6 Anticonvulsants
Magnesium sulphate is also an anticonvulsant used in the emergency treatment of eclampsia – see Chapter 25.
The student should be aware of:
conditions that require anticonvulsant therapy
local protocols for anticonvulsant therapy and specific medical conditions
recognition of fits, seizures and convulsions
resuscitative techniques and care of affected clients
maternal and fetal sequelae of absorption of these preparations.
Interactions
Phenytoin
Alcohol – high intake increases plasma phenytoin levels; chronic abuse decreases serum levels
Analgesics – NSAIDs increase the plasma phenytoin concentration
Antacids – reduce the absorption of phenytoin; cimetidine decreases the metabolism of phenytoin and therefore increases its plasma concentration
Antibiotics – metronidazole increases the plasma phenytoin concentration; plasma concentration and antifolate effect increased by co-trimoxazole and trimethoprim
Anticoagulants – probably reduce the effect of warfarin
Antidepressants – tricyclics decrease the phenytoin plasma concentration and the convulsive threshold
Antiepileptics – two or more antiepileptics enhance toxicity; monitoring of plasma concentrations required
Antiemetics – stemetil and derivatives lower convulsive threshold
Antihypertensives – nifedipine increases phenytoin plasma concentration; the effect of nifedipine is reduced
Anxiolytics and hypnotics – diazepam can increase or decrease plasma phenytoin concentration
Corticosteroids – metabolism is increased, so effect is decreased
Contraceptives – metabolism of oral contraceptives is increased, so their effect is decreased
Vitamins – the plasma phenytoin concentration is lowered by folic acid; vitamin D supplements may be required.
Phenobarbital
Alcohol – increased sedative effect
Antibiotics – metabolism of metronidazole is increased, so the effect is decreased
Anticoagulants – metabolism of warfarin increased, so effect is decreased
Antidepressants – tricyclics decrease the plasma concentration and the convulsive threshold
Antiepileptics – as for phenytoin; requires dose monitoring
Antiemetics – as for phenytoin
Antihypertensives – effect of nifedipine reduced
Corticosteroids – as for phenytoin
Contraceptives – as for phenytoin
Sodium valproate
Analgesics – aspirin enhances effect
Antacids – cimetidine increases plasma levels of valproate
Antibiotics – erythromycin increases plasma levels of valproate
Antiemetics – as for phenytoin
Antiepileptics – with two or more, close monitoring is required
Anticoagulants – increased anticoagulant effect – monitoring of PT required
Cholestyramine – decreases absorption of valproate
Zidovudine – antagonism of the metabolism of zidovudine, so increased toxicity
Carbamazepine
Alcohol – enhanced CNS effects
Antidepressants – tricyclics have an accelerated metabolism, so decreased effect; monitoring required
Antiepileptics – plasma concentration effected by concomitant use, so careful plasma monitoring required
Anticoagulants – decreased anticoagulant effect of warfarin
Cimetidine – inhibits the metabolism of carbamazepine, so increases plasma concentration
Corticosteroids – carbamazepine increases the metabolism of both β-prednisolone and dexamethasone
Dextropropoxyphene – increases effect of carbamazepine
Erythromycin – increased plasma concentrations of carbamazepine
Nifedipine – decreased antiepileptic effect
OCP – decreased contraceptive effect
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