Approaching the eeg

normal eeg

1. Approaching t he EEG Dr rzgar hamed abdwl

2. Approaching t he EEG: An Introduction to Visual Analysis EEG activity is described in the following terms: 1- frequency 2- Amplitude 3- distribution or location 4- Symmetry 5- Synchrony 6- reactivity 7- Morphology 8- Rhythmicity 9- regulation

3. 1- Frequency Frequency is defined as the number of complete waveforms that occur per second, expressed as cycles per second (cps) or Hz. we can determine the frequency by counting the number of waves between the 1-s vertical bars.

4. 2- Amplitude Amplitude is the size of the waveforms, measured in microvolts (µV). Amplitude is often measured “peak to peak,” i.e., from the highest to the lowest point of the sinusoidal wave. However, this can be misleading if there is a drift of the waveform due to slow oscillations (e.g., alpha waves superimposed on delta waves, as in Fig. 2-1B). Amplitude can be reported as a numerical range (e.g., 20 to 40 µV) or in descriptive terms as low (0 to 25 µV ), moderate (25 to 75 µV), or high (>75 µV ) amplitude. Some pathological conditions are associated with enormous amplitudes of hundreds of microvolts, such as hypsarrhythmia , a chaotic pattern seen in severe infantile epilepsies. Amplitude asymmetry ……. Confirmed by montage change

5. 3- Distribution or Location

6. FIGURE 1- 13. Commonly used montages for 10–20 electrode placement. A:Standard 10–20 electrode positions. B:Ipsilateral ear electrode referential montage. In this and subsequent figures, the exploring electrode (input 1) is at the origin of the arrowand the reference electrode (input 2) is at the arrow tip; numbers represent the channel at which that derivation is recorded. C:Vertex ( Cz ) referential montage. D:Longitudinal bipolar (“double banana”) montage . E:Transverse bipolar montage. F:“Hatband” bipolar montage.

7. 4- Symmetry Symmetry refers to a comparison of the amplitudes and frequencies on either side of the midline. Activity may be asymmetrical due to differences in frequency components between hemispheres, with similar overall amplitudes, or due to a significant difference in amplitude (defined as >50% difference between sides) but similar frequencies, or both.

8. 5- Synchrony Synchrony is the simultaneous occurrence of similar waveforms over each hemisphere. Normal EEG activity is usually synchronous over the left and right hemispheres, both for relatively continuous background activities and more sporadic waveforms (e.g., “K complexes,” the large amplitude biphasic slow waves seen in stage 2 sleep, should have simultaneous onset over both hemispheres). Loss of synchrony can occur when communication between the hemispheres is impaired by damage to the corpus callosum or in severe disorders of cortical function. Lack of synchrony can also indicate that the location where the waveform appears first may be closer to the origin of that activity, and thus help with the localization of abnormal or epileptiform activity.

9. 6- Reactivity Reactivity is a change in EEG activity in response to sensory stimulation or a sudden change in the internal state. Examples: >> When the eyes open, the alpha activity in the occipital leads disappears, and when the eyes close again, it reappears. >> slowing of the background frequencies during hyperventilation >> blocking of the mu rhythm (an alpha frequency activity over the centrotemporal region often unmasked by eye opening) by moving, or even thinking about moving, the opposite arm. >> changes in EEG activity in comatose patients induced by painful stimulation and occipital waveforms induced by a flashing strobe light known as the posterior photic driving response. Reactivity is usually a normal response, but its absence is not always abnormal unless it is asymmetrical. For example, absence of the photic driving response over one hemisphere suggests a lesion interrupting the optic tract on that side, which is known as Bancaud’s phenomenon

10. 7- Morphology Morphology is a physical description of : >> the waveform, which includes its shape (e.g., sinusoidal, saw-toothed, cone-shaped, spindle-shaped, and epileptiform ) >> the number of phases (e.g., biphasic, triphasic , polyphasic ) >> the polarity of those phases.

