The human body communicates and functions through a complex network of electrical signals. Electroencephalography (EEG) and Electromyography (EMG) are distinct medical tests that measure this bioelectrical activity to diagnose conditions affecting the nervous system. While both procedures use electrodes to record electrical impulses, they focus on entirely different anatomical structures and physiological processes. Understanding the specific signals each test captures is the first step in distinguishing these diagnostic tools.
Understanding the Electroencephalogram (EEG)
The Electroencephalogram (EEG) measures the spontaneous electrical activity generated by the brain’s nerve cells. This activity represents the synchronous firing of neurons and is visualized as wavy lines. It is a non-invasive procedure where a technician attaches small electrodes to the patient’s scalp using a conductive paste.
The core data collected are brain waves, categorized by frequency into rhythms like delta, theta, alpha, and beta waves. Delta waves are associated with deep sleep, while faster beta waves dominate during alertness. Specialists analyze the patterns, amplitude, and frequency of these waves to identify abnormal electrical discharges.
The primary application is the diagnosis and management of epilepsy and other seizure disorders, which manifest as rapid, abnormal spiking waves. The test also evaluates sleep disorders and assesses brain dysfunction in cases of stroke, tumors, or encephalopathy. It is sometimes used to confirm brain death.
Understanding Electromyography (EMG)
Electromyography (EMG) evaluates the health of skeletal muscles and the motor neurons that control them, focusing on the peripheral nervous system. This test measures the electrical potentials generated by muscle cells when they are activated. EMG is often performed with a Nerve Conduction Study (NCS) for a complete picture of neuromuscular function.
The EMG portion involves inserting a fine needle electrode directly into the muscle to record its electrical activity at rest and during contraction. This allows the specialist to detect abnormal spontaneous activity, such as fibrillation potentials, which can indicate muscle fiber denervation. Muscle action potentials are analyzed to assess muscle health.
The associated NCS uses surface electrodes to stimulate a peripheral nerve with a mild electrical current. This measures the speed and strength of the electrical signal as it travels along the nerve. EMG and NCS diagnose conditions like carpal tunnel syndrome, peripheral neuropathy, and neuromuscular diseases such as muscular dystrophy or amyotrophic lateral sclerosis (ALS).
Core Differences in Physiological Measurement and Application
The fundamental distinction lies in the anatomical structures they target. The EEG focuses on the central nervous system, recording electrical activity from the brain. Conversely, the EMG and NCS focus on the peripheral nervous system, evaluating the integrity of specific nerves and the muscles they innervate.
The output data reflects this difference in focus. An EEG produces continuous, rhythmic waveforms interpreted based on frequency and pattern, providing a functional snapshot of the brain’s state. In contrast, the EMG and NCS yield data centered on discrete electrical events.
The NCS output is measured by nerve conduction velocity and the latency of the signal. The needle EMG measures the amplitude and duration of muscle action potentials. This provides specific, localized information about nerve-to-muscle communication and muscle fiber health.
Patient Experience and Test Preparation
The preparation and physical experience vary significantly between the two procedures. For an EEG, patients are asked to wash their hair the night before and avoid using conditioners, sprays, or gels, as these interfere with electrode adhesion. Some EEG tests may require the patient to be sleep-deprived to increase the likelihood of capturing abnormal activity.
The EEG is entirely non-invasive, involving no electric shocks or needle insertions, and is painless. In contrast, the EMG/NCS procedure is more physically involved and can cause mild discomfort. Patients should avoid applying lotions or oils to the skin before an EMG, as these interfere with signal conduction.
The NCS component involves brief, mild electrical stimulation, which patients may describe as a quick, shocking or tingling sensation. The EMG component requires the insertion of thin needle electrodes into the muscle. This insertion can cause a slight prick or dull ache.