Phenytoin Toxicity
Phenytoin toxicity manifests primarily as central nervous system dysfunction, with nystagmus, ataxia, and dysarthria being the hallmark features, progressing to confusion, lethargy, and coma at higher serum levels. 1
Clinical Manifestations by Serum Level
The clinical presentation of phenytoin toxicity follows a predictable pattern based on serum concentrations 1:
- 20 mcg/mL: Nystagmus on lateral gaze appears
- 30 mcg/mL: Ataxia develops
- >40 mcg/mL: Dysarthria and lethargy emerge
- >50 mcg/mL: Severe toxicity with potential coma
However, marked individual variation exists—some patients tolerate concentrations up to 50 mcg/mL without toxicity 1.
Common Clinical Features
The most frequently observed symptoms include 2, 3:
- Neurological: Nystagmus (most common initial finding), ataxia, unsteady gait, dizziness/vertigo, confusion, tremor, hyperreflexia, slurred speech, diplopia
- Gastrointestinal: Nausea and vomiting
- Progressive symptoms: Lethargy, decreased consciousness, coma in severe cases
- Seizures: Paradoxically can occur in severe intoxication
Ataxia occurred in 63% of hospitalized patients in one series, with 18 patients falling and 9 sustaining injuries requiring medical care 4.
Cardiac Manifestations
Cardiac complications are rare with oral phenytoin overdose 2, 4. In a review of 94 cases, no hemodynamic instability occurred, and electrocardiographic monitoring revealed no phenytoin-related abnormalities in 76% of cases with available records 4.
However, parenteral administration carries significant cardiac risk, including dysrhythmias, hypotension, and cardiac arrest, particularly with intravenous phenytoin (less so with fosphenytoin) 5.
Causes of Toxicity
The most common precipitants include 3, 6, 7:
- Excessive self-medication or dosing errors
- Drug interactions: Particularly with isoniazid (blocks CYP2C19), omeprazole, and other CYP2C9 inhibitors
- Pharmacogenetic factors: CYP2C9 polymorphisms (*3/*3 genotype reduces metabolism by 50%) 7, 8
- Saturation kinetics: Zero-order pharmacokinetics at therapeutic doses means small dose increases can cause disproportionate serum level rises
Management
Treatment is entirely supportive—there is no antidote for phenytoin toxicity 1, 2, 9.
Immediate Management
- Discontinue phenytoin immediately
- Ensure airway protection and respiratory support if consciousness is depressed
- Monitor vital signs (though cardiac complications are unlikely with oral overdose)
- Prevent fall-related injuries due to ataxia—this is the primary source of morbidity 4
- Manage nausea and vomiting symptomatically
Decontamination
Activated charcoal may be considered if presentation is early (within 1-2 hours), but there is no evidence it improves outcomes 2, 9. Multiple-dose activated charcoal has shown increased clearance rates in experimental studies but no proven clinical benefit 2, 9.
Enhanced Elimination
No invasive enhanced elimination method is recommended 2, 9. Hemodialysis, hemoperfusion, plasmapheresis, and forced diuresis have all proven ineffective and carry their own risks, as phenytoin is highly protein-bound with a large volume of distribution 2, 9.
Clinical Course and Prognosis
The clinical course is typically prolonged but benign 2, 3, 4:
- Half-life is greatly increased in overdose due to zero-order kinetics, resulting in symptoms lasting a week or more 2, 9
- Deaths are extremely rare with phenytoin ingestion alone 2
- All patients in published series recovered with supportive care alone 3, 4
- Hospital admission is indicated until serum levels are declining and ataxia resolves 4
Common Pitfalls
Misdiagnosis: Phenytoin toxicity is under-recognized in emergency settings, with 33% diagnostic inaccuracy in one series. The most common misdiagnosis is brainstem or cerebellar stroke (14% of cases) 3.
Routine cardiac monitoring is unnecessary for oral overdose—no evidence supports this practice 4.
Drug interactions are frequently overlooked: Always review concomitant medications, particularly isoniazid, which can cause catastrophic toxicity even at therapeutic phenytoin doses 6.
Genetic polymorphisms: Consider CYP2C9 testing in patients with unexplained toxicity at therapeutic doses, particularly CYP2C9*3/*3 genotype 7, 8.