TOXBASEŽ - Updated 08/2002

 

Type of product:

Anticonvulsant which inhibits voltage dependent sodium channels.

 

Ingredients:

Phenytoin sodium
Tablets - 50 mg, 100 mg
Capsules - 25 mg, 50 mg, 100 mg, 300 mg
Phenytoin
Infatabs - 50 mg
Suspension - 30 mg/5 mL

 

Toxicity:

Deaths after acute ingestion are rare. Adults have survived up to 21 g. A 4 year old child died after ingestion of 2 g.

In naive adults symptoms are unlikely if they have taken less than 1 g. There is currently inadequate evidence on which to define safe dose limits for children.

The half-life varies from 7-60 hours and is dose-dependant.

 

Features:

Nausea, vomiting, ataxia and fine, rapid nystagmus on lateral gaze.

Less frequently, dysarthria with increasing incoordination of the limbs and trunk which, after gross overdosage, may be so severe that the patient cannot lift head or body from the bed without risk of injuring himself. Divergent gaze, impairment of consciousness, hypotension, apnoea and respiratory depression may also occur.

Cardiovascular toxicity may follow rapid IV injection (bradycardia, hypotension and arrhythmia) but is very rarely seen after poisoning by the oral route.

 

Management:

The benefit of gastric decontamination is uncertain.
1. Consider activated charcoal (50 g for adults; 10-15 g for children) if the patient presents within 1 hour of ingestion of more than 20 mg/kg body weight.

2. Consider repeated doses of oral activated charcoal, provided the patient is not vomiting and the airway can be protected. There are no controlled trials which demonstrate benefit in overdosage but evidence in volunteers does suggest it may increase the elimination of phenytoin.

3. Most patients with acute phenytoin overdose do not require measurement of plasma phenytoin concentration. An urgent phenytoin concentration is helpful (but not essential) if multiple dose activated charcoal is being contemplated, particularly if the diagnosis is in doubt, e.g. in patients with coma, respiratory depression or arrhythmias. The usual target range for phenytoin is 8-15 mg/L (32 - 59 micromol/L). Symptomatic toxicity is usually associated with concentrations in excess of 20 mg/L (79 micromol/L), while concentrations > 40 mg/L (159 micromol/L) suggest serious toxicity.

4. Observe for at least 4 hours. Patients who have not developed symptoms by this time are unlikely to do so.

5. Give intravenous fluids if vomiting is severe.

6. Correct hypotension by raising the foot of the bed or in severe cases by expanding the intravascular volume.

7. Protect the patient from falling if ataxia is gross.

8. Other measures as indicated by the patient's clinical condition.

9. If the patient is an epileptic monitor plasma phenytoin concentrations daily to help decide when to re-start therapeutic doses