Seizures occurring during another critical illness usually represent a central nervous system manifestation of a systemic disorder. In a prospective study of neurological complications of critical medical illnesses, seizures were the second most common problem encountered (and were almost as frequent as metabolic encephalopathy). The most frequent causes were cerebrovascular disease, central nervous system infections, metabolic encephalopathies, neoplasms, hypoglycemia, and osmolar disorders (including non-ketotic hyperglycemia). A retrospective analysis found that drug withdrawal was the most common etiology of seizures. Table 1 summarizes common causes of seizures in ICU patients.
Table 1 Common etiologies of seizures in critically ill patients
Iatrogenic precipitant seizures should be rapidly detected and the cause removed, if possible, instead of treating the patient only with antiseizure agents. Seizures occurring because of the abrupt withdrawal of GABA agonists (e.g. hypnosedatives such as barbiturates and benzodiazepines) are managed by using a drug of the same class, followed by a very slowly decreasing dose to prevent subsequent withdrawal seizures. The actual agent selected may be different to that from which the patient is withdrawing; the choice depends on the other circumstances of the patient. Alcohol withdrawal seizures constitute a special type of hypnosedative withdrawal, because several neurochemical systems are involved in addition to GABA and other withdrawal phenomena may be anticipated once seizures begin. Although seizures due to withdrawal of alcohol (and other hypnosedative agents) are usually thought to be primarily generalized, a substantial minority of alcohol withdrawal seizures have focal components. These localized cortical disturbances probably reflect pre-existing areas of cortical hyperexcitability, as might follow remote head trauma, which are rendered more excitable by the loss of inhibition.
Acute metabolic disturbances commonly produce seizures in ICU patients, but one must consider that seizures may be symptoms of structural disorders which are made manifest by metabolic disorders. Generalized convulsions are typical of metabolic disturbances, but two common exceptions are seizures due to non-ketotic hyperglycemia or hypoglycemia which often have focal components. Acute hyposmolar states (i.e. those developing over hours) frequently present with generalized convulsions, as well as evidence of increased intracranial pressure. Hyposmolar states developing over days are rarely associated with seizures; these patients more commonly present with weakness or confusion without seizures or loss of consciousness. Hypocalcemia is frequently listed as a cause of seizures in children and adults, but it is rarely the sole cause of convulsions beyond the neonatal period and must not be used as an excuse to avoid investigation for other etiologies, particularly structural lesions. The same is true of hypomagnesemia.
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