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Pediatric Hypermagnesemia

Background

Magnesium is the second most abundant intracellular cation and the fourth most abundant cation overall. Almost all enzymatic processes using phosphorus as an energy source require magnesium for activation.
Magnesium is involved in most biochemical reactions such as glycolysis and oxidative phosphorylation. Because magnesium is bound to adenosine triphosphate (ATP) inside the cell, shifts in intracellular magnesium concentration may help to regulate cellular bioenergetics.

Extracellularly, magnesium ions block neurosynaptic transmission by interfering with the release of acetylcholine. Magnesium ions also may interfere with the release of catecholamines from the adrenal medulla. In fact, magnesium has been proposed as being an endogenous endocrine modulator of the catecholamine component of the physiologic stress response.

Approximately 60% of total body magnesium is located in the bone, and the remainder is in the soft tissues. In this soft tissue intracellular compartment, which comprises about 38% of total body magnesium, relatively higher concentrations are found in the skeletal muscle and the liver. Because less than 2% is present in the extracellular fluid (ECF) compartment, serum levels do not necessarily reflect the status of total body stores of magnesium.

The reference range of serum concentration of magnesium is 1.8-2.5 mEq/L. Approximately one third of this magnesium is protein-bound. Analogous to plasma calcium, the free (ie, unbound) fraction of magnesium is the active component. Unfortunately, ionized serum magnesium cannot accurately be assessed at this time.

Less than 40% of dietary magnesium is absorbed; absorption takes place throughout the small intestine (predominantly in the ileum) and in the colon. A minimum daily intake of magnesium of 0.3 mEq/kg of body weight has been suggested to prevent deficiency. However, infants and children tend to have higher daily requirements of magnesium.

Elimination is predominantly renal; the threshold for urinary excretion nears the reference range of serum concentration. Thus, when serum levels are greater than 2.5 mEq/L, magnesium excretion dramatically increases. Conversely, the kidney retains a strong capacity to resorb magnesium, and the main site for reabsorption is the thick ascending loop of Henle.

(A) Magnesium reabsorption in the thick ascending

(A) Magnesium reabsorption in the thick ascending limb of the loop of Henle. The driving force for the reabsorption against a concentration gradient is a lumen-positive voltage gradient generated by the reabsorption of NaCl. FHHNC = Familial hypomagnesemia with hypercalciuria and nephrocalcinosis. ADH = autosomal dominant hypocalcemia. FHH/NSHPT = Familial hypomagnesemia/neonatal severe hyperparathyroidism. (B) Magnesium reabsorption in the distal convoluted tubule. Active transcellular transport is mediated by an apical entry through a magnesium channel and a basolateral exit, presumably via a Na+/Mg2+ exchange mechanism. HSH = Hypomagnesemia with secondary hypocalcemia. GS = Gitelman syndrome. IDH = Isolated dominant hypomagnesemia. Source: Konrad M, Schlingmann KP, Gudermann T: Insights into the molecular nature of magnesium homeostasis. Am J Physiol Renal Physiol 2004; 286: F599-F605.

Renal reabsorption is impaired by several factors such as volume expansion, ethanol ingestion, hypercalcemia, and diuretic administration (eg, osmotic, thiazide, loop). Of these 3 types of diuretics, loop diuretics have a greater effect on renal magnesium wasting because of their site of action.

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