A 28-year-old woman with generalized tonic-clonic seizures has been seizure-free for 2 years on phenytoin monotherapy. She now presents to the neurology clinic planning pregnancy. Her current phenytoin level is 15 µg/mL (therapeutic range 10–20 µg/mL). On examination, she has mild gingival hyperplasia and coarse facial features. What is the most appropriate next step in her antiepileptic management?

Explanation

## Clinical Context This is a preconception counselling scenario for a woman on phenytoin with well-controlled seizures. The question tests knowledge of antiepileptic teratogenicity, enzyme induction, and pregnancy planning. ## Key Pharmacological Issues with Phenytoin in Pregnancy **Key Point:** Phenytoin is a known teratogen associated with fetal hydantoin syndrome (cleft palate, cardiac defects, microcephaly, growth restriction, intellectual disability). **High-Yield:** Phenytoin is a potent inducer of CYP450 enzymes, which increases metabolism of oral contraceptives, reducing their efficacy — a critical concern for women of childbearing age. ## Recommended Antiepileptics in Pregnancy | Drug | Teratogenic Risk | Enzyme Induction | Pregnancy Category | Preferred? | | --- | --- | --- | --- | --- | | Phenytoin | High (fetal hydantoin syndrome) | Yes (CYP3A4, CYP2C9) | D | No | | Lamotrigine | Low | No | C | Yes | | Levetiracetam | Very low | No | C | Yes | | Valproate | Very high (neural tube defects, developmental delay) | No | X | Contraindicated | | Carbamazepine | Moderate (similar to phenytoin) | Yes | D | Relative caution | **Clinical Pearl:** Lamotrigine and levetiracetam are the preferred first-line agents in women of childbearing potential because they have low teratogenic risk and do not induce CYP450 enzymes. ## Management Strategy 1. **Preconception:** Switch from phenytoin to lamotrigine or levetiracetam 3–6 months before planned conception. 2. **Dose adjustment:** Lamotrigine levels drop significantly in pregnancy (increased clearance); dosing may need to be increased by 50–100% during pregnancy and adjusted postpartum. 3. **Folic acid supplementation:** All women on antiepileptics planning pregnancy should receive high-dose folic acid (4–5 mg daily) to reduce neural tube defect risk. 4. **Seizure control:** Uncontrolled seizures pose greater risk to the fetus than the medication itself; seizure freedom must be maintained. **Mnemonic: LAMP** — **L**amotrigine, **A**cetazolamide (limited data but safer), **M**onitoring (frequent levels), **P**regnancy planning. ## Why This Patient Needs a Switch - Phenytoin causes fetal hydantoin syndrome. - Enzyme induction reduces oral contraceptive efficacy (unplanned pregnancy risk). - Lamotrigine or levetiracetam offer seizure control with lower teratogenic burden. - Switching before conception allows stabilization on the new agent.