A 24 year old nursing student is admitted to hospital. He complains of muscle cramps that have been getting progressively worse since they started six weeks ago. He also complains of increasing weakness over the last few days. He is normally fit and well, and takes no regular medication, prescribed or otherwise. He is a non-smoker and drinks around 10 units of alcohol a week. He denies any illicit drug use.
He is seen on the medical assessment unit. Examination reveals global weakness. Pulse is 68 bpm, with a blood pressure of 115/70 mmHg.
Initial investigations reveal the following:
Na 138 mmol/L
K 2.3 mmol/L
Bicarbonate 34 mmol/L
Creatinine 80 micromol/L (0.9mg/dL)
Calcium 2.4mmol/L (9.6 mg/dL)
Phosphate 0.8 mmol/L (2.48 mg/dL)
He is treated with potassium and you see him on the ward round the following day. You suggest checking his magnesium level which is low at 0.4 mmol/L (0.9 mg/dL).
- What is the differential diagnosis of hypomagnesaemia?
- How will you proceed with your investigations in this man?
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Many thanks to our Programme Director, Professor Neil Turner for this case. We had an interesting discussion in the classroom.
We discussed the differential diagnosis of hypomagnesaemia, and stressed that it rarely occurs as an isolated biochemical abnormality. This patient’s urinary magnesium levels were measured, which confirmed ongoing renal losses in the face of hypomagnesaemia.
So, hypokalaemia, hypomagnesaemia, a modest alkalosis and a clinical picture of weakness and fatigue would all fit with Gitelman’s syndome. It is an autosomal recessive condition where there is a loss-of-function mutation in the sodium-chloride co-transporter (NCCT) of the distal convoluted tubule. This is where thiazide diuretics act, so it is no coincidence that over treatment with thiazides can lead to a similar picture. Several of you mentioned this, and it is important to remember especially given his occupation. Though you would perhaps see less marked hypomagnesaemia. The diagnosis of Gitelman’s would need to be confirmed with genetic testing.