Which of the following statements regarding the use of bowel for urinary diversion is TRUE?
A. Mild metabolic acidosis may be expected in up to half of the patients after ileal conduit diversionAnswer B
Metabolic consequences of the use of bowel for urinary diversion are mainly related to bowel type and length. Considering complications of urinary contact with bowel, the length of time urine is retained, concentration of urinary solutes, urinary pH and osmolality are also important. In an ileal conduit, hydrogen is normally secreted into the lumen in exchange for sodium, whereas bicarbonate is secreted into the lumen in exchange for chloride. In the presence of diluted or hypo-osmolar urine, often seen in the early postoperative period due to low salt intake, one may see a hypovolemic salt-losing state with subsequent acidosis, hyperchloremia and hypokalaemia. Mild metabolic acidosis may be expected in up to 15% of patients after ileal conduit diversion. Due to increased urine contact time and surface the incidence of metabolic acidosis in orthotopic diversions is greater by up to 50%. The principal mechanism leading to the production of acidosis is thought to be ammonium reabsorption in hyperosmolar urine. Chloride and sodium are absorbed, pH in reservoir increases and volume decreases. Absorption of ammonium chloride leads to a hyperchloremic acidosis. Over time, chronic acidosis can lead to bone demineralisation because excess protons in the serum would be buffered against bone minerals. Acidosis may also cause osteoclast activation and potential impairment of vitamin D synthesis. It is important to recognise the clinical signs of metabolic acidosis, such as nausea, lack of appetite, fatigue, weakness and ultimately vomiting. Patients with impaired hepatic and renal function, large bowel surface and long urine-bowel contact are at increased risk of developing this disorder. One should have a high index of suspicion if patients with urinary diversions have non-specific illnesses. Acidosis and electrolyte disturbance should be excluded early. Normal serum pH and bicarbonate do not exclude a severely compensated metabolic acidosis, and blood gas analysis and body weight measurements are required. But many metabolic effects may be subtle and only recognised with continued follow-up (particularly of patients at risk). Metabolic acidosis can be best detected by regular blood gas analysis. The terminal ileum is the sole site of vitamin B12 and of bile acid absorption. If more than 100 cm of distant ileum is resected, lipid malabsorption and therefore also fat-soluble vitamins (A, D, E and K) malabsorption will occur. All patients should be monitored regularly for B12 deficiency following the use of terminal ileum for urinary diversion. If deficiency is confirmed, lifelong supplementation with monthly intramuscular injection is required. The morbidity of radical cystectomy and urinary diversion is up to 75% diversion related.