Minimal recovery of drug from extracorporeal treatment in a case of phenytoin toxicity associated with coma and hypothermia

ABSTRACT

Background: The Extracorporeal Treatments in Poisoning (EXTRIP) workgroup provides a weak conditional recommendation in support of hemodialysis (HD) for select patients with severe phenytoin poisoning. Despite this recommendation, the HD clearance of phenytoin is poorly studied. We present a patient who developed phenytoin toxicity that was treated with hemodialysis and report on the efficacy of phenytoin removal during HD. Case report An 87-year-old man with epilepsy who was maintained on a stable dose of 300mg phenytoin extended-release daily was admitted to the hospital for treatment of Coronavirus Disease 2019 and congestive heart failure. On hospital day 14, the patient had a gradual onset of depressed mental status with hypothermia (nadir 35 degrees Celsius). At this time, he had a rising total blood phenytoin concentration (peak 49.3 mcg/mL [therapeutic 10-20mcg/mL] with an albumin of 3.8 g/dL [normal 3.4-5.4 g/dL]). The patient's other medications included furosemide, aspirin, atorvastatin, digoxin, doxycycline, metoprolol tartrate, and warfarin;he was also receiving albumin and crystalloid for hypovolemia (albumin nadir on hospital day 14 2.5 g/dL). Free phenytoin concentrations were not available. Alternate etiologies of hypothermia (endocrine, infectious) were excluded. The Poison Control Center was consulted and recommended HD because of the concern for prolonged coma, as per EXTRIP guidelines. The patient received three sessions of HD over a period of 6 days at 2.5-3 h per session using an F160 Optiflux membrane filter (Fresenius Medical Care, Waltham, MA, USA), with a blood flow rate of 350mL/min and a dialysate flow rate of 700mL/min. After the first session of HD (2.5 h) on hospital day 21, his hypothermia resolved and his phenytoin concentration fell from 39.2mcg/mL to 34.2 mcg/mL with only mild improvement in his mental status. After 6 days (hospital day 27), his phenytoin concentration decreased to 19.5 mcg/mL and his mental status normalized. Effluent from the first HD session had phenytoin concentrations below the limit of detection (0.50mcg/mL). Thus, no greater than 52mg of phenytoin was removed during a 2.5-h session of hemodialysis. Discussion(s) The reason for the sudden increase in blood phenytoin concentrations in this patient is unclear in the absence of drug-drug interactions or dosing changes to the phenytoin. Although uncommonly reported, patients with phenytoin toxicity can experience hypothermia. In this case, the patient's hypothermia resolved during HD, although it is unclear if this was related to changes in phenytoin concentration or (more likely) direct extracorporeal warming via the HD machine. If the patient's phenytoin clearance from the first session were extrapolated to subsequent sessions an estimated maximum of 166.4mg of phenytoin would be removed in 8 total hours of HD, which is far less than previously reported phenytoin clearances on the order of grams. This difference may be related to the use of high cutoff dialysis membranes in prior studies, which are not routinely used. Conclusion(s) Although HD rapidly resolved this patient's hypothermia, a minimal amount of phenytoin was recovered in the patient's dialysate. Prior studies suggesting consequential clearance and efficacy of phenytoin removal by extracorporeal treatment may not apply to routine HD methods. Further studies on the utility of extracorporeal treatment for phenytoin toxicity are needed.