11. 8- Rhythmicity is the continuous repetition of similar waveforms and frequencies over time. The rhythmic repetition of waves creates the background activity upon which sporadic waveforms are superimposed. When such superimposed waves occur at regular intervals, they are called periodic activity(if the period is irregular, the term pseudoperiodicis used). Waveforms that occur without any regular period can be called aperiodicor arrhythmic. Such activity is sometimes also called polymorphic 9- Regulation is the degree to which amplitude and frequency change over time. A rhythm can be described as “well-regulated” if the amplitude varies smoothly in a waxing and waning pattern and the average frequency does not vary more than ±0.5 Hz during a 2-sepoch (without an obvious change of state). The gradual variation in amplitude and frequency is also known as “modulation,”

12. Alpha Rhythm T he starting point of analysing awake EEG 8-13 Hz activity occurring during wakefulness 20-60 mV, max over posterior head regions P resent when eyes closed ; blocked by eye opening or alerting the patient 8 Hz is reached by 3 years of age and progressively increases in a stepwise fashion until 9-12 Hz is reached by adolescence V ery stable in an individual, rarely varying by more than 0.5 Hz. With drowsiness, a lpha activity may decrease by 1-2 Hz A difference of greater than 1 Hz between the two hemispheres is significant. 10 % of adult have little or no alpha

13. Normal Alpha Rhythm

14. Normal Alpha Rhythm

15. Alpha Rhythm: Reactivity Should attenuate bilaterally with eye opening alerting stimuli mental concentration Some alpha may return when eyes remain open for more than a few seconds. Failure of the alpha rhythm to attenuate on one side with either eye opening or mental alerting indicates an abnormality on the side that fails to attenuate

16. Normal Alpha Reactivity

17. Normal Alpha Reactivity Eyes Closed

18. Beta Activity F requency of over 13 Hz ; if > 30-35 Hz  g amma activity or exceedingly fast activity by Gibbs. A verage voltage is 10-20 microvolts T wo main types in adults: O ften enhanced during drowsiness or when present over a skull defect S hould not be misinterpreted as a focus of abnormal fast activity.

19. Beta Activity F requency of over 13 Hz ; if > 30-35 Hz  g amma activity or exceedingly fast activity by Gibbs. A verage voltage is 10-20 microvolts T wo main types in adults: The precentral type : predominantly over the anterior and central regions ; related to the functions of the sensorimotor cortex and reacts to movement or touch. The generalized beta activity : induced or enhanced by drugs ; may attain amplitude over 25 microvolts. O ften enhanced during drowsiness or when present over a skull defect S hould not be misinterpreted as a focus of abnormal fast activity.

20. Generalized Beta Activity

21. Beta Activity F requency of over 13 Hz ; if > 30-35 Hz  g amma activity or exceedingly fast activity by Gibbs. A verage voltage is 10-20 microvolts T wo main types in adults: O ften enhanced during drowsiness or when present over a skull defect S hould not be misinterpreted as a focus of abnormal fast activity.

22. Theta Activity The term theta was coined by Gray Walter in 1944 when it was believed that this rhythm was related to the function of the thalamus. O ccurs as a normal rhythm during drowsiness In young children between age 4 months  8 years : predominance over the fronto-central regions during drowsiness In adolescents: sinusoidal theta activity can occur over the anterior head regions during drowsiness. In adults, theta components can occur diffusely or over the posterior head regions during drowsiness. Single transient theta waveforms or mixed alpha-theta waves can be present over the temporal regions in older adults.

23. Theta Activity The term theta was coined by Gray Walter in 1944 when it was believed that this rhythm was related to the function of the thalamus. O ccurs as a normal rhythm during drowsiness In young children between age 4 months  8 years : predominance over the fronto-central regions during drowsiness In adolescents: sinusoidal theta activity can occur over the anterior head regions during drowsiness. In adults, theta components can occur diffusely or over the posterior head regions during drowsiness. Single transient theta waveforms or mixed alpha-theta waves can be present over the temporal regions in older adults.

24. Theta Activity The term theta was coined by Gray Walter in 1944 when it was believed that this rhythm was related to the function of the thalamus. O ccurs as a normal rhythm during drowsiness In young children between age 4 months  8 years : predominance over the fronto-central regions during drowsiness In adolescents: sinusoidal theta activity can occur over the anterior head regions during drowsiness. In adults: theta components can occur diffusely or over the posterior head regions during drowsiness. Single transient theta waveforms or mixed alpha-theta waves can be present over the temporal regions in older adults.

