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Residency Exam > Magnesium Disorders (Panopto Video File) > Flashcards

Magnesium Disorders (Panopto Video File) Flashcards

1
Q

Describe the abundance of magnesium in the body

A

Magnesium is the fourth most abundant cation in the body

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2
Q

Magnesium is responsible for catalysing and/or activating more than 300 separate enzymes including which important set of reactions?

A

All phosphate transfer reactions involving ATP.

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3
Q

What is the reference range for magnesium?

A

0.75 to 0.95 mmol/L

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4
Q

Where area 99% of total body stores of magnesium found?

A

In the bone and intracellular space

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5
Q

Approximately what percentage of magnesium is found in bone?

A

60%

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6
Q

Approximately what percentage of magnesium is found in skeletal muscle?

A

20%

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7
Q

What is the second most abundant cation in the intracellular fluid?

A

Magnesium

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8
Q

Which organs is magnesium most prominent in?

A

Organs with a high energy requirement such as the kidney, liver, and heart

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9
Q

Describe the prevalence of magnesium in the vascular space.

A

Only 1% of total body magnesium is found in the vascular space

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10
Q

What percentage of magnesium is protein bound?

A

33%

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11
Q

Describe the applicability of serum magnesium as a marker of total body magnesium.

A

Since only 1% of total body magnesium is located in the extracellular fluid, the serum magnesium concentration provides only a rough estimation of total body stores.

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12
Q

Describe the role of hormones in maintaining magnesium homeostasis.

A

There is no hormone directly involved in magnesium homeostasis, although there is some indirect effect of aldosterone.

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13
Q

What is the main regulatory control mechanism for magnesium homeostasis?

A

Renal excretion

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14
Q

How is magnesium homeostasis balanced?

A

It is a factor of the efficiency of intestinal absorption and renal excretion.

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15
Q

What is the normal dietary intake of magnesium?

A

300 to 400 mg a day

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16
Q

What is the oral bioavailability of magnesium?

A

Between 20 and 80%

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17
Q

Where is magnesium predominantly absorbed?

A

In the small intestine

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18
Q

What is a complication of magnesium supplementation?

A

If given orally, due to the limited bioavailability, magnesium can cause an osmotic laxative effect in the large intestine, which can further reduce its absorption.

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19
Q

Describe the activity of magnesium in the kidneys.

A

Normally, 95% of the filtered magnesium is reabsorbed by the nephron.

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20
Q

Where is the primary site of magnesium reabsorption in the kidneys?

A

The loop of Henle

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21
Q

What is a drug-related implication of magnesium primarily being reabsorbed in the loop of Henle?

A

This gives loop diuretics the propensity to cause hypomagnesaemia.

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22
Q

Why can only two thirds of magnesium be filtered by the kidneys?

A

As the remainder is complexed or bound to proteins.

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23
Q

How are magnesium concentrations maintained?

A

By renal mechanisms

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24
Q

What is the role of magnesium?

A

To regular neuromuscular excitability

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25
Q

What is an abnormality of plasma magnesium typically indicative of?

A

An intracellular abnormality

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26
Q

Describe the flow on effect of magnesium disorders.

A

Calcium, potassium and phosphate levels are often also abnormal if magnesium homeostasis is disrupted.

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27
Q

What are 5 potential aetiologies of hypermagnesaemia?

A
  1. Renal failure (in association with iatrogenic magnesium load)
  2. Hypoaldosteronism
  3. Adrenal insufficiency
  4. Lithium toxicity
  5. Hyperparathyroidism
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28
Q

What is a common source of magnesium which may contribute to hypermagnesaemia?

A

Mylanta

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29
Q

What are the 2 main classes of hypermagnesaemia symptoms?

A
  1. Neuromuscular
  2. Cardiovascular
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30
Q

What are 4 neuromuscular symptoms of hypermagnesaemia?

A
  1. Loss of deep tendon reflexes
  2. Paralysis of voluntary muscles
  3. Respiratory depression and failure (> 5 mmol/L)
  4. Stupor/coma
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31
Q

What are 3 cardiovascular symptoms of hypermagnesaemia?

