Hypomagnesemia 

Abnormalities of magnesium levels, such as hypomagnesemia, can result in disturbances in nearly every organ system and can cause potentially fatal complications (eg, ventricular arrhythmia, coronary artery vasospasm, sudden death). Despite the well-recognized importance of magnesium, low and high levels have been documented in ill patients,^[1] as a result of which, magnesium has occasionally been called the "forgotten cation." (See Pathophysiology.)^{[2, 3]}

Magnesium is the second-most abundant intracellular cation and, overall, the fourth-most abundant cation. It plays a fundamental role in many functions of the cell, including energy transfer, storage, and use; protein, carbohydrate, and fat metabolism; maintenance of normal cell membrane function; and the regulation of parathyroid hormone (PTH) secretion. Systemically, magnesium lowers blood pressure and alters peripheral vascular resistance. (See Etiology, Clinical and Workup.)

Almost all enzymatic processes using phosphorus as an energy source require magnesium for activation. Magnesium is involved in nearly every aspect of biochemical metabolism (eg, deoxyribonucleic acid [DNA] and protein synthesis, glycolysis, oxidative phosphorylation). Almost all enzymes involved in phosphorus reactions (eg, adenosine triphosphatase [ATPase]) require magnesium for activation. Magnesium serves as a molecular stabilizer of ribonucleic acid (RNA), DNA, and ribosomes. Because magnesium is bound to adenosine triphosphate (ATP) inside the cell, shifts in intracellular magnesium concentration may help to regulate cellular bioenergetics, such as mitochondrial respiration.

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. Magnesium has been proposed as an endogenous endocrine modulator of the catecholamine component of the physiologic stress response.

Magnesium homeostasis

The total body magnesium content of an average adult is 25 g, or 1000 mmol. Approximately 60% of the body's magnesium is present in bone, 20% is in muscle, and another 20% is in soft tissue and the liver. Approximately 99% of total body magnesium is intracellular or bone-deposited, with only 1% present in the extracellular space. Eighty percent of plasma magnesium is ionized or complexed to filterable ions (eg, oxalate, phosphate, citrate) and is available for glomerular filtration, while 20% is protein-bound. Normal plasma magnesium concentration is 1.7-2.1 mg/dL (0.7-0.9 mmol, or 1.4-1.7 mEq/L).^[4]

The main controlling factors in magnesium homeostasis appear to be gastrointestinal absorption and renal excretion. The average American diet contains approximately 360 mg (ie, 15 mmol) of magnesium; healthy individuals need to ingest 0.15-0.2 mmol/kg/d to stay in balance. Magnesium is ubiquitous in nature and is especially plentiful in green vegetables, cereal, grain, nuts, legumes, and chocolate. Vegetables, fruits, meats, and fish have intermediate values. Food processing and cooking may deplete magnesium content, thus accounting for the apparently high percentage of the population whose magnesium intake is less than the daily allowance.

The plasma magnesium concentration is kept within narrow limits. Extracellular magnesium is in equilibrium with that in the bones and soft tissues (eg, those of the kidneys and intestines). In contrast with other ions, magnesium is treated differently in 2 major respects: (1) no hormonal modulation of urinary magnesium excretion occurs, and (2) bone, the principal reservoir of magnesium, does not readily exchange with circulating magnesium in the extracellular fluid space. This inability to mobilize magnesium stores means that in states of negative magnesium balance, initial losses come from the extracellular space; equilibrium with bone stores does not begin for several weeks.

Magnesium absorption

Magnesium is absorbed principally in the small intestine, through a saturable transport system and via passive diffusion through bulk flow of water. Absorption of magnesium depends on the amount ingested. When the dietary content of magnesium is typical, approximately 30-40% is absorbed. Under conditions of low magnesium intake (ie, 1 mmol/d), approximately 80% is absorbed, while only 25% is absorbed when the intake is high (25 mmol/d). The exact mechanism by which alterations in fractional magnesium absorption occur has yet to be determined. Presumably, only ionized magnesium is absorbed. Increased luminal phosphate or fat may precipitate magnesium and decrease absorption.

