Febrile convulsions are seizures associated with fever (bodily temperature exceeding 100.4˚F) and are the most common neurologic disorder seen in infants and young children. They are an age-dependent phenomenon, occurring in children aged between 6 months and 5 years.
Febrile convulsions are relatively rare and will affect 2 to 4 percent of children in this age bracket. Febrile seizures are generally benign and are associated with only a slightly higher risk of epilepsy later in life. They are thought to be related to the vulnerability of infants’ developing immune systems combined with genetic factors, and are not considered a form of epilepsy.
This article discusses the different types of febrile seizures as well as information on the risk factors and management of febrile seizures for parents and caregivers.
Table of contents
Types of febrile seizures
Febrile seizures are classified as either simple or complex.
Simple febrile seizures: Seizures that last less than 15 minutes and do not recur within a 24-hour period are classified as simple febrile seizures. They usually last less than 5 minutes and make up the majority of cases of febrile convulsions. They will recur in approximately one-third of children in early childhood.
Complex febrile seizures: Complex febrile seizures are defined as being focal in onset (shaking in one limb or one side of the body), prolonged (lasting longer than 15 minutes), and occurring more than once in a 24-hour period. They carry a higher risk of recurring during childhood and a higher risk of seizures later in life. Complex febrile seizures are the less common type.
Criteria for febrile convulsions
Febrile convulsions are confirmed using the following criteria:
- Convulsions associated with fever (100.4˚F +).
- Age between 6 months and 5 years.
- Absence of Central Nervous System (CNS) infection or inflammation (involving the brain, spinal cord, or optic nerves).
- Absence of systemic metabolic abnormalities (e.g. diabetes, heart disease).
- No history of afebrile seizures (seizures unrelated to fever).
Common risk factors
- High fever: Febrile seizures have been shown to occur most frequently in children whose temperature reaches around 104˚F. It is believed that the maximum temperature of the fever, rather than the rate of temperature increase, is the main determining factor in febrile seizures. Additionally, the seizure threshold is lower in infants and can be further lowered due to hyponatremia (below-average sodium levels).
- Viral infection: Viral infections are more likely than bacterial infections to cause febrile seizures. The temperature increase caused by viruses increases the likelihood of fever and thus febrile seizures. In the United States, the most common virus associated with febrile seizures is the Human Herpesvirus 6 (HHV-6). It is found in one-third of children under 2 years of age experiencing first-time seizures and is associated with unusually high temperatures. The type of virus responsible for the seizure does not influence the likelihood of future seizures.
- Recent immunization: Certain vaccines increase the risk of febrile seizures, including diphtheria, tetanus toxoid, and measles, mumps, and rubella (MMR). Factors that contribute to this risk are the vaccine preparation, the child’s age, and possible genetic susceptibility.
- Genetic susceptibility: It has long been recognized that there is a genetic susceptibility to febrile seizures, meaning that children can genetically inherit a predisposition. For a child with febrile seizures, 10 to 20 percent of their first-degree relatives (such as parents and siblings) will also have a history or predisposition to febrile seizures. In some cases, this will represent an early manifestation of Generalized Epilepsy with Febrile Symptoms (GEFS+) which is a genetic form of epilepsy. Another form of epilepsy associated with febrile seizures is Severe myoclonic epilepsy of infancy (Dravet syndrome).
Other risk factors
Factors linked to febrile seizures but associated with a lower risk include:
- Prenatal exposure to nicotine (but not alcohol or coffee consumption)
- Iron insufficiency
- Allergic rhinitis (hay fever)
What do febrile convulsions look like?
Simple febrile seizures: Febrile convulsions manifest as jerking movements or rhythmic twitching in the face, arms, or legs. They usually occur while the child is passed out and, although they can last for up to 15 minutes, the median duration is 3 to 4 minutes. Afterward, the child may be lethargic, drowsy, or confused, but the seizure will not recur in the next 24 hours.
Complex febrile seizures: Febrile convulsions will last more than 15 minutes and may cause short-term weakness in the child’s arms or legs. Convulsions will usually recur within 24 hours. Complex seizures are more likely to affect younger children or those with developmental delays.
When to see a doctor
A child who has febrile convulsions should be taken to the doctor as soon as possible in order to determine the cause of the fever. Children younger than 12 months may need to be tested for meningitis; a serious infection of the lining of the brain.
Children who suffer from prolonged complex seizures may require further testing to determine their risk of epilepsy later in childhood.
- Hesdorffer DC, Benn EK, Bagiella E, Nordli D, Pellock J, Hinton V, Shinnar S; FEBSTAT Study Team. Distribution of febrile seizure duration and associations with development. Ann Neurol. 2011 Jul;70(1):93-100. doi: 10.1002/ana.22368. Epub 2011 Mar 17. PMID: 21437934; PMCID: PMC3134598.
- Nelson KB, Ellenberg JH. Predictors of epilepsy in children who have experienced febrile seizures. N Engl J Med. 1976 Nov 4;295(19):1029-33. doi: 10.1056/NEJM197611042951901. PMID: 972656.
- Subcommittee on Febrile Seizures; American Academy of Pediatrics. Neurodiagnostic evaluation of the child with a simple febrile seizure. Pediatrics. 2011 Feb;127(2):389-94. doi: 10.1542/peds.2010-3318. PMID: 21285335.
- MILLICHAP JG. Studies in febrile seizures. I. Height of body temperature as a measure of the febrile-seizure threshold. Pediatrics. 1959 Jan;23(1 Pt 1):76-85. PMID: 13613867.
- Shah SS, Alpern ER, Zwerling L, Reid JR, McGowan KL, Bell LM. Low risk of bacteremia in children with febrile seizures. Arch Pediatr Adolesc Med. 2002 May;156(5):469-72. doi: 10.1001/archpedi.156.5.469. PMID: 11980552.
- Hall CB, Long CE, Schnabel KC, Caserta MT, McIntyre KM, Costanzo MA, Knott A, Dewhurst S, Insel RA, Epstein LG. Human herpesvirus-6 infection in children. A prospective study of complications and reactivation. N Engl J Med. 1994 Aug 18;331(7):432-8. doi: 10.1056/NEJM199408183310703. PMID: 8035839.
- Millichap JG, Millichap JJ. Role of viral infections in the etiology of febrile seizures. Pediatr Neurol. 2006 Sep;35(3):165-72. doi: 10.1016/j.pediatrneurol.2006.06.004. PMID: 16939854.
- Rowhani-Rahbar A, Fireman B, Lewis E, Nordin J, Naleway A, Jacobsen SJ, Jackson LA, Tse A, Belongia EA, Hambidge SJ, Weintraub E, Baxter R, Klein NP. Effect of age on the risk of Fever and seizures following immunization with measles-containing vaccines in children. JAMA Pediatr. 2013 Dec;167(12):1111-7. doi: 10.1001/jamapediatrics.2013.2745. PMID: 24126936.
- Kugler SL, Stenroos ES, Mandelbaum DE, Lehner T, McKoy VV, Prossick T, Sasvari J, Swannick K, Katz J, Johnson WG. Hereditary febrile seizures: phenotype and evidence for a chromosome 19p locus. Am J Med Genet. 1998 Oct 12;79(5):354-61. doi: 10.1002/(sici)1096-8628(19981012)79:5<354::aid-ajmg5>3.0.co;2-j. PMID: 9779801.