Mohammed T. Abou-Saleh

58 PRINCIPLES AND PRACTICE OF GERIATRIC PSYCHIATRYdiscrimination between demented patients and elderly controls16,17 . Subtle cognitive impairment has been related to thepresence of EEG abnormalities in some groups of otherwisehealthy elderly subjects, supporting the view that many of thechanges found in the EEGs of elderly subjects are due to specificsubclinical pathologies 18 . No changes are observed in highly selectgroups of ‘successfully aged’ individuals 14,19 .NORMAL AGEINGIn an early study on an elderly population, Silverman et al. 20recorded EEGs of 90 healthy subjects aged over 60 and reported adiffuse slowing of the background rhythm in 26% and focalabnormalities in 43% of the subjects. These findings have beenconfirmed more recently 10,21,22 . The slowing of the alpha rhythmwith age was clearly demonstrated by Hughes and Cayaffa 23 , whorecorded the EEGs of 420 subjects aged 5–80 years. All subjectshad been hospitalized and had undergone extensive neurologicalassessment in order to exclude the presence of brain pathology. Inthis group, the alpha peak frequency, which, up to the age of 60,had been between 10–11 Hz, fell to 9–10 Hz in subjects aged over60. Some authors, however, claim a decreased slow and anincreased fast activity with ageing 24 . The annual changes in nondementedelderly are minimal compared to the alterations ofalpha and theta power in demented individuals 25 . There is nocorrelation between alpha and theta power and the degree ofbrain atrophy in the non-demented elderly 26 .Hubbard et al. 27 examined the EEGs of 10 centenarians aged100–105, seven of whom were healthy, with no clinical evidence ofdegenerative brain disease. In this group, posterior dominantrhythms were in the lower part of the alpha range, and slow wavefoci over temporal regions were common. These changes weresimilar to those found in subjects aged 80, and the study providedno evidence for a progressive decrease in alpha frequency or foran increase in focal temporal slow waves in subjects aged 80–100years.Changes in the EEG of elderly subjects, which have beenattributed to early cerebrovascular insufficiency, include adiminished response of slow-wave activity to hyperventilationand the development of focal abnormalities, particularly over theanterior temporal regions. In a young person, hyperventilation fora period of 3 or 4 minutes usually produces a gradual increase indiffuse slow activity in the theta and delta range, which settlesback to standard level within approximately 1 minute aftercessation of over-breathing. This response is age-dependent and ismost striking in children, who display delta activity at very highvoltages. In contrast, old people show diminished or absentresponse to over-breathing, which may, in part, be due todiminished alteration in P CO2 when hyperventilating 42 .Bursts of rhythmic theta activity over the temporal regionsfrequently appear in late adulthood, and these are associated withcognitive and memory deterioration 18,28 . In a recent study, Visseret al. 29 measured the EEG and performed computed tomography(CT) brain scans in a group of clinically healthy subjects aged 65–83 years. In this group of elderly subjects, those with focal EEGdelta wave activity, recorded over the left anterior temporalregion, performed poorly on neuropsychological tests of wordfluency (thought to address temporal lobe function) and also hadsignificant ventricular dilatation measured on the CT scan. It wasconcluded that such left-sided temporal slow-wave abnormalitiesfound in the EEGs of some elderly subjects may be a valuableearly indicator of temporal lobe pathology.The probability that the EEGs of average adults do not changemuch throughout life and may, indeed, be relatively normal inotherwise healthy centenarians is thus raised 27 . The slowing ofEEG frequency with age could be explained by changes incerebral blood flow. Regional cerebral blood flow shows a stronginverse correlation with the appearance of EEG slow waves and isdirectly correlated with posterior alpha activity 30,31 . A directcausal relationship between a reduced cerebral blood flow, anincrease in slow waves and a reduced alpha frequency in the EEGof elderly subjects could, therefore, be postulated 32 .EVENT-RELATED POTENTIALS (ERPs) ANDAGEINGElectrical cerebral responses to discrete stimuli, such as visual,acoustic or contact stimuli, cannot, by and large, be detected inthe scalp-recorded EEG. Electrical response to such events issmall in comparison to their cerebral background activity, andaveraging techniques are required for their visualization. Suchtechniques have proved of enormous value to neurologists andpsychiatrists studying brainstem and higher cerebral function, bypermitting the detection of tiny voltages generated in response tospecific stimuli. To extract the time-locked activity generated by agiven stimulus, a repeated series of stimuli is presented, andepochs of EEG, captured after each presentation, are summed andstandardized/averaged. The random background EEG will tendto decrease in amplitude on summation, whereas the desiredevent-related potential will remain the same in size.EXOGENEOUS EVENT-RELATED POTENTIALSERPs offer a means to assess peripheral nerve and brainstemfunction by using different sensory modalities. Early evokedpotentials, generated within about 80 milliseconds (ms) after astimulus, are described as exogenous because they seem to dependon the nature of the stimulus itself rather than any subjectiveresponse the subject may make to the stimulus. Auditorybrainstem potentials, generated within the first 10 ms after aclicking sound, are evoked in a routine procedure to provideinformation about the functioning of auditory nerve andbrainstem structures in the auditory pathway. Somatosensoryevent-related potentials (ERPs), evoked by electrical stimulationof, for example, the median nerve at the wrist, include the mediannerve action potential, recorded at the brachial plexus, andactivity generated in neurones of the spinal dorsal horn and dorsalcolumn. Later peaks probably reflect activity in the mediallemniscus and primary somatosensory cortex. Early visual evokedresponses to light flashes reflect activity in the visual path betweenthe retina and the visual striate cortex.From the second to the ninth decade of life, there is a linearincrease in the latency of exogenous potentials 33 . The latency ofthe median nerve compound action potential, recorded at thebrachial plexus (the ‘N10 waveform’), increases from an averageof 10 ms in the second decade to approximately 12 ms inoctogenarians. With few exceptions, a similar rise in latencywith increasing age is found in all exogenous potentials of all threesensory modalities addressed and renders age corrections clinicallyimportant. The central conduction time of auditory evokedpotentials (AEPs) increases by 1–4 ms/year, the latencies of visualevoked potentials (VEPs) by 2–4 ms/decade after age 40 years 34 .Many age-related anatomical, physiological and biochemicalchanges may contribute to the slowing of nerve conductionimplied by these latency delays.ENDOGENOUS EVENT-RELATED POTENTIALSERPs generated more than 80 ms after a stimulus may reflect thepsychic condition of an individual. Such responses are termed