thalamus and to the sensory cortex. It is mediated by the release of a variety of chemical substances: histamine, seritonin, tryptase papain, and probably substance P.
Thus, itching is a predominant symptom of skin diseases and a frequent manifestation of systemic or psychogenic diseases. Itching, associated with skin diseases, is usually accompanied by a distinctive eruption (lichen planus, psoriasis, urticaria etc) but not always (atopic dermatitis, pre-bullous pemphigoid, etc.). In systemic diseases that causes itching, such as liver disease, uremia , hypo and hyperthyroidism, and iron deficiency anemia, the only skin findings may be excoriations due to scratching scratching. It is noteworthy that several malignant diseases are sometimes accompanied by severe pruritus and even skin burning: polycythemia vera, lymphoma (especially Hodgkin’s disease), leukemia, myeloma, carcinoid, and other internal malignancies. Other common forms of itching include senile pruritus and drug-induced itching.
Batya Davidovici, MD