Loss of deep tendon reflexes occurs at approximately what serum magnesium concentration?

Loss of deep tendon reflexes occurs at approximately what serum magnesium concentration?

• A. 8 mEq/L
• B. 10 mEq/L
• C. 12 mEq/L
• D. 15 mEq/L

Answer: (B)

Hypermagnesemia depresses neuromuscular transmission by magnesium blocking calcium entry at the presynaptic terminal, which reduces acetylcholine release and lowers motor neuron excitability. As magnesium rises, reflexes are preserved until a threshold is reached, then disappear. Loss of deep tendon reflexes occurs around 10 mEq/L, signaling a high, toxic magnesium level. Beyond this, further depression can cause hypotension and bradycardia, followed by respiratory depression and potential cardiac issues at even higher concentrations. This makes reflex loss an important clinical warning sign when magnesium sulfate is used, indicating the need to adjust therapy.

Hypermagnesemia depresses neuromuscular transmission by magnesium blocking calcium entry at the presynaptic terminal, which reduces acetylcholine release and lowers motor neuron excitability. As magnesium rises, reflexes are preserved until a threshold is reached, then disappear. Loss of deep tendon reflexes occurs around 10 mEq/L, signaling a high, toxic magnesium level. Beyond this, further depression can cause hypotension and bradycardia, followed by respiratory depression and potential cardiac issues at even higher concentrations. This makes reflex loss an important clinical warning sign when magnesium sulfate is used, indicating the need to adjust therapy.