Chapter: Abnormal Nonepileptiform EEG
Introduction
Interictal electroencephalography (EEG) is an essential tool for detecting nonepileptiform neurophysiological dysfunction. While neuroimaging provides structural detail, EEG reveals functional abnormalities — even when imaging is normal. Nonepileptiform EEG abnormalities often correlate with the degree of cerebral dysfunction and can track clinical improvement or deterioration over time.
EEG abnormalities may be focal or diffuse, reflecting localized or generalized disturbances. Although these abnormalities lack specificity for underlying etiology, they provide valuable diagnostic, monitoring, and prognostic information.
SECTION 1: CATEGORIES OF NONEPILEPTIFORM EEG ABNORMALITIES
Nonepileptiform EEG abnormalities are grouped into: - Focal abnormalities - Diffuse abnormalities
Each category highlights distinct patterns of dysfunction involving white matter, gray matter, or both.
SECTION 2: FOCAL NONEPILEPTIFORM ABNORMALITIES
Focal EEG abnormalities represent localized cerebral dysfunction. These abnormalities may arise from: - Structural lesions (tumor, ischemia, hemorrhage, trauma, infection) - Postictal states - Transient physiological disturbances
Key Features Suggesting Structural Etiology
· Persistence
· Poor reactivity to stimuli
· Morphological distortion
· Concomitant loss of faster frequencies
2.1 Alpha Asymmetries
Alpha asymmetry indicates hemispheric dysfunction. - A persistent asymmetry >1 Hz in posterior dominant rhythm is abnormal. - Amplitude asymmetry >50% is significant.
Alpha asymmetry (slower over the right hemisphere) in a patient with an acute right fronto-parietal ischemic infarction.
Figure 10A – Alpha asymmetry with right hemisphere slowing.
Right hemisphere suppression in a patient with a right hemisphere intracerebral hemorrhage. Note the dramatic difference in amplitude at standard sensitivity settings of 7 uV/mm
Figure 10B – Right hemisphere suppression due to intracerebral hemorrhage.
2.2 Focal Polymorphic Delta Activity
Focal delta activity restricted to one or two electrodes suggests localized white matter dysfunction. - Heightened significance when faster frequencies are absent. - Persistent focal delta strongly correlates with structural lesions.
Diagram Placeholder: Figure 11A – Right temporal delta slowing in ganglioglioma.
Diagram Placeholder: Figure 11B – Intracranial EEG showing focal delta.
2.3 Temporal Intermittent Rhythmic Delta Activity (TIRDA)
TIRDA consists of rhythmic bursts of delta activity localized to temporal lobes. - Strongly associated with temporal lobe epilepsy. - Often occurs alongside epileptiform discharges.
Diagram Placeholder: Figure 12 – Left temporal TIRDA.
2.4 Intermittent Irregular Slowing
This pattern is less predictive of structural pathology compared to continuous slowing. - Suggests mild or transient cerebral dysfunction.
Diagram Placeholder: Figure 13 – Polymorphic delta in left temporal–parietal region.
2.5 Continuous Regional Delta Slowing
This pattern has high correlation with underlying structural lesions. - Seen in stroke, tumors, trauma, hemorrhage, and infection. - May resolve if pathology is transient (e.g., transient ischemic attack).
Diagram Placeholder: Figure 14A – Left hemispheric delta slowing in ischemic infarct.
Diagram Placeholder: Figure 14B – Reversible regional slowing in transient aphasia.
2.6 Lateralized Polymorphic Delta Slowing
Delta frequencies 3.5 Hz with variable morphology. - Indicates supratentorial lesion affecting ipsilateral white matter. - Persistence increases likelihood of structural lesions. - May also appear transiently (TIA, migraine, postictal states, head injury).
Diagram Placeholder: Figure 15A – Right hemisphere polymorphic delta in infarct.
Diagram Placeholder: Figure 15B – Drug-induced slowing during Wada test.
2.7 Asymmetry of Sleep Spindles
Normal sleep spindles are synchronous by age 2. - Persistent unilateral reduction or slowing indicates thalamocortical dysfunction.
Clinical Example: Right thalamic glioma.
Diagram Placeholder: Figure 16 – Asymmetric sleep spindles.
2.8 Fast Focal Midline Central Rhythm
This pattern may assist in diagnosing orthostatic tremor. - Frequency range: 14–24 Hz. - Maximal at Cz. - Correlates with EMG tremor activity.
Diagram Placeholder: Figure 17 – Midline fast rhythm in orthostatic tremor.
SECTION 3: DIFFUSE NONEPILEPTIFORM ABNORMALITIES
(Content will expand once diffuse abnormality section from slides is added.)
Diffuse abnormalities often indicate generalized encephalopathy and may reflect: - Metabolic disorders - Hypoxic injury - Drug toxicity - Systemic illness affecting cerebral function
SECTION 4: CLINICAL APPLICATIONS OF NONEPILEPTIFORM EEG FINDINGS
Nonepileptiform abnormalities help clinicians: - Monitor progress of encephalopathy - Identify lateralization of dysfunction - Differentiate transient vs structural causes - Evaluate postictal states - Support diagnosis when neuroimaging is normal
SECTION 5: SUMMARY
Nonepileptiform EEG abnormalities offer critical insights into cerebral dysfunction, aiding diagnosis and management in both acute and chronic neurological conditions. Although nonspecific, their patterns provide valuable localization, prognostic, and monitoring information.
DIAGRAM INSERTION NOTE
Diagrams referenced in this chapter correspond to the user’s PowerPoint file: Abnormal Nonepileptiform EEG fileciteturn0file0.
They can be inserted during book formatting at the specified placeholders above.
