chronically low blood pressure is accompanied by a variety of complaints including fatigue , reduced drive , faintness , dizziness , headaches , palpitations , and increased pain sensitivity [ 14 ] . in addition , hypotensive individuals report cognitive impairment , above all deficits in attention and memory . nevertheless , it is generally the case that in research , as well as in clinical practice , relatively little importance is ascribed to hypotension . one reason for this is that , despite mental symptoms , cerebral dysfunction generally is not taken into account . this is a consequence of the current doctrine that low systemic blood pressure is compensated by autoregulatory processes which prevent reduced blood perfusion of the brain [ 5 , 6 ] . some recent findings challenge this doctrine : reduced cognitive performance in hypotension has been demonstrated by neuropsychological testing , and eeg studies have revealed diminished cortical activity . moreover , the assumption of unimpaired brain perfusion in hypotension no longer holds . in the present review the necessity of a reappraisal concerning hypotension is discussed in light of the relationship between blood pressure and cerebral functioning . hypotension has been defined by the world health organization ( who )   as a low blood pressure with a systolic reading below 110  mmhg in males and below 100  mmhg in females , regardless of diastolic blood pressure . the concept of constitutional hypotension refers to a chronic condition of inappropriately reduced blood pressure independent of the presence of further pathological factors . both secondary hypotension ( e.g. due to blood loss or medication ) and orthostatic hypotension ( caused by circulatory problems when assuming an upright position ) are distinguished from the constitutional form [ 1 , 6 ] . constitutional hypotension is relatively widespread in the general population . in a representative german sample of more than 7,000 subjects in contrast to elevated blood pressure , which constitutes a significant risk factor for cardiovascular disease , hypotension is commonly not regarded as a severe condition . nevertheless , its impact on personal well - being and quality of life has been shown in several epidemiological studies . , as well as pilgrim et al .   demonstrated impaired physical and mental health in the case of low blood pressure . reduced health - related quality of life   and a heightened degree of depressiveness   were also reported by hypotensive individuals . , several studies focusing on the elderly population have reported associations between low blood pressure and the prevalence and incidence of alzheimer s and vascular dementia [ c.f . 1114 ] . whilst a few longitudinal studies examining this relationship have been published [ 15 , 16 ] , the causal role of low blood pressure in degenerative brain disorders has thus far not been proven [ e.g. 13].1 concerning the etiology of hypotension , reduced liquid intake and low body weight may be considered [ c.f . 1 , 17 ] various findings on reduced electrodermal activity   and increased heart rate variability   reflect diminished sympathetic tone and habitually heightened parasympathetic activity in persons with low blood pressure values . this is corroborated by findings on the effects of sympathomimetic drugs [ 20 , 21 ] . in mixed samples of persons with constitutional and orthostatic hypotension ,  and  adrenergic substances were shown to lead to blood pressure elevation accompanied by a reduction of subjective symptoms . responding to mechanical stretch of the vessel walls , the baroreceptors form part of a negative feedback loop (  baroreflex  ) compensating phasic blood pressure fluctuations . increased sensitivity of the baroreceptor system may result in stabilizing blood pressure at a lower level . in addition to findings in animals , this hypothesis is supported by a still unpublished study of our own group which yielded increased baroreflex sensitivity in the case of low blood pressure . in addition to the main results , the table includes information concerning the respective samples and assessment instruments . table  1studies on cognitive performance in hypotension ( sbp , systolic blood pressure ; dbp , diastolic blood pressure)authorsmain focussamplesassessment instrumentsmain resultsrichter - heinrich   et al . attentional performance in   hypotension and hypertension30 hypotensives ( sbp  <  106  mmhg ) ;   40 hypertensives ( sbp  >  140  mmhg ) ;   31 normotensive controls ; age 1640  yearskonzentrations - verlaufs - test ( course   of concentration test ) reduced attentional performance in the   case of both lowered and elevated   blood pressurestegagno et al . attentional , memory and   arithmetic performance   in hypotension17 female hypotensives ( sbp  <  100  mmhg ) ;   19 normotensive controls ; mean   age 23  yearsreaction times to acoustic stimuli ;   digit span ; recall of word   lists ; serial subtractionsprolonged reaction times , poorer   performance on the verbal memory   and arithmetic tasks in hypotension ,   no effect for digit spancosta et al . attentional and memory   performance in hypotension26 female hypotensives ( sbp  <  105 ,   dbp  <  65  mmhg ) , 22 normotensive   controls ; mean age 29  yearsaufmerksamkeits - belastungs - test   ( attentional strain test ) ;   zahlen - verbindungs - test ( trail making   test ) ; recall of word lists poorer attentional and memory   performance on each of the three   tests in hypotensionmorris et al . relationship between blood   pressure and cognitive   performance in elderly   personsrepresentative population sample covering   the total spectrum of blood pressure   ( n  =  5,816 ; age over 64  years)east boston memory test ; symbol   digit modalities test ; mini - mental   state examination weak u - shaped relationship between   blood pressure and cognitive   performanceweisz et al . attentional performance in   hypotension25 female hypotensives ( mean   sbp  =  102  mmhg ) ; 25 normotensive   controls ; age 1944  yearsattentional and cognitive   efficiency battery reduced performance on a subtest   assessing cognitive flexibility   in hypotensionduschek et al . attentional performance in   moderate hypotension26 borderline hypotensives ( mean   sbp  =  112  mmhg ) ; 29 normotensive   controls ; mean age 26  yearsreaction times to acoustic stimuli ;   aufmerksamkeits - belastungs - test ( attentional   strain test ) ; zahlen - verbindungs - test   ( trail making test ) prolonged reaction times and poorer   performance on the    aufmerksamkeits - belastungs - test  in   moderate