How does EEG compare to MRI for the evaluation of dementia and encephalopathy?

Updated: Oct 09, 2019
  • Author: Eli S Neiman, DO, FACN; Chief Editor: Selim R Benbadis, MD  more...
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EEG often is compared with MRI; when this comparison is made, it usually refers to clinical MRI rather than functional MRI. This comparison is puzzling, in that a primarily anatomic test is being compared with a functional one.

In general, MRI is good at telling us where the lesion is, whereas EEG is good at separating normal and abnormal primarily cortical function. The topologic usefulness of EEG is limited, although it may be improved with computerization. The purpose of MRI is to provide precise localization of a lesion, usually one that has passed a certain stage of evolution. The EEG, on the other hand, captures the changing electrical characteristics of a functioning brain, primarily those of the cortex.

Conditions can be identified with EEG that as a rule cannot be seen on the MRI; therefore, the use of these studies is not exclusive but complementary. The EEG may be used for the following purposes:

  • To exclude nonconvulsive status epilepticus

  • To identify focal interictal epileptiform activity to confirm clinical suspicion that seizures may contribute to the condition in question

  • To attempt to record functional disturbance in individuals whose brain MRI is "normal" but in whom brain dysfunction is clinically evident (eg, metabolic encephalopathies)

  • To attempt to record disease-specific patterns in the proper clinical setting, such as progressive myoclonic epilepsies, CJD, SSPE

  • To help a psychiatrist with the multitude of complex disorders masking as potential epilepsy or encephalopathy (eg, lithium intoxication may present with BiPEDs)

  • To identify focal or lateralized changes that suggest a structural cause to the encephalopathy

The truism that EEG is nonspecific and cannot diagnose etiology or localization well (eg, the cause of coma) is often cited. However, in general medical practice, nonspecificity is often not the question, because most of the referrals in general neurology are individuals in whom the cause is fairly clear, or reasonably suspected, on the basis of clinical history and laboratory chemistry. The questions from the clinician are whether the brain is involved and what is the extent of brain damage (if any). At present, for answering these questions, no clinical tool is more useful than the EEG.

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