Like potassium, magnesium is one of the major cations of the body responsible for neurotransmission and neuromuscular excitability. Regulation of magnesium balance is mainly by the kidneys.
Hypomagnesaemia may result from failure to supply adequate intake, from excess NG drainage or suctioning or in acute pancreatitis. It is usually accompanied by a loss of potassium. The patient may become confused and irritable, with muscle twitching.
Hypomagnesaemia should also be suspected in association with other fluid and electrolyte disturbances when the patient develops unexpected neurological features or cardiac arrhythmias.
Magnesium sulphate has long been the mainstay of treatment for pre-eclampsia/eclampsia in America, but the practice in the UK until recently has been to use more specific anticonvulsant and antihypertensive agents. A large international collaborative trial shows a lower risk of recurrent convulsions in eclamptic mothers given magnesium sulphate compared with those given diazepam or phenytoin.