25. Temporal Slowing Of The Elderly O ccur chiefly over the age of 60 years C onfined to the temporal regions and are usually maximal anteriorly O ccur more frequently on the left side D o not disrupt background activity U sually have a rounded morphologic appearance V oltage is usually less than 60-70 microvolts A ttenuated by mental alerting and eye opening and increased by drowsiness and hyperventilation O ccur sporadically as single or double waves but not in longer rhythmic trains P resent for only a small portion of the tracing (up to 1%) of the recording time when the patient is in a fully alert state

26. Normal awake EEG: 1- review technical aspect of recording: Low frequency filter (LFF) is set to 1 Hz High frequency filter (HFF) is set to 70 Notch filter is off Sensetivity = 7 microvolt/mm Time base = 30 mm/sec 2- review montage: Anterior posterior bipolar montage is the standard one. 3- interpreting the alpha rhythm -Reactivity to eye closure and opening Take amplitude in different regions then Say the range. Calculate amplitude symmetry between Left and right. Which is called PDR

27. They are generated by inspection of the dots on the ceiling

28. They found in the occipital region

29. lambda waves are blocked by eye closure and replaced by alpha rhythm

30. Resembles letter mu

31. Normal Wakefulness: The Posterior Dominant Rhythm The most important EEG pattern of wakefulness is the posterior dominant rhythm, or PDR. This activity is primarily located at the occipital poles but can be prominent more anteriorly, particularly as the patient becomes drowsy or with a sudden arousal from sleep. It is rhythmic and usually sinusoidal in character. it is often the highest amplitude activity observed in normal awake subjects. The frequency of the PDR gradually increases during development from infancy through childhood and reaches a plateau in the early teen years. In adults and children older than 9 years of age, this activity should be in the alpha frequency range (8 to 12.5 Hz). Thus, PDR frequencies slower than 9 Hz in young adults represent an abnormality > 13 Hz or faster are occasionally seen, and are not considered abnormal.

32. The PDR frequency should ideally be measured at two symmetrical occipital derivations (e.g., T5–O1 and T6–O2) within the same 1-s period, and should be counted “by hand,” not relying on digital frequency measurements If the subject is drowsy, the technician should stimulate alertness by asking the patient to perform a mental alerting task (count from one to ten, name the Great Lakes, etc.) with eyes closed. Other faster and slower frequencies may be represented in the waking EEG. Faster beta frequencies are sometimes seen, particularly over frontal and central regions, but they should be low in amplitude. Beta activity of more than 25 µV is considered abnormal (the benzodiazepines or barbiturates) Theta activity is frequently present, often in the context of a transition to drowsiness, and is almost never abnormal. Slow wave (delta) activity is generally considered abnormal in awake adults. A certain amount of delta activity is acceptable in younger children through the teenage years, particularly in the occipital region, where individual delta slow waves with overriding alpha are frequently observed in normal children.

33. PDR Frequency Changes in Childhood

34. PDR Amplitude, Synchrony, and Symmetry The PDR amplitude : 15 - 45 µV Higher in children and lower in elderly The waveforms should be synchronous between hemispheres and symmetrical in amplitude amplitude differences of up to 20% are not uncommon in normal patients. Most often, such differences in amplitude are due to variations in skull thickness, which is usually greater on the left side causing right side amplitudes to be slightly higher

35. Slow Alpha Versus Slow Alpha Variant If the PDR is slower than 9 Hz in young adults, this is generally considered abnormal but nonspecific and suggests a mild diffuse disturbance of cortical function, as seen in metabolic encephalopathies and primary neuronal disorders. However, patients with normal alpha frequencies will sometimes have brief episodes (several seconds) in which the PDR is suddenly reduced by half and increased in amplitude. This is “slow alpha variant,” a subharmonic of the normal alpha frequency, which is thought to be of no clinical significance.

