Phenytoin Flashcards
1
Q
What is the method of action of phenytoin?
A
- ↓ Neuronal excitability and electrical conductance among brain cells
- = inhibits the spread of seizures
- Bings to neuronal Na+ channels in their inactive state
- = prolongs inactivity
- Prevents Na+ flux and changes in the membrane potential
2
Q
What are the indications for phenytoin?
A
- Control seizures in status epilepticus – when benzodiazepines are ineffective
- Reduce frequency of generalised or focal seizures in epilepsy
3
Q
What are the contraindications for phenytoin?
A
- Hepatic impairment - ↓ therapeutic index
* Pregnancy – craniofacial abnormalities, reduced IQ
4
Q
What are the side effects of phenytoin?
A
- Changes in appearance – skin coarsening, acne, hirsutism (excessive hair growth), gun hypertrophy
- Cerebellar toxicity – nystagmus, ataxia, discoordination
- Impaired cognition/consciousness
- Haematological disorders
- Osteomalacia
- Cardiovascular collapse – phenytoin toxicity due to overdose/IV infusion
- Respiratory depression – phenytoin toxicity due to overdose/IV infusion
5
Q
What are the drug interactions with phenytoin?
A
- Warfarin - ↓ plasma concentration
- Oestrogen/progesterone - ↓ plasma concentration
- Amiodarone - ↑ conc. of phenytoin
- Diltiazem - ↑ conc. of phenytoin
- Fluconazole - ↑ conc. of phenytoin
- Other antiepileptic drugs
- SSRI’s – lowers the seizure threshold
- Tricyclic antidepressants – lowers the seizure threshold
- Anti-psychotics – lowers the seizure threshold
- Tramadol – lowers the seizure threshold
6
Q
What organ eliminates phenytoin?
A
Kidney
7
Q
What information should you give to a patient taking phenytoin?
A
- Medication should not be stopped suddenly due to risk of seizure recurrence
- Take with or after food
- Do not miss any doses
- Do not drive unless seizure free for 12 months (or asleep only seizures over 3 years) and for 6 months after changing/stopping treatment
