MAGNESIUM DISORDERS

Magnesium is the second most important intracellular ion after potassium, responsible for maintenance of normal cellular enzyme activity for protein and fat metabolism as well as electric activity in nerves and muscles.

Sources: Magnesium is present in plant chlorophyll, with green vegetables, e.g. legumes, nuts and whole grains as predominant source of dietary magnesium. Milk is a poor source.

Physiology: Since gt;99% of magnesium is intracellular, serum levels are unreliable indicators of magnesium status. Dietary intake usually exceeds normal require­ments and physiological magnesium balance is mainly determined by its renal excretion from thick ascending loop of Henle and regulated by: (a) parathormone, (b) thyrocalcitonin, and (c) serum calcium levels. Para­thormone enhances magnesium absorption from the gut and decreases excretion in the urine.

Hypomagnesemia: Although clinical severity may not correlate with serum levels, the term hypomagnesemia denotes S. magnesium levels lt;1.3 mg/dl.

Causes: Magnesium deficiency is rarely dietary except in cases of severe malnutrition or excessive consumption of junk foods (magnesium is eliminated during food processing). Secondary deficiency is common in malabsorption states and renal diseases (Table 7.11).

Clinically hypomagnesemia induces skeletal resistance to parathormone, leading to hypocalcemia. Consequently, hypomagnesemia and hypocalcemia frequently co-exist and present with:

• Neuromuscular irritability, e.g. tetany, seizures and tremors, and

• Cardiac arrhythmia or ECG changes.

Severe hypomagnesemia may lead to mental changes, e.g. irritability and disorientation as well as respiratory paralysis. Hypokalemia and acidosis is also common in severe hypomagnesemia. Hypomagnesemia should always be suspected in a case of tetany, which does not respond to IV calcium therapy.

Treatment: Severe hypomagnesemia or hypomagnesemic tetany is treated with slow infusion of IV magnesium sulphate 50% as 25-50 mg/kg (2.5-5 mg/kg of elemental

TABLE 7.11: Causes of hypomagnesemia

7.6.1