Pathological EEG Patterns PDF

Società Italiana di Neurofisiologia Clinica Percorso di Formazione e Certificazione Modulo Elettroencefalografia (EEG) Pathological EEG Patterns Oriano Mecarelli Department of Human Neurosciences Sapienza University of Rome...

Società Italiana di Neurofisiologia Clinica Percorso di Formazione e Certificazione Modulo Elettroencefalografia (EEG) Pathological EEG Patterns Oriano Mecarelli Department of Human Neurosciences Sapienza University of Rome Rome, Italy Text modified from: Mecarelli O. Pathological EEG Patterns. In: Clinical Electroencephalography. Springer Nature, 2019 (in press) Any unauthorized reproduction is prohibited Mecarelli 2019 1 1. Changes in background rhythms Regarding background EEG abnormalities ,it is necessary to consider: the amplitude reduction or the poor synchronization of the physiological rhythms the unilateral or bilateral slowings the reduced or absent reactivity to eye opening or alerting  Many healthy subjects have constitutionally low amplitude EEG tracings, while others show a poorly appreciable posterior alpha rhythm (low amplitude or desynchronized tracings), in relation to anxiety or non-relaxing registration environment. These patterns have not to be considered as abnormal, unless they are observed monolaterally.  The amplitude of the posterior alpha rhythm may be asymmetrical in the two hemispheres, with a lower voltage over the dominant hemisphere.However, this difference usually does not exceed 20%. An asymmetry of the posterior alpha rhythm is considered to be abnormal when its difference is greater than 50% or more when compared to contralateral side, and the abnormality is localized on the side where the amplitude is lower. Unilateral attenuation or absence of the alpha rhythm usually occurs in lesions of the occipital cortex and anterior ventral thalamus; posterior subcortical lesions may also induce the absence of alpha reactivity to eyes opening (Bancaud phenomenon). Mecarelli 2019 2 2. Slowings EEG slowings include all types of activity with a frequency less than the alpha band (i.e. theta and delta bands). Slow-wave abnormalities can be defined in multiple domains, considering the location (focal or regional vs diffuse), the morphology (mono- or polymorphic), the rhythmicity (irregular vs rhythmic), the amplitude (low or high voltage) and the continuity (intermittent/subcontinuous vs continuous). Diffuse non-specific EEG slowings can be observed in many pathological conditions that include consciousness disorders of various etiology, cerebral inflammations, neurodegenerative processes, post-ictal status, metabolic and toxic disorders, etc. In these cases EEG recording is important because, on the basis of the extent of the slowings and their reactivity to external stimuli, deductions can be made on the severity of the underlying pathology and also to express prognostic indications. Hypersynchronous diffuse slowings may be induced by drowsiness or be the result of hyperventilation and not necessarily, therefore, represent a pathological pattern. Mecarelli 2019 3 EEG slowings can also be focal, localized in a restricted scalp area. These slowimgs are highly correlated with localized structural brain lesions, such as tumors, intraparenchymal hematoma, stroke, encephalitic focus, etc. In these cases, the characteristics of the slowing sequences (delta or theta frequency, duration, morphology, amplitude) and their correlation with the clinical symptomatology are very important for a correct interpretation. EEG slow changes induced by the above-mentioned structural brain lesions probably reflect alterations in the cortical function caused by direct or indirect local neuronal networks dysfunction. Though the electrographic pattern is important for prognostic purposes, EEG is generally not useful in distinguishing among different types of brain lesions. The EEG diffuse or focal slowings tend to repeat periodically or in a rhythmic way Mecarelli 2019 4 A: burst of diffuse monomorphic delta waves. B: sequence of focal polymorphic delta waves in the left fronto-temporal region Mecarelli 2019 5 3. Epileptiform abnormalities There is still a terminological confusion regarding the definition of the epileptic abnormalities among electroencephalographers. Not all those who interpret and report an EEG use the same terms or give them a similar meaning. Often, we do not give identical meanings to terms like "discharge", "interictal", "ictal", "subclinical". For example, some electroencephalographers