36. Mu Rhythm Mu is an alpha frequency activity with arciform (arc-shaped) appearance located over the central regions that is not blocked by eye opening. It may be unilateral or bilateral, may be synchronous or asynchronous, and may or may not be present at any given time. It is likely generated in the sensorimotor cortex and can be suppressed by moving a contralateral extremity (or sometimes by thinking of moving a contralateral extremity)

37. Lambda Waves Lambda waves are sharply contoured surface-positive waves observed in the occipital leads during wakefulness with eyes open, which correlate with visual fixation to a target after a saccadic eye movement. They usually have a prominent initial positive component followed by a downstroke that may go past the baseline giving it a biphasic appearance Since they occur with eyes open, when the alpha PDR is suppressed, they stand out clearly from the background. They have no clinical significance.

38. EEG in Sleep

39. D rowsiness and Sleep ( stage 1 sleep): characterized by 1- Slowing and anterior spread of alpha activity, followed by Dropout of the posterior dominant rhythm 2- Slow lateral eye movements seen in the lateral eye and frontal leads 3- Vertex sharp waves 4- O ccurrence of theta activity over the posterior regions 5- Positive Occipital Sharp Transients of Sleep (POSTs)

40. Best appreciated in occipital region

41. Post Occipital Sharp Transients of Sleep (POSTs) S harp-contoured , mornophasic, surface-positive transients O ccurring singly or in trains of 4-5 Hz over the occipital head regions M ay have a similar appearance to the lambda waves during the awake record but are of higher voltage and longer duration U sually bilaterally synchronous but may be asymmetric over the two sides P redominantly seen during drowsiness and light sleep

42. Vertex Sharp Transient - V-Wave In young adults, the V-waves may have sharp or spiky appearance and attain rather high voltages During the earlier stages of sleep these may occur in an asymmetric fashion S hould be careful not to mistake V-waves for abnormal epileptiform activity Sometimes trains or short repetitive series, clusters, or bursts of V-waves may occur in quick succession In older adults the V-waves may have a more blunted appearance

43. Stage II Sleep Sleep Spindles K Complex

44. Sleep Spindles I n adults, a frequency of 13- 14 Hz occur in a symmetric and synchronous fashion over the two hemispheres Usually these occur at intervals between 5-15 seconds, S pindle trains ranging from 0.5-1.5 seconds in duration M ore prolonged trains or continuous spindle activity may be seen in some patients on medication, particularly benzodiazepams

45. K-Complex A broad diphasic or polyphasic waveform (>500 msec ) F requently associated with spindle activity K-complexes can occur in response to afferent stimulation and may be linked to an arousal response

46. Slow-Wave Sleep (stage 3 and stage 4 sleep)

47. Arousal Patterns Arousals from light drowsiness (stage 1) are often marked only by return of the alpha PDR. From stage 2 or deeper NREM sleep, there is often a high-amplitude biphasic or triphasic transient lasting 0.5 s or longer, reminiscent of an exaggerated K complex often followed by a burst of diffuse alpha activity and intermixed muscle artifact In children, arousals tend to occur more slowly with the diffuse alpha activity gradually slowing into the theta and delta range before being replaced by the PDR.

48. Mental Alerting Alerting is performed by the technician when the patient appears to be drowsy and the background alpha frequency is slower than expected. To enhances the PDR alpha frequency. This procedure is thus designed to determine whether a slower-than-expected PDR is due to drowsiness (i.e., state-dependent) or is pathological. Vertex waves followed by an arousal. Sharply contoured repetitive vertex waves in light sleep, reversing at the Cz electrode, are followed by a high-amplitude slow transient and then a diffuse alpha frequency pattern indicative of arousal. Calibration bar is 1 s, 50 µV. Counting slowly from one to ten, serial subtractions, spelling “world” forward and backward, or simply calling the patient’s name may each be appropriate for different patients. The verbal answer will create speech related artifacts involving temporal and tongue muscles, but between words or after completion, the patient should be at maximal alertness.

