Double Dose, Double Trouble – Dilantin Toxicity

98 y/o female had a long history of seizures was treated with Dilantin (phenytoin) 100 mg twice daily.  The Dilantin level was routinely drawn every 6 months and had been in the 6-10 range for quite sometime (normal total level is 10-20, but there are multiple variables that can make the value less than accurate).  The most recent level was 5 and the primary provider was concerned it was too low and increased the dose from 100 mg BID to 200 mg BID.

Keep in mind this patient had not had a seizure for years.  This patient’s albumin was lower as well, which actually increases the corrected Dilantin value as well.  An increase in a maintenance dose like this with Dilantin should scare you.  I have seen toxicity result several times due to inappropriate increases.

Dilantin is metabolized by a few different enzymes, and when those enzymes get saturated, the amount of Dilantin in the body can skyrocket quickly.  Think of a hockey stick type curve.  So clinically what this means is that when you start to hit the upward slope of that curve, small increases in dose is the usual practice.  MODERATE TO LARGE INCREASES IN DILANTIN CAN LEAD TO HUGE JUMPS IN LEVELS!  Pharmacokinetics is an ugly word for some, but not knowing the kinetics of Dilantin can harm patients.

Within a week or two, this patient began displaying signs of Dilantin toxicity – GI symptoms, difficulty with walking, lethargy, and confusion.  She was hospitalized and was diagnosed with Dilantin toxicity with a total level of 28.

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Written By Eric Christianson

July 23, 2014

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