In the gut, calcium and magnesium intakes influence each other's absorption; a high calcium intake may decrease magnesium absorption, and a low magnesium intake may increase calcium absorption. PTH appears to increase magnesium absorption. Glucocorticoids, which decrease the absorption of calcium, appear to increase the transport of magnesium. Vitamin D may increase magnesium absorption, but its role is controversial.

Renal handling of magnesium

Unlike most ions, the majority of magnesium is not reabsorbed in the proximal convoluted tubule (PCT). Micropuncture studies, in which small pipettes are placed into different nephron segments, indicate that the thick ascending limb (TAL) of the loop of Henle is the major site of reabsorption (60-70%). The PCT accounts for only 15-25% of absorbed magnesium, and the distal convoluted tubule (DCT), for another 5-10%.^[5] There is no significant reabsorption of magnesium in the collecting duct.^{[6, 7]} Inherited disorders of magnesium transport, although rare, may present through an array of underlying biochemical abnormalities.^{[8, 9]}

In the TAL, magnesium is passively reabsorbed with calcium through paracellular tight junctions; the driving force behind this reabsorption is a lumen-positive electrochemical gradient, which results from the reabsorption of sodium chloride. Claudin-16, also known as paracellin-1, has been identified as the renal tight junction protein in the TAL, where the reabsorption of magnesium occurs.^{[10, 11]} Mutations in the paracellin-1 gene cause a human hereditary disease, familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC), which is characterized by excessive renal magnesium and calcium wasting, bilateral nephrocalcinosis, and progressive renal failure.^{[12, 13, 14]}

In the distal convoluted tubule (DCT), magnesium is reabsorbed via an active, transcellular process that is thought to involve TRPM6, a member of the transient receptor potential (TRP) family of cation channels.^{[15, 16]} Mutations in TRPM6 have been identified as the underlying defect in patients with hypomagnesemia with secondary hypocalcemia (HSH),^{[5, 17, 18, 19]} an autosomal-recessive disorder that manifests in early infancy with generalized convulsions refractory to anticonvulsant treatment or with other symptoms of increased neuromuscular excitability, such as muscle spasms or tetany. Laboratory evaluation reveals extremely low serum magnesium and serum calcium levels.

Interestingly, mutations of the epithelial growth factor (EGF) have been associated with reduced expression of TRPM6^[20] and thus, with hypomagnesemia; colorectal cancer treatment with cetuximab (an EGF receptor inhibitor) also causes hypomagnesemia.^{[21, 22, 23]}

The mechanism of basolateral transport into the interstitium is unknown. Magnesium has to be extruded against an unfavorable electrochemical gradient. Most physiologic studies favor a sodium-dependent exchange mechanism driven by low intracellular sodium concentrations; these concentrations are generated by Na⁺/K⁺ - ATPase, also known as the sodium-potassium pump.

A mutation in the gamma subunit of Na⁺/K⁺ -ATPase is responsible for isolated dominant hypomagnesemia (IDH), an autosomal-dominant condition associated with few symptoms other than chondrocalcinosis.^[24] Patients always have hypocalciuria and variable (but usually mild) hypomagnesemic symptoms. This mutation in the gamma subunit is thought to produce a disturbed routing of the Na⁺/K⁺ -ATPase complex to the basolateral membrane, leading to reduced expression of the Na⁺/K⁺ -ATPase on the cell surface.^{[25, 26]} Consequently, the entry of K⁺ is reduced and the cell depolarizes to some extent, leading to closing of the TRPM6 channel and magnesium wasting.

A variety of factors influence the renal handling of magnesium.^[4] For example, the expansion of the extracellular fluid volume increases the excretion of calcium, sodium, and magnesium. Magnesium reabsorption in the loop of Henle is reduced, probably due to increased delivery of sodium and water to the TAL and a decrease in the potential difference that is the driving force for magnesium reabsorption.

