Related Subjects:
|Status Epilepticus (Epilepsy)
|Coma management
|Lorazepam
|Phenytoin
|Levetiracetam
|Epilepsy - General Management
|First Seizure
|Epilepsy in Pregnancy
|Febrile seizures
⚠️ Phenytoin Toxicity – Red Flag Symptoms
🚨 Ataxia • Diplopia • Nystagmus • Sedation • Dysarthria • Confusion
➡️ Check serum levels and review dosing immediately.
💊 About Phenytoin
Always
check the BNF here
for up-to-date prescribing information.
- 🧠 Phenytoin is a sodium-channel blocking antiepileptic drug (AED).
- 💉 Now mainly used intravenously (e.g. status epilepticus).
- 📉 Less favoured for long-term oral use due to toxicity and interactions.
- 🧪 Therapeutic trough level: 10–20 mg/L (40–80 µmol/L).
⚙️ Mode of Action
- 🚪 Blocks voltage-gated Na⁺ channels in neurons.
- ⛔ Limits repetitive neuronal firing.
- 📉 Stabilises neuronal membranes.
🧬 Pharmacokinetics (High-Yield)
- ⚠️ Zero-order kinetics at therapeutic doses:
- Small dose increases → disproportionate level rise.
- High risk of toxicity.
- 🩸 ~90% protein bound.
- 💊 Displaced by valproate, aspirin → ↑ free fraction.
- 🏭 Strong hepatic enzyme inducer.
🧠 Clinical pearl: In hypoalbuminaemia, liver disease, pregnancy, or critical illness, measure or calculate free phenytoin.
🎯 Indications
- ⚡ Status epilepticus (IV loading).
- 🧠 Focal and generalised tonic-clonic seizures.
- 😖 Trigeminal neuralgia (second-line).
- 🏥 Post-neurosurgical seizure prophylaxis (selected cases).
🚫 Contraindications & Cautions
- ⛔ Porphyria.
- ❤️ Sinus bradycardia.
- 🫀 Heart block.
- ⚠️ Severe hepatic impairment.
- 🤰 Pregnancy (specialist advice required).
⚠️ Side Effects
- 🚨 Dose-related toxicity: ataxia, diplopia, nystagmus, sedation, dysarthria.
- 🧬 Gingival hyperplasia.
- 🧔 Hirsutism, coarsened facial features, acne.
- 🩸 Folate deficiency → macrocytosis.
- 🧠 Cerebellar atrophy (long-term use).
- 🦴 Vitamin D deficiency → osteomalacia.
- 👶 Teratogenicity (fetal hydantoin syndrome).
- 🔥 IV extravasation → tissue necrosis (“purple glove syndrome”).
🔁 Drug Interactions
- ⬇️ Reduced efficacy of OCP, steroids, thyroxine, TCAs, theophylline.
- ⬆️ Levels increased by valproate, amiodarone, omeprazole.
- 🍺 Acute alcohol ↑ levels; chronic alcohol ↓ levels.
🚫 Drugs That Reduce Phenytoin Breakdown
- Erythromycin
- Metronidazole
- Allopurinol
- Amiodarone
- Omeprazole
- Sulphonamides
💉 Loading Dose (Status Epilepticus)
- 📏 Standard loading: 20 mg/kg IV (if not already on phenytoin).
- 💧 Dilute in 0.9% saline only (never dextrose).
- 🫀 Continuous ECG and BP monitoring.
- 🧪 Use inline filter if available.
- 🚦 Maximum rate: 50 mg/min (25 mg/min in elderly/cardiac disease).
🚑 Emergency Loading Dose Guide
| Weight (kg) |
Dose (Saline) |
| 40–49 | 750 mg in 100 ml |
| 50–64 | 1000 mg in 100 ml |
| 65–78 | 1250 mg in 250 ml |
| 79–92 | 1500 mg in 250 ml |
| >92 | 1750 mg in 250 ml |
💊 Maintenance Dosing
| Drug |
Dose |
Frequency |
Route |
| Phenytoin | 200–300 mg | OD | PO |
| Phenytoin | 100 mg | TDS | PO |
📚 References
