ANTIEPILEPTIC DRUGS IN THE TREATMENT OF
NEUROPATHIC PAIN: DRUG-TO-DRUG
INTERACTION IN ELDERLY PEOPLE
To the Editor: Pain is a common complain in elderly people,
often linked to neuropathic syndromes that affect old
people particularly, such as trigeminal neuralgia, painful
diabetic neuropathy, and postherpetic neuralgia.1 The effectiveness
of anticonvulsant drugs in the management of
neuropathic pain has been proven; for example, carbamazepine
has been extensively employed and evaluated for
pain relief in trigeminal neuralgia and has received Food
and Drug Administration approval for this use.2–4 Nevertheless,
carbamazepine has a narrow therapeutic window
and has caused, albeit rarely, severe side effects, including
hepatic failure, agranulocytosis, aplastic anemia, StevensJohnson
syndrome, and toxic epidermal necrolysis. In
addition, in polymedicated elderly patients, there is a high
potential for adverse effects from drug-to-drug interactions,
carbamazepine being a powerful inducer of the hepatic
P450 cytochrome system. This could lead to reduced effectiveness
of the drugs co-prescribed with carbamazepine.
CASE PRESENTATION
A 90-year-old woman was admitted to the hospital with
exacerbation of long-standing arterial hypertension. She
was also known to have symptomatic facial neuralgia,
which had been treated with carbamazepine over many
years. During her hospital stay, secondary causes of hyJAGS
JANUARY 2009–VOL. 57, NO. 1 LETTERS TO THE EDITOR 181
pertension, such as pheochromocytoma and renal artery
stenosis, were excluded. High systolic blood pressure measurements
(range 240–260 mmHg) were noted several times
daily despite pentatherapy (calcium channel blocker, loop
diuretic, thiazide diuretic, angiotensin II receptor antagonist,
and beta-blocker). Left ventricular hypertrophy was
diagnosed according to echocardiogram. Other treatments
with drugs that are not metabolized by the liver, in this case
glyceryl trinitrate and moxonidine, appeared to be more
effective. This observation suggested that the treatment
resistance of the hypertension might be due to an effect of
carbamazepine at the level of the antihypertensive drugs’
hepatic metabolism. After discontinuing carbamazepine
(replaced by gabapentin) and allowing sufficient time for
its elimination, significant improvement in the blood pressure
profile was observed; mean arterial blood pressures
decreased from 177/64 mmHg (38 measures/24 h) to 157/
65 mmHg (17 measures/24 h). Blood pressure measurements
greater than 200 mmHg disappeared completely
(410 peaks/24 h previously), allowing the antihypertensive
treatment regime to be reduced from five to three drugs.
Some months after discharge home, carbamazepine
was reintroduced for neurological symptoms, leading again
to an uncontrolled elevation in blood pressure and hospital
readmission.