HYPOMAGNESAEMIA DEFINITION A serum magnesium concentration below the normal range (typically 0.8 to 1.2 mmol/L, 1.6 to 2.4 mEq/L, 2.0 to 2.6 mg/dL). TOXIC CAUSES Alcoholism Amphotericin Aminoglycosides Beta agonists Cisplatin Cyclosporin Diuretics Hydrofluoric acid Laxatives Pentamidine Theophylline NON-TOXIC CAUSES Congenital renal magnesium wasting Diabetes mellitus Hyperparathyroidism Hyperthyroidism Inadequate magnesium intake Pancreatitis Primary hyperaldosteronism Prolonged diarrhoea Prolonged nasogastric suction Renal transplantation Total parenteral nutrition CLINICAL FEATURES The clinical features are non-specific, variable and not well correlated with serum magnesium concentration. Initially, anorexia, nausea, vomiting, lethargy and weakness may develop. The principal symptoms of magnesium deficiency consist of paresthesiae, muscular cramps, irritability, decreased attention span and mental confusion. Physical findings reflect associated hypocalcaemia and may include positive Trousseau's and Chvostek's signs, tremor, hyperreflexia, peculiar movements of the fingers described as "athetoid tetany" and, sometimes, convulsions. Cardiac arrhythmias, disturbances of conduction, ventricular fibrillation and cardiac arrest can occur in patients with coexisting hypokalaemia. DIFFERENTIAL DIAGNOSIS Hypocalcaemia Hypokalaemia Neurologic disorders with increased deep tendon reflexes (e.g. progressive primary muscular atrophy) Seizures due to other causes RELEVANT INVESTIGATIONS Serum magnesium Serum calcium, phosphorus Serum sodium, potassium, chloride, bicarbonate Renal function tests (urea, creatinine) ECG Arterial blood gas analysis TREATMENT In treating magnesium deficiency, it is important to detect and correct any associated potassium and calcium deficiencies. In mild magnesium deficiency, restoration of body stores occurs quickly after providing a diet high in magnesium. In more severe magnesium deficiency, parenteral administration of magnesium salts is safe and effective but must be used cautiously in patients with renal insufficiency. Initial treatment requires 8 to 12 g of intravenous magnesium sulfate in divided doses over the first 24 hours, followed by 4 to 5 g daily for 3 to 4 days. It is important to replete magnesium stores in patients with hypomagnesaemia but not to provide an excess. Magnesium oxide is typically supplied as 600 mg tablets containing 30 mEq/L of magnesium per tablet. Several days of 4 to 6 tablets per day should be sufficient to restore the deficit in most patients. Administration of oral magnesium can cause diarrhoea. CLINICAL COURSE AND MONITORING The management of moderate-to-severe hypomagnesaemia should include admission to hospital with monitoring of haemodynamic status, neurologic status and serum electrolytes. During replacement therapy, serum magnesium and deep tendon reflexes should be monitored closely, especially in patients with renal insufficiency. If the patient becomes weak or loses deep tendon reflexes, stop the infusion immediately. LONG-TERM COMPLICATIONS None AUTHOR(S)/REVIEWERS Authors: Dr Tim Meredith and Dr Yeong-Liang Lin Center for Clinical Toxicology Vanderbilt University Medical Center Nashville, USA. Reviewers: Rio de Janeiro 9/97: J.N. Bernstein, E. Birtanov, H. Hentschel, T.M. Meredith, Y. Ostapenko, P. Pelclova, C.P. Snook, J. Szajewski Birmingham 3/99: B. Groszek, H. Kupferschmidt, N. Langford, K. Olson, J. Pronczuk.