Changes in the glomerular filtration rate (GFR) also influence tubular magnesium reabsorption. When the GFR and, thus, the filtered load of magnesium in chronic renal failure are reduced, fractional reabsorption is also reduced, such that the plasma magnesium value remains normal until the patient reaches end-stage renal disease (ESRD).

Hypercalcemia and hypermagnesemia inhibit magnesium reabsorption through activation of the calcium-sensing receptor (CaSR), a member of the family of G-protein–coupled receptors. The CaSR is expressed in the basolateral membrane of the TAL. When calcium or magnesium activates the receptor, there is a resultant enhancement in the formation of arachidonic acid ̶ derived 20-hydroxyeicosatetraenoic acid (20-HETE), which reversibly inhibits apical potassium channels (ROMK2 channels).^[27]

Secretion of potassium into the lumen via these channels has 2 functions: it provides potassium for sodium chloride reabsorption by the Na-K-2Cl cotransporter (NKCC2), and it makes the lumen electropositive, which permits passive calcium and magnesium reabsorption.^[28] Thus, inhibition of ROMK2 channels in the TAL will reduce active sodium transport and passive calcium and magnesium reabsorption.

Activating mutations of the CaSR result in autosomal-dominant hypocalcemia with hypercalciuria (ADHH), a condition characterized by hypocalcemia, hypercalciuria, and hypomagnesemia and by low, but detectable, levels of PTH.^{[29, 30]}

Phosphate depletion can also increase urinary magnesium excretion, through a mechanism that is not clear.

Chronic metabolic acidosis results in renal magnesium wasting, whereas chronic metabolic alkalosis is known to exert the reverse effects. Chronic metabolic acidosis decreases renal TRPM6 expression in the DCT, increases magnesium excretion, and decreases serum magnesium concentration, whereas chronic metabolic alkalosis results in the exact opposite effects.^[31]

No single hormone has been implicated in the control of renal magnesium reabsorption. In experimental studies, a number of hormones have been shown to alter magnesium transport in the TAL. These include PTH, calcitonin, glucagon, arginine vasopressin (AVP), and the beta-adrenergic agonists, all of which are coupled to adenylate cyclase in the TAL. Postulated mechanisms include an increase in luminal positive voltage (via activation of basolateral membrane chloride conductance and NKCC2) and an increase in paracellular permeability (possibly by the phosphorylation of paracellular pathway proteins). It is not known if these effects have an important role in normal magnesium hemostasis.

Related metabolic abnormalities

Hypokalemia is a common event in patients with hypomagnesemia, occurring in 40-60% of cases.^[1] This is partly due to underlying disorders that cause magnesium and potassium losses, including diuretic therapy and diarrhea.

The mechanism for hypomagnesemia-induced hypokalemia relates to the intrinsic biophysical properties of renal outer medullary K (ROMK) channels mediating K⁺ secretion in the TAL and the distal nephron. ROMK channels represent the first (Kir1.1) of 7 subfamilies making up the 2-transmembrane segment inward-rectifier potassium channel family. The channels are designated as inward rectifiers because they have a greater inward conductance of potassium ions than they do an outward conductance of them at negative membrane potentials (if external and internal K⁺ concentrations are equivalent).^[28]

The mechanism for this differential conductance results from the binding and subsequent cytoplasmic blocking of the outward K⁺ movement through the inward-rectifier conduction pathway by cytoplasmic magnesium and polyamines. A reduction in intracellular magnesium (in the absence of polyamines) results in the loss of inward rectification, thus causing the greater outward conductance of K⁺ ions through the channel pore. Therefore, a decrease in intracellular magnesium concentration in the TAL and collecting duct cells results in increased K⁺ secretion through the ROMK channels.

Evidence also suggests that this wasting may be due to a hypomagnesemia-induced decline in adenosine triphosphate (ATP) and the subsequent removal of ATP inhibition of the ROMK channels responsible for secretion in the TAL and collecting duct.

