Typical febrile convulsions are:
- age 6 months to 6 years
- Normal neurodevelopment
- generalized, tonic-clonic
Most important differential is CNS infection eg encephalitis, meningitis. These tend to present with posturing, impaired conscious level, or focal seizures. 15% of patients presenting with status epilepticus with fever have meningitis (observational study) – although low rate of LP so underestimate? I suspect there would have been other features to suggest meningitis beforehand. Stiff neck? Fear of doing LP due to RICP from fit and/or meningitis, so do CT first if in ICU or abnormal neurology else as soon as no contraindication. If in doubt, treat empirically for meningitis (+/- herpes encephalitis, although risk unknown) with antibiotics and steroids. [Chin RFM, Arch Dis Child 2005;90:66-9.(Ed by Kneen)]
About 30-35% of febrile seizures in the absence of CNS infection however have one or more complex features:
- focal onset,
- duration >10 minutes,
- or multiple seizures during the illness episode
Febrile status epilepticus, a subgroup of complex febrile seizures with seizures lasting more than 30 minutes, occur in about 5% of cases. [BMJ 2015; 351 doi: http://dx.doi.org/10.1136/bmj.h4240 ]
One third of children with febrile convulsion/seizure (FS) will experience further FS; age would appear to be the single, strongest, and most consistent risk factor. Most recurrences will occur during the first year after the initial FS and over 90% recur within two years. Other risk factors for recurrence are –
- family history of febrile seizures (but not epilepsy) in a first degree relative,
- children whose initial FS occurred with a relatively low fever,
- multiple initial seizures occurring during the same febrile episode.
Surprisingly, status in an otherwise normal child does not appear to significantly increase the risk for further febrile seizures or the development of epilepsy.
Following a first FS, 2-4% of children will have an unprovoked ie afebrile seizure (4x risk in general population) and most of these children will subsequently develop epilepsy. Risk factors for epilepsy include –
- family history of epilepsy (not just febrile convulsions),
- atypical/complex features,
- presence of early onset neurodevelopmental abnormalities (NB look for neurocutaneous syndromes eg neurofibromatosis, tuberosclerosis, Sturge-Weber syndrome). Febrile convulsions often first manifestation of Dravets syndrome, often prolonged, often partial.
There is clearly a genetic basis for FS, but multiple chromosomes have been identified, complex! Current opinion supports an association between prolonged FS and pre-existing lesions within the temporal lobe, but not specifically FS and temporal lobe epilepsy.
No indication for EEG or anti-epileptic treatment in febrile convulsions – see SIGN 81.
Risk of dying slightly raised for 2yr after febrile seizure but only for complex ie more than 15min duration or recurrent within 24hr (RR 1.99, but absolute risk v low).(Denmark, Lancet 08;372)