Drug prescribed for seizures can cause lower sodium levels

DEAR DR. ROACH: I am a 71-year-old woman in generally good health. The only prescription drugs I take are carbamazepine for seizures and ibandronate for osteoporosis. I’ve taken carbamazepine since 1996, after I suffered a second seizure. (I’ve had only two seizures in my life, the first in 1992.)

I tended to have low sodium readings for several years. It was never a concern of doctors until 2019 when my sodium reading was 120 mEq/L. This was determined to be due to the fact that I’d had the flu and had been drinking excess amounts of hot liquids. Since that time, my sodium readings returned to the 130-135 levels. Doctors (both general and neurologist) have told me it’s probably due to the carbamazepine and that I should just increase my use of salt.

This summer, I was diagnosed with hyponatremia to 127. I had been consciously increasing my salt use, so that lower reading was unexpected. I was advised to decrease my fluid intake to 45 ounces a day and increase my sodium intake to 2,500 milligrams a day.

Could you please discuss my diagnosis? Is it just something to watch, or should I be worried? Should I see a specialist other than my general doctor (M.D.-board-certified in family medicine) and neurologist (D.O.-certified in neurology)?

Could you also clarify the fluid limitations? First, are all liquids, milk included, to be counted? I have been able to limit my liquids, including milk, only to 60-65 ounces a day this summer, which I assume is because I sweat when I walk or even perform chores around the house. I walk briskly and strive for 10,000 steps daily; my walks average from 30-50 minutes in hilly terrain. I walk during cooler evenings, but it still can be warm.

Second, can limiting fluids this much cause other problems? It is the exact opposite of what we are usually told in terms of how much fluid we need each day. I am wondering if I should discuss changing to a different seizure drug with my neurologist. — G.M.

ANSWER: Carbamazepine (Tegretol) is a common epilepsy medicine that is well known to cause low sodium levels. It can cause inappropriately high levels of anti-diuretic hormone in about 4% of people who take it. The syndrome of inappropriate anti-diuretic hormone (SIADH) is treated mainly by fluid restriction: 1.5 liters/day (45 ounces) is usually enough to keep the sodium level at the goal of 130 or higher, but some people do need extra salt to achieve this number.

The fluid number consists of all liquids, including milk. Foods that are mostly water (watermelon comes to mind) are another source of water. You need to ignore the recommendations to drink lots of water (most people don’t need as much is recommended), since your situation is uniquely yours. Only by regular checks of your sodium level can your doctor make recommendations about water restriction and dietary sodium. A nephrologist is the expert on SIADH, but your internist sounds like they are treating you appropriately.

I think discussing changing medications with your neurologist is an excellent idea. You’ve only had two seizures, and none in more than 25 years, so a discussion about discontinuing medication entirely wouldn’t be unreasonable. Everybody on carbamazepine should receive regular sodium checks.

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Dr. Roach regrets that he is unable to answer individual letters, but will incorporate them in the column whenever possible. Readers may email questions to ToYourGoodHealth@med.cornell.edu or send mail to 628 Virginia Dr., Orlando, FL 32803.

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