Understanding Dialysis Disequilibrium Syndrome (DDS) - "Reverse Urea Effect"

Urea usually has a lag of inherent delay of entry into the brain, means that urea can function as a temporary effective osmole, which couples the faster movement of water in the opposite direction. This effect is called the “urea effect”.

Conversely, when urea is removed from the blood after equilibration between the blood and the brain has been achieved, the lag in the exit of urea from the brain into the blood can draw water into the brain, thus engendering cerebral edema. This process has been termed the “reverse urea effect”. To make things worse, the lag in urea exit from the brain may be magnified in patients with renal failure.
DDS is related to the “reverse urea effect”. The syndrome was attributed to the delayed exit of urea from the brain in the face of a rapid dialysis-induced decline in blood urea level, thus creating an osmotic gradient that favored the shift of water into the brain from the blood.