Hypermagnesemia is an electrolyte disorder in which serum magnesium levels are above the reference range. The most common cause is renal insufficiency in combination with increased magnesium intake (e.g., from laxatives). Other causes include magnesium therapy, hypothyroidism, and rhabdomyolysis. Mild hypermagnesemia is often asymptomatic but higher magnesium levels (e.g., above 4–5 mg/dL) can cause neuromuscular, gastrointestinal, and cardiovascular symptoms, including ECG abnormalities. In very severe cases, hypermagnesemia can lead to cardiac arrest and death. Concomitant electrolyte imbalances (e.g., hypocalcemia) should be assessed, as they may worsen symptoms. The most important treatment step is the discontinuation of magnesium intake, which is sufficient for most asymptomatic or stable patients. Symptomatic patients may be treated with IV isotonic fluids, IV calcium, and loop diuretics. Acute dialysis should be considered in severe cases.
Hypermagnesemia is a serum magnesium concentration above 2.2–2.4 mg/dL (0.91–1.0 mmol/L). 
Decreased magnesium elimination 
- Renal insufficiency; (e.g., resulting from severe hypovolemia): most common cause
- Familial hypocalciuric hypercalcemia
- Adrenal insufficiency, e.g., Addison disease
Increased magnesium intake 
- Magnesium therapy, e.g., for eclampsia, premature labor, cardiac arrhythmias
- Overdose of drugs containing magnesium, e.g., laxatives including magnesium hydroxide, antacids
Increased tissue breakdown 
- Trauma, e.g., burns, surgery
- Sepsis, shock, cardiac arrest
Renal insufficiency in combination with increased magnesium intake (e.g., from laxatives) is the most common cause of hypermagnesemia.
Mild hypermagnesemia is often asymptomatic; symptoms typically occur if magnesium levels are above 4–5 mg/dL. 
- Reduced deep tendon reflexes
- Blurry vision 
- Muscle paralysis (flaccid quadriplegia)
- Respiratory failure
Because most of the total magnesium in the body is located intracellularly, serum magnesium levels may not accurately reflect total body magnesium levels, and symptoms may not correlate with specific serum levels. 
Very high magnesium levels (e.g., ≥ 10 mg/dL) may lead to respiratory failure, heart block, and/or cardiac arrest! 
Laboratory studies 
- Serum electrolytes
- Renal function tests: often show findings of renal impairment, e.g., ↑ creatinine, ↓ estimated GFR
- Coagulation studies: may show decreased clotting time
- Further studies: Consider depending on the suspected cause of hypermagnesemia (e.g., endocrine testing for adrenal insufficiency).
Hypocalcemia may worsen symptoms of hypermagnesemia because calcium usually antagonizes the effects of magnesium. 
ECG changes are usually seen if magnesium levels are > 7 mg/dL. Findings are nonspecific and may include: 
- ↑ PR interval and AV block
- ↑ QRS duration
- ↑ QT interval
Stable and asymptomatic patients with normal renal function usually do not require medical therapy.
All patients 
- Discontinue magnesium-containing medication and any other magnesium intake.
- Identify and treat the underlying cause of hypermagnesemia, e.g:
Significant and/or symptomatic hypermagnesemia 
- Place under continuous cardiac monitoring.
- Administer IV isotonic crystalloids (e.g., normal saline) to dilute extracellular magnesium.
- Consider loop diuretics, e.g., furosemide.
- If magnesium intake was oral: Consider magnesium-free laxatives or enemas.
- Evaluate for indications for acute dialysis (especially in patients with renal insufficiency).
- In patients with life-threatening hypermagnesemia : Consider calcium administration to antagonize magnesium effects.
Prevention is key in hypermagnesemia. Caution should be used in all patients treated with magnesium-containing medication, especially those with renal insufficiency.