Focal EEG Abnormalities PDF

12 Focal EEG Abnormalities THORU YAMADA and ELIZABETH...

12 Focal EEG Abnormalities THORU YAMADA and ELIZABETH MENG General Characteristics of Focal EEG Abnormalities Focal EEG abnormalities were first described in 1936 by Gray Walter in brain tumor patients.1 The abnormalities were characterized by localized slow waves, which he termed “delta waves.” Since then, EEG has served as an important, noninvasive diagnostic tool for localized cerebral lesions. However, this has changed with the advent of computerized tomography (CT) and magnetic resonance imaging (MRI) as these studies proved to be more accurate in the anatomical localization of lesions. However, EEG has remained an important tool for functional assessment of localized cerebral lesions. The potential for improved anatomical/spatial accuracy of EEG lies with the use of computerized quantitative methods. Although positron emission tomography (PET) scan and functional MRI can reveal functional alterations of the brain, EEG is superior in its temporal resolution. For example, focal cerebral dysfunction can be revealed almost instantaneously after the clamping of a carotid artery during endarterectomy surgery if the hemisphere on the side of clamped carotid artery encounters a risk of ischemia (see Video 11-1). Poor spatial resolution of scalp- recorded EEG is in part due to the distortion by volume conductors that lie between the cortex and scalp, that is, CSF (cerebrospinal fluid), dura, skull, and scalp. By the time the electrical activity reaches the scalp, the current is attenuated and distorted. Combining computerized EEG data and MRI will provide more accurate anatomical localization. The Copyright © 2017. Wolters Kluwer. All rights reserved. more accurate anatomical assessment of electrical source of interest from the brain can be made by MEG (magnetoencephalography). Focal EEGs are represented by the following findings: 1. Focal/unilateral amplitude depression or slowing of basic background activity (alpha, beta waves) without an increase in theta–delta slow waves (Fig. 12-1) 2. Focal/unilateral amplitude depression or slowing of basic background activity associated with an increase in theta–delta slow waves (Fig. 12-2) 3. Focal/unilateral theta–delta slow waves with preserved basic background activity (Fig. 12-3) 4. Focal/unilateral enhancement of basic background activity with or without associated Yamada, T., & Meng, E. (2017). Practical guide for clinical neurophysiologic testing : Eeg. ProQuest Ebook Central http://ebookcentral.proquest.com Created from mccollege-ebooks on 2021-01-17 13:12:11.  slow waves (Fig. 12-4) 5. Focal/unilateral epileptiform activity with or without associated slow waves (Fig. 12-5) FIGURE 12-1 | A 6-year-old boy with a history of focal seizures involving left arm. There was depression and intermittent slowing of alpha rhythm over the right hemisphere but without significant focal delta–theta slow. Copyright © 2017. Wolters Kluwer. All rights reserved. Yamada, T., & Meng, E. (2017). Practical guide for clinical neurophysiologic testing : Eeg. ProQuest Ebook Central http://ebookcentral.proquest.com Created from mccollege-ebooks on 2021-01-17 13:12:11.  FIGURE 12-2 | A 62-year-old woman with acute aphasia and right-sided weakness secondary to left middle cerebral artery infarct. EEG showed decreased background activity along with more or less continuous polymorphic delta activity over the left hemisphere, while EEG on the right side is entirely normal. Copyright © 2017. Wolters Kluwer. All rights reserved. Yamada, T., & Meng, E. (2017). Practical guide for clinical neurophysiologic testing : Eeg. ProQuest Ebook Central http://ebookcentral.proquest.com Created from mccollege-ebooks on 2021-01-17 13:12:11.  FIGURE 12-3 | A 27-year-old woman with a history of partial complex seizures secondary to arachnoid cystic lesion in right frontal lobe. Note polymorphic delta from right anterior temporal region, but with preserved symmetric background alpha rhythm. A portion of this example is shown in the box. Copyright © 2017. Wolters Kluwer. All rights reserved. Yamada, T., & Meng, E. (2017). Practical guide for clinical neurophysiologic testing : Eeg. ProQuest Ebook Central http://ebookcentral.proquest.com Created from mccollege-ebooks on 2021-01-17 13:12:11.  