The brain is a complex organ which controls all the functions of the body. By receiving, processing and transmitting electrical impulses, the brain is able to carry out its diverse functions. At any given time, there is extensive electrical activity in the normal human brain due to the incoming, outgoing and crossing over of impulses. The brain therefore has controlling mechanisms which prevent overactivity or underactivity. However, sometimes these mechanisms may be impaired and the activity is out of control. If there is any abnormal electrical activity in the brain, it can disrupt various processes and functions of the body. This may manifest as a seizure.
What is a Seizure?
A seizure is a clinical event associated with abnormal electrical activity in the cortex of the brain. It usually manifests as altered sensations, convulsions, and temporary loss of consciousness, with the person often falling to the ground. Symptoms of a seizure will depend upon the part of the brain involved. There may be different types of seizures and it may be caused by various factors such as infection, head injury, brain tumor high fever, or other factors. In case of partial or focal seizures, there is paroxysmal neuronal activity limited to one part of the brain, while in generalized seizures there is abnormal activity throughout the cortex of the brain.
Seizure vs Epilepsy vs Convulsion
Although epilepsy is thought to be synonymous with a seizure, it may be defined more correctly as a tendency to have recurrent, spontaneous seizures. Epilepsy is more of a symptom of brain dysfunction than a disease itself and the cause of recurring seizures may or may not be known. Another term that is often confused with seizure is convulsion. A convulsion is an uncontrollable muscle spasm throughout the body which may be a manifestation of a seizure, although some seizures may occur without obvious convulsions. The word “fits” is often used loosely to describe a seizure, epileptic attack and/or convulsion.
How does a seizure occur?
Pathophysiology of a Seizure
The brain has 3 major divisions :
- Forebrain which consists of the diencephalon and telencephalon. The diencephalon contains the thalamus and hypothalamus, while the telencephalon contains the cerebrum, which is divided into the right and left hemispheres. The cerebrum is covered by a layer of gray matter known as the cerebral cortex.
- Midbrain which together with the hindbrain, makes up the brainstem.
- Hindbrain which consists of the metencephalon and myelencephalon. The metencephalon contains the pons and cerebellum, while the myelencephalon is made up of the medulla oblongata.
In the cortex of a normal brain, there are two types of neurotransmitters, excitatory and inhibitory, which balance each other so that synchronous discharge amongst neighboring groups of neurons are limited and controlled. The inhibitory neurotransmitter, gamma-aminobutyric acid (GABA), plays an important role in inhibiting excessive neuronal discharge and it is seen that drugs that block GABA receptors can trigger seizures. Of the excitatory neurotransmitters, acetylcholine and the amino acids glutamate and aspartate are particularly important in causing excessive neuronal discharge.
It has been suggested that during a seizure there is a reduction in inhibitory activity as well as excessive excitatory activity, which together result in abnormal electrical activity in the brain. When there are recurrent or epileptic discharges involving large groups of neurons, the cells may undergo morphological and physiological changes which makes it prone to future abnormal discharges. This is known as kindling.
Types of Seizures
Physiologically, seizures may be of two types :
- Partial seizures or focal seizures where the abnormal neuronal activity is confined to one part of the brain. A partial seizure may be termed as simple or complex, depending on whether consciousness is maintained or lost. When there is further spread of a partial seizure throughout the brain by the diencephalic activating pathway, it is known as secondary generalized seizure.
- Generalized seizures where the abnormal electrical activity involves both sides of the brain simultaneously. A primary generalized seizure is one that originates from the diencephalic activating system and spreads simultaneously throughout the cortex.
Causes of Seizure
Some causes of seizures remain unknown. The known causes and certain trigger factors of seizure may include :
- Infection – meningitis, encephalitis, syphilis, HIV, and cerebral cysticercosis (parasitic infection of the brain).
- Seizures due to high fever or febrile seizures (febrile convulsions) may occur in young children. Though alarming, a febrile seizure does not usually cause brain damage, and in most cases will not cause epilepsy.
- Brain injury – caused by a stroke or head injury.
- Brain tumor.
- Lead, carbon monoxide, and other poisoning.
- Family history of seizures.
- Alzheimer’s disease.
- Congenital defects in the brain.
- Vascular malformations.
- Birth injuries.
- Alcohol – particularly alcohol withdrawal.
- Drug abuse.
- Certain medication.
- Withdrawal from certain drugs, such as benzodiazepines and barbiturates.
- Kidney or liver failure.
- Abnormal metabolic state, such as increased or decreased sodium levels.
- Hypoglycemia (low blood sugar).
- Stopping or irregular intake of anti-epileptics.
- Sleep deprivation.
- Physical and mental exhaustion.
- Hormonal changes linked with the menstrual cycle.
- Flickering lights may be as innocuous as those from television or computer screens.
- Loud noise or music.
- Eclampsia is a life-threatening complication of pregnancy, where a pregnant woman, who had been diagnosed with pre-eclampsia (edema, hypertension and protein in the urine), develops seizures.
- Phenylketonuria (PKU) is an inherited disorder that increases the levels of phenylalanine, an amino acid, in the blood. It can cause seizures in children.
Article reviewed by Dr. Greg. Last updated on December 4, 2011