HYPERMAGNESAEMIA DEFINITION A serum magnesium concentration above 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 States of magnesium excess usually result from overzealous therapeutic administration of magnesium or from conventional doses in the presence of impaired renal function. NON-TOXIC CAUSES Adrenal insufficiency End-stage renal disease Familial benign hypocalciuric hypercalcaemia. Rhabdomyolysis CLINICAL FEATURES Magnesium reduces neuromuscular transmission and acts as a central nervous system depressant. Nausea usually appears at 3 to 5 mEq/L. Sedation, hypoventilation with respiratory acidosis, decreased deep tendon reflexes and muscle weakness appear at 4 to 7 mEq/L. Hypotension, bradycardia and diffuse vasodilatation appear at 5 to 10 mEq/L. Respiratory paralysis occurs at 10 to 15 mEq/L. DIFFERENTIAL DIAGNOSIS Bradycardia due to other causes (e.g. beta-adrenoceptor antagonists, digoxin, myocardial infarction). Disturbance of consciousness due to other causes (e.g. drug overdose). Muscle weakness due to other causes (e.g. muscular dystrophies). RELEVANT INVESTIGATIONS Serum electrolytes, blood urea, creatinine, glucose and arterial blood gases are helpful in determining the diagnosis and in evaluating the severity of this disorder. An ECG may be indicated to assess any cardiac dysrrhythmias. TREATMENT Treatment of hypermagnesaemia is directed firstly at removal of the source of magnesium and secondly at enhancing removal if the serum concentration of magnesium poses a threat to survival. Infusion of calcium will produce a rapid but short-lived reduction in the serum magnesium and often dramatic improvement in the patient's clinical condition. High serum concentrations of magnesium in the presence of impaired renal function may require haemodialysis. CLINICAL COURSE AND MONITORING Magnesium concentrations should be monitored during therapeutic intervention. If the concentration does not return to the normal range, further correction may be necessary. Fluid balance, electrolytes, cardiac status and acid-base balance should also be monitored. LONG-TERM COMPLICATIONS None. AUTHOR(S)/REVIEWERS: Authors: Tim Meredith and Yeong-Liang Lin Center for Clinical Toxicology Vanderbilt University Medical Center Nashville, USA. Reviewers: Rio de Janeiro 9/97: J.N. Bernstein, E. Birtanov, R. Fernando, H. Hentschel, T.J. Meredith, Y. Ostapenko, P. Pelclova, C.P. Snook, J. Szajewski.