FIGURE 12-4 | A 49-year-old man with a history of right subdural hematoma and status post craniotomy on the right. EEG showed increased amplitude of background activity as well as underlying polymorphic delta slow waves over the right hemisphere. Increased background amplitude over the right is likely secondary to skull defect. A portion of this example is shown in the box. Copyright © 2017. Wolters Kluwer. All rights reserved. Yamada, T., & Meng, E. (2017). Practical guide for clinical neurophysiologic testing : Eeg. ProQuest Ebook Central http://ebookcentral.proquest.com Created from mccollege-ebooks on 2021-01-17 13:12:11.  FIGURE 12-5 | A 6-year-old boy with a history of focal seizure with secondary generalization. Note spike maximum at C3 or P3 electrode along with irregular delta activity from left hemisphere (shown in boxes). ALTERATION OF BACKGROUND ACTIVITY EEG recorded directly from the surface of a brain tumor shows no EEG activity. Slow waves appear some distance from the tumor location. Recorded from the scalp, depression, slowing, and disruption of the background rhythm are common findings near a lesion. In evaluating focal abnormalities, the EEG should be examined for symmetry of amplitude and frequency, Copyright © 2017. Wolters Kluwer. All rights reserved. continuity, and reactivity of the background activity between homologous areas. An amplitude asymmetry alone, without frequency asymmetry, should be treated cautiously as technical errors (i.e., unequal interelectrode distances between homologous electrode pairs or cancelation effect due to equipotential distribution between two electrodes) must be considered (see “Technical Pitfalls and Errors,” Chapter 15; see also Figs. 15-47 and 15-48). The destructive lesions or lesions involving cortex and white matter tend to show attenuated background activity associated with delta activity (see Fig. 12-2). Slowed background rhythm with preservation of amplitude tends to occur in chronic lesions (see Fig. 12-3). Slowing of the alpha rhythm may occur in lesions not necessarily involving the occipital lobe. Slowing of background rhythm often accompanies theta–delta waves, disrupting normal continuity of background rhythm. Focal delta slow waves in a preserved background activity Yamada, T., & Meng, E. (2017). Practical guide for clinical neurophysiologic testing : Eeg. ProQuest Ebook Central http://ebookcentral.proquest.com Created from mccollege-ebooks on 2021-01-17 13:12:11.  may be seen in subcortical lesions. Focal or unilateral depression of beta rhythm, either intrinsic or drug induced, is also an important parameter in interpretation of a focal abnormality (Fig. 12-6). Beta depression is a sensitive indicator for cerebral ischemia. Beta depression may also be seen in the region of an epileptic focus. In sleep, unilateral depression of sleep spindles, vertex sharp waves, or other sleep patterns may be present on the side of a lesion (Figs. 12-7A and B and 12-8A and B). When EEG shows bilateral slowing, the side of slower frequency and/or decreased background activity is the worse hemisphere irrespective of amplitude asymmetry (Fig. 12- 9). Copyright © 2017. Wolters Kluwer. All rights reserved. FIGURE 12-6 | A 19-year-old man with left frontal intraparenchymal hemorrhage secondary to head trauma. EEG showed bilaterally diffuse and bifrontal dominant delta–theta activity. There was consistent depression of beta activity over the left frontal region and this was the only lateralizing EEG finding (compare the channels shown by the box and oval circle). Yamada, T., & Meng, E. (2017). Practical guide for clinical neurophysiologic testing : Eeg. ProQuest Ebook Central http://ebookcentral.proquest.com Created from mccollege-ebooks on 2021-01-17 13:12:11. Copyright © 2017. Wolters Kluwer. All rights reserved. Yamada, T., & Meng, E. (2017). Practical guide for clinical neurophysiologic testing : Eeg. ProQuest Ebook Central http://ebookcentral.proquest.com Created from mccollege-ebooks on 2021-01-17 13:12:11.  