The Electroencephalogram (EEG) is a medical test that measures the electrical activity produced by the brain. It utilizes small sensors, or electrodes, placed on the scalp to record the synchronized communication between billions of neurons in the form of brain waves. These waves are categorized by frequency, measured in Hertz (Hz), and grouped using Greek letters to denote different states of consciousness.
The fastest frequencies include Beta waves (greater than 13 Hz), associated with active, focused thinking, and Alpha waves (8–13 Hz), seen when a person is awake but relaxed. Slow waves specifically refer to activity in the Theta (4–7.5 Hz) and Delta (4 Hz or less) frequency ranges. The presence of these slower frequencies is entirely dependent on the context of the recording, meaning they are normal and expected in certain physiological conditions.
Normal Slow Wave Activity
The most common time to observe slow wave activity is during sleep, particularly in the deeper stages of non-rapid eye movement (NREM) sleep. Stages III and IV of NREM sleep are known as slow wave sleep, characterized by synchronized, high-amplitude Delta activity across the brain. This Delta activity reflects the deep restorative processes occurring and is a normal feature of a healthy adult sleep cycle.
Theta and Delta waves are also a normal feature of brain development in children and adolescents. In infants, Delta is the dominant rhythm, and Theta waves are common in the waking EEG of young children. As the brain matures, the dominant frequency gradually increases, and these slower waves typically diminish by adulthood.
Even in an awake adult, brief periods of Theta wave activity may be seen during moments of drowsiness or a relaxed resting state. The transition from Alpha to Theta frequencies is a natural sign that the individual is becoming less alert. Therefore, an EEG reading must be interpreted with the patient’s level of alertness and age in mind to distinguish normal physiological slowing from a potential problem.
Pathological Causes of Diffuse Slowing
When slow waves appear diffusely and persistently across both hemispheres of an awake adult, it signals encephalopathy, a generalized disturbance of cerebral function. This generalized slowing reflects impairment of the brain’s metabolism or chemical environment. The degree of slowing, moving from Theta to continuous Delta, often correlates with the severity of the underlying systemic illness.
Metabolic disturbances are common causes of diffuse slowing because the brain is sensitive to fluctuations in its chemical supply. Conditions like severe hypoglycemia, which starves the brain of glucose, or advanced kidney and liver failure, which allow toxic waste products to accumulate, impair neuronal function. In cases of hepatic or renal encephalopathy, a specific pattern called triphasic waves may emerge, suggesting metabolic disarray.
Various drugs and toxins can also induce global slowing by depressing the central nervous system. Sedative medications, such as benzodiazepines or opioids, can cause a generalized increase in Theta and Delta activity that mimics encephalopathy. Severe systemic infections, including sepsis or meningitis, can also lead to global cerebral dysfunction through inflammation.
Global hypoxia or ischemia, often resulting from cardiac arrest, can damage the brain’s regulatory systems, leading to profound and continuous diffuse Delta activity. Lesions affecting deeper brain structures, such as the thalamus or upper brainstem, can disrupt cortical timing mechanisms. This disruption projects a generalized slow pattern like Frontal Intermittent Rhythmic Delta Activity (FIRDA). This rhythmic slowing is often seen in metabolic or toxic states.
Pathological Causes of Localized Slowing
In contrast to diffuse patterns, slow waves confined to a specific region typically indicate a focal lesion or structural problem in that area. This localized slowing means the underlying brain tissue is dysfunctional while surrounding areas remain electrically normal. Continuous focal slow waves strongly indicate a structural abnormality disrupting the cerebral cortex or its underlying white matter.
Brain tumors are a common cause of focal slowing, particularly if they infiltrate the white matter beneath the cortex. The slowing is caused by the tumor mass compressing or invading the surrounding functional brain tissue. An acute stroke, whether an infarct or hemorrhage, also causes immediate and pronounced focal Delta activity due to the severe injury or death of local neurons.
Localized trauma, such as a contusion, or a focal infection, like a brain abscess, will also produce slow waves restricted to the site of injury. Localized Delta activity often suggests a more significant or deeper structural lesion, while focal Theta activity may indicate a less severe disruption or superficial cortical irritation. The precise location of the slow waves on the EEG tracing helps the neurologist pinpoint the exact area of cerebral dysfunction.
This localization is valuable for directing further diagnostic imaging, such as a CT or MRI scan, to identify the source of the structural damage. The persistence of the slowing, whether continuous or intermittent, also helps determine the nature and seriousness of the underlying issue.
Interpreting the Findings
Detecting slow waves on an EEG indicates cerebral dysfunction, but it does not provide a definitive diagnosis alone. The test results must be integrated with the patient’s clinical picture, including their age, medication history, and current mental status. For example, a small amount of Theta in a drowsy adult is expected. However, continuous Delta activity in an alert patient is a serious finding that requires immediate investigation.
EEG findings are used alongside other diagnostic tools to establish a cause. If diffuse slowing is noted, blood work is typically ordered to check for liver, kidney, or glucose abnormalities to rule out metabolic encephalopathy. If the slowing is focal, neuroimaging studies like MRI or CT scans are required to identify structural lesions such as tumors or strokes.
In the context of epilepsy, slow waves can be observed between seizures, known as interictal slowing, often over the area where the seizure originates. This localized slowing points toward an abnormally functioning brain region that acts as a seizure focus. In severe cases, such as profound anoxic brain injury or coma, continuous, severe slowing with no reactivity can reflect widespread and irreversible brain damage.