EMG, nerve conduction studies, and evoked responses
How neurophysiology maps the nervous system outside the brain
Beyond EEG, neurophysiology covers several examination types that map the nervous system's function outside the brain: nerve conduction study (ENG/neurography), electromyography (EMG) and evoked potentials. These tests are often used together because they complement each other when the doctor needs to distinguish between diseases of the nerves, muscles or the central nervous system, see the Doctor's Handbook on sundhed.dk.
§Nerve conduction examination (ENG/neurography)
With a nerve conduction study, you electrically stimulate a nerve through the skin and measure how quickly and powerfully the signal reaches another point on the nerve or the muscle it supplies. The examination is used especially to clarify nerve compression e.g. carpal tunnel syndrome in the wrist and polyneuropathy where many nerves are affected at the same time, typically due to diabetes or other systemic diseases.
§Electromyography (EMG).
EMG measures electrical activity directly in a muscle and is used to assess whether a muscle's nerve supply is intact and whether there are signs of muscle disease. Needle electromyography is performed by a doctor, while you as an assistant often prepare the patient, set up the equipment, and support the examination. Surface-based EMG, where electrodes are attached to the skin instead of inserted into the muscle, is also included in certain examinations and can be carried out by the assistant.
§Evoked potentials
| Abbreviation | Examination. | Used for |
|---|---|---|
| VEP | Visually evoked response | Investigates the visual pathway from the eye to the brain's visual center. |
| SEP | Somatosensory evoked response | Investigates the sensory pathway through spinal cord and brain. |
| AEP/ABR | Auditory Evoked Response | Investigates hearing pathway function including in infants. |
Common to the evoked responses is that you give repeated, controlled stimulation (light, electric shock or sound) and measure the nervous system's response millisecond by millisecond. Because each individual response is very weak, stimulation is repeated many times, and the device combines the recordings (so-called 'averaging'), so random background noise averages out and the signal itself becomes clear.
§Your role as an assistant
- 01Prepare the patient and explain what the examination involves including that some examinations give a slight electrical shock or a tingling sensation.
- 02Place surface electrodes correctly and ensure good skin contact.
- 03Operate the equipment and monitor signal quality throughout.
- 04Documenting the examination so the doctor has a clear basis for assessment.
“An evoked response is a whisper from the nervous system — the job is to listen precisely enough to hear it.”