You are in: eMedicine Specialties > Neurology > Electroencephalography and Evoked Potentials Focal EEG Waveform AbnormalitiesArticle Last Updated: Aug 27, 2007AUTHOR AND EDITOR INFORMATIONSection 1 of 8
  Author: Joseph F Hulihan, MD, Vice President, Medical Affairs, Ortho-McNeil Janssen Scientific Affairs, LLC Joseph F Hulihan is a member of the following medical societies: American Academy of Neurology, American Clinical Neurophysiology Society, American Epilepsy Society, American Headache Society, and American Medical Association Editors: Edward B Bromfield, MD, Associate Professor of Neurology, Faculty Member, Division of Sleep Medicine, Harvard Medical School; Chief, Division of EEG, Epilepsy and Sleep Neurology, Consulting Neurologist, Brigham and Women's Hospital; Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine; Norberto Alvarez, MD, Assistant Professor, Department of Neurology, Harvard Medical School; Consulting Staff, Department of Neurology, Boston Children's Hospital; Selim R Benbadis, MD, Professor of Neurology, Director of Comprehensive Epilepsy Program, Departments of Neurology and Neurosurgery, University of South Florida College of Medicine, Tampa General Hospital; Nicholas Lorenzo, MD, Chief Editor, eMedicine Neurology; Consulting Staff, Neurology Specialists and Consultants Author and Editor Disclosure Synonyms and related keywords: electroencephalogram, EEG, transient EEG abnormalities, epileptiform patterns, spikes, sharp waves, continuous focal abnormalities, alterations of ongoing EEG background activity, attenuation, enhancement, focal slow-wave abnormalities, periodic EEG patterns, rhythmic and repetitive sharp wave or spike patterns, epileptiform discharges, epileptiform EEG patterns, EEG background rhythms, focal slowing, periodic EEG patterns, periodic lateralized epileptiform discharges, PLEDs, bilateral independent periodic lateralized epileptiform discharges, BIPLEDs INTRODUCTIONSection 2 of 8
  The electroencephalogram (EEG) is a mainstay of diagnosis for patients with epilepsy, suspected seizures, or impaired consciousness. Prior to the availability of cerebral imaging techniques, such as computed tomography (CT) scanning and magnetic resonance imaging (MRI), EEG was critical for diagnosis of mass lesions in the cerebral hemispheres. For example, in a comatose patient whose EEG manifested decreased amplitude over one cerebral hemisphere, subdural hematoma was strongly suspected. Such EEG diagnosis of cerebral lesions is no longer necessary. Nonetheless, EEG is a critical tool for defining the physiology of focal abnormalities of the cerebrum; such physiologic foci may have no neuroimaging correlates. EEG can further define the clinical significance of regions with subtle or undetected changes on CT scan or MRI. Focal EEG abnormalities may be either transient or continuous. Transient EEG abnormalities include epileptiform patterns, such as spikes or sharp waves; these must be distinguished from other transient patterns representing benign variants that may be mistaken for pathological findings. Continuous focal abnormalities include alterations of ongoing EEG background activity (either attenuation or enhancement), focal slow-wave abnormalities, or periodic EEG patterns that consist of rhythmic and repetitive sharp wave or spike patterns. Each of these types of abnormalities typically is associated with underlying focal pathology. More widespread central nervous system (CNS) physiologic derangements, such as those due to metabolic disturbances, also can occasionally produce such focal EEG abnormalities; most often, these are superimposed on a structural abnormality, though the lesion may not be visualized on brain imaging. Patient Education For excellent patient education resources, visit eMedicine's Procedures Center. Also, see eMedicine's patient education article Electroencephalography (EEG). EPILEPTIFORM DISCHARGESSection 3 of 8
