What does it mean when slow waves are seen on an electroencephalogram (EEG) in patients, particularly in geriatric individuals with dementia or younger patients with brain injury?

Slow Waves on EEG: Clinical Significance

Slow waves on EEG indicate diffuse cortical dysfunction and are pathological markers of various acute and chronic brain disorders, including delirium, metabolic encephalopathy, dementia, encephalitis, and structural brain injury.

Primary Clinical Associations

Delirium and Acute Encephalopathy

• Delirium is characterized by increased delta (<4 Hz) and theta (4-8 Hz) oscillations with decreased alpha (>8 Hz) oscillations on EEG 1.
• These slow wave changes correlate directly with cognitive performance and delirium severity, with quantitative EEG distinguishing delirious from non-delirious patients with 100% sensitivity and 99% specificity 1.
• Slow waves in delirium reflect impaired functional connectivity and less integrated brain networks following various stressors 1.

Metabolic Encephalopathies

• Triphasic waves (a specific pattern of slow waves) are characteristic of metabolic encephalopathies including hepatic encephalopathy, uremia, hyponatremia, and drug intoxications, not epileptic activity 2.
• Progressive slowing of EEG with increasing amplitude initially, followed by decreased amplitude, appearance of triphasic waves, and eventually discontinuous or isoelectric patterns indicates worsening hepatic encephalopathy severity 3.
• Generalized rhythmic delta activity (GRDA) represents diffuse encephalopathy and requires investigation for infectious, autoimmune, or metabolic causes 4.

Structural Brain Pathology

• Bilaterally synchronous slow wave bursts in an otherwise normal waking EEG indicate diffuse encephalopathy in the majority of cases, not focal deep midline lesions 5.
• Focal slow waves, particularly in temporal regions, localize to epileptogenic zones in patients with mesial temporal lobe epilepsy and correlate with the side requiring surgical resection 6.
• Sharp slow waves (SSWs) occur primarily in the first two decades of life and indicate either partial/generalized epilepsy or underlying chronic CNS pathology, particularly congenital CNS anomalies 7.

Dementia and Neurodegenerative Disease

• In Alzheimer's disease and dementia, abnormally prominent theta (4-7 Hz) or delta (1-4 Hz) rhythms during resting state are considered signs of brain dysfunction 1.
• High amplitude low-frequency alpha rhythms (8-10 Hz) may reflect low levels of general brain arousal and attention in quiet vigilance 1.
• Slow wave abnormalities in dementia patients help stratify disease severity and monitor progression 1.

Encephalitis

• Diffuse slow wave activity is a sensitive but nonspecific indicator of cerebral dysfunction in encephalitis, often appearing early in the disease course 1.
• Temporal focus with periodic lateralizing epileptiform discharges (occurring at 2-3 second intervals) on days 2-14 after symptom onset is seen in approximately 80% of herpes simplex encephalitis cases 1.
• Rapidly improving EEG findings, including resolution of slow waves, often indicate good prognosis in encephalitis 1, 4.

Post-Cardiac Arrest

• Rhythmic and periodic patterns including generalized periodic discharges at 0.5-2.5 Hz without evolution are common after cardiac arrest but their treatment significance remains unclear 1.
• These patterns may represent markers of injured brain rather than requiring specific antiseizure treatment 1.

Diagnostic Approach Algorithm

Initial Assessment

• Obtain detailed clinical context: acute versus chronic onset, level of consciousness, metabolic derangements, medication exposures, and neurological examination findings 2, 4.
• Perform brain MRI with contrast to exclude structural lesions, infectious processes, or autoimmune encephalitis 4.

Pattern Recognition

• Delta (<4 Hz) and theta (4-8 Hz) predominance: Consider delirium, metabolic encephalopathy, or diffuse brain injury 1.
• Triphasic waves: Evaluate for hepatic, uremic, or other metabolic encephalopathies; perform benzodiazepine trial to distinguish from non-convulsive status epilepticus 2.
• Focal slow waves: Investigate for structural lesions, focal epilepsy, or localized brain injury 6, 7.
• Generalized rhythmic delta activity: Rule out infectious or autoimmune encephalitis with lumbar puncture and CSF analysis 4.

Prognostic Implications

• Presence of delta slow waves without spikes more often associates with uncontrolled seizures, while absence of slow waves despite many spikes typically indicates controlled epilepsy 8.
• Serial EEG monitoring tracks treatment response and disease progression, with improving slow wave patterns indicating favorable prognosis 1, 4, 3.

Critical Pitfalls to Avoid

• Do not mistake triphasic waves for epileptiform activity—they require metabolic correction, not antiseizure medications 2.
• Do not assume all slow waves indicate the same pathology—focal versus generalized patterns have distinct etiologies requiring different workups 5, 6.
• Do not delay lumbar puncture when GRDA is present with altered mental status—infectious and autoimmune encephalitis require urgent specific treatment 4.
• Do not overlook congenital CNS anomalies in young patients with sharp slow waves—brain MRI is essential in this population 7.