FIGURE 12-7 | A 47-year-old woman with a left chronic subdural hematoma. EEG showed only minimal slowing on left side (shown by rectangular boxes) (A), but sleep record showed consistent depression of V wave (shown by rectangular box) over the left hemisphere (B). Copyright © 2017. Wolters Kluwer. All rights reserved. Yamada, T., & Meng, E. (2017). Practical guide for clinical neurophysiologic testing : Eeg. ProQuest Ebook Central http://ebookcentral.proquest.com Created from mccollege-ebooks on 2021-01-17 13:12:11. Copyright © 2017. Wolters Kluwer. All rights reserved. Yamada, T., & Meng, E. (2017). Practical guide for clinical neurophysiologic testing : Eeg. ProQuest Ebook Central http://ebookcentral.proquest.com Created from mccollege-ebooks on 2021-01-17 13:12:11.  FIGURE 12-8 | A 46-year-old man with partial complex seizures secondary to left frontoparietal infarct 13 years ago. Awake EEG showed only decreased amplitude of background activity (A). Sleep record showed consistent depression of sleep spindles and POSTs (shown by rectangular box) (B). Copyright © 2017. Wolters Kluwer. All rights reserved. Yamada, T., & Meng, E. (2017). Practical guide for clinical neurophysiologic testing : Eeg. ProQuest Ebook Central http://ebookcentral.proquest.com Created from mccollege-ebooks on 2021-01-17 13:12:11.  FIGURE 12-9 | A 6-year-old boy with developmental delay and right hemiparesis. EEG showed bilateral delta activity with slower and lower amplitude delta over the left hemisphere, indicating that left hemisphere is worse than right. Although amplitude depression and paucity of background activity are common findings of focal abnormality, higher-than-normal alpha rhythm amplitude on the side of a lesion may be seen in some cases, especially in slowly progressive or chronic lesions (Fig. 12-10A and B). Similarly, beta rhythm, mu rhythm, or sleep spindles may be augmented ipsilateral to the side of a lesion (Fig. 12-11).2–6 Both ends of the spectrum, either depression or accentuation of sleep spindles, have been reported in patients with Sturge–Weber syndrome (Fig. 12-12).5 Copyright © 2017. Wolters Kluwer. All rights reserved. Similarly, marked attenuation or accentuation of beta rhythm has been reported on the side of a porencephalic cyst.7 Yamada, T., & Meng, E. (2017). Practical guide for clinical neurophysiologic testing : Eeg. ProQuest Ebook Central http://ebookcentral.proquest.com Created from mccollege-ebooks on 2021-01-17 13:12:11. Copyright © 2017. Wolters Kluwer. All rights reserved. Yamada, T., & Meng, E. (2017). Practical guide for clinical neurophysiologic testing : Eeg. ProQuest Ebook Central http://ebookcentral.proquest.com Created from mccollege-ebooks on 2021-01-17 13:12:11.  FIGURE 12-10 | Patient was a 60-year-old man presenting with sudden onset of slurred speech and right facial droop. MRI showed large parietal mass lesion with small hemorrhagic extending from posterior periventricular region (A). EEG in awake showed left>right alpha rhythm without significant slow waves (A). In sleep, there were irregular delta slow waves focused at left posterior head region (B, shown by rectangular box). This case illustrates very unusual EEG features showing enhanced alpha on the side of lesion (in common sense, right side should be considered abnormal side). Sleep record, however, showed delta slow waves in left posterior quadrant corresponding to the site of legion (B). This is also unusual Copyright © 2017. Wolters Kluwer. All rights reserved. since focal slow waves are usually more clear in awake than in sleep record (see Fig. 12-14). (This patient did not have skull defect or craniotomy at the time of this EEG recording.) Yamada, T., & Meng, E. (2017). Practical guide for clinical neurophysiologic testing : Eeg. ProQuest Ebook Central http://ebookcentral.proquest.com Created from mccollege-ebooks on 2021-01-17 13:12:11.  FIGURE 12-11 | An 11-year-old boy with a history of stable cyst in right frontal region. EEG showed focal polymorphic delta in right frontal region. Note increased beta activity in the same region (shown in boxes). There was no skull defect in this patient. Copyright © 2017. Wolters Kluwer. All rights reserved. Yamada, T., & Meng, E. (2017). Practical guide for clinical neurophysiologic testing : Eeg. ProQuest Ebook Central http://ebookcentral.proquest.com Created from mccollege-ebooks on 2021-01-17 13:12:11.  