A
  1. Hypotension
  2. Electrocardiogram abnormalities (particularly prolonged PR interval)
  3. Complete heart block (> 7.5 mmol/L)
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32
Q

In what 2 instances is treatment for hypermagnesaemia indicated?

A
  1. If the patient is symptomatic with concentrations 2.5 to 4 mmol/L
  2. Any patient > 4 mmol/L
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33
Q

What are the 3 pillars of treatment of hypermagnesaemia?

A
  1. Cease any magnesium administration.
  2. Give calcium to protect from membrane excitability.
  3. Dialysis
34
Q

Describe the relationship between calcium and magnesium.

A

Calcium directly antagonizes the neuromuscular and cardiovascular effects of magnesium.

35
Q

What are 3 causes of hypomagnesaemia secondary to decreased intake?

A
  1. Decreased dietary consumption
  2. Alcohol dependence
  3. Parenteral nutrition
36
Q

What are four causes of hypomagnesaemia secondary to redistribution from the extracellular compartment to the intracellular compartment?

A

Treatment of diabetic ketoacidosis
Hungry bone syndrome
Refeeding syndrome
Acute pancreatitis

37
Q

Causes of hypomagnesaemia secondary to gastrointestinal losses include small bowel bypass surgery, nasogastric suction, malabsorption and what four others?

A

Fistulas
Diarrhoea
Vomiting
Proton pump inhibitors

38
Q

What are 3 familial causes of renal losses of magnesium?

A
  1. Bartter syndrome
  2. Gitelman syndrome
  3. Familial hypomagnesaemia with hypercalciuria and nephrocalcinosis
39
Q

What are 3 acquired causes of hypomagnesaemia ?

A
  1. Medication-induced
  2. Alcohol dependence
  3. Hypercalcaemia
40
Q

What are 7 medications implicated in increasing renal losses of magnesium?

A
  1. Thiazide diuretics
  2. Aminoglycoside antibiotics
  3. Amphotericin B
  4. ciSplatin
  5. pentamidine
  6. TACrolimus
  7. cyclosPORIN
41
Q

How can parenteral nutrition cause hypermagnesaemia?

A

If the concentration of magnesium is too high

42
Q

How can parenteral nutrition cause hypomagnesaemia?

A

If the concentration of magnesium is too low

43
Q

What are 5 aetiologies of hypomagnesaemia ?

A
  1. nutritional
  2. gastrointestinal
  3. endocrine
  4. renal (including drug-induced)
  5. paediatric
44
Q

What are 8 neuromuscular/nervous system effects of hypomagnesaemia?

A
  1. Positive Chvostek’s and Trousseau’s signs
  2. Tremor
  3. Fasciculations
  4. Tetany
  5. Headaches
  6. Seizures
  7. Fatigue
  8. Asthenia
45
Q

What are 4 cardiovascular effects of hypomagnesaemia?

A
  1. Atherosclerotic vascular disease/coronary artery disease
  2. Arrhythmias
  3. Hypertension
  4. Congestive heart failure
46
Q

What are 4 arrhythmias which can occur with hypomagnesaemia?

A
  1. Torsades de pointes
  2. PR prolongation
  3. Progressive QRS widening
  4. Diminution of T-waves
47
Q

What are 2 endocrine effects of hypomagnesaemia?

A
  1. Altered glucose homeostasis/diabetic complications
  2. Osteoporosis
48
Q

What are 2 biochemical effects of hypomagnesaemia?

A
  1. Hypokalaemia
  2. Hypocalcaemia
49
Q

What are 2 biochemical effects of hypomagnesaemia?

A
  1. Hypokalaemia
  2. Hypocalcaemia
50
Q

What is a respiratory effect of hypomagnesaemia?

A

Asthma

51
Q

What is a renal effect of hypomagnesaemia?

A

Nephrolithiasis

52
Q

Why may magnesium be given in asthma?

A

It may cause bronchodilation. Asthma is associated with hypomagnesaemia.

53
Q

What is Trousseau’s sign?

A

Muscular spasm of the hand elicited by compression of the upper arm (as with a blood pressure cuff); a sign of latent tetany.

54
Q

What is Chvostek’s sign?

A

When tapping the facial nerve produces facial muscle spasms.