still use the term "irritative" (a term to be nowadays discouraged) to indicate an EEG transient that may be correlated with epilepsy. The various editions of IFCN glossary often derives from compromise choices between the various positions, but it is more universally accepted; it focuses the attention on the morphology of EEG graphoelements, without interpreting them in relation to the clinical aspects. Therefore, the principal characteristics of epileptiform EEG abnormalities as described in the revised glossary of IFNC (2017) will be described. Mecarelli 2019 6 The term epileptiform pattern (synonyms: interictal epileptiform discharge, epileptiform activity) refers to interictal transients distinguishable from background activity, with - but no exclusively - spiky morphology. Even the term paroxysm indicates a graphoelement phenomenon with a sudden onset and abrupt termination, clearly distinguished from the background activity. During the routine EEG recording in the laboratory the majority of epileptic patients do not experience a seizure. Consequently, the interictal epileptiform discharges are the main tool in defining a specific diagnosis Mecarelli 2019 7 The ictal activity is referred then to seizure EEG patterns, consisting of: repetitive epileptiform EEG discharges, with relatively abrupt onset and termination, lasting several seconds; electrodecremental period (usually with superimposed fast activity) ; fast paroxysmal activity. The epileptiform EEG discharges have a characteristic frequency with spatio- temporal evolution (i.e. gradual change in frequency, amplitude, morphology and location). When the seizure EEG pattern is related to concurrent clinical symptoms, the term electroclinical seizure can be used; when the seizure EEG pattern is instead without clinical correlates, it should be referred to as electrographic or subclinical seizures. However, always consider that the demonstration of a clinical correlation with the seizure EEG pattern may not be always possible. Mecarelli 2019 8 The sporadic epileptiform discharges (non-rhythmic and non-periodic) can be quantified and categorized as follows: abundant : > 1/10 s; frequent: < 1/10 s > 1/min; occasional: < 1/min > 1/h; rare: < 1/h The epileptiform graphoelements, according to revised glossary of IFCN 2017, are described as follows:  Spike: a transient with pointed peak, clearly distinguished from the background activity, and with a duration from 20 to less than 70 ms (1/50 to 1/14 sec). Amplitude is variable, but typically > 50 mV. As regard to morphology, the spike can be mono- or multiphasic, with a dominant negative component.  Polyspike complex: a sequence of two or more spikes.  Sharp wave: an epileptiform transient clearly distinguished from the background activity, with pointed peak and a duration of 70-200 ms (1/14 to 1/5 sec). Usually the sharp wave (di-triphasic) has a variable amplitude (100-200 mV); the main component is negative and the ascending phase is steeper when compared to the descending phase. The term “sharp wave” does not apply to vertex sharp transients of sleep, lambda waves, positive occipital sharp transients of sleep, sharp transients poorly distinguished from background activity. Mecarelli 2019 9  Spike-and-slow-wave complex (SSWc): single or multiple epileptiform pattern consisting of a spike and an associated following slow wave, clearly distinguished from background activity. Based on the frequency, it is necessary to distinguish the classical (typical) SSWc at 3 Hz from the slow (atypical) SSWc (1.5-2.5 Hz)  Polyspikes-and-slow-wave complex: an epileptiform pattern consisting of two or more spikes associated with one or more slow waves  Sharp-and -slow-wave complex: an epileptiform pattern (single or multiple) consisting of a sharp wave and an associated following slow wave, clearly distinguished from background activity. Mecarelli 2019 10  Graphic representation of epileptiform abnormalities: a – spike; b – polyspikes; c – sharp wave; d – spike-and-slow wave complex (SSWc); e: polyspikes-and- slow wave complex; f: sharp- and- slow wave complex; g: typical SSWc (3 Hz); h: atypical slow SSWc (< 3 Hz) Mecarelli 2019 11  Raw EEG examples of epileptiform abnormalities (a, b, c, d, e, f as in previous figure) Mecarelli 2019 12 Typical discharge of spike-and-slow wave complexes (SSWc) at 3 Hz in a child with absence seizures Mecarelli 2019 13 Atypical discharge of spike-and slow wave complexes, at 2 Hz, in a patient with Lennox-Gastaut syndrome. Mecarelli 