The interictal marker of a seizure focus is the spike or sharp wave. The distinction between these 2 patterns has no etiologic significance, the only difference being one of EEG pattern morphology. A spike is defined as being less than 70 milliseconds in duration, and a sharp wave has a duration of 70-200 milliseconds. The terms spike and sharp wave, while having particular meaning to the electroencephalographer, often are used interchangeably. Spikes and sharp waves are almost always of negative polarity at the scalp surface. These epileptiform discharges may arise from any region of the cerebral hemispheres but most commonly are manifested in the anterior temporal, frontal, or centrotemporal regions. An anterior temporal spike or sharp wave is associated strongly with the occurrence of clinical focal-onset seizures. When this pattern is seen on the EEG, the likelihood of the individual manifesting clinical seizures is over 90%. However, the converse is not necessarily true. While the EEG of most patients with temporal lobe seizures demonstrates anterior temporal spikes, an EEG without this finding does not exclude a diagnosis of epilepsy. Often, repeated EEG recordings or prolonged EEG monitoring is required to demonstrate an interictal epileptiform pattern. In some patients with focal-onset seizures, EEG abnormalities occur only during a seizure. Frontal spikes and sharp waves also are highly associated with clinical seizures but not to the same degree as temporal discharges. Approximately 70-80% of individuals whose EEG demonstrates frontal spikes have clinical seizures. Frontal spikes or sharp waves are more likely to be associated with mass lesions such as neoplasms, traumatic lesions, or congenital cerebral malformations. Centrotemporal or rolandic sharp waves are often a marker for a particular epilepsy syndrome of childhood known as benign rolandic epilepsy or benign focal epilepsy of childhood with centrotemporal spikes. This is a disorder in which a child, typically aged 4-12 years, develops focal sensory or motor seizures in the mouth or face region. These children also may have generalized seizures; typically, these seizures are nocturnal. The EEG pattern is unusual in that a simultaneous negative waveform often occurs in the centrotemporal region and a positive one in the frontal region. When seen in a child with seizures, this EEG finding is virtually diagnostic of benign rolandic epilepsy. However, rolandic spikes are commonly seen in children with no seizure symptoms, and most children whose EEG shows rolandic spikes do not have seizures. FOCAL ALTERATIONS IN EEG BACKGROUND ACTIVITYSection 4 of 8
Visual analysis of the EEG includes evaluation of certain background rhythms, particularly the posterior predominant alpha rhythm, in terms of amplitude, frequency, reactivity, and symmetry. Any of these may be affected by regional alterations in brain function, usually due to focal intracranial lesions. Amplitude abnormalities Amplitude differences need to be interpreted with caution since isolated differences in amplitude may occur as a normal finding. The alpha rhythm may be increased in amplitude on one side, most often the right, by as much as a 2:1 ratio. Less commonly, the alpha rhythm of the left hemisphere is increased by as much as a 3:2 ratio. More pronounced differences in background amplitude are abnormal. Markedly diminished background amplitude on one side of the EEG, compared to homologous channels of the contralateral hemisphere, is found with abnormalities of cortical gray matter, or with excess fluid between the cortex and recording electrodes. This finding is characteristic of ischemic stroke with gray matter involvement or subdural hematoma. In patients with gray matter involvement, concurrent white matter involvement causing polymorphic delta activity is typical. Decreased background amplitude also may occur with congenital lesions, such as porencephalic cysts, or with Sturge-Weber syndrome. Transient background attenuation also is characteristic of the postictal EEG of patients with focal-onset seizures. Less commonly, increased amplitude of background waveforms (alpha rhythm, sleep spindles, beta activity, or mu rhythm) can be seen ipsilateral to cerebral lesions. Amplitude abnormalities seen in the presence of skull defects also should be mentioned in the context of focal EEG disturbances. The breach rhythm is an accentuation of EEG amplitude in the region of a skull defect. This results from decrease in the filtering effect of the skull, and affects primarily faster frequencies, so that theta, alpha, and especially beta are accentuated more than delta, and waveforms appear unusually sharp. Frequency Alterations in EEG background frequency typically are most useful in the assessment of diffuse rather than focal cerebral disturbances. The EEG background frequency of the 2 hemispheres in the adult EEG should be within 1 Hertz (Hz). Any greater difference is indicative of a lateralized EEG abnormality on the side with the slower background. Reactivity In focal cerebral lesions, the posterior predominant frequency may show unilateral impairment of reactivity to eye opening (Bancaud phenomenon) or alerting. Lesions do not need to be in the occipital lobes to produce these abnormalities of EEG reactivity. FOCAL SLOWINGSection 5 of 8
