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.