2019 14  a: discharge of polyspikes-and-slow wave complexes b: discharge of sharp wave-and-slow wave complexes Mecarelli 2019 15 4. Periodic patterns According to revised glossary of IFCN (2017), the term periodic is applied to EEG waves or complexes occurring in a sequence or intermittently, at an approximately regular rate or intervals (generally of one to several seconds). When the waves or complexes occur at random or non-regular intervals the “quasi- periodic” (preferable than pseudoperiodic) term is used, defined as having a period varying by 25-50% from one cycle to the next. The distinction between periodic and quasi-periodic is to be applied only if determined by quantitative computer analysis. For periodic discharges (PDs) we mean the repetition of a waveform with a relatively uniform morphology and duration, with a quantifiable inter-discharge interval between consecutive waveforms, and a recurrence of waveform at nearly regular intervals. On the basis of old terminology the periodic EEG patterns have been classified in Diffuse and Lateralized. The Diffuse Periodic Discharges included: the Periodic Short-Interval Diffuse Discharges (PSIDDs) the Periodic Long-Interval Diffuse Discharges (PLIDDs). Mecarelli 2019 16  The PSIDDs are represented by diffuse graphoelements (spikes, polyspikes, sharp waves, etc) occurring periodically at interdischarge interval of 0.5 - 4 sec and they are more frequently associated to anoxic encephalopathy. A subset of PSIDDs may be considered the Triphasic Waves (TWs). TWs are defined as high amplitude (> 70 mV) positive sharp transients, which are preceded and followed by relatively low amplitude negative waves. TWs are usually bilateral (with an antero-posterior or a postero-anterior lag) and they have a repetition rate of about 1- > 2 Hz. TWs occur in sequences and can be induced or regressed by alerting or noxiuos stimuli. They are non-specific, since they can be found in various metabolic and degenerative encephalopathies, usually in comatose patients. TWs may decrease with sleep and after intravenous benzodiazepines. Periodic Long-Interval Diffuse Discharges (PLIDDs) refer to repetitive polimorphic waveforms separated by intervals of 4-20 sec of duration. PLIDDs are the hallmark finding of subacute sclerosing panencephalitis (SSPE). Mecarelli 2019 17 According to the old nomenclature, the best-known periodic lateralized pattern is represented by Periodic Epileptiform Lateralized Discharges (PLEDs). Typically, PLED waveform contains sharp or spike-like features bi-triphasic; the amplitude is variable (100-300 mV) and the duration is between 100 and 400 msec. When PLEDs are present, they are represented almost thoroughly the EEG tracing, approximately every 1 to 4 seconds. The word lateralized is used in place of the word focal, in consideration that the field of a PLED spreads across a whole hemisphere. Reiher et al (1991) proposed a subclassification of PLEDs in two major groups: PLEDs proper, defined as simply configured and uniform discharges, PLEDs plus, required an accompanying low amplitude rhythmic fast activity between periodisms. The few literature data show that clinical seizures are more frequent in patients with PLEDs-plus, but this finding remains still controversial. PLEDs that occur in both hemispheres simultaneously, but asynchronous or Independent, are termed according to old terminology: Bilateral Indipendent Periodic Epileptiform Discharges (BIPLEDs). Mecarelli 2019 18 PLEDs and BIPLEDs are associated with multiple etiologies (stroke, infections, tumor, etc) and they are in general considered an interictal pattern, usually resistant to the treatment with antiepileptic drugs. However, there are also significant evidences that PLEDs are sometimes ictal or simply just postictal. The wide variety of etiologies that determine the PLEDs and their electrographic heterogeneity suggests considering these findings along a continuum between interictal and ictal phases. Electrographic features of Periodic Lateralized Discharges may help determine seizure risk in patients with acute structural lesions. Sharply contoured morphology, overlying fast activity, or rhythmicity are related with higher risk of seizures during continuous EEG monitoring, whereas blunt delta morphology had the lowest risk of seizures. Mecarelli 2019 19 New Standardized Terminology (2013) In 2013, the American Clinical Neurophysiology Society (ACNS) published a new standardized terminology of periodic and rhythmic EEG patterns to be used