Encephalopathy is a term describing a state of generalized brain dysfunction, resulting in changes in mental status, cognition, or consciousness. It is not a single disease but rather a manifestation of an underlying issue affecting the brain’s overall function. The electroencephalogram (EEG) is a non-invasive tool for assessing the health of the cerebral cortex. The EEG provides a rapid, objective measure of brain activity, helping to determine the severity of the generalized brain impairment.
Defining Encephalopathy and the Role of EEG
Encephalopathy encompasses a wide spectrum of issues, ranging from mild confusion and delirium to profound coma. This diffuse cerebral dysfunction is caused by various systemic problems, including toxic, metabolic, or infectious derangements. Symptoms often involve lethargy, cognitive impairment, and a disturbed sleep-wake cycle.
The EEG measures the electrical impulses generated by the brain’s neurons. By placing electrodes on the scalp, the EEG records these signals, translating them into wavy lines that represent brain function. The EEG is sensitive to the functional status of the gray matter.
The EEG’s role is to objectively assess the degree of cerebral impairment, reflected by a slowing of the brain’s dominant electrical rhythms. It can also help differentiate true encephalopathy from non-convulsive seizures, which require different immediate treatment. The hallmark of most acute encephalopathies is a generalized slowing of the background electrical activity.
The EEG Procedure and Patient Preparation
The EEG procedure is painless. To ensure the best electrical contact, patients are asked to wash their hair the night before or the morning of the test, avoiding conditioners, oils, and styling gels. A doctor may request a period of sleep deprivation to provoke certain abnormal brain patterns.
During the test, a trained technologist measures the patient’s head to mark the placement points for the electrodes. These spots are gently scrubbed with a mild cream before approximately 16 to 25 small metal discs are attached to the scalp using a conductive paste. The electrodes are connected by wires to the EEG machine, which amplifies and records the brain’s electrical signals.
A routine EEG usually involves a recording time of 20 to 40 minutes, with the entire appointment lasting about an hour to an hour and a half. The patient is asked to relax and remain as still as possible. The technologist may introduce various activating procedures, such as asking the patient to open and close their eyes, breathe deeply and rapidly (hyperventilation), or look at a flashing light.
Interpreting Key EEG Wave Patterns
The most common finding in encephalopathy is a generalized slowing of the background rhythm, which correlates with the severity of the brain dysfunction. In a healthy, awake adult, the dominant electrical frequency is typically in the alpha range (8–13 Hz). As encephalopathy progresses, this dominant rhythm slows down, first into the theta range (4–7 Hz), and then into the slower delta range (less than 4 Hz).
A recognizable pattern in metabolic encephalopathy is the appearance of Triphasic Waves (TWs). These waves are characterized by a triphasic morphology—a sharp, positive deflection preceded and followed by negative deflections. They are generally seen bilaterally across both hemispheres.
Triphasic Waves are often frontally dominant and associated with severe toxic-metabolic conditions, most notably hepatic encephalopathy. Their presence prompts an investigation for systemic derangements like uremic encephalopathy or severe electrolyte imbalances. Triphasic Waves represent generalized brain irritation rather than a seizure, as they lack the characteristic evolution seen in true epileptic events.
Periodic Discharges
Other specific patterns include periodic discharges. Generalized Periodic Discharges (GPDs), which occur at regular intervals across the entire brain, can be seen in conditions like sporadic Creutzfeldt-Jakob disease. Lateralized Periodic Discharges (LPDs), which are confined to one hemisphere, are a characteristic finding in conditions such as herpes simplex encephalitis.
Applying EEG Results to Treatment and Prognosis
The EEG findings are directly applied to the patient’s clinical management to guide treatment and predict long-term outlook. The degree of background slowing is used to grade the severity of the encephalopathy. For example, established grading systems use EEG findings to classify the severity of hepatic encephalopathy into four stages.
Serial EEG monitoring is frequently used to assess the effectiveness of therapeutic interventions. A positive response to treatment, such as correcting a severe metabolic imbalance, is reflected by a progressive normalization of the EEG, with the background rhythm gradually speeding up. Conversely, a worsening EEG pattern suggests the patient is not responding to therapy or that the underlying condition is advancing.
EEG patterns are important in determining the patient’s prognosis in severe cases. Patients whose EEGs show severe abnormalities, such as Grade 3 or 4 findings in hepatic encephalopathy, indicate higher mortality rates. The presence of certain patterns, like Triphasic Waves following an anoxic brain injury (from lack of oxygen), is associated with a poor long-term outlook. The EEG provides an objective physiological measure that helps clinicians make informed decisions about the course of care.