FIGURE 12-13 | A 5-year-old boy with diagnosis of Sturge–Weber syndrome and history of seizures. MRI showed atrophy of left frontal, parietal, and occipital lobes, enlarged left lateral ventricle, and left frontal calcification. This sleep EEG showed consistent depression of sleep spindles, V waves, and beta activity over the left hemisphere. Skull defect or burr hole secondary to craniotomy or head injury causes local enhancement of underlying EEG activity, especially beta rhythm. The focal amplitude accentuation secondary to skull defect is referred to as “breach rhythm” and is usually discreetly localized to one or two electrodes near the skull defect or burr hole site (Fig. 12- Copyright © 2017. Wolters Kluwer. All rights reserved. 13; see also Fig. 7-20). This beta enhancement may result in a “spiky”-appearing activity, which includes “spike-like” or “sharp-like” transients. In fact, these are often difficult to differentiate from true spike or sharp discharges. If the “spiky” discharge is disproportionately larger or wider spread than the breach rhythm, it is likely true spike. Yamada, T., & Meng, E. (2017). Practical guide for clinical neurophysiologic testing : Eeg. ProQuest Ebook Central http://ebookcentral.proquest.com Created from mccollege-ebooks on 2021-01-17 13:12:11.  FIGURE 12-12 | A 26-year-old man with a history of closed head injury and left epidural hematoma, and status post craniotomy 6 months prior to the EEG. Note increased beta and “spiky” alpha and enhanced mu rhythm over the left central and midtemporal region (breach rhythm). FOCAL DELTA ACTIVITY Delta activity can be divided into two types by morphological characteristics, one is arrhythmic (polymorphic) and the other is rhythmic (monomorphic or monorhythmic). Arrhythmic delta activity (ADA) consists of serial waves of irregular shape with variable Copyright © 2017. Wolters Kluwer. All rights reserved. duration and amplitude, which occur continuously or intermittently. ADA reacts little to eye opening or alerting stimuli. Focal ADA is usually most evident in the awake state and becomes less distinct in non-REM sleep (Fig. 12-14A and B). Yamada, T., & Meng, E. (2017). Practical guide for clinical neurophysiologic testing : Eeg. ProQuest Ebook Central http://ebookcentral.proquest.com Created from mccollege-ebooks on 2021-01-17 13:12:11. Copyright © 2017. Wolters Kluwer. All rights reserved. Yamada, T., & Meng, E. (2017). Practical guide for clinical neurophysiologic testing : Eeg. ProQuest Ebook Central http://ebookcentral.proquest.com Created from mccollege-ebooks on 2021-01-17 13:12:11.  FIGURE 12-14 | A 44-year-old woman with right spastic hemiparesis secondary to head injury 11 years ago. MRI showed encephalomalacia on left temporal lobe and enlarged left lateral ventricle. EEG in awake state showed intermittent delta waves and decreased alpha rhythm over the left hemisphere (A). In sleep, delta waves appear bilaterally and focal delta activity noted in awake state becomes unclear (B). Focal ADA usually indicates a lesion involving subcortical white matter. Greater irregularity in waveforms, slower frequency, greater persistence, and less amount of superimposed or intermixed fast activity generally indicate a more severe and acute lesion. In Copyright © 2017. Wolters Kluwer. All rights reserved. determining the most affected area, the same rule applies. The region of slower frequency and lower amplitude delta activity is more severely affected (see Fig. 12-9). Faster and higher amplitude delta, often mixed with alpha or theta, is more common at a distance further from the lesion. Continuous ADA, especially when associated with loss of background activity, correlates highly with acute or rapidly progressive destructive lesions (see Fig. 12-2). Intermittent or less continuous ADA intermingled with theta or alpha background activity may be a sign of a chronic lesion or may occur during the recovery process of focal damage (see Fig. 12-3). In contrast to the irregularly formed ADA, intermittent rhythmic delta activity (IRDA) or RDA* has a rhythmic sinusoidal waveform occurring as bursts or paroxysms. Classically, Yamada, T., & Meng, E. (2017). Practical guide for clinical neurophysiologic testing : Eeg. ProQuest Ebook Central http://ebookcentral.proquest.com Created from mccollege-ebooks on 2021-01-17 13:12:11.  