Implications of the EEG The eleetroencephalogram (EEG)was first described in 1929 by the psychiatrist Hans Berger.Be discovered that the electrical activity of the humn brain could be neasured through the skull.Since this time the EEG has becooe a valuable investigative and diagnostic tool used widely in both psychiatry and neurology. Despite its widespread use,the physiological basis of the EEG is not entirely understood and the interpretation of EEGs is a highly skilled and difficult task. The EEG measures the voltage difference between electrodes attached to the scalp. This voltage is related to the electrical activity of the underlying neurones of the cerebral cortex:the tiny electrical signals are arplified and recorded. Intracellular recordings from individual neurones my measure several millivolts, whereas the EEG recorded on the skull may measure between 10 and 100 microvolts. The EEG wave characteristics recorded depend on several factors including physical factors such as the degree of folding of the underlying gyri of the cerebral cortex and the degree of bone thickness (and therefore electrical resistance)of the skull.The type of wave form recorded on the EEG also depends on the amount of synchronization of the underlying neurones:the greater the degree of synchronization the greater the voltage recorded and therefore the greater the amplitude of the EEG wave. High degrees of synchromization of cortical activity result in EEG spikes.These are defined as waves lasting 70 nilliseconds or less.EEG spikes are seen in disease states such as epilepsy.which is oftem diagnosed using electroencephalography in addition to a clinical history of convulsions. The physiology of what happens at the cellular level to produce an ECG is still unclear but it is known that the pyramidal cells in the cortex have afferent inhibitory and excitatory connections which result in the conduction of changes in the neuronal action potential as axon spikes.Axon spikes are coeducted to the postsynaptic merbranes causing electrical changes.and it is assumed that the summation of these can be recorded on the sealp as the EEG
Implications of the EEG The electroencephalogram (EEG) was first described in 1929 by the psychiatrist Hans Berger. He discovered that the electrical activity of the human brain could be measured through the skull. Since this time the EEG has become a valuable investigative and diagnostic tool used widely in both psychiatry and neurology. Despite its widespread use, the physiological basis of the EEG is not entirely understood and the interpretation of EEGs is a highly skilled and difficult task. The EEG measures the voltage difference between electrodes attached to the scalp. This voltage is related to the electrical activity of the underlying neurones of the cerebral cortex; the tiny electrical signals are amplified and recorded. Intracellular recordings from individual neurones may measure several millivolts, whereas the EEG recorded on the skull may measure between 10 and 100 microvolts. The EEG wave characteristics recorded depend on several factors including physical factors such as the degree of folding of the underlying gyri of the cerebral cortex and the degree of bone thickness (and therefore electrical resistance) of the skull. The type of wave form recorded on the EEG also depends on the amount of synchronization of the underlying neurones; the greater the degree of synchronization the greater the voltage recorded and therefore the greater the amplitude of the EEG wave. High degrees of synchronization of cortical activity result in EEG spikes. These are defined as waves lasting 70 milliseconds or less. EEG spikes are seen in disease states such as epilepsy, which is often diagnosed using electroencephalography in addition to a clinical history of convulsions. The physiology of what happens at the cellular level to produce an ECG is still unclear but it is known that the pyramidal cells in the cortex have afferent inhibitory and excitatory connections which result in the conduction of changes in the neuronal action potential as axon spikes. Axon spikes are conducted to the postsynaptic membranes causing electrical changes, and it is assumed that the summation of these can be recorded on the scalp as the EEG
The normal EEG In the normal individual the amplitude and frequency of the EEG are dependent on hehavioural state and age.The EEG of an infant under I year typically has a dominant slow occipital rhythn with generalized slow-wave delta and theta activity. This is a very different picture from that of a normal adult where there is usually dominant alpha activity over the posterior quadrant of the skull.Slow-vave delta and theta activity is normal in the very young and in adults during sleep. The sboorsal E6G Certain EEG patterns are indicative of distinct clinical disorders.The following are descriptions of a smll sample of clinically important disorders that display characteristic EEG abnormalities: Petit sl epilepsy classically has a 3 Hz spike and wave pattern. Grand ar/epilepsy is identified by large 8-12 Hz spikes in groups during tonie-clonfe selgures. Wigraine can be difficult to distinguish fron cerebrovascular disease. especially if the presentation is with acute heniplegia.The EEG in these migraine sufferers shows localized slow wave activity.Generally in migraine the EEG changes are non-specific. Creutzfeldt-fakob disease is a neurodegenerative disorder caused by prion disease which presents clinically with dementia and ayoclonie jerks (often elicited by startle).The EEG contains periodic repetitive discharges,typically at a rate of 1-2/second,occurring bilaterally.This feature,while mot always present early in the illness,is strongly suggestive of the diagmosis. Mmtingron's chorea shows a generalized flattening of the EEG trace that is thought to be due to loss of basal ganglia cells. A/zheiser's dementia is characterized by reduced alpha activity on EEG. Delirjue usually shows slow alpha activity and increased delta activity on EEG