The classic sign of severe hypomagnesemia (< 1.2 mg/dL) is hypocalcemia. The mechanism is multifactorial. Parathyroid gland function is abnormal, largely because of impaired release of PTH. Impaired magnesium-dependent adenyl cyclase generation of cyclic adenosine monophosphate (cAMP) mediates the decreased release of PTH.^[32] Skeletal resistance to this hormone in magnesium deficiency has also been implicated. Hypomagnesemia also alters the normal heteroionic exchange of calcium and magnesium at the bone surface, leading to an increased bone release of magnesium ions in exchange for an increased skeletal uptake of calcium from the serum.

Arrhythmia

The cardiovascular effects of magnesium deficiency include effects on electrical activity, myocardial contractility, potentiation of digitalis effects, and vascular tone. Epidemiologic studies also show an association between magnesium deficiency and coronary artery disease (CAD).

Hypomagnesemia is now recognized to cause cardiac arrhythmia.^[33] Changes in electrocardiogram findings include prolongation of conduction and slight ST depression, although these changes are nonspecific. Patients with magnesium deficiency are particularly susceptible to digoxin-related arrhythmia. Intracellular magnesium deficiency and digoxin excess act together to impair Na⁺/K⁺ -ATPase. The resulting decrease in intracellular potassium disturbs the resting membrane potential and repolarization phase of the myocardial cells, enhancing the inhibitory effect of digoxin. Intravenous magnesium supplementation may be a helpful adjunct when attempting rate control for atrial fibrillation with digoxin.^[34]

Non–digitalis-associated arrhythmias are myriad. The clinical disturbance of greatest importance is the association of mild hypomagnesemia with ventricular arrhythmia in patients with cardiac disease. At-risk patients include those with acute myocardial ischemia, congestive heart failure, or recent cardiopulmonary bypass, as well as acutely ill patients in the intensive care unit.^[33]

The ionic basis of the effect of magnesium depletion on cardiac arrhythmia may be related to impairment of the membrane sodium-potassium pump and the increased outward movement of potassium through the potassium channels in cardiac cells, leading to shortening of the action potential and increasing susceptibility to cardiac arrhythmia.^[35] Torsade de pointes, a repetitive, polymorphous ventricular tachycardia with prolongation of the QT interval, has been reported in conjunction with hypomagnesemia, and the American Heart Association now recommends that magnesium sulfate be added to the regimen used to manage torsade de pointes or refractory ventricular fibrillation.

Hypertension

It has been suggested that magnesium plays a role in blood pressure regulation, its therapeutic efficacy in the hypertensive syndromes of pregnancy having been demonstrated in the 19th century. Hypertension appears to be uniformly characterized by a decrease in intracellular free magnesium that, due to increased vascular tone and reactivity, causes an increase in total peripheral resistance.

At a cellular level, increased intracellular calcium content is believed to account for this increased tone and reactivity. This increased cytosolic calcium concentration may be secondary to decreased activation of calcium channels, which may enhance calcium current into cells, decrease calcium efflux from cells, increase cellular permeability to calcium, or decrease sarcoplasmic reticulum reuptake of intracellularly released calcium.

Whatever the cause, intracellular accumulation leads to activation of actin-myosin contractile proteins, which increase vascular tone and total peripheral resistance. In contrast to experimental cellular physiology data supporting a role for magnesium deficiency in hypertension, results from clinical epidemiologic studies have failed to confirm a relationship, and results from clinical trials examining the hypotensive effects of magnesium supplementation have been conflicting. Large, carefully performed, randomized clinical trials are needed.

Coronary artery disease

In epidemiologic studies, patients with CAD have a higher incidence of magnesium deficiency than do control subjects.^{[36, 37]} Mounting evidence suggests that magnesium deficiency may play a role in the pathogenesis, initiation, morbidity, and mortality associated with myocardial infarction.

In experimental animals, arterial atherogenesis varied inversely with dietary magnesium intake. In humans, the level of serum magnesium is inversely related to the serum cholesterol concentration. Therefore, magnesium deficiency is associated with hypertension and hypercholesterolemia, which are well-recognized risk factors for atherogenesis and CAD. Magnesium deficiency is also known to be accompanied by thrombotic tendencies, increased platelet aggregatability, and increased coronary artery responsiveness to contractile stimuli. These factors are important in the initiation of acute myocardial infarction.

