An Electroencephalogram, commonly known as an EEG, is a non-invasive test that measures the electrical activity of the brain. Tiny metal discs, or electrodes, are placed on the scalp to detect the electrical signals produced by brain cells. These signals are then recorded by a computer, appearing as wavy lines that reflect ongoing brain wave patterns. The primary purpose of an EEG is to detect and monitor brain conditions, particularly those related to abnormal electrical activity such as seizure disorders. It also helps in identifying causes for other neurological issues like sleep disorders or changes in behavior.
Understanding Brain Waves
Brain activity is characterized by electrical signals that form distinct patterns, known as brain waves, which vary in speed and strength. These patterns are measured in Hertz (Hz) for frequency and microvolts for amplitude. Different brain wave types are associated with various states of consciousness and mental activities.
Delta waves, ranging from 0.5 to 4 Hz, are the slowest and highest in amplitude, observed during deep, dreamless sleep, playing a role in physical restoration. Theta waves, with a frequency of 4 to 8 Hz, are present during light sleep, deep relaxation, and states of creativity or meditation. Alpha waves, at 8 to 12 Hz, reflect a relaxed but alert mental state, seen when eyes are closed or during quiet introspection.
Beta waves, operating between 12 and 30 Hz, are characteristic of an alert, focused, and actively thinking state, prevalent during daily activities like conversation or problem-solving. Gamma waves, the fastest at 30 to 100 Hz, are associated with heightened perception, intense focus, and complex problem-solving, indicating peak mental performance. Understanding these wave types forms the basis for interpreting EEG recordings.
Interpreting EEG Patterns
Interpreting EEG patterns involves observing several characteristics of the recorded brain waves, including their frequency, amplitude, location on the scalp, and how symmetrically and synchronously they appear. Normal EEG patterns differ between awake and sleeping states. For instance, a normal awake adult EEG shows predominant alpha and beta activity, without significant delta activity.
During drowsiness and the initial stages of sleep, the alpha rhythm may become less prominent, and theta waves emerge, sometimes accompanied by slow rolling eye movements. As sleep deepens, specific patterns like sleep spindles and K-complexes become visible in Stage 2 sleep, and delta waves dominate in deep NREM sleep stages. Normal patterns also exhibit reactivity, meaning they change predictably in response to stimuli, such as the suppression of alpha waves when eyes open or during mental effort.
Abnormal patterns on an EEG can indicate underlying brain dysfunction. For example, epileptiform activity, which includes sharp waves, spikes, and spike-and-wave complexes, suggests cortical hyperexcitability and an increased risk for seizures. Diffuse or focal slowing, characterized by an excess of slower delta or theta waves in an awake adult, can signal generalized or localized cerebral dysfunction due to various conditions like tumors, stroke, or encephalopathy.
Factors Influencing EEG Readings
Several variables, both internal and external, can influence the appearance of an EEG recording. These unwanted signals, known as artifacts, can originate from physiological sources other than the brain or from external electrical interference.
Common physiological artifacts include electrical activity from eye movements, such as blinks or lateral eye movements, which generate large waveforms, especially in frontal electrodes. Muscle movements, like jaw clenching or facial muscle tension, can produce high-frequency, irregular noise that can obscure brain activity. Cardiac activity or sweat can also contaminate the recording.
The patient’s state during the recording also affects the EEG; drowsiness, alertness, or anxiety can alter baseline brain wave patterns. Furthermore, various medications can impact EEG readings. For example, certain drugs can cause an increase in beta activity or induce generalized slowing with theta and delta waves, potentially mimicking or obscuring pathological findings.
Limitations and Professional Interpretation
While understanding basic EEG patterns offers insight into brain activity, EEG interpretation has limitations. The technique primarily measures electrical activity at the scalp surface, making it less sensitive to signals originating from deeper brain structures. Pinpointing the exact location of brain activity can be challenging due to how electrical signals spread through brain tissue, the skull, and the scalp.
The presence of various artifacts, whether physiological or external, can also complicate interpretation, as they may mimic or mask genuine brain activity. Therefore, a simplified understanding of EEG patterns is not a substitute for clinical diagnosis.
Accurate interpretation and diagnosis based on EEG recordings require specialized training and extensive experience. Trained neurologists or neurophysiologists perform comprehensive analysis, integrating EEG findings with a patient’s medical history and other clinical information. Their expertise differentiates normal variations or artifacts from pathological activity, ensuring appropriate clinical decisions.