1232 - Feeling Funny-toin: A rare case of Phenytoin (Dilantin) Toxicity

An 84-year-old female with a past medical history of glaucoma presented with altered mental status. She was found to have a large mass in her right frontal lobe as well as smaller masses in her bilateral cerebellar peduncles and left frontal and right cerebral cortices. She was placed on steroids then underwent right frontal craniotomy for tumor biopsy.  Post-operatively, her steroids were tapered, and she was placed on Dilantin for 30 days for seizure prophylaxis. On rehab admission, she was noted to have poor concentration, impulsivity, and imbalance. There was concern for increasing cerebral edema due to her steroid taper, however, CT head showed an interval improvement in swelling. Her labs were significant for an elevated Dilantin level of 48.7 mcg/mL (normal 10 - 20 mcg/mL). Dilantin was discontinued for Keppra with symptomatic improvement as blood concentrations of Dilantin decreased (24.3 mcg/mL after 4 days and 11.2 after one week).

Discussions: Dilantin is an effective anti-seizure medication. There are few side effects when dosed appropriately. As blood concentration increases, side effects can include mild nystagmus (10 - 20 mcg/mL); nystagmus (20 - 30 mcg/mL); ataxia, slurred speech, nausea, and vomiting (30 - 40 mcg/mL); lethargy and confusion (40 - 50 mcg/mL); and coma ( > 50 mcg/mL). It is important to clinically monitor patients for Dilantin toxicity, and blood levels should be measured if there is a decline in neurologic exam or any suspicion of overdose.

Conclusions: This case highlights a rare side effect of a commonly used anti-seizure medication. Although generally well tolerated, Dilantin can become toxic at elevated levels. Providers should monitor patients on Dilantin for nystagmus, ataxia, slurred speech, nausea, confusion, and lethargy.  Dilantin blood levels should be measured if there is any concern for overdose.