Clinical Neurophysiology Electroencephalography PDF

Clinical Neurophysiology Part IV Electroencephalography Karsten Schwerdtfeger Klinik für Neurochirurgie, Universitätsklinikum des Saarlandes. Studiengang: Neural Engineering Master WS 2024/2025 Electroencephalography Electroencephalography...

Clinical Neurophysiology Part IV Electroencephalography Karsten Schwerdtfeger Klinik für Neurochirurgie, Universitätsklinikum des Saarlandes. Studiengang: Neural Engineering Master WS 2024/2025 Electroencephalography Electroencephalography is the technique to record the electrical activity of the brain usually with several electrodes from the scalp over the brain. The electroencephalogram is the recorded electrical activity of the brain. A living brain always shows a continuous electrical activity. EEG = electroencephalography or electroencephalogram The first human EEG was recorded in 1924 by Hans Berger in Jena during a neurosurgical operation with opening of the skull. Electrode placement Electrodes - The International 10-20-system  Electroencephalogram (EEG) https://www.youtube.com/watch?v=XMizSSOejg0 EEG - The International 10-20-system Based upon the distances between four anatomically defined points of the head: Nasion Inion Left ear Right ear. Nomenclature of the electrode positions: Region where the electrode is placed Fp – frontoparietal F – frontal C – central P – parietal O – occipital T – temporal A – auricular (ears) Side Z – midline Odd – left Even - right EEG – what do we record? The electrical activity under the electrodes? EEG – what do we record? The electrical activity under the electrodes Potential differences between pairs of two electrodes. EEG – what do we record? The electrical activity under the electrodes Potential differences between pairs of two electrodes. What we will see depends upon the way we connect the electrodes to the amplifiers (channels) EEG – what do we record? Montages Bipolar Longitudinal Transversal Reference Ipsilateral ear Cz ……. Average reference Laplacian montage EEG – Phase reversal in bipolar montages  EEG Each montage has its advantages and disadvantages Reference montages are a bit more sensitive especially if the distance between the active electrode and the reference is larger than in bipolar montages. The source of a concrete signal is better localized in bipolar montages. Especially if the source is more widespread and affects the reference electrode the localisation becomes very difficult. EEG - frequency bands The brain shows a permanent electric activity In the records from the scalp it is made up of a more or less rhythmic activity within distinct frequency bands – (Gamma > 31 Hz) – Beta 13 – 30 Hz – Alpha 8 – 12 Hz – Theta 4 – 7 Hz – Delta < 4 Hz Note: The limits of the alpha- and beta bands may differ in the literature EEG - normal findings The normal EEG depends upon – Indivdual factors – Normal variants – The region it is derived from – External stimuli – The degree of attention Awake Sleep (five distinct sleep stages – four ones without and one stage with rapid eye movements - REM-sleep) – Age EEG - normal findings Alpha - EEG – Very regular not differing by more than 1 Hz – Individual specific frequency – Accentuated over the occipital and temporal lobe – Side differences in amplitude of less than 50% are normal. – Often modulated in form of a spindle – Blocked by eye opening – Restored after eye closing  EEG - normal findings Alpha - EEG – Blocked by eye opening – Restored after eye closing Eye closing EEG - normal findings Alpha - EEG – My - rhythm (central region) EEG - normal findings Beta-EEG – Only beta-activity in all channels – Alpha –activity sometimes seen for a short period after eye closing – Individual factors – Subject is drowsy or sleeping – Subject is strained (EMG artifacts!) – Pharmacologically induced EEG - abnormal findings Pathologically slowed EEG – Dominated by Theta- and/or Delta-activity – Elevated intracranial pressure – Pathologically impaired consciousness – Cave: Theta- or Delta is increased during some sleep stages – Cave: alpha-coma  EEG - abnormal findings Burst – suppression EEG EEG - abnormal findings Electrocerebral silence – Amplification – Filter settings – Without ECG – artifacts not valid – May be used to shorten waiting time in the diagnosis of brain death EEG - abnormal findings Focal abnormalities – Circumscribed slowing or epileptic activity – Tumors – Small Bleedings – Infarction – ……. EEG - abnormal findings Focal abnormalities – Circumscribed slowing or epileptic activity – Tumors – Small Bleedings – Infarction – ……. EEG - abnormal findings Epileptic activity - interictal – Spikes/sharp waves – Spike waves EEG - abnormal findings Epileptic activity - ictal EEG  EEG - generators Cortical afferents – Sensory input – Specific thalamic nuclei – second relay station – Primary cortex  EEG - generators Cortical afferents – Sensory input – Specific thalamic nuclei – second relay station – Primary cortex – Association cortex  EEG - generators Cortical afferents – Sensory input – Specific thalamic nuclei – second relay station – Primary cortex – Association cortex – Unspecific thalamic nuclei – Widespread cortical projections – Relation to the ascending reticular activating system (ARAS) – Cortical inhibition  EEG - generators Relation to cell potentials – Layer I has a dense population of synapses on peripheral dendrites of the cortical pyramid cells.  - EEG - generators - Relation to cell potentials + - – Layer I has a dense population of synapses on peripheral dendrites of the cortical pyramid cells. – EPSPs induce a greater current than IPSPs or APs thus inducing the biggest dipole with surface negativity perpendicular to the surface and smaller dipoles to adjacent regions after activation of the unspecific thalamo- + cortical projections – Potential differences at the scalp are determined by the geometric relation of the dipoles and the recording electrodes  EEG - generators And the anatomy of the cortex Dipole orientation differs between the tip of a gyrus and the slopes of a sulcus and even between the slopes of the sulcus.  EEG - generators The EEG we record is mainly generated by subsynaptic potentials in the uppermost layer (layer I) of the cortex. Afferents in this layer are part of the ARAS which regulates consciousness and attention.  EEG - generators The EEG we record is mainly generated by subsynaptic potentials in the uppermost layer (layer I) of the cortex. Afferents in this layer are part of the ARAS which regulates consciousness and attention. The frequency bands of the EEG are modulation of the synaptic activity in layer I. The rhythms in the alpha band or with even still lower frequencies express cortical inhibition due to cortical interneurons or subcortical modulators  EEG - generators The EEG we record is mainly generated by subsynaptic potentials in the uppermost layer (layer I) of the cortex. Afferents in this layer are part of the ARAS which regulates consciousness and attention. The frequency bands of the EEG are modulation of the synaptic activity in layer I. The rhythms in the alpha band or with even still lower frequencies express cortical inhibition due to cortical interneurons or subcortical modulators Beta activity indicates a cortical activation via the unspecific afferents focussing attention to the registration and processing of cortical input. To be continued