Because febrile seizures are usually brief, the diagnosis must be made from the history. Frequent diagnostic errors include rigors due to fever, febrile myoclonus, breath-holding, and syncope triggered by illness (Stevenson 1990). It is possible that many febrile seizures are febrile syncope, a diagnosis that should be suspected by sudden collapse with pallor and a reflex anoxic seizure.
The entity of an afebrile seizure associated with minor infection has come to attention (Lee and Ong 2004). The age group of the patients described by Lee and Ong was similar to children with febrile seizures, and there was a high prevalence of family history of febrile seizures. Children with this disorder frequently had febrile seizures on other occasions. The seizures occurred in children who were afebrile (temperature lower than 37.8° C), had definite signs or symptoms of illness, and normal metabolic and CSF findings with no other cause for seizures. Nearly 25% of the 125 children had more than 1 recurrent seizure within the next 24 hours. After a follow-up of 6 years, the risk of epilepsy (recurrent afebrile seizures) was 7.8%, compared with 1.6% after a first unprovoked (simple) febrile seizure. The “afebrile-febrile seizure” scenario was also noted in a study of 39 children from Seattle, Washington and in 2 studies of benign afebrile convulsions after mild gastroenteritis (Zerr et al 2005; Verrotti et al 2011; Kang et al 2013); both had a similar profile and excellent psychomotor development and medical outcome. Compared to febrile seizures, these illness-related seizures appear to be associated more frequently with gastroenteritis. The benign course of childhood development, and low risk of relapse or epilepsy after gastroenteritis-associated seizures, was recently confirmed in a large, retrospective study (Verrotti et al 2014).
Other provoking causes for the seizure must be excluded, especially a CNS infection. About 15% of children with meningitis will have seizures, but virtually none are neurologically normal shortly after the seizure (Gerber and Berliner 1981). In older children with meningitis, there are constitutional symptoms such as headache, and signs such as nuchal rigidity. Any child with meningeal signs and/or symptoms who presents with a fever and a seizure should have a lumbar puncture. However, children under 1 year of age may not have such obvious signs of meningeal irritation. For children aged 6 to 12 months who present with a seizure and fever, a lumbar puncture should be considered, especially if the child has not received Haemophilus influenzae type B or Streptococcus pneumoniae vaccinations (American Academy of Pediatrics 2011). In addition, if the child has previously been treated with antibiotics, the clinician should be aware that the signs and symptoms of CNS infection may become masked, and lumbar puncture should be strongly considered in such cases.