An electroencephalogram (EEG) and an electromyogram (EMG) are distinct diagnostic tests that both measure electrical signals within the body. While their names sound similar, they investigate different physiological systems and serve unique diagnostic purposes. This article will clarify the function, purpose, and key differences of each test.
The Electroencephalogram (EEG) Explained
An electroencephalogram (EEG) records the electrical activity generated by the brain. It is primarily used to detect and investigate epilepsy, a condition characterized by repeated seizures, by identifying abnormal electrical discharges. An EEG also helps assess sleep disorders, evaluate brain function after a head injury, and investigate conditions such as brain tumors, encephalitis, and dementia.
During an EEG, electrodes are attached to the scalp with glue or paste. Between 16 and 25 electrodes are typically used, connecting to a machine that records brain signals. The non-invasive procedure is generally painless, as electrodes only record activity. Patients sit or lie down and may be asked to relax, open/close eyes, breathe deeply, or look at flashing lights to observe brain responses. A routine EEG typically lasts 20 to 60 minutes.
The Electromyogram (EMG) Explained
An electromyogram (EMG) assesses the health of muscles and their controlling motor neurons, revealing issues with nerve or muscle function, or nerve-to-muscle signal transmission. Common diagnostic uses for an EMG include identifying conditions such as carpal tunnel syndrome, pinched nerves, muscular dystrophy, amyotrophic lateral sclerosis (ALS), and myasthenia gravis.
An EMG often involves two main parts. The first is a nerve conduction study (NCS), which uses surface electrodes placed on the skin. Mild electrical pulses are delivered to a nerve, and the speed and strength of the signals traveling between points are measured. This helps determine if nerves are damaged or if signals are moving too slowly.
The second part is the needle EMG, where a fine, sterile needle electrode is inserted directly into a muscle. It records the muscle’s electrical activity at rest and during contraction. While surface electrodes are typically painless, needle insertion may cause quick, sharp pain or mild discomfort. Soreness or tingling may persist for up to two days after the test.
Comparing EEG and EMG
The primary distinction between an EEG and an EMG lies in the area of the body they measure. An EEG records electrical activity originating from the brain, focusing on the central nervous system. In contrast, an EMG assesses the electrical activity of muscles and the peripheral nerves that control them, exploring the peripheral nervous system and neuromuscular connections.
Their primary purposes also differ significantly. An EEG is used to diagnose conditions affecting brain function, such as seizure disorders, sleep abnormalities, or the effects of head injuries. An EMG, however, is employed to identify disorders related to muscle health or nerve damage, including carpal tunnel syndrome, muscular dystrophy, or nerve root compression.
The procedural methods also vary. An EEG is a non-invasive test where electrodes are placed on the scalp’s surface, capturing brain wave patterns. An EMG, on the other hand, typically involves both surface electrodes for nerve conduction studies and minimally-invasive needle electrodes inserted directly into muscles.
Combined Use in Diagnostic Testing
Both EEG and EMG tests can be used together to provide a more comprehensive diagnostic picture in certain scenarios. A notable example is during a polysomnogram (sleep study). In this context, the EEG component tracks brain wave activity to identify different sleep stages, such as light sleep, deep sleep, and rapid eye movement (REM) sleep. This allows detection of sleep architecture abnormalities or disruptions.
Simultaneously, the EMG component monitors muscle activity throughout the sleep study. Electrodes are typically placed on chin and leg muscles. This allows detection of muscle paralysis during REM sleep, or involuntary movements like restless leg syndrome or periodic limb movements that disturb sleep. Combined data helps differentiate sleep disorders originating from brain activity versus those involving muscle or nerve control. These tests can also be combined in complex neuromuscular cases to investigate or rule out central nervous system involvement, offering a broader assessment of neurological function.