in the critically ill patients under continuous EEG monitoring. The main purpose is to avoid terms with clinical connotations and to completely define the terms to maximize inter-rater reliability. In the new definition of the periodic pattern, the term “epileptiform” has then been avoided, as it is an interpretative term, since these periodic patterns may or may be not associated with clinical seizures. According to this new nomenclature, the Periodic Discharges (PDs) may be cassified as: Generalized (GPDs) Lateralized (LPDs) Bilateral Independent (BIPDs) Multifocal (MfPDs) Mecarelli 2019 20 The GPDs refer to any bilateral bisynchronous and symmetric pattern, even if it has a restricted field. GPDs may be predominant in frontal, occipital or in the midline regions. The pattern predominance in anterior, posterior or midline derivations is defined as having the amplitude of the pattern at least 50% greater than that in the other derivations. Generalized periodic discharges (GPDs) with triphasic morphology (diffuse triphasic waves - TWs) in a patient with hepatic encephalopathy 21  A 5 sec B 5 sec 22 Mecarelli 2019 Periodic long-interval diffuse discharges (PLIDDs) in a patient with subacute sclerosing panencephalitis Mecarelli 2019 23 The LPDs include focal, regional and hemispheric patterns. For the description of LPDs it is important to specify: if the patterns are purely unilateral or bilateral and synchronous but clearly more prominent on one side (unilateral vs bilateral asymmetric); lobe or lobes most involved. BIPDs refer to the presence of 2 independent/asynchronous lateralized patterns, one in each hemisphere; MfPDs refer to the presence of at least three independent lateralized patterns with at least one in each hemisphere. For BIPDs and MfPDs it is necessary to specify: symmetric vs asymmetric (asymmetric when the pattern is clearly more prominent on one side); lobes most involved in both hemispheres. Mecarelli 2019 24 LPDs localized in left centro-temporal region in a patient with homolateral ischemic stroke. Mecarelli 2019 25 Lateralized periodic discharges (LPDs) in left hemisphere, with tendency of contralateral spread, in a patient with massive left hemorrhage Mecarelli 2019 26 PLEDs proper PLEDs plus Mecarelli 2019 27 6. Rhythmic Patterns The term rhythmic is applied to regular waves occurring at a constant period and of relatively uniform morphology (old term discouraged: monomorphic). 1.Rhythmic Delta Activity (RDA) Rhythmic Delta Activity (RDA) is represented by a repetition of rhythmic slow signals (< 4 Hz) without interval between the consecutive waves. When the RDA occurs in bursts (usually lasting no longer than a few seconds) and it is interrupted by an interval, it is defined Intermittent Rhythmic Delta Activity (IRDA). IRDA are classified in: FIRDA, OIRDA, or TIRDA. Frontal Intermittent Rhythmic Delta Activity (FIRDA) is characterized by bursts of fairly regular and sinusoidal waves at 1.5-2.5 Hz, synchronously over the frontal areas of two hemispheres, occasionally unilateral. This rhythmic activity is associated with non-specific encephalopathies and in older patients the hyperventilation response may mimic the appearance of FIRDA. Mecarelli 2019 28 Bursts of intermittent rhythmic delta activity (IRDA), predominantly in frontal areas (FIRDA) in an old patient with dysmetabolic encephalopathy 29 Occipital Intermittent Rhythmic Delta Activity (OIRDA) is represented by bursts of regular slow waves at 2-3 Hz in the occipital areas of one or both hemispheres. This abnormal pattern, frequently blocked or attenuated by eye opening, is observed more frequently in children than in adults, associated or not with generalized epilepsies. In younger subjects a pattern that mimics OIRDA is hyperventilation-induced occipital slowings. Temporal Intermittent Rhythmic Delta Activity (TIRDA) represents an EEG pattern characterized by short sequences (unilateral or bilateral) of rhythmic delta activity at 1-3.5 Hz, recorded predominantly in the anterior temporal regions. TIRDA occurs more frequently during drowsiness and light sleep and it is associated with temporal lobe epilepsy. Mecarelli 2019 30 Sequences of regular delta waves in the occipital areas (occipital intermittent rhythmic delta activity, OIRDA) in a child with absence epilepsy. Mecarelli 2019 31 2. Stimulus-Induced Rhythmic, Periodic, or Ictal Discharges (SIRPIDs) This EEG pattern was first described by Hirsch et al (2004) in critically ill patients. Stimulus-Induced Rhythmic, Periodic, or