Introduction

The first line treatment of trigeminal neuralgia (TN) include carbamazepine or oxcarbazepine. The side effects of oxcarbazepine include drowsiness, nausea, dizziness, ataxia, elevation of serum transaminase, and hyponatremia. We report a case of a patient with trigeminal neuralgia treated with the long-term administration of oxcarbazepine, where the patient showed symptomatic hyponatremia, presented with lower leg weakness, which was differentiated from the symptoms due to spinal stenosis.

Case report

A 78-year-old man was admitted to the hospital with a severe lancinating pain around his right cheek, eyes, and lip. The pain was evoked with mild touch and occurred more than ten times a day with visual analogue scale (VAS) score 10 out of 10 points.
Under the diagnosis of TN with the involvement of both right maxillary and mandibular branch, oxcarbazepine 600 mg/d, milnacipran 25 mg/d, oxycodone 20 mg + naloxone 10 mg/d were prescribed. The pain improved to VAS score 5 and the medications were maintained. Ten months later, the patient visited the pain center due to leg weakness, claiming that had falling events for several months.
Lumbar spine computed tomography (CT) scan showed multilevel bulging discs along with central canal stenosis and neural foraminal stenosis. Epidural steroid injection was performed to relieve the back pain of the patient. The effect of the nerve block intervention was not significant, and the blood test showed hyponatremia with sodium level of 127 mmol/L.
Thus oxcarbazepine was replaced with topiramate 75 mg per day until the next visit in 2 weeks, but the pain persisted. Hence the dose of topiramate was increased to 100 mg per day. Follow-up of serum electrolyte levels revealed normalized (143 mmol/L). The patient came back to the clinic one week afterwards with improved VAS score of 4 points without any side effects. Leg weakness had dramatically improved.

Discussion

In conclusion, even in patients with chronic pain who have continued certain pain medication for a period of time, a routine serum laboratory exam would be required as the patient may start another medication at any time due to concomitant underlying diseases, which may cause drug interaction with the pain medication.