Neurological studies: EEG, EMG and nerve conduction, evoked potentials
- Topic
Tests that record the electrical activity of the brain, nerves and muscles to locate a problem the examination has found.
What it is
Neurophysiology extends the neurological examination. The electroencephalogram (EEG) records cortical activity and is central in epilepsy and altered consciousness. Nerve conduction studies and electromyography (EMG) tell apart nerve, neuromuscular junction and muscle disease, and axonal from demyelinating neuropathy. Evoked potentials time the response of the visual, auditory and sensory pathways to a stimulus and reveal silent lesions, as in multiple sclerosis. Lumbar puncture and nerve or muscle biopsy complete the toolbox when needed.
Key facts
- A normal EEG between seizures does not rule out epilepsy; repeating it, sleep deprivation or prolonged video-EEG raise the yield.
- Hyperventilation and flashing light can provoke seizures in susceptible patients during the EEG, and the staff are ready to respond.
- Cold limbs slow nerve conduction, so the limb is warmed before testing.
- Nerve conduction reflects the best-surviving fibres, so it can be normal in mild or small-fibre neuropathy.
- In amyotrophic lateral sclerosis, EMG shows widespread denervation with normal sensory conduction.
- Pacemakers and other implanted devices must be reported before nerve stimulation.
Step by step
- Explain the test, its duration and any discomfort (electric pulses, needle insertion).
- For EEG: clean hair without oil or spray, no caffeine for several hours, and sleep reduction the night before if requested; bring the medicine list.
- For nerve conduction and EMG: no lotions on the skin, warm limbs, and report anticoagulants and implanted devices.
- Do not stop antiseizure or other medicines unless the neurologist says so.
- During EEG, keep seizure precautions and record any event with its time.
- Afterwards, expect mild soreness or bruising at EMG needle sites.
Neurophysiological studies and their main findings.
| Study | How it is done | Main findings | Chosen when |
|---|---|---|---|
| EEG | Scalp electrodes record at rest with eyes closed, with hyperventilation, flashing light and sometimes sleep; long or video-EEG for days | Spikes and sharp waves (epileptiform); 3 per second spike-and-wave in absence epilepsy; focal slowing over a lesion; diffuse slowing in encephalopathy | Seizures and epilepsy classification, unexplained altered consciousness, suspected non-convulsive status, encephalitis |
| Nerve conduction studies | Surface electrodes and brief electrical pulses over a nerve; speed, latency and amplitude are measured | Demyelination: slow velocity, long latencies, conduction block. Axonal loss: low amplitude with near-normal speed. Entrapment: focal slowing (median nerve at the wrist) | Numbness, tingling or weakness; carpal tunnel syndrome; Guillain-Barré syndrome; polyneuropathy |
| Needle EMG | A thin needle electrode records muscle at rest and during contraction | Neurogenic: fibrillations and fasciculations at rest, large long units with reduced recruitment. Myopathic: small short polyphasic units with early recruitment | Weakness of unclear origin, motor neuron disease, myopathy, radiculopathy |
| Repetitive nerve stimulation and single-fiber EMG | Trains of stimuli to a nerve; very fine needle recording of single fibres | Decrement of the response in myasthenia; increment after exercise in Lambert-Eaton; increased jitter in junction disorders | Fatigable weakness, ptosis, double vision |
| Evoked potentials | Visual: checkerboard pattern; auditory brainstem: clicks in headphones; somatosensory: pulses to the median or tibial nerve | Delayed visual P100 after optic neuritis; delayed brainstem waves with brainstem lesions or hearing loss; absent somatosensory cortical responses predict poor outcome in coma | Suspected multiple sclerosis, hearing in infants, spinal cord monitoring during surgery, coma prognosis |
Test yourself
Think of the answer, then tap the question to see it.
Does a normal EEG between seizures rule out epilepsy?
No. Interictal EEGs are often normal; repeating the test, recording sleep or using prolonged video-EEG increases the yield.
How do demyelinating and axonal neuropathies differ on nerve conduction studies?
Demyelinating: slow velocity, prolonged latencies and conduction block. Axonal: reduced amplitude with near-normal velocity.
Which EMG pattern is neurogenic?
Fibrillations and fasciculations at rest, with large, long motor units and reduced recruitment on effort.
Why are visual evoked potentials useful in suspected multiple sclerosis?
A delayed P100 shows demyelination in the visual pathway, even without symptoms, which supports dissemination of lesions.
Elsewhere in DurgiPedia
Connections
Educational content on the clinical exam; it is learned and practised under supervision. It does not replace assessment by a health professional and is not for self-diagnosis.
Sources: EEG: MedlinePlus Medical Encyclopedia (MedlinePlus, U.S. National Library of Medicine, 2026); Electromyography: MedlinePlus Medical Encyclopedia (MedlinePlus, U.S. National Library of Medicine, 2026); Nerve conduction velocity: MedlinePlus Medical Encyclopedia (MedlinePlus, U.S. National Library of Medicine, 2026); Evoked potential (Wikipedia, 2026).
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Representative model, for educational purposes only.