Interpretation of paroxysmal slow activity--"subcortical signs"--in the electroencephalography of adults

Slow paroxysmal EEG activity, also referred to as "subcortical signs", offers an indication of impaired subcortico-cortical functional interaction that may be set off as a result of intracranial processes of a wide range of localisation. For this reason, the occurrence of this EEG pattern should not be taken to indicate a primary localisation in the range of the medical subcortical structures, nor does its absence imply any improbability of extensive lesions in the region. Taking into consideration the form and frequency of the waves during paroxysms and their local distribution, even this aspecific pattern yields more information for the clinical diagnosis. In particular, generalised paroxysms from monomorphic delta waves are usually associated with an existing primary or secondary brain illness, and would suggest the need for further diagnostic clarification. In interpreting the slow-wave groups restricted to the temporal regions, frequently counted among the "subcortical signs", the wave frequency and the patient's age must be taken into account. In the second half of life they frequently occur unaccompanied by any pathological process.