The incidence of cardiac arrhythmia also correlates with the degree of magnesium deficiency in patients with CAD. Preliminary data suggest that magnesium supplementation may reduce the frequency of potentially fatal ventricular arrhythmia, although this finding has not been conclusively proven.

Hypomagnesemia can also develop during cardiopulmonary bypass and predispose the patient to arrhythmia.^[38] Intravenous magnesium given after the termination of cardiopulmonary bypass has resulted in significantly fewer incidences of supraventricular and ventricular dysrhythmia in relatively small trials of adult^{[39, 40]} and pediatric^[41] patients. Considering the above data, carefully assessing magnesium status in patients with CAD and supplementing patients deficient in magnesium seem prudent. The use of routine magnesium supplements in myocardial infraction remains controversial in the era of thrombolytics and percutaneous coronary interventions.

Neuromuscular manifestations

The earliest manifestations of magnesium deficiency are usually neuromuscular and neuropsychiatric disturbances, the most common being hyperexcitability. Neuromuscular irritability, including tremor, fasciculations, tetany, Chvostek and Trousseau signs, and convulsions, has been noted when hypomagnesemia has been induced in volunteers. Other manifestations include the following:

• Convulsions
• Apathy
• Muscle cramps
• Hyperreflexia
• Acute organic brain syndromes
• Depression
• Generalized weakness
• Anorexia
• Vomiting

Magnesium is required for stabilization of the axon. The threshold of axon stimulation is decreased and nerve conduction velocity is increased when serum magnesium is reduced, leading to an increase in the excitability of muscles and nerves. The cellular basis for these changes is due to increased intracellular calcium content, by mechanisms similar to those described above for hypertension.

Osteoporosis

Magnesium deficiency has also been implicated in osteoporosis.^[42] The magnesium content in trabecular bone is significantly reduced in patients with osteoporosis, and magnesium intake in people with osteoporosis reportedly is lower than it is in control subjects.^[43] (Magnesium intake frequently is lower than the recommended dietary intake in many groups, especially elderly persons.)^[44]

Postmenopausal women are encouraged to consume at least 1000 mg of calcium per day, which leads to altered dietary calcium-to-magnesium ratios. This calcium supplementation may reduce the efficacy of magnesium absorption and further aggravate the consequences of diminished estrogen and the greater demineralizing effects of PTH. The H⁺ -K⁺ -ATPase pump in the cells of the periosteum is magnesium dependent, which may lead to decreased pH in the bone extracellular fluid and increased demineralization. In addition, because the formation of calcitriol involves a magnesium-dependent hydroxylase enzyme, calcitriol concentrations are reduced in magnesium deficiency, possibly affecting calcium reabsorption.

Magnesium supplementation may be beneficial in osteoporosis and may increase bone density, arrest vertebral deformity, and decrease osteoporotic pain. In the large joints, chondrocalcinosis is associated with long-term magnesium depletion.^[45]

Nephrolithiasis

Urinary magnesium is an inhibitor of urinary crystal formation in vivo, and some studies have shown a lower urinary excretion of magnesium in patients with stones. Magnesium deficiency due to etiologies other than renal wasting is associated with hypomagnesuria and, theoretically, could play a role in predisposition to urinary calculus formation.

Diabetes

Patients with diabetes mellitus are often magnesium deficient, expressed by hypomagnesemia.^{[46, 47]} Magnesium deficiency decreases insulin sensitivity and secretion.^{[48, 49]} Moreover, magnesium deficiency is inherently related to the pathogenesis and development not only of diabetic microangiopathy but also of lifestyle-related diseases, such as hypertension and hyperlipidemia.^[50]

Generally, modern people tend to live in a state of chronic dietary magnesium deficiency.^[51] There is a possibility that one of the major factors contributing to the drastic increase of type 2 diabetes mellitus is the drastically decreased intake of grains, such as barley or cereals rich in magnesium.^[52] This implies an association between the volume of dietary magnesium intake and the onset of type 2 diabetes, raising expectations that in the future, clinical trials will be performed to investigate the efficacy of magnesium supplementation therapy.

