A 64-year-old man comes to the office with 2 months of episodic facial pain. He describes sudden-onset, severe, electric shock-like pain over the right cheek and ear lasting several seconds. The patient reports that activities like shaving and washing his face can trigger the pain and it is becoming more frequent. He is a retired veteran and has a history of posttraumatic stress disorder. Physical examination, including neurologic examination, is normal. Laboratory studies for blood cell counts, electrolytes, and hepatic and renal function are normal. The patient is started on a single drug therapy. On a follow-up visit 3 months later, he reports symptom improvement but is found to have the following blood counts: Which of the following is the most likely mechanism of action of the drug used in this patient?

Question

A 64-year-old man comes to the office with 2 months of episodic facial pain.  He describes sudden-onset, severe, electric shock-like pain over the right cheek and ear lasting several seconds.  The patient reports that activities like shaving and washing his face can trigger the pain and it is becoming more frequent.  He is a retired veteran and has a history of posttraumatic stress disorder.  Physical examination, including neurologic examination, is normal.  Laboratory studies for blood cell counts, electrolytes, and hepatic and renal function are normal.  The patient is started on a single drug therapy.  On a follow-up visit 3 months later, he reports symptom improvement but is found to have the following blood counts:  Which of the following is the most likely mechanism of action of the drug used in this patient?

Quizplus · Accepted Answer

This patient has trigeminal neuralgia (tic douloureux), a neuropathic disorder that typically presents with episodic, severe, unilateral, electric shock-like pain in the distribution of CN V. Symptoms are often triggered by innocuous stimuli such as shaving or washing the face. The first-line treatment of trigeminal neuralgia is carbamazepine, a medication that inhibits neuronal high-frequency firing by reducing the ability of sodium channels to recover from inactivation (ie, sodium channel blockade). Carbamazepine may cause the following important adverse effects: (Choice A) Ketamine, felbamate, and memantine are examples of medications that block NMDA receptors. Ketamine is used as a general anesthetic agent, felbamate is an anticonvulsant, and memantine may be effective in the treatment of Alzheimer dementia. (Choice B) Selective serotonin reuptake inhibitors (eg, fluoxetine, sertraline) are typically used to treat depression, anxiety, and posttraumatic stress disorder. Side effects may include headache, worsening anxiety/agitation, and insomnia. (Choice C) Ethosuximide blocks T-type Ca²⁺ channels and decreases calcium current in thalamic neurons. This anticonvulsant is the agent of choice for absence seizures. Major side effects include gastrointestinal symptoms and drowsiness. (Choice E) Various antiseizure medications (eg, benzodiazepines, barbiturates) act on the GABA-A receptor to increase chloride conductance; however, carbamazepine does not significantly affect chloride channels. Educational objective: The first-line treatment for trigeminal neuralgia is carbamazepine, a medication that inhibits neuronal high-frequency firing by blocking sodium channels. Carbamazepine can cause bone marrow suppression; therefore, complete blood cell counts should be monitored periodically.

A 64-Year-Old Man Comes to the Office with 2 Months