Polymorphic delta activity (PDA, see Media file 1) consists of arrhythmic slow waves that vary in frequency, amplitude, and morphology. PDA can occur in either a focal or generalized distribution. Continuous PDA is indicative of abnormalities involving subcortical white matter. One of the shortcomings of standard scalp EEG recordings is their limited spatial resolution. This holds true for the relationship of PDA to an underlying structural abnormality. Not only is the inherent localizing value of the scalp EEG limited, but also the PDA of a structural lesion is referable not to the lesion itself but to the surrounding brain tissue. Because of this limitation, the area of a lesion is indicated not by the maximal amplitude of PDA but rather by a region of relatively low-amplitude slowing. Continuous, rather than intermittent, PDA is associated with large lesions, mass effect, and impairment of consciousness. Rhythmic delta activity consists of sinusoidal waveforms of approximately 2.5 Hz that occur intermittently in the EEG recording. It is most often symmetric but can be lateralized. In adults, the delta activity has a frontal predominance (frontal intermittent rhythmic delta activity [FIRDA]). In children, it is maximal posteriorly (occipital intermittent rhythmic delta activity [OIRDA]). Intermittent rhythmic delta activity is associated with structural lesions, most commonly diencephalic, infratentorial, or intraventricular tumors, or with diffuse encephalopathies. FIRDA occurring in patients with a normal EEG background suggests that the pattern is due to a structural lesion; when associated with EEG background abnormalities, it is likely to be due to encephalopathy. In cases of encephalopathy with FIRDA, the pathophysiologic processes are believed to involve cortical and subcortical gray matter. OIRDA is associated with absence epilepsy in children aged 6-10 years. Focal theta activity is less likely to reflect a macroscopic structural lesion than is focal delta. Theta, however, commonly is associated with a functional disturbance, such as epileptogenic cortex, especially postictally, after amplitude suppression and focal delta have resolved. In addition, localized theta usually is superimposed on focal delta to some degree; the relative proportion of delta and theta reflects the size and/or severity of the underlying structural or functional cerebral abnormality. PERIODIC EEG PATTERNSSection 6 of 8
Periodic lateralized epileptiform discharges (PLEDs, see Media file 2) are EEG abnormalities consisting of repetitive spike or sharp wave discharges, which are focal or lateralized over one hemisphere, recur at intervals of 0.5-5 seconds, and continue through most of the duration of the EEG study. They are seen most frequently in the setting of acute unilateral lesions such as cerebral infarctions. They also may occur in other cerebral diseases, such as encephalitis or tumors, or in the setting of chronic lesions or long-standing epileptic disorders. Bilateral independent PLEDs (BIPLEDs) are periodic complexes over both hemispheres. BIPLEDS are not synchronous and may differ in morphology and site of maximal expression on each side. This is an uncommon EEG finding. In a series of 18 patients, the most common etiologies were anoxic brain injury (28%) and CNS infection (28%). While BIPLEDs have been associated with herpes simplex encephalitis, the pattern can occur in other CNS infections as well. The clinical correlates of BIPLEDs differ somewhat from those of PLEDs. With BIPLEDs, incidence of coma is higher compared to PLEDs (72% vs 24%), mortality rate is higher (61% vs 29%), and likelihood of focal seizures or focal neurological deficits is lower. Brenner and Schaul have reviewed the classification of periodic EEG patterns based on the interval between the discharges, topographic distribution, and synchrony between hemispheres (see Media file 3). MULTIMEDIASection 7 of 8
REFERENCESSection 8 of 8
Focal EEG Waveform Abnormalities excerpt Article Last Updated: Aug 27, 2007 | |||||||||||||||||||||
