Phenytoin Toxicity — Management

Pharmacokinetic Background (Relevant to Toxicity)

• Oral absorption is slow, variable, and often incomplete, especially in overdose; peak levels typically occur 3–12 hours after a single oral dose — serial serum levels may be needed
• Phenytoin follows zero-order (saturable) kinetics at therapeutic and supratherapeutic levels: small increases in dose produce disproportionately large increases in serum concentration
• Protein binding ~90% (primarily albumin); free fraction increases with hypoalbuminemia or uremia — adjust interpretation accordingly (Sheiner-Tozer formula)
• Elimination half-life: 7–42 hours at therapeutic doses, but prolonged unpredictably in overdose

Clinical Features of Toxicity

• Nystagmus (earliest sign), ataxia, dysarthria, diplopia
• Progressive lethargy → confusion → coma
• Paradoxical seizures at very high concentrations

• Hypotension, bradycardia
• PR prolongation, QRS widening, ventricular arrhythmias
• Heart block, asystole
• Rate must not exceed 50 mg/min IV (25 mg/min in elderly/cardiac patients) — fosphenytoin (150 mg phenytoin equivalents/min) is safer parenterally

Diagnosis

• Serum phenytoin level is the primary test; therapeutic range is 10–20 mcg/mL (40–79 µmol/L)
• Free (unbound) phenytoin level ~1–2 mcg/mL is more accurate in hypoalbuminemia or renal failure
• Corrected phenytoin level in hypoalbuminemia: Corrected = measured ÷ [(0.2 × albumin g/dL) + 0.1]
• ECG: assess QRS, PR, QTc (especially after IV administration)
• Metabolic panel, glucose, LFTs (phenytoin can rarely cause hepatotoxicity)

Management

1. Stabilization (ABCDE)

• Airway management for obtunded/comatose patients
• IV access, cardiac monitoring, pulse oximetry
• 12-lead ECG

2. Decontamination

• Activated charcoal (1 g/kg PO/NG): consider if within 1–2 hours of oral ingestion and patient is not obtunded (aspiration risk)
• Multidose activated charcoal (MDAC) can decrease phenytoin half-life but has not been shown to change clinical outcome or shorten duration of toxicity in trials — routine use is not recommended
• Gastric lavage is rarely indicated

3. Cardiovascular Toxicity (IV phenytoin-related)

• Stop or slow the infusion immediately
• Hypotension: IV fluids, vasopressors if refractory
• Bradycardia/heart block: atropine; temporary pacing if severe
• Ventricular arrhythmias: standard ACLS; sodium bicarbonate for QRS widening is reasonable (although phenytoin is not a sodium channel blocker in the classical TCA sense, the propylene glycol vehicle causes cardiovascular toxicity)
• Fosphenytoin should be preferred over phenytoin IV when parenteral loading is clinically required, due to its much safer cardiovascular profile

4. Seizures Paradoxically Caused by Phenytoin Toxicity

• Phenytoin is not effective for controlling seizures it induces
• Use benzodiazepines (lorazepam, diazepam) as first-line
• Consider phenobarbital or propofol for refractory cases
• Consult medical toxicology for serious toxicity

5. Enhanced Elimination

• Hemodialysis is generally not effective due to high protein binding (~90%)
• Activated charcoal hemoperfusion has been used in severe cases but clinical evidence is limited
• Supportive care with time is the primary strategy for oral overdose

6. Supportive Care

• Most patients with oral phenytoin overdose recover fully with supportive care alone
• Serious complications are extremely rare after intentional oral overdose
• Monitor neurological status serially; symptoms resolve as levels decline

Disposition

Fosphenytoin vs. Phenytoin (IV Safety)

• Aqueous solution (pH 8.8, no propylene glycol) — far fewer cardiovascular adverse effects
• Can be given IM
• Can be infused faster (150 mg PE/min vs. 50 mg/min for phenytoin)
• Dosed in phenytoin equivalents (PE)

No recent systematic reviews or meta-analyses (last 2 years) were identified in PubMed specifically on phenytoin toxicity management, so the above guidance reflects established emergency medicine practice from authoritative textbooks.

IVIg Dose in Guillain-Barré Syndrome (GBS)

Standard Dose

2 g/kg total, divided over 5 days (0.4 g/kg/day × 5 days)

2 g/kg over 2 days (1 g/kg/day × 2 days) — used when a faster course is needed or tolerated

Indications for IVIg

• Moderate to severe GBS: patients unable to walk independently or worse
• Should be started within 2 weeks of symptom onset (benefit is clearest in this window; some benefit up to 4 weeks)
• Equally effective as therapeutic plasma exchange (TPE) — the choice depends on availability, patient factors, and contraindications
• Combination of IVIg + plasma exchange offers no additional benefit over either alone
• Corticosteroids alone are not effective in GBS (methylprednisolone + IVIg has a slight initial advantage but no long-term benefit vs. IVIg alone — generally not recommended given steroid risks)

Second Dose (Retreatment)

The 2024 systematic review (PMID: 38553651) on IVIg retreatment in poor responders also confirms this — routine retreatment is not recommended.

Summary Table