The drugs employed for the prevention and management of seizures lack an appropriate name. Most of these agents only suppress seizures and do not treat epilepsy itself, and for that reason ought not be called antiepileptics. The term anticonvulsant ignores their effects on seizure types other than convulsions. For lack of a better choice, they are described here as antiseizure agents. The important pharmacological properties of these drugs are summarized in Table..,!,.
Table 1 Pharmacological properties of selected antiseizure agents in critical care practice
In general, patients receiving antiseizure agents prior to developing a critical illness unrelated to their epilepsy should continue with the same drug(s). Patients controlled on an agent only available for oral administration (e.g. carbamazepine) whose illness prevents enteral medication should either be switched to phenytoin or covered with a benzodiazepine (e.g. lorazepam or clonazepam) until enteral medication is possible. Abrupt termination of GABA A agonists (e.g. benzodiazepines and barbiturates) is associated with withdrawal convulsions even in patients without a history of seizures, and so these drugs should either be tapered slowly or, in the rare case of allergy or a serious adverse reaction, substituted with another GABA A agonist. It is less certain whether phenytoin, valproate, and other drugs acting by different mechanisms also produce withdrawal convulsions, but it is prudent to assume that patients receiving these drugs need them and therefore to continue treatment.
Only drugs available for parenteral administration are discussed here. The reader is referred to standard texts on antiseizure agents and epilepsy for more detail or for information about enterally administered drugs.
One major source of confusion in the use of antiseizure agents is the rational use of plasma concentrations ('levels'). In out-patient practice, the appropriate concentration of an agent is one in which seizures are controlled without adverse effects. Measurement of serum concentrations is undertaken to ensure compliance and absorption, and to guide changes in the regimen dictated by breakthrough seizures; they are also useful in determining the cause of a patient's apparent intoxication when more than one antiseizure agent is being used. Since seizures are often infrequent events, it is difficult to know what the appropriate concentration of an antiseizure agent will be in a critically ill patient. The laboratory's 'therapeutic range' is generally used in such circumstances, despite the lack of evidence that this is useful in the critically ill patient.
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