Pharmacology · patient case with 1 question
Electric-shock facial pain
Patient: Female, 58 years old
Chief Complaint: “I get lightning bolts through my cheek when I brush my teeth or feel a breeze.”
Background: Episodes lasting a few seconds, in the right V2 distribution, triggered by light touch to a specific area of the cheek. No neurological deficit. Two teeth have been extracted elsewhere without relief. Her physician has started carbamazepine.
Vital Signs: BP 130/78 mmHg · HR 74 bpm
Findings: Teeth in the affected quadrant are caries-free, respond normally to cold and are non-tender to percussion. Radiographs are unremarkable.
Question 1
What is the mechanism of action of carbamazepine?
Show answer and explanation
Correct answer: C. Blockade of voltage-gated sodium channels
- A. GABA-A potentiation describes the BENZODIAZEPINES and barbiturates, which increase chloride conductance and produce sedation and anxiolysis.
- B. That is GABAPENTIN and pregabalin, used for post-herpetic neuralgia and as second-line agents in trigeminal neuralgia. Both drugs turn up in facial pain questions, so keeping the two mechanisms apart matters — sodium channels for carbamazepine, calcium channel subunit for gabapentin.
- C. CORRECT. Carbamazepine blocks VOLTAGE-GATED SODIUM CHANNELS, binding preferentially to the inactivated state. This stabilises hyperexcitable neuronal membranes and suppresses high-frequency repetitive firing, which is exactly the pathophysiology of trigeminal neuralgia. It is the FIRST-LINE drug for trigeminal neuralgia as well as an antiepileptic, and a good response to it is itself supportive of the diagnosis.
- D. NMDA receptor antagonism describes ketamine and, in part, memantine and dextromethorphan.
Note: The dental relevance of carbamazepine is substantial. It is a CYP3A4 ENZYME INDUCER, so it accelerates metabolism of other drugs. It causes XEROSTOMIA. And it carries a risk of BLOOD DYSCRASIAS including agranulocytosis and aplastic anaemia, so a full blood count should be reviewed before elective surgery. It also causes hyponatraemia and STEVENS-JOHNSON SYNDROME, with a strong association with HLA-B*1502 in patients of Han Chinese and South-East Asian ancestry, who should be screened before starting. Oxcarbazepine is the better-tolerated alternative.