[Pruritus and dryness of the skin in chronic kidney insufficiency and dialysis patients - a review].

The uremic pruritus is a very painful symptom suffered by chronic haemodialysis patients and is observed in 22 to 74% of the subjects. The causes of uremic pruritus have not yet been clarified. During the last 20 to 30 years it has been focused on altogether 5 different pathophysiological hypotheses: stimulating influences (e.g. calcium phosphate deposits in the epidermis), stimuli (e.g. secondary hyperparathyroidism), neuropathic injuries (e.g. disturbance of the cutaneous innervation in patients with uremic peripheral neuropathy), and central nervous changes (e.g. accumulation of endorphins in uremic patients which is associated with increasing pruritus), and immunologic conditions. The last mentioned immunological hypothesis has increasing importance, not at least based on the fact that the application of a topical calcineurin inhibitor (tacrolimus) improves the uremic pruritus. However, this fact could not be confirmed in a recent prospective placebo-controlled study from the USA. Only after kidney transplantation with a functioning transplant the uremic pruritus is stopped. That is why no causal therapy exists so far. Actually, the uremic pruritus has to be treated by topical and systemic means in a symptomatic and polypragmatic way only. Urea represents one of the most important "natural moisturizing factors" which are responsible for the hydration of the skin. It has been demonstrated that older patients have decreased urea levels within the stratum corneum of the epidermis, whereas in patients with terminal kidney insufficiency - despite dryness of the skin - as a paradox finding elevated levels of urea have been assessed in the stratum corneum. Because of this reason, the meaning of urea as part of the "natural moisturizing factors" system is not understood, until now. However, there are very promising results of clinical phase II studies showing a significant effect of topical application of 2.5% L-arginine hydrochlorid ointment - a semi-essential amino acid - on improvement of dryness and, in particular, on improvement of pruritus in haemodialysis patients.