A seizure is the temporary manifestation of abnormal, synchronous electrical activity in the brain. Infections can cause seizures by altering the brain’s electrical balance through several biological pathways. They disrupt normal brain function either by direct invasion of the central nervous system or indirectly through the body’s inflammatory response to illness.
Understanding the Seizure Triggers
Infections trigger seizures by lowering the threshold for electrical overactivity in the brain. This occurs via two primary pathways.
The direct pathway involves pathogens, such as viruses or bacteria, crossing the blood-brain barrier and invading brain tissue. This invasion causes irritation, localized damage, or the formation of scar tissue (gliosis), which acts as a focus for abnormal electrical discharges.
The indirect pathway involves the systemic immune response to an infection located elsewhere in the body (e.g., lungs or urinary tract). When fighting a pathogen, the body releases inflammatory mediators like cytokines into the bloodstream. These substances can cross the blood-brain barrier, leading to neuroinflammation. This inflammatory state increases neuronal excitability, making the brain prone to seizures even without the pathogen present in the brain.
Infections Directly Affecting the Brain and Spinal Cord
The most severe infection-induced seizures occur when the pathogen directly targets the central nervous system (CNS). These events are often acute symptomatic seizures, resulting directly from immediate brain injury or severe inflammation. Conditions include meningitis, encephalitis, and brain abscesses.
Meningitis and Encephalitis
Meningitis involves inflammation of the meninges, the protective membranes surrounding the brain and spinal cord, caused by bacterial, viral, or fungal agents. The resulting inflammation and toxins can lead to seizures in many patients. Encephalitis is more severe, involving inflammation of the brain tissue itself, where pathogens directly damage neurons. Seizures are a common symptom of encephalitis, reflecting widespread neuronal irritation and swelling.
Brain Abscesses
A brain abscess is a localized collection of pus and infected material within the brain tissue. These often result from infections spreading from nearby areas like the sinuses or ears. The mass effect and surrounding inflammation from the abscess can trigger seizures, sometimes being the first sign of the condition. Damage from these infections can leave behind scar tissue, increasing the risk of long-term epilepsy.
The Role of Systemic Illness and Fever
Systemic infections that do not invade the CNS can trigger seizures through the indirect pathway involving fever and inflammation. The most common example is a febrile seizure, which typically occurs in children between six months and five years old. These seizures are triggered by a rapid rise in body temperature, usually above 100.4°F (38°C), associated with common childhood illnesses.
Febrile seizures are linked to routine infections like the flu, ear infections, or viral upper respiratory illnesses. The fever and the associated release of inflammatory cytokines are the trigger, not the pathogen itself in the brain. Simple febrile seizures are brief, lasting less than 15 minutes, and carry an excellent prognosis. Importantly, having a febrile seizure does not mean a child has epilepsy, which involves recurrent seizures without an immediate trigger.
Clinical Evaluation and Treatment
When a seizure occurs due to an infection, treatment focuses on eliminating the pathogen and controlling the seizure activity. For CNS infections like bacterial meningitis, this requires the immediate administration of appropriate antibiotics or antiviral agents to halt brain damage.
Diagnosis often requires a comprehensive workup. This may include a lumbar puncture (spinal tap) to analyze cerebrospinal fluid for infection or inflammation. Brain imaging, such as an MRI, is also used to look for swelling, abscesses, or structural damage. The long-term outlook varies: simple febrile seizures carry a benign prognosis with no lasting neurological effects. However, seizures from severe CNS infections indicate significant brain injury, and recovery depends on the extent of the damage and the speed of treatment.