55
Q

What are 2 nutritional causes of hypomagnesaemia?

A
  1. Alcoholism
  2. Protein calorie malnutrition
56
Q

What are 3 gastrointestinal causes of hyponatraemia?

A
  1. Chronic diarrhoea
  2. Laxative abuse
  3. Malabsorption syndromes
57
Q

What are 5 renal causes of hypomagnesaemia?

A
  1. Renal tubular acidosis
  2. Diuretic phase of acute tubular necrosis
  3. Diuretics
  4. Antibiotic induced tubular dysfunction
  5. ciSplatin
58
Q

What percentage of long-term diuretic users are thought to develop hypomagnesaemia?

A

Up to 50%

59
Q

What are 2 antimicrobials implicated in causing antibiotic-induced tubular dysfunction?

A
  1. Aminoglycosides
  2. Amphotericin B
60
Q

What are 4 endocrine and metabolic causes of hypomagnesaemia?

A
  1. Hyperaldosteronism
  2. Malignancy induced hypercalcaemia
  3. Pregnancy
  4. Excessive lactation
61
Q

How does low magnesium affect calcium?

A

Low plasma magnesium leads to a failure of magnesium to exchange with bone calcium

62
Q

How does the body respond to an inability of magnesium to exchange with bone calcium in hypomagnesaemia?

A

There is an initial decrease in plasma calcium, and increase in parathyroid hormone secretion

63
Q

Why does end-organ resistance to parathyroid hormone occur in hypomagnesaemia?

A

Target tissue response to parathyroid hormone is modulated by cAMP, which needs magnesium to work properly

64
Q

What are the primary target tissues for parathyroid hormone?

A

Adenylate cyclase in the bone and kidney

65
Q

Why are the bones and the kidneys the two organs most affected by hypomagnesaemia?

A

As the they are the primary target tissues for parathyroid hormone

66
Q

How does hypomagnesaemia affect potassium?

A

Hypomagnesaemia causes failure of the kidneys to reabsorb potassium

67
Q

Why does hypomagnesaemia cause failure of the kidneys to reabsorb potassium?

A

Due to decreased activity of sodium and potassium-ATPase

68
Q

How does hypomagnesaemia affect reflexes?

A

Hypomagnesaemia causes hyperactive reflexes

69
Q

How does hypermagnesaemia affect reflexes?

A

Hypermagnesaemia causes paralysis

70
Q

How does renal function affect magnesium replacement?

A

Patients with renal dysfunction will have a more pronounced response to magnesium replacement

71
Q

What are 6 factors to monitor when replacing magnesium?

A
  1. Serum magnesium
  2. Blood pressure
  3. Respiratory rate
  4. Urinary output
  5. CrCl/eGFR
  6. Patellar reflex
72
Q

Why should you monitor serum magnesium when replacing magnesium?

A

For signs of clinical improvement

73
Q

Why should you monitor blood pressure when replacing magnesium?

A

As magnesium replacement may produce vasodilation and result in hypotension if administered too quickly

74
Q

Why should you monitor respiratory rate when replacing magnesium?

A

Due to the associated risk of respiratory depression

75
Q

Why should you monitor urinary output when replacing magnesium?

A

As magnesium is renally excreted

76
Q

Why should you monitor renal function when replacing magnesium?

A

There is an increased risk of magnesium toxicity in renal impairment

77
Q

Why should you monitor patellar reflex when replacing magnesium?

A

Loss of patellar reflex is indicative of magnesium toxicity

78
Q

What is a local side effect of intravenous magnesium administration?

A

Magnesium may be irritant to veins, extravasation may cause tissue damage

79
Q

Describe the time to onset of intravenous magnesium

A

Following intravenous administration the onset of action is immediate and lasts approximately 30 minutes

80
Q

Describe the time to onset of intramuscular magnesium

A

Following intramuscular administration the onset of action is approximately one hour and lasts approximately 3-4 hours

81
Q

What are 2 potential drug-drug interactions with magnesium replacement?

A
  1. Hypotension in patients taking calcium channel blockers
  2. Respiratory depression with high does of barbiturates, opioids or hypnotics
82
Q

What is the antidote for magnesium overdose?

A

Intravenous calcium gluconate

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