Miscellaneous conditions

Magnesium deficiency has also been implicated in many other conditions. Low intracellular magnesium levels in the brain have been reported in migraine headache.

In addition, magnesium status may have an influence on asthma, because magnesium deficiency is associated with increased contractility of smooth muscle cells. Magnesium supplementation in asthma remains controversial,^[53] but it has been shown to reduce bronchial hyperreactivity to methacholine and other measures of allergy.^[54]

Magnesium deficiency has also been linked to chronic fatigue syndrome, sudden death in athletes, impaired athletic performance, and sudden infant death syndrome.

Causes of hypomagnesemia related to decreased magnesium intake include the following^[4] :

• Starvation
• Alcohol dependence
• Total parenteral nutrition

Causes related to the redistribution of magnesium from extracellular to intracellular space include the following:

• Hungry bone syndrome
• Treatment of diabetic ketoacidosis
• Alcohol withdrawal syndromes
• Refeeding syndrome
• Acute pancreatitis

Causes related to gastrointestinal magnesium loss include the following:

• Diarrhea
• Vomiting and nasogastric suction
• Gastrointestinal fistulas and ostomies
• Hypomagnesemia with secondary hypocalcemia (HSH)

Causes related to renal magnesium loss include the following, including inherited renal tubular defects^{[12, 14, 55]} and drugs^[56] :

• Gitelman syndrome
• Classic Bartter syndrome (Type III Bartter syndrome)
• Familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC)
• Autosomal-dominant hypocalcemia with hypercalciuria (ADHH)
• Isolated dominant hypomagnesemia (IDH) with hypocalciuria
• Isolated recessive hypomagnesemia (IRH) with normocalcemia
• HSH
• Diuretics - Loop diuretics, osmotic diuretics, and chronic use of thiazides
• Antimicrobials - Amphotericin B, aminoglycosides, pentamidine, capreomycin, viomycin, and foscarnet
• Chemotherapeutic agents - Cisplatin
• Immunosuppressants - Tacrolimus and cyclosporine
• Proton-pump inhibitors^{[57, 58]}
• Ethanol^[59]
• Hypercalcemia
• Chronic metabolic acidosis
• Volume expansion
• Primary hyperaldosteronism
• Recovery phase of acute tubular necrosis
• Postobstructive diuresis

Decreased magnesium intake

Alcoholics and individuals on magnesium-deficient diets or on parenteral nutrition for prolonged periods can become hypomagnesemic without abnormal gastrointestinal or kidney function. The addition of 4-12 mmol of magnesium per day to total parenteral nutrition has been recommended to prevent hypomagnesemia.

Redistribution of magnesium from extracellular to intracellular

The transfer of magnesium from extracellular space to intracellular fluid or bone is a frequent cause of decreased serum magnesium levels. This depletion may occur as part of hungry bone syndrome,^[60] in which magnesium is removed from the extracellular fluid space and deposited in bone following parathyroidectomy or total thyroidectomy.

Hypomagnesemia may also occur following insulin therapy for diabetic ketoacidosis and may be related to the anabolic effects of insulin driving magnesium, along with potassium and phosphorus, back into cells.

Hyperadrenergic states, such as alcohol withdrawal, may cause intracellular shifting of magnesium and may increase circulating levels of free fatty acids that combine with free plasma magnesium. The hypomagnesemia that sometimes is observed after surgery is attributed to the latter.

Hypomagnesemia is a manifestation of the refeeding syndrome, a condition in which previously malnourished patients are fed high carbohydrate loads, resulting in a rapid fall in phosphate, magnesium, and potassium, along with an expanding extracellular fluid space volume, leading to a variety of complications.

Acute pancreatitis can also cause hypomagnesemia. The mechanism may represent saponification of magnesium in necrotic fat, similar to that of hypocalcemia. However, postoperative states^[61] or critical illnesses in general are associated with low magnesium levels,^[62] without pancreatitis necessarily being present.

