Other frequent complications include Restless legs syndrome Cramps Pruritus Nausea and vomiting Headache Chest and back pain In most cases, these complications occur for unknown reasons, but some may be part of a first-use syndrome (when the patient's blood is exposed to cuprophane or cellulose membranes in the dialyzer) or dialysis dysequilibrium syndrome, a syndrome thought to be caused by too-rapid removal of urea and other osmolytes from the serum, causing osmotic movement of fluid into the brain.