Phenytoin

Question 1

MoA

Answer 1

Voltage-gated neuronal Na+ channels
Antagonist
Inhibits influx of Na+ into neuronal cells
Slows rate of recovery of Na+ channels from inactivation
Stabilises neuronal membrane + prevents hyper excitability
Limits spread of seizure activity
Reduces seizure propagation

Question 2

Indications

Answer 2

Epilepsy except absent seizures

Question 3

ADR

Answer 3

P = P450 interactions
H = Hirsutism
E = Enlarged gums
N = Nystagmus
Y = Yellow-browning of skin
T = Teratogenic
O = Osteomalacia
I = Interferes with Folate metabolism --> Anaemia
N = Neuropathies: Vertigo, Ataxia, Headaches

Question 4

Prescribing Advice

Answer 4

CYP450 INDUCER
OCP (reduced contraceptive effect)
Theophylline (reduced plasma conc of both drugs)
Cimetidine (reduced plasma phenytoin conc)
Amiodarone (increased plasma phenytoin conc)