Hyperchloraemic acidoses

Based on their associated biochemical derangements, which one of the following

surgical pathologies is the odd one out?

a) Pyloric stenosis

b) Enteric fistula

c) Ureterosigmoidostomy

d) Toxic megacolon

e) Villous adenoma of the rectum

Answer: a

Explanation

Pyloric stenosis is associated with a hypochloraemic metabolic alkalosis. Enteric fistulas,

ureterosigmoidostomies, severe diarrhoea (e.g. that caused by inflammatory bowel

disease flare) and villous adenomas are all responsible for hyperchloraemic acidoses.

In pyloric stenosis, persistent vomiting with loss of stomach contents rich in hydrochloric

acid causes the alkalaemia and hypochloraemia. Potassium ions are lost at

the kidneys in favour of preserving hydrogen ions to correct the alkalosis. This, as

well as potassium ion loss in the vomitus, means hypokalaemia is also a problem.

Hypernatraemia may be present if the patient is very dehydrated but hyponatraemia

may also be seen. In health, the acidic stomach contents are usually rendered neutral

soon after arrival in the duodenum by the abundant bicarbonate ions secreted by

the pancreas, biliary system and duodenum. These bicarbonate ions are effectively

reabsorbed in the jejunum so minimal bicarbonate is ultimately lost in the faeces. Any

diarrhoeal state where this process is jeopardised results in bicarbonate loss.

Alternatively if bowel contents, biliary or pancreatic juices are externally drained,

bypassing reabsorption, a similar consequence ensues. Villous adenomas secrete

bicarbonate-rich mucus. Renal chloride retention, resulting in hyperchloraemia, is a

compensatory response to the bicarbonate loss.