Gastrointestinal losses

Impaired gastrointestinal magnesium absorption is a common underlying basis for hypomagnesemia, especially when the small bowel is involved, due to disorders associated with malabsorption, chronic diarrhea, or steatorrhea, or as a result of bypass surgery on the small intestine. Because there is some magnesium absorption in the colon, patients with ileostomies can develop hypomagnesemia.

HSH is a rare autosomal-recessive disorder characterized by profound hypomagnesemia associated with hypocalcemia.^[63] Pathophysiology is related to impaired intestinal absorption of magnesium^[64] accompanied by renal magnesium wasting as a result of a reabsorption defect in the DCT. Mutations in the gene coding for TRPM6, a member of the transient receptor potential (TRP) family of cation channels, have been identified as the underlying genetic defect.^{[17, 18, 19]}

Renal losses

Several inherited tubular disorders are responsible for urinary magnesium wasting. Gitelman syndrome is an autosomal-recessive condition caused by mutations of the SLC12A3 gene, which encodes the thiazide-sensitive NaCl cotransporter (NCCT).^[65] This syndrome is characterized by hypokalemia, hypomagnesemia, and hypocalciuria.^[66] Hypomagnesemia is found in most patients with Gitelman syndrome and is assumed to be secondary to the primary defect in the NCCT, but some data points to magnesium wasting as a primary abnormality.^[67] Some studies have indicated that magnesium wasting in Gitelman syndrome may be due to down-regulation of TRPM6 in the DCT.

Classic Bartter syndrome is caused by mutations in CLCNKB ’s encoding of the basolaterally located renal chloride channel ClC-Kb, which mediates chloride efflux from the tubular epithelial cell to the interstitium along the TAL and DCT. It is unknown how hypomagnesemia is produced in this syndrome.

In FHHNC, an autosomal-recessive disorder, there is profound renal magnesium and calcium wasting. The hypercalciuria often leads to nephrocalcinosis, resulting in progressive renal failure.^{[12, 14, 55]} Other symptoms that have been reported in patients with FHHNC include urinary tract infections, nephrolithiasis, incomplete distal tubular acidosis, and ocular abnormalities.^[68] This syndrome is caused by mutations in the gene CLDN16, which encodes for paracellin-1 (claudin-16),^[13] a member of the claudin family of tight junction proteins that form the paracellular pathway for calcium and magnesium reabsorption in the TAL.

ADHH is another disorder of urinary magnesium wasting.^[29] Individuals who are affected present with hypocalcemia, hypercalciuria, and polyuria, and about 50% of these patients have hypomagnesemia. ADHH is produced by mutations of the CASR gene, the gene that encodes for the calcium-sensing receptor (CaSR) located basolaterally in TAL and DCT, which is involved in renal calcium and magnesium reabsorption.^[30] Activating mutations shift the set point of the receptor to a level of enhanced sensitivity by increasing the apparent affinity of the mutant receptor for extracellular calcium and magnesium. This results in diminished PTH secretion and decreased reabsorption of divalent cations in the TAL and DCT, which leads to loss of urinary calcium and magnesium.

IDH with hypocalciuria^[24] is an autosomal-dominant condition associated with few symptoms other than chondrocalcinosis. Patients always have hypocalciuria and variable (but usually mild) hypomagnesemic symptoms. A mutation in the gene FXYD2, which codes for the gamma subunit of the basolateral Na+/K+-ATPase in the DCT, has been identified. This mutation in the gamma subunit is thought to produce a disturbed routing of the Na+/K+-ATPase complex to the basolateral membrane, leading to reduced expression of the Na+/K+-ATPase on the cell surface.^{[25, 26]} Consequently, the entry of potassium is reduced and the cell depolarizes to some extent, leading to closing of the TRPM6 channel and magnesium wasting.