49. Hyperventilation The underlying mechanisms for the EEG changes remain controversial. The effects on EEG may be subtle or quite dramatic, particularly in children. After 30 to 60 s of hyperventilation, there can be slowing of the PDR into the theta range with diffuse spread of rhythmic polymorphic theta activity. With continued hyperventilation, the amplitude of the rhythmic slowing may increase dramatically to several hundred µV, and waves may slow into the delta frequency range (known as “buildup,” hyperventilation hypersynchrony , or hyperventilation-induced high-amplitude rhythmic slowing (HIHARS). This effect persists for tens of seconds after hyperventilation ceases evidence of abnormality : epileptiform discharges c lear -cut focal or lateralized slowing or asymmetry of activity

50. The movements associated with breathing may cause artifacts ( the patient leans forward and back during deep breaths, and increased temporalis muscle tone) The technician will encourage the patient to continue overbreathing for up to 3 min Contraindications: COPD chronic lung or heart disease Pregnancy recent stroke subarachnoid hemorrhage sickle cell disease moyamoya disease

51. Sleep Sleep is activating for nearly all forms of epilepsy, The most striking example is electrical status epilepticus of sleep, in which spike-and-wave discharges occur during more than 85% of slow-wave sleep. Children with benign childhood epilepsy with centrotemporal spikes may have a normal EEG in wakefulness, but characteristic centrotemporal spikeand -wave discharges appear once the patient is asleep. Seizures are also more likely during sleep or upon awakening, particularly in some epilepsy syndromes like juvenile myoclonic epilepsy. Chloral hydrate was often the drug of choice due to its relatively minor effect on EEG patterns compared to other sedatives (benzodiazepines, etc.). Chloral hydrate was fairly well tolerated in most patients, but rare cases of respiratory depression and even mortality have occurred with accidental overdoses the use of behavioral techniques including sleep deprivation prior to the study and timing studies to correspond to nap times has largely replaced the use of sedatives, with nearly the same success rate.

52. Eye Opening and Closure We have already mentioned the ability of eye opening to block the PDR and the return of the PDR with a slightly faster frequency in the first second after eye closure (called “squeak”). Eye opening and closure should be performed several times during the course of the study to assess reactivity of the PDR. In children who have childhood epilepsy with occipital paroxysms (CEOP), the occipital spikes are sometimes suppressed by eye opening.

53. perform this twice, once with eyes open and once with eyes closed the absence of a response is only abnormal if it is unilateral. It is important to distinguish photic driving from the photomyoclonic response (stimulated blinking or facial twitching in response to each flash, seen in the frontal electrodes, which is usually not epileptic) and the photoelectric response (a rare finding in which the light itself triggers an electrical impulse from its interaction with the frontal electrodes).

54. FIGURE 2- 14.Photic driving and photomyoclonic response. A:Posterior photic driving response seen at 10-Hz, 14-Hz, and 18-Hz stimulation rates. Note that although the posterior dominant rhythm is close to 10 Hz, driving is still clearly evident as synchronization of the alpha activity with the light stimulus (marked by the bottom trace). B:Low-frequency photic stimulation (3 Hz) evokes a repetitive frontal slow transient associated with flash-induced blinking, termed a photomyoclonic response. No posterior photic driving is observed. Calibration bar is 1 s, 20 µV.

55. Photoparoxysmal Response Photic stimulation can induce epileptiform activity, termed a photoparoxysmal response (PPR), particularly associated with idiopathic generalized epilepsies (IGE). The PPR consists of flash-induced spike-and-wave or polyspike -and-wave discharges tracking each flash, initially in the occipital leads but often spreading anteriorly as photic stimulation continues . Such discharges can evolve into a clinical seizure, known as a photoconvulsive response, with spike discharges that outlast the photic stimulation and clinical features ranging from loss of awareness to convulsions. Patients with juvenile myoclonic epilepsy and the progressive myoclonic epilepsies appear to be particularly susceptible to photic stimulation.

56. Painful Stimulation In unresponsive or comatose patients, sternal rub or forceful pinching of the fingernail or toenail may cause a partial arousal that alters the EEG background. The change can be subtle or dramatic and varies from patient to patient. Reactivity to pain is usually a favorable prognostic sign suggesting that the cortex can respond to sensory stimuli.