Electroencephalogram and epilepsy
Electroencephalogram and Epilepsy
Overview of EEG, significance, method of recording, EEG waves, EEG during sleep, epilepsy, definitions and types of epilepsy.
Overview of EEG
Electroencephalography is the study of electrical activities of brain.
Electroencephalogram (EEG) is the graphical recording of electrical activities of brain.
Electrical activity of the brain is complicated compared to that of a single nerve fiber or neuron. It is due to the involvement of large number of neurons and glial cells.
Berger Waves
German psychiatrist Hans Berger was the first one to analyze the EEG waves, hence the EEG waves are called Berger waves.
Electroencephalograph
Electroencephalograph is the instrument used to record EEG.
Significance of EEG
Electroencephalogram is useful in the diagnosis of neurological and sleep disorders.
EEG pattern is altered during following neurological disorders:
- Epilepsy, which occurs due to excess discharge of impulses from cerebral cortex.
- Disorders of midbrain which affect ascending reticular activating system.
- Subdural hematoma during which there is collection of blood in subdural space over cerebral cortex.
Method of Recording EEG
Electrodes called scalp electrodes from the instrument (electroencephalograph) are placed over unopened skull or over brain after opening the skull or by piercing into electrodes.
Types of Electrodes
Electrodes are of two types:
- Unipolar electrodes
- Bipolar electrodes
Unipolar Electrodes
When unipolar electrodes are used, active electrode is placed over cortex and the indifferent electrode is kept on some part of the body away from cortex.
Bipolar Electrodes
With using bipolar electrodes, both the terminals are placed in different parts of brain.
Waves of EEG
Electrical activity recorded by EEG may have synchronized or desynchronized waves.
Synchronized waves are the regular and invariant waves, whereas desynchronized waves are irregular and variant.
EEG has three frequency bands:
- Alpha rhythm
- Beta rhythm
- Delta rhythm
In children, in addition to above waves, theta rhythm appears.
Properties of EEG Waves
| Rhythm | Frequency (per second) | Amplitude (µV) |
|---|---|---|
| Alpha | 8 to 13 | 50 |
| Beta | 15 to 80 | 5 to 10 |
| Delta | 1 to 4 | 20 to 200 |
| Theta | 4 to 7 | 10 |
Alpha Rhythm
Alpha waves are rhythmical waves, which appear with a frequency of 8 to 13 waves/second and with the amplitude of 50 µV.
Alpha waves are synchronized waves.
Occurrence of Alpha Waves
Alpha waves are obtained in inattentive brain or mind in drowsiness, light sleep or narcosis with closed eyes.
Alpha waves are abolished by any form of sensory or mental effort and diminished when eyes are opened.
Distribution
Alpha waves are most marked in parieto-occipital area.
Alpha Block
Alpha block is the replacement of synchronized alpha waves in EEG by desynchronized and low voltage waves when the eyes are opened.
Desynchronized waves do not have specific frequency and appear due to any form of sensory stimulation or mental concentration, such as solving arithmetic problems.
Desynchronization is the common term used for replacement of regular waves with irregular low voltage waves.
Beta Rhythm
Beta waves are high frequency waves of 15 to 80 per second.
Their amplitude is low, i.e., 5 to 10 µV.
Beta waves are desynchronized waves and are recorded during mental activity or mental tension or arousal state.
Beta waves are not affected by opening the eyes.
Delta Rhythm
Delta waves are low frequency and high amplitude waves.
Frequency of these waves is 1 to 4 per second and the amplitude is 20 to 200 µV.
Occurrence
Delta waves are common in early childhood during waking hours.
In adults, delta waves appear mostly during deep sleep.
Clinical Significance
Presence of delta waves in adults during conditions other than sleep indicates pathological process in brain such as tumor, epilepsy, increased intracranial pressure and mental deficiency or depression.
Delta waves are not affected by opening the eyes.
Theta Rhythm
Theta waves are obtained generally in children below 5 years of age.
Theta waves are of low frequency and low voltage waves.
Frequency of theta waves is 4 to 7 per second and the amplitude is about 10 µV.
Epilepsy
Definition
Epilepsy is a brain disorder characterized by convulsive seizures or loss of consciousness or both.
Convulsion and Convulsive Seizure
Convulsion refers to uncontrolled involuntary muscular contractions.
Convulsive seizure means sudden attack of uncontrolled involuntary muscular contractions.
Convulsive seizure occurs due to paroxysmal (sudden and usually recurring periodically) and uncontrolled discharge of impulses from neurons of brain, particularly cerebral cortex.
Epileptic
Patient affected by epilepsy is called epileptic.
Epileptic person remains normal in between seizures.
Epileptic attack develops only when excitability of the neuron is increased, causing excess neuronal discharge.
Types of Epilepsy
Epilepsy is divided into two categories:
- Generalized epilepsy.
- Localized epilepsy.
Generalized Epilepsy
Generalized epilepsy is a type of epilepsy that occurs due to excess discharge of impulses from all parts or large parts of brain.
Generalized epilepsy is subdivided into three types:
- Grand mal.
- Petit mal.
- Psychomotor epilepsy.
1. Grand Mal
Grand mal is characterized by sudden loss of consciousness followed by convulsion.
At the onset of convulsions, the person feels warning sensation in the form of some hallucination.
It is called aura.
EEG Changes
EEG shows fast waves with a frequency of 15 to 30 per second.
Later slow and large waves appear.
In between seizures, the EEG shows delta waves in all types of epileptics.
Cause for Grand Mal
Grand mal epilepsy occurs due to excess neural activity in all parts of the brain.
2. Petit Mal
In petit mal, the person becomes unconscious suddenly without any warning.
Unconsciousness lasts for a very short period of 3 to 30 seconds.
Convulsion does not occur.
However, muscles of face show twitch-like contractions and there is blinking of eyes.
Afterwards, the person recovers automatically and becomes normal.
Occurrence
Frequency of attack may be once in many months or many attacks may appear in rapid series.
It occurs in late childhood and disappears completely at the age of 30 or above.
EEG Changes
EEG recording shows slow and large waves during the attack.
Each wave is followed by a sharp spike.
Delta waves appear in between the seizures.
Causes for Petit Mal
Causes of petit mal are head injury, stroke, brain tumor and brain infection.
3. Psychomotor Epilepsy
It is characterized by emotional outbursts such as abnormal rage, sudden anxiety, fear or discomfort.
There is amnesia and decreased mental state for some period.
Some persons have the tendency to attack others bodily or rub their face vigorously.
In most cases, the persons are not aware of their activities.
EEG Changes
EEG recordings show low frequency rectangular waves, ranging between 2 and 4 per second.
Causes for Psychomotor Epilepsy
Causes of psychomotor epilepsy are abnormalities in temporal lobe and tumor in hypothalamus and other regions of limbic system such as amygdala and hippocampus.
Localized Epilepsy
Localized epilepsy is a type of epilepsy that occurs because of excess discharge of impulses from a localized area of brain.
It is otherwise known as local or focal epilepsy or local seizure or Jacksonian epilepsy.
Spread of Abnormality
Abnormality starts from a particular area and spreads to adjacent areas, developing slow-spreading muscular contractions.
Contractions usually start in the mouth region and spread down towards the legs.
Causes for Localized Epilepsy
Localized epilepsy is caused by tumor, abscess or vascular defects in cerebral cortex or deeper parts of cerebellum.