Chapter 26: Acute oncology
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Hypomagnesaemia
Definition
Hypomagnesaemia is defined as serum magnesium of less than 0.75 mmol/L; normal values of serum magnesium are between 0.75 and 1.5 mmol/L (Efstratiadis et al. [69]).
Related theory
Magnesium is the second in abundance intracellular ion and is known to have a significant role in the cardiovascular system, arterial tension, central nervous system, skeletal muscles and pregnancy (Efstratiadis et al. [69], Martin et al. [166]). Hypomagnesaemia is a life‐threatening condition as it affects nearly every organ of the body but it is usually picked up through routine testing when the patient is being assessed for other reasons. Therefore, most patients are asymptomatic as levels are only mildly depressed (> 0.5 mmol/L). Due to the underlying pathophysiology of hypomagnesaemia it is common for patients to have associated hypokalaemia (occurring in 40–60% of cases), partly due to underlying disorders that cause magnesium and potassium losses, including diuretic therapy, chemotherapy and diarrhoea (Fulop [88]). It is important to acknowledge that in many chemotherapy regimens diuretic therapy is standard, therefore monitoring of magnesium is imperative and, if patients present at risk of dehydration or oedema, review of their magnesium level should be included (Martin et al. [166]).
Assessment
As most cases of hypomagnesaemia are identified through routine blood tests, most patients are asymptomatic on presentation. Hypomagnesaemia is also commonly found in association with hypocalcaemia, hypokalaemia and hyponatraemia, therefore these must be investigated through the addition of these tests within the blood profile. In any patient who presents with an acute deterioration of their condition with unknown or unclear cause the standard assessment should be carried out as stated in the introduction of this chapter. Review of the patient's history, including cancer treatment, will guide further investigations such as inclusion of magnesium in the bloods and undertaking an ECG to look for arrhythmias (Box 26.1).
Box 26.1
Symptoms arising once serum magnesium levels fall below 0.5 mmol/L
Neuromuscular
- Muscular weakness
- Cramping
- Tremors
- Seizure
- Paraesthesias
- Tetany
- Positive Chvostek sign (metacarpal hyperflexion) and Trousseau sign (facial nerve hypersensitivity)
- Vertical and horizontal nystagmus
- Ataxia
- Depression
- Hyperactive deep tendon reflexes
- Altered mental state (in severe cases)
Cardiac
- Tachycardia
- Palpitations
- Non‐specific T‐wave changes – U waves
- Prolonged QT and QU interval
- Repolarization alternans
- Premature ventricular contractions – monomorphic ventricular tachycardia
- Torsades de pointes
- Ventricular fibrillation
- Enhanced digitalis toxicity
Metabolic
- Hypokalaemia
- Hypocalcaemia
Management
Management of hypomagnesaemia is dependent on grade (Table 26.6) however patients with recurrent low magnesium should be encouraged to have a diet rich in sources of magnesium such as green vegetables (spinach), beans and peas, nuts and seeds, and whole, unrefined grains and seafood (Guerrera et al. [99]).
Table 26.6 Grading and management of hypomagnesaemia
| Grade (UKONS classification) | Serum magnesium | Management |
|---|---|---|
| 1 (Green) | <0.5 mmol/L | If patient is asymptomatic consider oral Mg replacement and advise a Mg‐rich diet |
| If patient is symptomatic review other electrolytes and treat as per grade 3 or 4 (red) | ||
| 2 (Amber) | <0.5–0.4 mmo/L |
Consider oral replacement and advise a Mg‐rich diet.
Recheck bloods in 24–48 hours
Correct any other electrolyte imbalance as necessary |
| If patient is symptomatic review other electrolytes and treat as per grade 3 or 4 (red) | ||
| 3 (Red) | <0.4–0.3 mmol/L |
Administer IV Mg 10–20 mmol diluted in 0.9% sodium chloride over 3–6 hours
Correct any other electrolyte imbalance as necessary |
| 4 (Red) Life threatening | <0.3 mmol/L |
Admit for slow IV Mg
If arrhythmias on ECG then admit to ICU for bolus IV Mg |