The normal EEG In the normal individual the amplitude and frequency of the EEG are dependent on behavioural state and age. The EEG of an infant under 1 year typically has a dominant slow occipital rhythm with generalized slow-wave delta and theta activity. This is a very different picture from that of a normal adult where there is usually dominant alpha activity over the posterior quadrant of the skull. Slow-wave delta and theta activity is normal in the very young and in adults during sleep. The abnormal EEG Certain EEG patterns are indicative of distinct clinical disorders. The following are descriptions of a small sample of clinically important disorders that display characteristic EEG abnormalities: • Petit mal epilepsy classically has a 3 Hz spike and wave pattern. • Grand mal epilepsy is identified by large 8-12 Hz spikes in groups during tonic-clonic seizures. • Migraine can be difficult to distinguish from cerebrovascular disease, especially if the presentation is with acute hemiplegia. The EEG in these migraine sufferers shows localized slow wave activity. Generally in migraine the EEG changes are non-specific. • Creutzfeldt-Jakob disease is a neurodegenerative disorder caused by prion disease which presents clinically with dementia and myoclonic jerks (often elicited by startle). The EEG contains periodic repetitive discharges, typically at a rate of 1-2/second, occurring bilaterally. This feature, while not always present early in the illness, is strongly suggestive of the diagnosis. • Huntington's chorea shows a generalized flattening of the EEG trace that is thought to be due to loss of basal ganglia cells. • Alzheimer's dementia is characterized by reduced alpha activity on EEG. • Delirium usually shows slow alpha activity and increased delta activity on EEG
Merpes sfoplex encepholirfs is characterized by discharges occurring every 1-3 seconds with slow-wave activity prominent in the temporal areas.These abnormalities on the EEG typically occur within the first 2 weeks of illness. Psychiatric disorders unfortunately do not show EEGs of unvarying character that could be used diagnostically.There are,however,a few useful associations: schizopdrew/a no characteristic pattern is found uniformly,although many non-specific patterns have been reported depress/on the normal characteristic,of a latent period of about I hour before an REM sleep EEG pattern occurs,is disturbed,vith REM pattern occurring soretimes in a matter of minutes after falling asleep deeewtjas the associations in luntington's chorea and Alzheiner's disease are described above. As might be expected.drugs can alter EEGs: Aypmotics and sedatives increase beta wave activity rrfeve/fe aorfdepressants increase theta and delta waves and reduce alpha waves a/colo/increases theta waves. Clinical simnificance The EEG is a useful diagnostic test but it should be noted that corroborative clinical features are required.EEG is particularly helpful in that it can suggest the possibility of abnormality in the function of the brain despite normal structure (e.g.a normal CT scan in a patient with severe epilepsy). In addition to its role in the diagnosis of disease states,EEG has proved to be a useful tool in the study of the physiology of sleep and sleep disorders. It should be noted that despite widespread use in nedicine.EEG has its limitations.Up to 30%of patients with epilepsy have a normal EEG between fits, while up to 15%of normal individaals show abnormalities in their EEG.Therefore all EEG results need to be interpreted in the coatext of the overall clinical picture
• Herpes simplex encephalitis is characterized by discharges occurring every 1-3 seconds with slow-wave activity prominent in the temporal areas. These abnormalities on the EEG typically occur within the first 2 weeks of illness. Psychiatric disorders unfortunately do not show EEGs of unvarying character that could be used diagnostically. There are, however, a few useful associations: • schizophrenia - no characteristic pattern is found uniformly, although many non-specific patterns have been reported • depression - the normal characteristic, of a latent period of about 1 hour before an REM sleep EEG pattern occurs, is disturbed, with REM pattern occurring sometimes in a matter of minutes after falling asleep • dementias - the associations in Huntington's chorea and Alzheimer's disease are described above. • As might be expected, drugs can alter EEGs: • hypnotics and sedatives - increase beta wave activity • tricyclic antidepressants - increase theta and delta waves and reduce alpha waves • alcohol - increases theta waves. Clinical significance The EEG is a useful diagnostic test but it should be noted that corroborative clinical features are required. EEG is particularly helpful in that it can suggest the possibility of abnormality in the function of the brain despite normal structure (e.g. a normal CT scan in a patient with severe epilepsy). In addition to its role in the diagnosis of disease states, EEG has proved to be a useful tool in the study of the physiology of sleep and sleep disorders. It should be noted that despite widespread use in medicine, EEG has its limitations. Up to 30% of patients with epilepsy have a normal EEG between fits, while up to 15% of normal individuals show abnormalities in their EEG. Therefore all EEG results need to be interpreted in the context of the overall clinical picture