Seizures may occur (a) as a manifestation of acute porphyria, where they may be secondary to the hyponatraemia which develops in up 35% of acute attacks or (b) due to a cause unrelated to porphyria. Treatment firstly involves terminating the seizure and then assessing the likely cause and planning the most appropriate therapy. In the case of hyponatraemia this involves slow correction of the electrolyte imbalance by fluid restriction and isotonic or hypertonic saline where necessary.
A major problem in the management of seizures is that many of the commonly used anticonvulsants can precipitate or worsen acute attacks. Therefore where a primary seizure disorder is suspected this should be fully investigated by an epilepsy expert to ensure that treatment is absolutely necessary.
Termination of an acute convulsion should be with an intravenous benzodiazepine such as lorazepam, diazepam or clonazepam. The choice of diazepam may be controversial but it is almost certainly safe as a single intravenous dose.
Under no circumstances should phenytoin or phenobarbitone be used.
Where benzodiazepine treatment fails, paraldehyde or magnesium sulphate should be considered.
Where general anaesthesia is required propofol is the drug of choice.
Wherever possible an anticonvulsant should be chosen from the list of safe alternatives below. When control of epilepsy is particularly difficult or no safe alternative is effective, discussion with a recognised porphyria expert may help in the selection of an appropriate drug regime that minimises risk to the patient (see porphyria specialist centres). Close biochemical monitoring by regular measurent of urine porphobilinogen is advisable under these circumstances (click to monitoring).
Where the patient is on an anticonvulsant that carries a particularly high risk (e.g. carbamazepine, phenytoin, phenobarbitone, primidone, ethosuximide) changing to a safer alternative is advisable. However change of therapy may occasionally provoke either acute porphyria or epilepsy and patient should always be informed of the risk and carefully monitored (click to monitoring).
In some cases the risk of changing anticonvulsant therapy, eg in a patient whose seizures are difficult to control, may outweigh that of inducing an acute attack. In these circumstances, the patient should be informed of the risk and advised to seek medical help should symptoms of acute porphyria develop.
(P; denotes porphyrogenic in cell culture/animal model, C; denotes drug known to cause acute attacks in patients)
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