IRDA/RDA* was thought to represent a projected rhythm, thus correlating with deep midline lesions such as tumors in the thalamus, hypothalamus, or brainstem. IRDA/RDA* is now recognized to occur more commonly in toxic/metabolic encephalopathy than in focal intrinsic brain lesions. IRDA/RDA* has two characteristic distributions, one is frontally predominant RDA*/(FIRDA) and the other is occipitally predominant RDA (OIRDA). FIRDA/frontally predominant RDA* is usually seen in adult patients (see Fig. 6-3; see also Fig. 8-14), whereas OIRDA/ occipitally predominant RDA* occurs more commonly in children (see Fig. 8-15, see also Video 10-6). Although IRDA is more common in metabolic, toxic, encephalopathy, it can occur in infratentorial lesions and is usually bilaterally symmetrical or shows shifting asymmetries. With supratentorial lesions, IRDA may be asymmetric, usually with greater amplitude on the side of the lesion.8,9 Unlike ADA which is resistant to reactivity, IRDA/RDA* tends to be augmented by eyes closing or hyperventilation and is attenuated by alerting stimuli. Focal or lateralized IRDA (LRDA*) implies potential seizure activity rather than focal structural lesion (see Chapter 10, section “Indication for CCEEG.” See also Fig. 13-2A and B, Video 13-4). FOCAL EPILEPTIFORM ACTIVITY Cortical scarring following a cortical insult can give rise to epileptiform activity. Therefore, spike or spike-wave discharges occur more commonly in slowly progressive or static lesions than in rapidly destructive lesions. Epileptiform activity is seen in about 10% to 20% of patients with cerebral tumors. It is more common in low-grade tumors (i.e., astrocytoma) than in rapidly growing tumors (i.e., glioblastoma).10 By the same token, the occurrence of focal spike activity is rare in acute and localized cerebrovascular accidents (CVAs), either infarction or hemorrhage. Focal spikes tend to appear in months or years after a CVA. When epileptiform activity appears in CVA, it commonly occurs in massive hemorrhage or infarction and EEG usually takes the form of PLEDs (periodic lateralized epileptiform activity or LPD*) (see “Periodic Lateralized Epileptiform Discharges,” Chapter 10; see also Figs. 10-38 to 10-41). PLEDs/LPD*s are usually seen in patients with impaired Copyright © 2017. Wolters Kluwer. All rights reserved. consciousness, secondary to a massive and acute hemispheric lesion (infarction, anoxic cerebral insult, brain abscess, acute exacerbation of a glioblastoma, or herpes simplex encephalitis). These PLEDs/LPDs* could be asynchronous within the same hemisphere, either totally independent from each other (see Fig. 10-41A) or the discharges on one lobe could be time locked and lead the discharges on another lobe (see Fig. 10-41B, see also Video 13-2A and B). Of these, herpes encephalitis and acute cerebral infarct are the most common etiologies. PLEDs /LPD* seen in an acute infarction are usually self-limited and tend to disappear within 1 to 2 weeks after the illness. LPD* in herpes encephalitis may evolve to BiPLEDs or BIPD* (bilateral independent periodic discharge) (see Fig. 10-42A and B). Yamada, T., & Meng, E. (2017). Practical guide for clinical neurophysiologic testing : Eeg. ProQuest Ebook Central http://ebookcentral.proquest.com Created from mccollege-ebooks on 2021-01-17 13:12:11.  LOCALIZATION VALUES As stated earlier in this chapter, EEG is relatively poor in determining the accurate location of a lesion. The localization is at best left/right and anterior/posterior quadrant. False localization is not uncommon (Fig. 12-15). For example, lesions situated in the frontal lobe, parietal lobe, or even in the thalamus may show focal EEG abnormalities in the temporal region. Other false-positive findings commonly seen in the temporal lobes are focal EEG patterns without focal pathology, such as temporal slow waves of the elderly or wicket spikes. Conversely, EEG is highly sensitive for lesions in temporal lobe. EEG abnormalities may well precede CT or MRI findings in detecting brain tumors growing in the temporal lobe. Occipital lesions also show a high incidence of abnormalities with focal ADA and suppression or slowing of alpha rhythm.11 In contrast, false-negative findings are relatively common for lesions involving the parietal lobe. EEG changes are often minimal, if any, in these lesions. However, focal abnormalities in the parietal lobe, especially with altered background activity, have a relatively high localizing value (Fig. 12-16). Frontal lobe lesions may show ADA over the frontal or temporal region. The frontally predominant RDA/FIRDA may be seen in frontal lobe lesions and may appear as bilateral, ipsilateral, or, less often, contralateral in its distribution. Copyright © 2017. Wolters Kluwer. All rights reserved. FIGURE 12-15 | A 35-year-old man with brain tumor at right frontal region extending to parietal region as shown in MRI scan (the left/right was intentionally Yamada, T., & Meng, E. (2017). Practical guide for clinical neurophysiologic testing : Eeg. ProQuest Ebook Central http://ebookcentral.proquest.com Created from mccollege-ebooks on 2021-01-17 13:12:11.  