Magnesium Flashcards
Magnesium and Potassium
Magnesium deficiency may also cause hypokalaemia. In such cases, normalizing the potassium level may be difficult until the magnesium deficiency has been corrected
Alcoholism and Hypomagnesaemia - Example Question
You are asked to review a 42- year- old alcoholic who has been admitted to the medical ward following 48hrs of vomiting, generalised muscle weakness and palpitations. Despite two calcium infusions, the most recent measured calcium is still 1.89 mmol/l. On examination his blood pressure is 95/60 mmHg, pulse is 95 beats per minute and regular. You note intermittent runs of SVT on his cardiac monitor.
Other urea and electrolytes are shown below:
Na+ 132 mmol/l K+ 3.7 mmol/l Urea 5.4 mmol/l Creatinine 82 µmol/l Glucose 5.2 mmol/l
Which of the following is the most appropriate next step?
IV calcium > IV magnesium IV potassium IV phosphate IV glucose
Long term alcoholism as well as leading to falls in serum calcium, can also lead to significant falls in magnesium, which can account both for the persistently decreased calcium despite replacement, and for the runs of SVT seen here. The most appropriate next step is magnesium replacement, which is likely to facilitate both an improvement in calcium and resolution of SVT.
Further IV calcium will be ineffective without first replacing magnesium, and potassium is within the normal range, as is glucose. Phosphate replacement may be required in the treatment of alcoholism, but this is usually in the context of refeeding syndrome.
Hypomagnesaemia leading to Hypocalcaemia - Example Question
A 45-year-old woman with chronic alcohol abuse admitted 3 days ago for nausea and severe diarrhoea now complains of peri-oral and finger tingling. She was admitted for hydration after 1 week of severe watery diarrhoea. She has been receiving intravenous hydration and dextrose but has not been able to take oral nutrition secondary to continued nausea. Her blood pressure is 130/74 mmHg, pulse is 68/min, and respiratory rate is 16/min. She is afebrile.
Physical examination is significant for facial twitching on percussion of her facial nerve just anterior to the ear, as well as the induction of carpal spasm after the inflation of a blood pressure cuff on her arm.
Which of the following is most likely to have caused these findings?
Hyperuricaemia Hypernatraemia > Hypomagnesaemia Hypophosphataemia Hypouricaemia
This patient is displaying classic signs of hypocalcaemia, including hyperexcitability of her facial nerve (Chvostek’s sign), induced carpal spasm (Trousseau’s sign), and tingling of the extremities and lips. Calcium homeostasis is a complicated process involving PTH, vitamin D, albumin and numerous electrolytes. Acquired hypoparathyroidism is the most common form of true hypocalcaemia, most often occurring transiently after thyroid surgery or after the removal of a parathyroid adenoma. Occasionally, hypomagnesaemia can produce hypocalcaemia by decreasing both the body’s production of PTH and its sensitivity to the hormone. In this case, it is likely that the patient became magnesium depleted from her course of watery diarrhoea, likely baseline poor nutritional status and alcohol abuse.
Choice 1: Hyperuricaemia is not a cause of hypocalcaemia. Chronic kidney disease, however can lead to hypocalcaemia in the setting of secondary hyperparathyroidism, but there is no evidence of renal failure in this patient.
Choice 2: Fluid balance (hyper- or hyponatraemia) does not play a role in calcium homeostasis.
Choice 4: Hypophosphataemia is not a cause of hypocalcaemia. Actually, hypocalcaemia often leads to hyperphosphataemia secondary to increased PTH-mediated bone resorption. Elevations in phosphate may also contribute to hypocalcaemia by complexing with circulating calcium and suppressing conversion of 25-OH to 1, 25-OH vitamin D.
Choice 5: Urate levels do not affect calcium homeostasis.
Hypomagnesaemia - Example Question
A 68-year-old Indian patient presents to the emergency department with facial tetany, muscle cramps and paraesthesia of her fingers and toes. This is her second admission with similar symptoms. Her past medical history includes diffuse cutaneous systemic sclerosis with gastrointestinal, cutaneous and pulmonary manifestations. She was also diagnosed with vitamin D deficiency two years ago and receives regular vitamin D supplements. Her blood tests are as follows:
Hb 124 g/l WBC 8.0 * 109/l Na+ 141 mmol/l K+ 4.3 mmol/l Urea 6.5 mmol/l Creatinine 90 µmol/l CRP 15 mg/l Corrected calcium 1.68 mmol/l Phosphate 1.4 mmol/l Magnesium 0.28 mmol/l PTH 2 pmol/L (normal range = 8.5-12) Amylase 14 u/l
Her symptoms improve with intravenous calcium replacement and intravenous magnesium replacement, correcting both electrolytes to within normal range. What is the underlying cause for these metabolic disturbances in this patient?
> Hypomagnesaemia Primary hypoparathyroidism Insufficient vitamin D supplementation Chronic kidney failure Chronic pancreatitis
This complex picture investigates the underlying cause of hypomagnesaemia and hypocalcaemia in a patient with significant GI disease. With regular vitamin D supplementation, it is unlikely this is the cause. Her renal function is also within normal range. Although her parathyroid hormone levels are low, the likely underlying cause is due to insufficient magnesium absorption due to GI systemic sclerosis, which results in reduces parathyroid hormone release. There is nothing in the history to suggest a primary hypoparathyroidism or chronic pancreatitis.
Hypomagnesaemia - Causes
Causes of Low Mg2+:
- Diuretics
- TPN
- Diarrhoea
- Alcohol
- Hypokalaemia
- Hypocalcaemia
- Conditions causing Diarrhoea = Crohns, UC
- Metabolic Disorders = Gitelman’s, Bartter’s
- Decreased absorption e.g. Limited Systemic Sclerosis
Hypomagnesaemia - Features
- Paraesthesiae
- Tetany
- Seizures
- Arrhythmias
- Decreased PTH secretion > Hypocalcaemia
- ECG Fx similar to those of Hypokalaemia
- Exacerbates digoxin toxicity
Hypomagnesaemia Mx
If < 0.4mmol/L = IV Replacement e.g. 40mmol IV Magnesium Sulphate over 24h
If > 0.4mmol/L = Oral Mg2+ Salts (10-20mmol PO/d)
NB Diarrhoea can occur with Magnesium oral salts
