What is the most appropriate next step for a pregnant woman with stable epilepsy taking Phenytoin (Dilantin) at 15 weeks gestation?

Management of Phenytoin in a Pregnant Woman with Stable Epilepsy at 15 Weeks Gestation

Continue the same medication (phenytoin) at the lowest effective dose as monotherapy, supplement with high-dose folic acid, and monitor closely throughout pregnancy. 1, 2

Rationale for Continuing Phenytoin

Abruptly stopping or switching antiepileptic drugs during pregnancy poses greater risks to both mother and fetus than continuing treatment. 1, 2 The FDA label explicitly warns that stopping phenytoin suddenly can cause status epilepticus (seizures that will not stop), which is life-threatening. 2 With the patient's last seizure occurring 6 years ago, she has excellent seizure control that should not be jeopardized. 3, 4

• Uncontrolled tonic-clonic seizures during pregnancy are potentially hazardous to both mother and fetus, and are generally assumed to be more harmful than antiepileptic drug exposure. 3, 5
• At 15 weeks gestation, the critical period of organogenesis (weeks 3-8) has already passed, making medication changes less beneficial for preventing major malformations while still risking seizure breakthrough. 3, 6
• Switching medications at this stage introduces uncertainty about seizure control with a new agent and may require polytherapy during the transition, which carries higher teratogenic risks than monotherapy. 3, 6, 5

Why Not Switch to a "Safer" Drug

While valproic acid should be avoided in pregnancy due to higher malformation rates and cognitive risks, phenytoin is an acceptable option when seizure control has been established. 7, 3, 6 The WHO guidelines specifically list phenytoin as a standard antiepileptic drug for convulsive epilepsy, noting it is compatible with breastfeeding. 7

• The decision about which antiepileptic drug to use should ideally have been made before conception, not during pregnancy. 3, 4
• Changing medications during pregnancy risks loss of seizure control, which poses immediate danger versus the theoretical benefit of switching to another drug with similar teratogenic potential. 1, 6

Essential Management Steps

Immediate Actions

• Supplement with high-dose folic acid (5 mg daily) to reduce the risk of neural tube defects, which is elevated with phenytoin exposure. 1, 4 Standard 400 mcg dosing is insufficient for women on antiepileptic drugs. 8, 4
• Use the lowest effective dose as monotherapy to minimize fetal exposure while maintaining seizure control. 1, 2, 3
• Obtain baseline phenytoin serum levels and establish a therapeutic monitoring schedule, as pregnancy alters drug metabolism. 2, 3

Ongoing Pregnancy Monitoring

• Monitor phenytoin levels monthly or as clinically indicated, as pregnancy can decrease serum concentrations through increased metabolism and volume of distribution. 2, 3, 4
• Perform detailed ultrasound examination and maternal serum alpha-fetoprotein testing to screen for phenytoin-associated birth defects, particularly neural tube defects and cardiac malformations. 2, 4
• Adjust dosing based on serum levels and clinical response, with postpartum restoration to pre-pregnancy dosing anticipated. 2

Third Trimester Preparations

• Begin maternal vitamin K (phytomenadione) supplementation 4 weeks before expected delivery to prevent neonatal bleeding complications related to phenytoin's interference with vitamin K-dependent clotting factors. 2, 4
• Ensure optimal seizure control during the third trimester to minimize risk of convulsions during labor and delivery. 4

Neonatal Care

• Administer vitamin K to the newborn immediately after birth to prevent potentially life-threatening bleeding disorder. 2, 4
• Monitor the infant for neonatal adaptation syndrome, which may include irritability, feeding difficulties, tremors, and rarely seizures, typically resolving within 1-4 weeks. 2, 9
• Assess the neonate for phenytoin-associated dysmorphology, including facial features, nail and digit hypoplasia, and growth parameters. 2, 9

Common Pitfalls to Avoid

• Do not abruptly discontinue phenytoin without neurologist consultation, as this can precipitate status epilepticus. 1, 2
• Do not use standard-dose (400 mcg) folic acid—women on antiepileptic drugs require 5 mg daily. 4
• Do not assume seizure control will remain stable—pregnancy-induced pharmacokinetic changes may necessitate dose adjustments. 2, 3
• Do not forget vitamin K supplementation—both maternal (late pregnancy) and neonatal (at birth) administration are required. 2, 4

Counseling Points

The patient should understand that phenytoin increases the risk of congenital malformations to approximately 10% (compared to 2-3% in the general population), but this risk is outweighed by the dangers of uncontrolled seizures. 2, 3 She should be informed about the North American Antiepileptic Drug Pregnancy Registry (1-888-233-2334) for enrollment. 2