Phenytoin when used as a loading dose in ED:
– loading dose: 18 mg /kg, max 1.2 g
– patients need to be cardiac monitored due to the risk of arrhythmias
– infusion rate is max 50 mg/min (or 1 mg/kg/min) as the propylene glycol it contains can cause arrhythmias and hypotension
– should not be mixed with glucose solutions (as it will precipitate)
Contraindications:
– known allergy to phenytoin
– 2nd or 3rd degree AV block
– seizure activity secondary to toxic ingestion
Complications:
– arrhythmias / hypotension
– toxic epidermic necrolysis / Stevens-Johnson syndrome
– if accidentally given intraarterially or subcutaneously, it can cause extensive soft tissue injury due to the alkaline pH
Toxicity (chronic use or acute ingestion):
– correlated with serum levels:
20 – 30 mg/L – nystagmus
30 – 50 mg/L – ataxia (NB check phenytoin levels on all patients on chronic treatment who present with ataxia)
> 50 mg/L – coma
– symptoms resolve slowly over 2 – 4 days
– there is no specific antidote – supportive care is key
– massive overdoses can cause seizures (paradoxically) / hypernatraemia / hyperglycaemia / permanent cerebellar injury