Hypomagnesemia

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Hypomagnesemia

• Ginny Barton, M3
• Gyn/Onc Presentation
• February 2006

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Magnesium

• 4th most common cation in the body
• 2nd most abundant intracellular cation after K
• plays a fundamental role in many functions of the
  cell
• Involved in regulation of PTH secretion
• Systemically, Mg lowers blood pressure and alters
  peripheral vascular resistance

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More on Magnesium

• The total body magnesium level of an average
  adult is 25 g
• Approximately 60 is present in bone, 20 in
  muscle, and 20 in soft tissue and the liver
• Approximately 99 of the total body magnesium is
  intracellular
• Normal plasma magnesium concentration is 1.7-2.1
  mg/dL

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Mg Homeostasis

• The main determinates are gastrointestinal
  absorption and renal excretion
• Healthy individuals need to ingest 0.15-0.2
  mmol/kg/d to stay in balance
• Extracellular magnesium is in equilibrium with
  that in the bone, kidneys, intestine, and other
  soft tissues
• There is no hormonal modulation of urinary
  magnesium excretion
• Bone, the principal reservoir of magnesium, does
  not readily exchange with circulating magnesium
  in the extracellular fluid space

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Causes of Hypomagnesemia

• Malabsorption of Mg in the ileum
• GI secretions in large amounts
• Malnutrition with low dietary intake of Mg
• Renal losses from primary renal disorders or
  secondary causes
• Extracellular volume expansion
• Redistribution of Mg into cells (Insulin)
• Pregnant women have been found to be Mg depleted
  (esp those who experience PTL)

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More on Renal losses of Mg

• Primary renal disorders cause hypomagnesemia by
  decreased tubular reabsorption of magnesium by
  the damaged kidneys
• Drugs may cause magnesium wasting
• Endocrine disorders may cause hypomagnesemia
• Osmotic diuresis results in magnesium loss in the
  kidney

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Evaluation of Hypomagnesemia

• History
• Physical
• Labs
• EKG and Cardiac Monitoring
• Other tests as needed

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History

• Clues to the presence of hypomagnesemia can be
  found by obtaining a history of potential causes
• Historical complaints related to hypomagnesemia
  are nonspecific
• Altered mental status may be present in severe
  cases

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Physical Findings

• Neuromuscular irritability
• -Hyperactive deep tendon reflexes
• -Muscle cramps
• -Muscle fibrillation
• -Trousseau and Chvostek signs
• -Dysarthria and dysphagia from esophageal
  dysmotility

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Physical Findings (cont)

• CNS hyperexcitability
• -Irritability and combativeness
• -Disorientation
• -Psychosis
• -Ataxia, vertigo, nystagmus, and seizures (at
  levels lt1 mEq/L)

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More Physical Findings

• Cardiovascular
• -Atrial Fibrillation
• -Ventricular arrhythmias
• -Repolarization Alternans
• -Coronary Artery Vasospasm
• -Sudden Death

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Neonatal Physical Findings

• Apnea
• Weakness
• Seizures
• Jitteriness

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Labs

• Serum magnesium, calcium, potassium, and
  phosphorus levels
• BUN and creatinine levels
• Blood glucose level

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Considerations

• Body stores of magnesium may be depleted markedly
  before the serum level drops, so a deficiency of
  Mg is clearly present if the serum level is low
• Because extracellular magnesium is protein bound,
  the patient's protein status is an important
  consideration in interpreting magnesium levels
• Hypocalcemia is caused by magnesium depletion,
  but the reason is not known
• Hypophosphatemia has been found in patients with
  hypomagnesemia

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Hypomagnesemia Hypokalemia

• Hypomagnesemia can cause Hypokalemia
• Mg is a cofactor for the Na/KATPase
• Without Mg, cellular homeostasis cannot be
  maintained and K loss occurs in the kidneys
• HYPOKALEMIA CANNOT BE CORRECTED UNLESS
  HYPOMAGNESEMIA HAS BEEN CORRECTED

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EKG and Cardiac monitoring

• Findings in hypomagnesemia are nonspecific
• Findings include ST segment depression tall,
  peaked T waves flat T waves or depression in the
  precordium U waves loss of voltage PR
  prolongation and widened QRS.

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Treatment of Mild or Chronic Hypomagnesemia

• Treated with 240mg elemental Mg PO qd-bid
• Mg oxide preparations include Mag-Ox 400 and
  Uro-Mag
• The major side effect of these is diarrhea

• Singer G Fluid and electrolyte management. In
  The Washington Manual of Therapeutics.
  Lippencott. 30th edition, 2001. p68-69.

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Treatment of Severe Symptomatic Hypomagnesemia

• Treated with 1-2g Mg sulfate (4mEq/ml) IV over 15
  min, followed by infusion of 6g Mg sulfate in 1L
  or more IV fluid over 24hrs
• B/c of the need to replenish intracellular
  stores, the infusion should be continued for 3-7
  days
• Serum Mg should be measured q24h and the infusion
  rate adjusted to maintain a serum Mg level of
  lt2.5 mEq/L

• Singer G Fluid and electrolyte management. In
  The Washington Manual of Therapeutics.
  Lippencott. 30th edition, 2001. p68-69.

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Considerations with Mg Replacement

• In pt with normal renal function, excess Mg is
  readily excreted, and there is little risk of
  causing hypermagnesemia with recommended doses.
  However, Mg must be given with extreme care in
  renal failure b/c of the risk of hypermagnesemia
• Reduced doses and more frequent monitoring must
  be used even in mild renal failure
• Tendon reflexes should be tested frequently as
  hyporeflexia suggests hypermagnesemia

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DONT FORGET

• HYPOKALEMIA CANNOT BE CORRECTED UNLESS
  HYPOMAGNESEMIA HAS BEEN CORRECTED

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References

• Agraharker M, Fahlen M Hypomagnesemia.
  www.emedicine.com
• Novello N, Blumstein HA Hypomagnesemia.
  www.emedicine.com
• Singer G Fluid and electrolyte management. In
  The Washington Manual of Therapeutics.
  Lippencott. 30th edition, 2001. p68-69.