The key abnormal value
The client started carbamazepine for trigeminal neuralgia three weeks ago, and her facial pain is controlled. Her new symptoms are headache, nausea, and confusion. Of the three results given, only one is abnormal: a serum sodium of 124 mEq/L, well below the normal 135–145 mEq/L. Her white cell count and carbamazepine level are both normal. When one value is clearly abnormal and it matches the symptoms, that value is the explanation.
How carbamazepine lowers sodium
Carbamazepine can produce an effect like the syndrome of inappropriate antidiuretic hormone (SIADH). It increases the action or release of antidiuretic hormone, so the kidneys retain free water. The extra water dilutes the blood and the sodium concentration falls. This is dilutional hyponatremia, and it can occur even when the drug level is in the therapeutic range. Low sodium draws water into brain cells, causing headache, nausea, confusion, dizziness, and, if severe, seizures. Older adults and clients taking diuretics are at higher risk.
| Result | Value | Interpretation |
|---|
| Sodium | 124 mEq/L (135–145) | Low: carbamazepine-induced hyponatremia |
| White blood cells | 6,800/mm³ (4,500–11,000) | Normal: no agranulocytosis |
| Carbamazepine level | 8 mcg/mL (4–12) | Therapeutic: not a high-level toxicity |
Why the other explanations fail
Agranulocytosis is a serious adverse effect of carbamazepine, but it lowers the white cell count and presents with fever, sore throat, and infection; her count is normal. Carbamazepine toxicity from a high level causes dizziness, ataxia, double vision, and drowsiness, but her level of 8 mcg/mL is in range. Spread of the neuralgia is unlikely because her pain is controlled, and trigeminal neuralgia does not cause confusion or a low sodium in any case.
Nursing actions
The nurse reports the sodium result so the physician can adjust therapy, which may include fluid restriction, a dose change, or a change of drug. The nurse checks her safety because confusion raises fall risk, monitors neurologic status for worsening confusion or seizure, and reviews other drugs that can lower sodium. Teaching for clients on carbamazepine includes reporting headache, confusion, fever, sore throat, unusual bruising, or rash, and keeping appointments for blood counts, sodium, and liver tests.
Exam takeaway
Watch out A therapeutic drug level does not exclude an adverse effect. Carbamazepine causes hyponatremia through an SIADH-like effect, and confusion with a low sodium is the clue.