Synopsis
Choose electroencephalography and peripheral neurophysiology for a defined clinical question, prepare patients safely and interpret sampled physiological data without treating normal or incidental findings as definitive diagnoses.
- EEG records cortical electrical potentials from scalp electrodes; it supports seizure classification, detects some electrographic seizures and evaluates encephalopathy, but it is not a brain scan and does not measure intelligence or thoughts.
- A normal routine EEG does not exclude epilepsy because interictal discharges are intermittent and deep or small generators may not reach the scalp. Epilepsy remains a clinical diagnosis supported by eyewitness history and appropriate tests.
- Incidental epileptiform or non-specific abnormalities do not prove that a collapse was epileptic. Interpret morphology, state, activation, medicines and pre-test probability with a clinical neurophysiologist.
- Sleep-deprived, sleep, ambulatory or video EEG increases sampling or links behaviour to electrical activity for selected questions; deliberate sleep deprivation carries driving and seizure-safety implications.
- Continuous EEG is used in critical care for suspected non-convulsive seizures, treatment response and some encephalopathic patterns, but artefact and sedation make specialist real-time interpretation essential.
- Nerve conduction studies stimulate peripheral motor or sensory nerves and record latency, amplitude and velocity. They principally assess large myelinated fibres and may be normal in isolated small-fibre neuropathy.
- Demyelination tends to prolong distal latencies, slow conduction, disperse responses or cause block; axonal loss more often reduces response amplitudes. Temperature, age, height, oedema and technical factors affect values.
- Needle EMG samples electrical activity within selected muscles at rest and during activation, identifying denervation, reinnervation and myopathic recruitment patterns; it does not examine every muscle or directly image a nerve root.
- Denervation changes can lag an acute root or nerve injury, so test timing and muscle selection matter. A normal early study may need clinical follow-up or repeat testing.
- Repetitive nerve stimulation and single-fibre EMG assess neuromuscular transmission in different ways. Abnormal jitter is sensitive but not specific and must be combined with fatigability, antibodies and the clinical syndrome.
- Before testing, state symptom onset, distribution, examination, suspected level, relevant surgery or trauma, pacemaker or implanted stimulator, bleeding risk, infection risk and medicines such as anticoagulants or pyridostigmine.
- Results should answer the referral question: localisation, pathophysiology, severity, chronicity and limitations. 'Abnormal neurophysiology' without clinical synthesis can misdirect care.
Key red flags
Evolving rhythmic EEG activity in frequency, distribution or morphology can represent seizure, including without major motor signs. Critical-care patterns may lie on an ictal–interictal continuum and need urgent specialist interpretation.
Marked non-uniform slowing, prolonged distal and F-wave latencies, temporal dispersion or conduction block across non-compressible segments supports acquired demyelination when technical and temperature factors are excluded.
Investigation priorities
Support epilepsy classification and identify focal or generalised cerebral dysfunction after a clinically assessed event.
Management branches
A transient event involving loss of awareness, abnormal movement, sensory phenomena or post-event change raises possible epilepsy.
- Obtain eyewitness and video evidence, examine, check ECG and provoking metabolic or structural factors and arrange urgent specialist assessment according to red flags and NICE timing.
- Request EEG only to support a clinically suspected epileptic event and classification; provide the event description, medicines and timing rather than asking the laboratory to 'exclude epilepsy'.
- Use MRI, sleep-deprived or ambulatory EEG and treatment decisions through the epilepsy specialist; provide seizure first aid, driving and safety advice while diagnosis remains uncertain.