EEG Lecture- Introduction.pptx

Introduction and General consideration in EEG. This presentation describes about initial understandings in clinical EEG.

1. EEG Lecture 1 General Considerations Dr. Md. Saiduzzaman Munna Medical Officer Department of Neurology Mymensingh Medical College Hospital Mymensingh, Bangladesh.

2. EEG (Electroencephalography): Recording of electrical activity of cerebral cortex. - Analog. - Digital.

3. Technical considerations: Electrodes: These are the means by which electrical potentials are conducted to amplification apparatus. Should be low resistance to both electrical activity and environmental artifact. Electrodes are applied to scalp with conductive paste, which acts as interface between scalp and electrode.

4. Electrodes……. Electrodes may be disc/cup shaped; made of gold, tin, platinum, silver. Cup electrodes Disc electrodes

5. Electrodes…...... Long term monitoring for mobile patients (Ambulatory EEG)- - Cup electrodes placed with glue. Newer electrodes are made of plastic (MRI compatible). Electrode placement: - International 10-20 system. (1958)

6. 10-20 system

7. 10-20 system….. Advantages: Simple, easy to apply, reproducible. Provides required information in majority of scalp EEG. Disadvantages: Lower spatial resolution. No coverage for basal & interhemispheric areas of brain.

8. EEG Generation EEG records the extracellular field potentials generated by- - EPSP (Excitatory Post-Synaptic Potentials) - IPSP ( Inhibitory Post-Synaptic Potentials ) Records electrical activity predominantly from cerebral convexities. Action potentials generated within neurons are very brief and small; so cannot be recorded.

9. EEG Amplifiers It is a device which amplifies very small electrical signals to be displayed on paper or computer screen. Electrical potentials are in the range of microvolts, further attenuated by intervening tissues. Amplifies desired signals and removes undesired potentials and noise.

10. Calibrations It is a way to accurately measure EEG potentials by administering a standard signal through each amplifier. The voltage of an EEG potential is compared against known voltage. It is built in the software if EEG machine and performed automatically. Display: 30 mm/sec with 10 seconds of EEG per display.

11. Polarity

12. Derivations and Montage Derivations: Strategies of connecting different electrodes to various channels are called derivations. Montage: Arrangement of different sequences of electrode connections. Types: 1) Bipolar- a) Longitudinal bipolar b) Transverse bipolar 2) Referential

13. Bipolar montage Longitudinal Transverse

14. Bipolar montage (Longitudinal)

15. Bipolar montage (Transverse)

16. Referential Montage

17. Referential montage

18. Amplitude adjustment Amplitude displayed as µV per mm or mV per cm. Adjusted by sensitivity. Sensitivity is defined as the ratio of input voltage to pen deflection. e.g ; sensitivity 7 µV/mm indicates that a voltage of 7 µV is required to move the pen 1 mm. So, increasing the sensitivity setting will reduce the amplitude.

19. Frequency adjustment Frequency is contaminated by- a) Higher frequency activities- - Myogenic artifact. - Electrical lines. b) Lower frequency activities- - Breathing. - Sweating.

20. High frequency filter: filters ≥ 70 Hz activity. 2. Low frequency filter: filters 0.5-1.0 Hz activity. 3. Notch filter: filters 50-60 Hz activity. Higher setting of LFF abolishes pathological slow waves. Higher setting of HFF/Notch filter attenuates or distorts sharp waves.

21. Activation procedures Def: Procedures used to increase the yield of EEG in detecting seizure activity. Activation procedures during EEG recording: Intermittent photic stimulation. Hyperventilation. Sleep and sleep deprivation. Special forms of stimulation (visual, auditory, somatosensory, olfactory stimulation, Caloric test) Drug activation and suppression.

22. Intermittent photic stimulation EEG response following series of regular light flashes. Objectives: To see the response following photic stimulation. Asymmetrical or symmetrical. To precipitate photosensitive epilepsy, absence or Janz epilepsy. To observe suppression of pre-existing EEG activities during photic stimulation.

23. Intermittent photic stimulation…… Procedure: Inbuilt stroboscope- placed 30 cm away from patient’s nasion . Photic stimulation must be terminated if epileptic attack starts. EEG machine must be running during photic stimulation.

24. Intermittent photic stimulation…… Patterns of response following IPS: 1. Photodriving : Occipital spikes (Physiological). 2. Photomyoclonic : Myogenic spikes over anterior region of head. (Physiological) 3. Photoparoxysmal : Epileptic spikes or spike-wave complex.

25. Hyperventilation Procedure: Deep breath (not rapid) usually for 3 minutes (may be extended for 5 minutes) Mechanism: Deep breath-----  Empties lungs--------- Hypocarbia--------- Cerebral vasoconstriction-------- Reduced oxygen and glucose supply to brain-------- Increased chance of seizure. Symptoms: Lightheadedness, Tingling and numbness of hands feet and mouth, Rarely tetany.

26. Sleep Sleep induction: Chloral hydrate is preferred over barbiturate and diazepam. - not suppresses epileptic activity. - not induces beta activity. Focal seizure occurs during drowsiness and light sleep. Generalized seizure occurs during deep sleep. Temporal lobe epilepsy may persist during REM sleep. Pseudoseizure disappears.

27. Sleep………… Sleep deprivation: Not sleeping the previous night, preferably without caffeine containing beverages. EEG is recorded in the morning during sleep. Patient should be accompanied by attendant. Focal seizure may develop. Enhanced sensitivity to other activation procedures.

28. Important points in visual analysis of EEG 1. Montage 2. Waveforms: Frequency Voltage. Morphology Distribution. Pattern of occurrence. 3 . Interhemispheric coherence 4. Reactivity to stimuli. 5. Abnormal discharges.