Objective: To determine plasma phenytoin levels and seizure outcome in women given phenytoin for seizure prophylaxis in severe pre-eclampsia and eclampsia.
Design: Prospective observational study comparing two phenytoin loading regimens.
Setting: Two UK teaching hospitals.
Subjects: Sixty-seven consecutive women with severe pre-eclampsia and five with eclampsia.
Interventions: The first 29 women were given a 15 mg/kg intravenous loading dose of phenytoin. The next 43 received 17.5 mg/kg. All were given 500 mg phenytoin 12 h after completion of the loading dose and then 250 mg every 12 h for four doses.
Main outcome measures: Total plasma phenytoin levels at 30 min, 6 h and 12 h after loading dose, 6 h after first maintenance dose and on days 2 and 3 of maintenance therapy; eclamptic seizures after starting phenytoin.
Results: Mean plasma phenytoin levels were higher at 30 min and 6 h after the 17.5 mg/kg loading dose. Nine of 29 (31%) phenytoin levels 30 min after the loading dose were above the therapeutic range in the 15 mg/kg group compared with 26/38 (68%) in the 17.5 mg/kg group (P < 0.01). Six of 27 (22%) phenytoin levels 12 h after the loading dose were subtherapeutic in the 15 mg/kg group compared with 2/38 (5%) in the 17.5 mg/kg group (P < 0.05). Three women, two in the 17.5 mg/kg group, developed seizures after starting phenytoin. All three had plasma levels within the therapeutic range.
Conclusions: Compared with a loading dose of 17.5 mg/kg, loading with 15 mg/kg phenytoin was associated with a lower incidence of high plasma levels at 30 min but a higher incidence of subtherapeutic levels at 12 h. Seizures occur in 2 to 3% of pre-eclamptics despite apparently therapeutic phenytoin levels.