107: Magnesium Disorders (Hypomagnesemia, Hypermagnesemia)

INTRODUCTION

As noted in Chapter 85, abnormalities of magnesium (Mg⁺²) and calcium (Ca⁺²) metabolism are commonly seen in the neonatal intensive care unit. Calcium disturbances may be mirrored by magnesium, as in hypocalcemia with hypomagnesemia or hypercalcemia with hypermagnesemia. Infants of diabetic mothers (IDMs) and infants with intrauterine growth restriction (IUGR) may present with hypocalcemia, hypomagnesemia, or both. Abnormalities in serum values for Ca⁺² and Mg⁺² are of concern in any infant and warrant further investigation.

I. HYPOMAGNESEMIA

1. Definition. Normal serum levels for Mg⁺² are typically 0.6–1.0 mmol/L (1.6–2.4 mg/dL). Hypomagnesemia is usually seen as any value <0.66 mmol/L (1.6 mg/dL); however, clinical signs do not manifest until levels drop below 0.5 mmol/L (1.2 mg/dL).

2. Incidence. True overall incidence in neonates is not well documented and remains to be determined; however, neonates appear to be more predisposed than other groups of patients, and the most frequent occurrence tends to follow that of those infants with hypocalcemia.

3. Pathophysiology. Mg⁺² is a key trace element for maintaining skeletal integrity, and it acts as a catalyst for intracellular enzymes for adenosine triphosphate (ATP) activation in skeletal and myocardial contractility. It has an important role in different processes related to cell physiology, hormonal and metabolic pathways, nerve conduction, and blood coagulation. It is also integral to protein synthesis, vitamin D metabolism, parathyroid function, and calcium homeostasis.

4. Risk factors

   1. Hypocalcemia

   2. Preterm and late-preterm infants

   3. Inadequate intake of magnesium

   4. Infant of diabetic mother (IDM), reflecting maternal Mg⁺² deficiency secondary to gestational diabetes

   5. IUGR, especially if mother had preeclampsia

   6. Inherited renal wasting (eg, Gitelman syndrome, Na⁺-K-ATPase mutation, others)

   7. Hypoparathyroidism

   8. Associated hypocalciuria and nephrocalcinosis

   9. Magnesuria secondary to furosemide or gentamicin

   10. Citrated blood exchange transfusions

5. Clinical presentation

   1. Similar to hypocalcemia (see Chapter 85, Section I.E) (eg, jitteriness, apnea, feeding intolerance), and may also present as seizures.

   2. Clinical signs may be masked as hypocalcemia. If symptoms persist after adequate calcium gluconate therapy, hypomagnesemia should be considered.

6. Diagnosis. Laboratory testing to establish serum levels.

   1. Serum magnesium level. Normal values are 0.6–1.0 mmol/L (1.6–2.4 mg/dL), although it may vary minimally with gestational age. Twin gestation, multiple births, or vaginal delivery may result in lower levels of Mg⁺². It is important to note that most methods to assess Mg⁺² levels measure total Mg⁺² concentration, while only free Mg⁺² is biologically active and almost 30% is inactive bound to albumin.

   2. Total and ionized calcium levels. Usually hypomagnesemia is associated with hypocalcemia, and hypercalcemia may inhibit magnesium reabsorption in the distal loop of Henle and cause hypomagnesemia.

7. Prevention. Adequate intake of magnesium should be assured in parenteral and enteral nutrition to prevent hypomagnesemia (recommend 8–15 mg/kg/d).

8. Management. Acute hypomagnesemia should be treated with intravenous magnesium sulfate (see Chapter 148 for specific dosing guidelines). Infusion must be monitored closely for cardiac arrhythmias and hypotension. Maintenance Mg⁺² can be by parenteral nutrition solutions or by oral feeds with 5-fold dilution of Mg⁺² salt solution. Mg⁺² infusion should be used cautiously ...