Introduction
Phenytoin is used in the management of seizures. It was one of the first antiepileptics to be developed.
Mechanism of action
Binds to sodium channels increasing their refractory period.
Adverse effects
Phenytoin is associated with a large number of adverse effects. These may be divided into acute, chronic, idiosyncratic and teratogenic. Phenytoin is also an inducer of the P450 system.
Acute
Chronic
Idiosyncratic
Teratogenic
Acute
- Initially: dizziness, diplopia, nystagmus, slurred speech, ataxia
- Later: confusion, seizures
Chronic
- Common: gingival hyperplasia (secondary to increased expression of platelet derived growth factor, PDGF), hirsutism, coarsening of facial features, drowsiness
- Megaloblastic anaemia (secondary to altered folate metabolism)
- Peripheral neuropathy
- Enhanced vitamin D metabolism causing osteomalacia
- Lymphadenopathy
- Dyskinesia
Idiosyncratic
- Fever
- Rashes, including severe reactions such as toxic epidermal necrolysis
- Hepatitis
- Dupuytren's contracture*
- Aplastic anaemia
- Drug-induced lupus
Teratogenic
- Associated with cleft palate and congenital heart disease
Monitoring
Phenytoin levels do not need to be monitored routinely but trough levels, immediately before dose should be checked if:
- Adjustment of phenytoin dose
- Suspected toxicity
- Detection of non-adherence to the prescribed medication