IRH with normocalcemia is an autosomal-recessive disorder in which the individuals who are affected present with symptoms of hypomagnesemia early during infancy. Hypomagnesemia due to increased urinary magnesium excretion appears to be the only abnormal biochemical finding. IRH is distinguished from the autosomal-dominant form by the lack of hypocalciuria.^[69] The mechanism of hypomagnesemia remains unknown.

HSH, also called primary intestinal hypomagnesemia, is an autosomal-recessive disorder that is characterized by very low serum magnesium and low calcium levels.^[63] Mutations in the gene encoding for TRPM6, the active magnesium transporter in the DCT, have been identified.^{[18, 19]} Patients usually present within the first 3 months of life with the neurologic symptoms of hypomagnesemic hypocalcemia, including seizures, tetany, and muscle spasms.

Untreated, HSH may result in permanent neurologic damage or may be fatal. Hypocalcemia is secondary to parathyroid failure and peripheral parathyroid hormone resistance as a result of sustained magnesium deficiency. Usually, the hypocalcemia is resistant to calcium or vitamin D therapy. Normocalcemia and relief of clinical symptoms can be attained by administration of high oral doses of magnesium, up to 20 times the normal intake. As large oral amounts of magnesium may induce severe diarrhea and noncompliance in some patients, parenteral magnesium administration has to sometimes be considered. Alternatively, continuous nocturnal nasogastric magnesium infusions have been proven to efficiently reduce gastrointestinal adverse effects.

Several drugs, such as loop diuretics (including furosemide, bumetanide, and ethacrynic acid), produce large increases in magnesium excretion through the inhibition of the electrical gradient necessary for magnesium reabsorption in the TAL. Long-term thiazide diuretic therapy also may cause magnesium deficiency. Chronic thiazide administration has enhanced magnesium excretion and has specifically reduced renal expression levels of the epithelial magnesium channel TRPM6.^[70] Many nephrotoxic drugs, including aminoglycoside antibiotics, cisplatin, amphotericin B, cyclosporine, and pentamidine, can produce urinary magnesium wasting by a variety of mechanisms, some of which are still unknown. For instance, tacrolimus causes hypomagnesemia through down-regulation of TRPM6 channels.^[71]

On the other side, aminoglycosides are thought to induce the action of the CaSR on the TAL and DCT, producing magnesium wasting.^[72] Some data suggest that magnesium loss associated with cisplatin treatment is mainly the result of lowered intestinal absorption rather than, as presently thought, the result of increased renal elimination.

Other causes of renal magnesium wasting include aldosterone excess, most likely through chronic volume expansion, thereby increasing magnesium excretion; hypercalcemia, due to stimulation of the CaSR and inhibition of magnesium reabsorption; hypophosphatemia, for unknown reasons; and alcohol, most likely due to alcohol-induced tubular dysfunction (which is reversible within 4 weeks of abstinence).^[59]

Finally, magnesium wasting can be seen as part of the tubular dysfunction that is observed with recovery from acute tubular necrosis or during a postobstructive diuresis.

Occurrence in the United States

Although the incidence of hypomagnesemia in the general population has been estimated at less than 2%, some studies have estimated that 75% of Americans do not meet the recommended dietary allowance of magnesium.^[73]

The risk of hypomagnesemia can be summarized as follows:

• 2% in the general population
• 10-20% in hospitalized patients
• 50-60% in intensive care unit (ICU) patients
• 30-80% in persons with alcoholism
• 25% in outpatients with diabetes

Age-related demographics

Although no comprehensive studies have addressed the actual incidence of hypomagnesemia stratified by age group, neonates may be more predisposed to develop the condition. The mechanism for this is unknown, although several studies suggest that neonates have an increased requirement for intracellular magnesium in growing tissues.

Patients should be counseled regarding modification of risks of hypomagnesemia. Such modifications may include maintaining a proper diet, ceasing alcohol consumption, improving diabetic controls, and taking supplements if the cause of hypomagnesemia is still present.

For patient education information, see the Digestive Disorders Center and the Thyroid and Metabolism Center, as well as Chronic Kidney Disease, Celiac Sprue, Alcoholism, and Thyroid Problems.