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EEG in neurological conditions other than epilepsy: when does it help, what does it add?
  1. S J M Smith
  1. Correspondence to:
 Dr Shelagh Smith
 Department of Clinical Neurophysiology, The National Hospital, Queen Square, London WC1N 3BG, UK;

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Although the electroencephalogram (EEG) is a reliable test to assess cerebral function, its value in diagnosis and evaluation of neurological conditions apart from epilepsy has been largely superceded in recent years by other investigations with greater specificity and sensitivity. Is EEG still worthwhile, and in which cases can it provide information that affects management? Broadly speaking, EEG is most important in patients with impaired consciousness or altered mental state (table 1):

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Table 1

 Strengths and weaknesses of the EEG

  • Where seizures or non-convulsive status epilepticus (NCSE) may be a contributing factor

  • To demonstrate functional disturbance when cerebral dysfunction is evident and structural imaging is normal

  • To detect focal or lateralised abnormalities which could suggest a structural basis for an encephalopathy

  • To identify diagnostic EEG patterns in appropriate clinical settings, such as sporadic Creutzfeldt-Jakob disease (CJD).


EEG changes in encephalopathies are similar, whether the cause is septic, metabolic, toxic, or structural. There is a progressive increase in slow wave activities, the degree of which parallels the severity of brain dysfunction. In mild encephalopathic states, slowing of normal alpha (α) rhythms occurs, and with more severe encephalopathy, the appearance of theta (θ) and continuous or non-continuous delta (δ) activities. A variety of additional EEG patterns can be seen, such as frontal intermittent rhythmic delta (FIRDA), periodic lateralised or bilateral epileptiform discharges (PEDs, BIPEDs), and triphasic waves. None of these patterns is specific to a particular pathophysiological process or diagnosis, but PEDs are most likely to occur in acute or subacute focal destructive pathologies or focal epileptogenic lesions; triphasic waves are typically found in metabolic encephalopathies; and some patients with mesial fronto-parietal lesions or third ventricle tumours show FIRDA in their EEG.

Metabolic encephalopathies

A patient with acute change in awareness whose EEG shows triphasic waves and diffuse slow activity will usually have a metabolic encephalopathy. Triphasic …

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