reversed in order to match with EEG topography). There were delta slow waves in right hemisphere, maximum at parietal region, as shown by delta frequency topography. The localization was not quite accurate as shown by MRI localization. FIGURE 12-16 | A 74-year-old man presenting with dyscalculia and right hemiparesis secondary to left parieto-occipital infarct. EEG showed irregular delta activity from entire left hemisphere, maximum at parietal electrode indicated by phase reversal at P3 in parasagittal electrodes chain. Copyright © 2017. Wolters Kluwer. All rights reserved. In deep, midline lesions such as tumors involving the basal forebrain or diencephalic structures, the incidence of EEG abnormality is less (at least 25% of patients have normal EEG) with less reliable localizing signs as compared to hemispheric lesions.12,13 FIRDA/frontally predominant RDA* is the most frequent abnormality in these deep, midline lesions. Subtentorial lesions involving the third ventricle or posterior fossa usually show a normal EEG. EEG becomes abnormal (characterized by frontally predominant or occipitally predominant RDA*, FIRDA, or OIRDA) only when increased intracranial pressure develops.12,13 OIRDA, the occipitally predominant RDA* is more common in children. Yamada, T., & Meng, E. (2017). Practical guide for clinical neurophysiologic testing : Eeg. ProQuest Ebook Central http://ebookcentral.proquest.com Created from mccollege-ebooks on 2021-01-17 13:12:11.  DIAGNOSTIC SPECIFICITY OF FOCAL EEG ABNORMALITIES Focal EEG abnormalities are rarely specific for diagnosing underlying focal pathologies. However, diagnostic probabilities are enhanced when clinical contexts are taken into account for interpreting EEG results. This applies even to normal EEG. For example, a normal EEG in awake patients with profound motor/sensory deficit rules out an extensive cortical lesion but points to a small deep or midline lesion such as a lacunar or capsular infarction. Similarly, disparity between extensive neurological deficits and minimal EEG changes may be seen in a brainstem lesion, for example, in a patient who is awake but mute and tetraplegic secondary to an infarction of bilateral ventral pons sparing the tegmentum (locked-in syndrome). The EEG in locked-in syndrome is usually normal with alpha rhythm that is reactive to various stimulations.14 With bilateral pontine lesions involving the tegmentum, the patient is in a deep comatose state, and EEG may show “alpha coma pattern” with posterior dominant normal alpha distribution,15 in contrast to diffuse and anterior dominant alpha seen in postanoxic state (see Figs. 11-13 and 11-14A–D). It is usually not possible to differentiate between CVA and brain tumor based on focal EEG abnormalities. However, the presence of a well-defined focal spike lowers the possibility of acute stroke since focal spikes are more common in chronic lesions. If focal delta activity appears in the frontal or frontotemporal region, the probability of CVA is high since the middle cerebral artery (which is the most commonly affected artery in stroke) covers the frontal and temporal lobe territory (Fig. 12-17). Copyright © 2017. Wolters Kluwer. All rights reserved. Yamada, T., & Meng, E. (2017). Practical guide for clinical neurophysiologic testing : Eeg. ProQuest Ebook Central http://ebookcentral.proquest.com Created from mccollege-ebooks on 2021-01-17 13:12:11.  FIGURE 12-17 | A 67-year-old woman with a history of aphasia secondary to infarct in the territory of left middle cerebral artery 7 months prior to this EEG. Note polymorphic delta activity over the left hemisphere with emphasis to frontotemporal region and decreased alpha rhythm over the left occipital region. Also note a sharp transient at left temporal region (shown by rectangular box). Focal EEG abnormalities associated with evidence of CSF pleocytosis suggest brain abscess or herpes simplex encephalitis. High-amplitude and exceedingly slow (