Ictal Discharges (SIRPIDs) are rhythmic, periodic or ictal-appearing EEG discharges of sharp transients, elicited by alerting stimuli (auditory, sensory or noxious stimulation). SIRPIDs may be of variable duration, lateralized or generalized. In the ACNS nomenclature (2013) the SI prefix indicate “Stimulus Induced” and the pattern elicited may be GPDs (SI-GPDs), LPDs (SI-LPDs), and SI-RDA. The relation between SIRPIDs and clinical seizures is unclear and this phenomenon usually represents an electrographic pattern without a clinical correlate. However, there are some reports that demonstrate the ictal nature of SIRPIDs. Although the clinical significance of this pattern remains to be clarified, patients with SIRPIDs had more severe brain injuries and poor prognosis. Mecarelli 2019 32 Stimulus-induced diffuse rhythmic delta activity (SI-RDA or SIRPIDs) in a comatose post-traumatic patient with poor outcome. Acoustic stimulus (arrow) induces a prolonged discharge of slow diffuse activity, with sometimes sharp morphology Mecarelli 2019 33 7. Attenuation/Suppression and Electrocerebral inactivity The attenuation, suppression or total loss of EEG activity may be considered as the expression of the reduced or absent cortical neuronal metabolic function, due to reversible or irreversible brain injuries of multiple etiologies. This EEG pattern may vary in degree and duration, until the electrocerebral inactivity (terms discouraged: electrocerebral silence, flat or isoelectric EEG) is reached, which is typical of brain death. Mecarelli 2019 34  Short periods of EEG attenuation/suppression, either diffuse or localized, may precede an epileptic seizure onset and thus be its first EEG sign, useful also for locating purposes. In comatose subjects it is sometimes possible to clearly distinguish short periods (0.5-4 sec) of flattening of the tracing, followed by a change in the pre-existing background activity. This feature, described as ELAEs (Episodic Low-Amplitude Events) is generally of poor prognostic significance. A clearly evident alternation of periods of attenuation/suppression of the tracing and of bursts of EEG activity at various morphology and amplitude is the so-called “burst-suppression” patterns. This is a feature that can be observed in various and severe brain diseases, but it also may be of iatrogenic origin (i.e. excessive sedative/anesthetic level). When the periods of attenuation/suppression are excessively prolonged, alternated with short bursts of residual activity, the finding is defined suppression-burst pattern Mecarelli 2019 35 According to Standardized Critical Care EEG terminology of ACNS (2013) and the Revised Glossary of terms most commonly used by clinical electroencephalographers of IFCN (2017): “attenuation” is defined by the reduction (transiently or permanently) in amplitude of EEG background activity < 50% , with period of lower voltage > 10 μV; “suppression” indicates instead that the period of lower voltage are < 10 μV. On the basis of its characteristics the background activity is classifiable into five main categories:  Continuous: background not interrupted by periods of attenuation/suppression  Nearly continuous: background continuous but with occasional (< 10% of the record) periods of attenuation/suppression (spontaneous or stimulus-induced)  Discontinuous: periods of attenuation/suppression consisting of 11-49% of the record  Burst-attenuation/burst-suppression: > 50% of the record is attenuated/suppressed (note 2)  Suppression: entirety of the record is suppressed (< 10 μV) Mecarelli 2019 36 Short periods of EEG attenuation in a comatose patients Mecarelli 2019 37 Pattern of burst-suppression of iatrogenic origin Mecarelli 2019 38 Pattern of suppression-burst of iatrogenic origin Mecarelli 2019 39 The EEG interpretation and EEG training The interpretation of an EEG is always subjective and that it can be influenced by multiple factors, mainly dependent on the experience of the technologists and of the clinical neurophysiologists. If the technician has not been able to recognize and eliminate from the registration an artefactual activity that can be easily mistaken for pathological changes, this can obviously lead to misinterpretations. It is also very important that the technician assesses the level of awareness of the patient during the recording session: focal and/or diffuse slowings may depend on variations in the subject’s state of arousal or attention, which could also lead to incorrect interpretation of the