Many metabolic disturbances cause an acquired encephalopathy in adults. The most frequently encountered are:
- Hypoxic Less commonly:
- Hypercapnoeic - Hyponatremia. Hypernatremia.
- Hypoglycaemic - Hypokalemia. Hyperkalaemia.
- Hyperglycaemic - Hypocalcaemia. Hypercalcaemia.
- Hepatic - Hypothyroidism. Lactic acidosis.
- Uraemic - Addison's disease.
Drugs and toxins producing encephalopathy are dealt with separately (page 512).
Laboratory assessment of suspected metabolic encephalopathy
All patients should have a basic biochemical screen:
- Serum urea and electrolytes.
- Liver function (albumin, globulin, bilirubin, alkaline phosphatase and enzymes) and random blood glucose.
- Serum ammonia.
- Electroencephalography - slow wave activity (theta or delta) supports the diagnosis of a diffuse dysfunction: hepatic encephalopathy shows a specific triphasic slow wave configuration.
- CT scan - if the above tests are normal or coexisting structural brain disease is suspected.
Calculation of the anion gap may be helpful in the diagnosis of encephalopathies, especially lactic acidosis. The sum of the anions (CP and HCO3) normally equals the sum of the cations (Na" and K + ). An increase in the gap in the absence of ketones, salicylates and uraemia suggests lactic acidosis.
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