Usually, a child having a febrile seizure shakes all over and loses consciousness. Sometimes, the child may get very stiff or twitch in just one area of the body.

A child having a febrile seizure may:

• Have a fever higher than 100.4 F (38.0 C)
• Lose consciousness
• Shake or jerk arms and legs

Febrile seizures are classified as simple or complex:

• Simple febrile seizures. This most common type lasts from a few seconds to 15 minutes. Simple febrile seizures do not recur within a 24-hour period and are not specific to one part of the body.
• Complex febrile seizures. This type lasts longer than 15 minutes, occurs more than once within 24 hours or is confined to one side of your child's body.

Febrile seizures most often occur within 24 hours of the onset of a fever and can be the first sign that a child is ill.

Usually, a higher than normal body temperature causes febrile seizures. Even a low-grade fever can trigger a febrile seizure.

Infection

The fevers that trigger febrile seizures are usually caused by a viral infection, and less commonly by a bacterial infection. Influenza and the virus that causes roseola, which often are accompanied by high fevers, appear to be most frequently associated with febrile seizures.

Post-immunization seizures

The risk of febrile seizures may increase after some childhood immunizations. These include the diphtheria, tetanus and pertussis or measles-mumps-rubella vaccinations. A child can develop a low-grade fever after a vaccination. The fever, not the vaccination, causes the seizure.

Factors that increase the risk of having a febrile seizure include:

• Young age. Most febrile seizures occur in children between 6 months and 5 years of age, with the greatest risk between 12 and 18 months of age.
• Family history. Some children inherit a family's tendency to have seizures with a fever. Additionally, researchers have linked several genes to a susceptibility to febrile seizures.

Most febrile seizures produce no lasting effects. Simple febrile seizures don't cause brain damage, intellectual disability or learning disabilities, and they don't mean your child has a more serious underlying disorder.

Febrile seizures are provoked seizures and don't indicate epilepsy. Epilepsy is a condition characterized by recurrent unprovoked seizures caused by abnormal electrical signals in the brain.

Recurrent febrile seizures

The most common complication is the possibility of more febrile seizures. The risk of recurrence is higher if:

• Your child's first seizure resulted from a low fever.
• The period between the start of the fever and the seizure was short.
• An immediate family member has a history of febrile seizures.
• Your child was younger than 18 months at the time of the first febrile seizure.