Short dialysis, middle molecules, and uremia.

The cause of the uremic syndrome remains unknown; the success of dialysis therapy suggests that retained, dialyzable, but unidentified toxic solutes may play a role. Since chronic peritoneal dialysis seems to prevent or improve uremic neuropathy as well as does hemodialysis, it has been suggested that retained solutes of "middle molecular weight" (500 to 5000 daltons) may be major toxins. Only body fluid concentrations of these larger solutes are presumed to be reduced by peritoneal dialysis as well as with hemodialysis, whereas small solute concentrations are relatively poorly controlled. There have been numberous hemodialysis studies to examine the toxic potential of "middle molecules" as compared with that of smaller solutes. Although the importance of "middle molecules" as toxins remains unproved, numberous factors influencing their concentrations in body fluid have been discovered. Studies have also shown a surprising tolerance of patients to many variations in dialysis strategies. Difficulties in defining adequate dialysis have been intensified.