Thursday, 19 June 2008

CT scanning, Magnesium Hydroxide and Collapse!

Dear Bloggers

An 80 year old patient was admitted with collapse. He was found to have severe bradycardia and hypotension requiring emergency transvenous pacing which was successful.

On later assessment, the patient was found to have new onset renal failure whereas two weeks prior his renal function had been normal.

The patient was found to have hypercalcaemia and hypermagnesaemia. The hypercalcaemia had been appropriately worked up with tests for PTH, myeloma screen, vitamin D, thyroid function and so on. However, there had been no assessment of the hypermagnesaemia.

The cardiac enzymes had been completely normal.

The patient had been previously admitted into the hospital two weeks prior with a urinary tract infection which was successfully treated and the patient was discharged back home.

So essentially, what were the causes of the complete heart block, the new onset renal failure, the hypercalcaemia and hypermagnesaemia?

This is why the history is so, so, so important. The chronological set of events can provide the answer and doing additional reading can help the physician understand what may have gone wrong.

Following the chronology here, the patient received a contrast CT scan prior to discharge and no further renal function tests were performed. Then on the re-admission, the FeNa test revealed an intra-renal defect rather than pre-renal defect. The renal ultrasound scan was within normal limits ruling out obstruction.

Hence, a contrast induced nephropathy was highly suspected in this case.

However, in most cases of contrast induced nephropathy, there is usually only a modest rise in creatinine and it is usually self-limiting after about 5 days. About 5% of patients who develop this problem actually have permanent renal damage requiring dialysis. Hence, the renal failure in this case could be due to another unseen problem.

Why the hypercalaemia? Well, renal failure can result in parathyroid hormone abnormalities and cause hypercalcaemia. On the other hand, it would seem to have occurred too soon for such an acute renal insult. Hypercalcaemia itself can result in renal failure. Conditions such as multiple myeloma can result in amyloid kidney, light chain deposition and hypercalcaemia all causing renal failure. The patient could simply have a parathyroid adenoma causing the problem! However, at the bedside the patient had digital clubbing suggesting that malignancy could be yet another cause!

Why the hypermagnesaemia??

Well, this is the most interesting part. When the drug list was examined in detail, the patient was using magnesium hydroxide for constipation. Levels of >5mmol/L (this patient's was >6mmol/L) can cause prolongation of the P-R interval, widening of the QRS interval, increased T-wave amplitude leading to cardiac arrest. Hypotension is yet another feature. 
Hypermagnesaemia occurs especially when there is a combination of taking magnesium containing products e.g. purgatives (as in this case), with the presence of, yes you guessed it, renal failure !

Hence, tracing the set of events backwards it was possible to work out the likely causes to therefore work out why the patient developed heart block and collapse.

The lessons learned here include:

  • Only do a CT scan when it is absolutely necessary; even then, in the elderly, be mindful about using contrast even in patients with seemingly normal renal function. Ensure that the patient remains well hydrated and follow-up blood tests are done to see if renal function deteriorates. 
  • Try and think if you can get the same information in a different way e.g. waiting to do a colonoscopy rather than CT abdomen in cases of lower GI haemorrhage.
  • Hypermagnesaemia can occur with magnesium hydroxide for treating constipation especially with concomitant renal failure ! 
  • Once the acute problem e.g. complete heart block, has been effectively treated (in this case transvenous pacemaker insertion), the underlying cause must be investigated.
  • Every problem whether it be a history problem, a physical examination problem, a radiological problem or a lab data abnormality MUST have an assessment and plan in order to avoid missing obvious and treatable causes of disease. Without an assessment there can be no treatment or further investigations.
It is entirely possible in this patient that the constipation was caused by the hypercalcaemia and use of purgatives resulted in the patient's heart block and collapse exacerbated by iatrogenic renal dysfunction.

My advice as always is to thoroughly analyse the history and its chronology, and moreover, check the drugs !!!!!!!!! In this case, a very commonly used drug caused a serious problem.

The current evidence suggests that either oral n-acetylcysteine or intravenous bicarbonate which are given before and after the contrast-requiring procedure, may reduce the incidence of contrast induced renal failure with similar numbers needed to treat of about 8. Please consider in at risk patients.

Treatment for hypermagnaesemia is circulatory support with intravenous fluid, cardioprotective calcium gluconate, furosemide (which causes magnesium loss via the kidneys) or failing that, dialysis.

Please consider...

p.s. the results to the latest mind boggling case will be available soon.... keep reading!

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