tracing. In a routine EEG laboratory the tracings should always be accompanied by an anamnestic data sheet containing complete information on the patient, from the personal data to the more specific clinical ones.. Mecarel 40 The EEG interpretation and EEG training Even if it is a laboratory test, the EEG must be interpreted in light of the patient's age, of their clinical history, the reasons for the request, and any pharmacological therapies taken, etc. Given that the tracing has been acquired correctly by technicians and that it is accompanied by all the necessary informations, the interpretation of the EEG recording as normal or abnormal depends on the clinical neurophysiologist, who will prepare a report based on his theoretical preparation and on his practical experience. The theoretical knowledge of clinical EEG is obviously indispensable and it would be preferable that those who dedicate themselves to the reporting of EEG examinations are specialised in Clinical Neurophysiology or have attended at least specific residential courses. The theoretical preparation, however, has very little value if not accompanied by extensive experience in EEG, which should be certified only after a large number of tracings actually evaluated under supervision. Mecarel 41 The EEG interpretation and EEG training Routine EEGs are frequently misinterpreted by neurologist without neurophysiology fellowship training (as are the majority of neurologists interpreting EEG). Misinterpretation leads to the misdiagnosis of epilepsy and the inappropriate use of antiepileptic drugs as well as an inadequate diagnosis and treatment of non- epileptic seizures. Only with appropriate practical experience it is possible to correctly differentiate a normal EEG recording from an abnormal one, taking into account that there are unusual and/or borderline patterns and that what can be considered “normal” for an individual is not necessarily for another. In for a correct EEG interpretation, it is essential a continuous exchange of information between the technician and neurophysiologist and that the technician participates directly, where possible, in the reporting phase. Equally important is that the neurophysiologist does not get stuck in his position and is able to acquire an EEG independently. In the initial training periods all medical doctors interested in the Electroencephalography should spend a period in a laboratory, in close contact with the patient, the technician and the equipment. Mecarelli 2019 42 The EEG interpretation and EEG training We should never forget that a normal EEG in basal conditions could become pathological during the activation tests and that they must be performed according to correct and universally accepted criteria. However, the activation tests can sometimes highlight EEG modifications with respect to the baseline tracing which, although they are completely physiological for that subject or that situation, can instead be interpreted as abnormal and therefore pathological. Equally, it is sometimes necessary to repeat the EEG recording because in some pathologies tracings performed serially give a greater probability of highlighting pathological changes: so it is not said that a first EEG considered normal is not followed by successive registrations with clear abnormalities. When it is not sure that a certain pattern is normal or pathological, it is always advisable to repeat the EEG, with the perspective that the repetition of a diagnostic exam is always better than an incorrect report. It can happen that the neurophysiologist is so uncertain in the assessment of an EEG that he requests the opinion of a second expert; in this case it is better that the second evaluation is carried out blindly, without knowing the clinical data. Subsequently, the two experts can collaborate together an electro-clinical correlation and reach a shared report. Mecarelli 2019 43 References Kane N, Acharya J, Benickzy S et al. A revised glossary of terms most commonly used by clinical electroencephalographers and updated proposal for the report format of the EEG findings. Revision 2017. Clinical Neurophysiology Practice 2017; 2: 170-185. Noachtar S, Binnie C, Ebersole J, Mauguiere F, Sakamoto A, Westmoreland B. A glossary of terms most commonly used by clinical electroencephalographers and proposal for the report form for the EEG findings. Electroenceph Clin Neurophysiol 1999; Suppl 52:21-41. Hirsch LJ, LaRoche SM, Gaspard N, et al. 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