Metabolic Encephalopathies
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Direct answer
Metabolic encephalopathies are diffuse brain dysfunction caused by systemic derangements such as uraemia, liver failure, hypoglycaemia, hyponatraemia and hypercapnia, rather than by a structural lesion. They present with confusion, drowsiness, asterixis, myoclonus or seizures, and the distinguishing clues lie in the bedside signs and simple blood tests. Correcting the underlying abnormality is the core treatment, and sodium must be corrected slowly to avoid osmotic demyelination.
What you must remember
- Uraemia: asterixis and myoclonus with raised urea and creatinine, treated definitively by dialysis.
- Hepatic encephalopathy: asterixis, foetor hepaticus and a precipitant, managed with lactulose and precipitant control.
- Hypoglycaemia: adrenergic symptoms such as sweating and tremor, then neuroglycopenic confusion or seizures, reversed by intravenous dextrose.
- Hyponatraemia: low serum sodium with headache, nausea, confusion and seizures, where the rise is capped at 8 mmol/L in 24 hours in high-risk patients.
- Hypercapnia: flushing, a bounding pulse and drowsiness with a raised pCO2, confirmed by arterial blood gas rather than pulse oximetry.
- Asterixis: a flapping tremor on wrist extension that is nonspecific, since uraemia and hypercapnia also produce it.
- Osmotic demyelination: the feared result of rapid sodium correction, with high-risk patients including those with hypokalaemia, alcoholism or malnutrition.
- Capillary glucose: checked in every unexplained encephalopathy, because hypoglycaemia is quick to reverse and costly to miss.
Matching the cause to its clue
Metabolic encephalopathies are recognised by the systemic cause rather than a brain lesion, so the work-up starts with bedside clues and a few blood tests. The table pairs each cause with its main clue, its confirmatory test and its first treatment.
| Cause | Distinguishing clue | Confirm and treat |
|---|---|---|
| Uraemia | Asterixis, myoclonus, high urea and creatinine | Renal function; dialysis |
| Hepatic failure | Asterixis, foetor hepaticus, ascites | Liver function; lactulose and precipitant control |
| Hypoglycaemia | Sweating and tremor, quick response to dextrose | Capillary glucose; intravenous dextrose |
| Hyponatraemia | Low sodium, headache, seizures | Serum sodium; slow, controlled correction |
| Hypercapnia | Flushing, bounding pulse, drowsiness | Arterial blood gas; ventilatory support |
Hypoglycaemia is the emergency most often missed, because the confusion can be mistaken for intoxication. Hyponatraemia must be corrected slowly, and severe symptomatic cases may first receive hypertonic saline. Hypercapnia is confirmed by arterial blood gas, because a normal saturation on oximetry does not exclude a rising pCO2 and the result guides ventilatory support.
Where the exam tries to catch you
Examiners test whether you can separate a systemic cause from a structural lesion and avoid the corrections that harm.
- Trap: correcting hyponatraemia briskly to normalise sodium quickly Correct: rapid correction risks osmotic demyelination, so the rise is capped at 8 mmol/L in 24 hours in high-risk patients.
- Trap: missing hypoglycaemia in a confused diabetic Correct: check a capillary glucose in every unexplained encephalopathy, since dextrose reverses the cause within minutes.
- Trap: attributing asterixis only to liver failure Correct: uraemia and hypercapnia also produce asterixis, so the sign must be tied to the blood tests.
Frequently asked questions
Which metabolic cause of encephalopathy must be excluded first?
Hypoglycaemia must be excluded first in any unexplained encephalopathy, because it reverses quickly with intravenous dextrose and causes lasting injury if prolonged. A capillary glucose check takes seconds and should be done at the bedside.
What is the characteristic neurological sign of hepatic encephalopathy?
Asterixis, a flapping tremor on wrist extension, is the characteristic neurological sign of hepatic encephalopathy. It is nonspecific, because uraemia and hypercapnia can produce the same sign, so the blood tests and history must be used alongside it.
How fast should hyponatraemia be corrected to avoid osmotic demyelination?
Hyponatraemia should be corrected slowly, with a rise of no more than 8 mmol/L in 24 hours in high-risk patients. High-risk patients include those with hypokalaemia, alcoholism, malnutrition or advanced liver disease, and severe symptomatic cases may first receive hypertonic saline.
How is hypercapnic encephalopathy diagnosed?
Hypercapnic encephalopathy is diagnosed by arterial blood gas analysis, which shows a raised pCO2 with respiratory acidosis. It presents with drowsiness, confusion, headache and flushing, and pulse oximetry alone cannot establish the diagnosis.
What is the first definitive step in managing uraemic encephalopathy?
Dialysis is the definitive treatment of uraemic encephalopathy, because it removes the solutes responsible for the neurological features. Myoclonus and asterixis usually improve as urea and other toxins are cleared.