by Susan T. Arnold, MD
Dr. Arnold is Assistant Professor of Neurology and Pediatrics at Washington University School of Medicine and Co-Director of the Pediatric Epilepsy Center at St. Louis Children’s Hospital.
Febrile seizures are the most common type of seizure encountered in infants and young children. While they are usually benign and self-limited, they frequently recur and a small number of children will subsequently develop epilepsy.
Approximately 4% of children will experience at least one febrile seizure, defined as an episode “occurring between 3 months and 5 years of age, associated with fever but without evidence of intracranial infection or defined cause” (NIH consensus statement). The majority (90%) of cases present between 6 months and 3 years, with a peak incidence between 18 and 24 months. A family history of febrile or afebrile seizures is present in 25% of patients.
Most febrile seizures are classified as “simple”, characterized by generalized tonic and/or clonic seizures lasting less than 15 minutes, with no focal features. “Complex” febrile seizures may have focal motor manifestations, last longer than 15 minutes, or recur more than once in a 24 hour period, and are associated with a higher risk of developing later epilepsy.
Febrile seizures typically occur at the onset of an illness. Although the risk of a seizure increases with higher fevers, half of all episodes occur at temperatures under 40 C.
Recurrent Febrile Seizures
One third of children will have recurrent febrile seizures, with 15% having more than one recurrence. The most important risk factor is age, with 50% of children less than one year experiencing recurrences compared to only 20% of children whose first febrile seizure occurs at greater than three years of age. Children with neurologic impairment or a family history of febrile or afebrile seizures are also at increased risk, as are children whose first febrile seizure is associated with a temperature of less than 40 C (presumably because of a lower threshold for seizures with fever). Complex partial seizures are not more likely to recur than simple ones, but complex features tend to repeat themselves, so a child with a prolonged initial seizure is also more likely to have a prolonged febrile seizure.
Risk of Developing Epilepsy
2-4% of children with febrile seizures will subsequently experience afebrile seizures. This risk is higher for children with complex febrile seizures, neurologic abnormalities, or a family history of epilepsy. In part, this reflects children diagnosed with febrile seizures who in retrospect actually had epilepsy, with their first seizures triggered by fever. However, febrile seizures may have a role in the pathogenesis of some types of epilepsy, particularly temporal lobe epilepsy where many patients have a history of febrile seizures in childhood. The nature of this relationship is still under investigation, and at this time there is no way to identify which children will develop temporal lobe epilepsy, or whether any form of treatment can prevent its occurrence.
Evaluation of Febrile Seizures
The initial evaluation of a child with febrile seizures should be aimed at excluding more serious conditions which may present with seizures and fever. The cornerstone of this process is a careful history and examination. Because signs of intracranial infection are difficult to evaluate in young infants, and febrile seizures are relatively uncommon in this group, infants under age 6 months should usually have a lumbar puncture to rule out meningoencephalitis. In older children with simple febrile seizures, the decision to perform an L.P. can be guided by the clinical presentation. Other causes for seizures such as electrolyte disturbances, intoxications, and trauma should be considered and screened for if not readily excluded by the history and exam.
Most children with complex febrile seizures merit a more extensive investigation, including a neuroimaging study and an EEG. An EEG may also be helpful in patients with multiple recurrent simple febrile seizures. These studies should be normal in febrile seizures, and if abnormal can help identify children with epilepsy whose seizures are triggered by fever.
Most patients with febrile seizures require no intervention other than counselling caregivers regarding care during a seizure and the use of antipyretics with illnesses. For children with severe or frequent recurrent febrile seizures, several options for seizure prophylaxis exist. The decision to treat must balance the risk of recurrent seizures against the potential adverse effects of antiepileptic medication.
Diazepam: Diazepam can be used in two ways to treat febrile seizures. First, the intravenous solution can be given rectally, using a dose of 0.5 mg/kg, during a seizure. The medicine is rapidly absorbed and may abort an ongoing seizure within one to two minutes of administration. This is an effective strategy for children with prolonged febrile seizures or febrile status epilepticus, as the caregiver can act to limit the duration of a seizure.
Diazepam can be given orally at the start of a febrile episode and within an hour reaches levels protective against seizures. The dose is 0.33 mg/kg every 8 hours for the first 24-48 hours of fever. 40% of patients will develop side effects of ataxia, lethargy or irritability. Prophylactic use of oral diazepam has been associated with a 44% decrease in recurrent febrile seizures, with most treatment “failures” happening when a child has a seizure before the caregiver realizes he is sick, and thus before diazepam is given.
Phenobarbital: Of the traditional antiepileptic drugs only phenobarbital and valproic acid appear to be effective in preventing febrile seizures. Due to the risks of valproic acid use in young children, phenobarbital is usually chosen for continuous prophylaxis. The benefit of its use is that it will be effective even before a fever is recognized, with the drawback being the need to treat children on a continuous basis even when they are not sick. Behavioral disturbances are not uncommon on phenobarbital, even at low daily doses. This intervention is reserved for children with prolonged or very frequent febrile seizures in whom a trial of diazepam has been ineffective.
Febrile seizures are a common and usually benign childhood occurrence. The goal of the initial evaluation is to exclude more serious conditions which may present with fever and seizures, and identify children at increased risk for recurrent or prolonged febrile seizures. While antipyretics are the only treatment necessary for most children, for a small number prophylactic use of diazepam may be helpful. Continuous prophylaxis with phenobarbital is indicated only when seizures are severe and diazepam has been ineffective.
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