hypotension , no effects   for the  zahlen - verbindungs - testduschek et al . attentional and working   memory performance in   hypotension with motor   performance and   mood controlled40 hypotensives ( sbp  <  105  mmhg in   women , sbp  <  110  mmhg in men ) ;   40 normotensive controls ; age   1945  yearstestbatterie zur aufmerksamkeitsprfung   ( battery for the assessment of attention )   ; motorische leistungsserie ( motor   performance series ) ; befindlichkeitsskala   ( mood scale ) poorer performance on six tests   assessing tonic and phasic alertness ,   selective , divided and sustained   attention as well as working memory   in hypotensives with fine motor   performance and mood controlled studies on cognitive performance in hypotension ( sbp , systolic blood pressure ; dbp , diastolic blood pressure ) a pioneering study investigating the relationship between blood pressure and cognitive abilities was conducted by richter - heinrich et al . . they showed reduced performance of individuals with both lowered and elevated blood pressure on a test of concentration . a more comprehensive assessment was carried out by stegagno et al . , in which poorer performances of hypotensive subjects on a verbal memory test and on an arithmetic task , as well as prolonged reaction times to acoustic stimuli were observed . found reduced scores on two standard german paper  pencil tests assessing selective attention and cognitive speed . weisz et al .   reported that female hypotensive subjects performed significantly worse than controls on a computer - based test measuring attentional flexibility . subjects with only a moderately decreased blood pressure were assessed by duschek et al . . in this sample , reduced attentional performance and prolonged reaction times were again found as compared to normotensive controls . investigated the relationship between low blood pressure and attentional abilities through the application of a multidimensional diagnostic approach . they presented their subjects with a battery of six computer - based tasks   focusing on tonic and phasic alertness , selective , divided and sustained attention , as well as working memory . additionally , in order to control for possible confounders , a test battery examining fine motor abilities   and a mood questionnaire   were presented . the significant differences between hypotensive and control subjects persisted even when the effects of motor performance and mood were controlled . altogether , the existing data provide strong evidence for cognitive deficits related to constitutional hypotension , especially in the domains of attention and memory . the reduced performance seems to be a direct consequence of low blood pressure rather than an effect of impaired well - being related to this state . up to now there have been no empirical data concerning specific effects of hypotension - related deficits on everyday life . nonetheless , these findings raise concern regarding the impact of the deficits on attention - demanding activities ( e.g. professional or academic ) , including public health - related functions ( e.g. traffic safety ) . it would appear that both extremes of the blood pressure spectrum are accompanied by a decrease in cognitive ability . a number of studies concerned with elevated blood pressure showed reduced performance on various cognitive tests [ 43 , 44 ] . this is in line with epidemiological studies reporting an inverted u - shaped relationship between blood pressure and cognitive performance in elderly persons [ c.f . therefore , it may further be hypothesized that there is a relatively small range of normal blood pressure in which the brain exerts its optimal function . cognitive deficits in low blood pressure states are also reflected in decreased cortical activity . in accordance with earlier results [ 22 , 29 ] , duschek et al .   found the amplitude of the contingent negative variation ( cnv ) to be reduced in a hypotensive sample defined according to the who criteria   ( c.f . 1the cnv occurs during the period between a warning signal ( s1 ) and a second stimulus ( s2 ) demanding a motor , verbal or cognitive response . as can be seen in the figure , its amplitude was found to be reduced in hypotensive subjects ( n  =  40 ) as compared to normotensive controls ( n  =  40 ) ; modified from   the cnv occurs during the period between a warning signal ( s1 ) and a second stimulus ( s2 ) demanding a motor , verbal or cognitive response . as can be seen in the figure , its amplitude was found to be reduced in hypotensive subjects ( n  =  40 ) as compared to normotensive controls ( n  =  40 ) ; modified from   the cnv is an evoked potential which is generally viewed as a neuroelectrophysiological correlative of attentional processing . this is supported by pharmacological studies showing that the effects of various stimulant and sedative drugs on attention involve an increase or decrease , respectively , of the cnv amplitude [ 49 , 50 ] . moreover , attentional deficits due to brain lesions and psychiatric diseases are associated with a reduced cnv [ 51 , 52 ] . in the duschek et al . study , the amplitude of the cnv proved to be negatively correlated with reaction time , which in turn was prolonged in the hypotensive group . this highlights that the cnv constitutes a brain electrical correlate of an aspect of cognitive functioning which is affected in hypotension . the relationship between blood pressure and cortical activity is also reflected in a negative correlation between blood pressure and  wave activity in the spontaneous eeg . this suggests that low blood pressure is associated with a reduced tonic cortical arousal . on the behavioral level , higher degrees of  activity are known to be associated with generally diminished vigilance and preparedness to react . considering the physiological processes mediating the relationship between blood pressure and brain function , it must be taken into account that blood pressure can influence cortical activation processes via afferent projections [ 47 , 5557 ] . the brain continuously receives information about the state of the cardiovascular system by means of viscero - afferent fibers . from there , ascending pathways continue via hypothalamic and thalamic regions to cortical areas such as the anterior cingulate , the insula and the prefrontal lobe . the latter areas , in particular the prefrontal cortex and the anterior cingulate are of crucial importance in the regulation of cortical arousal , as well as for attentional processes [ 59 , 60 ] . also neurochemical mediators should be regarded in the relationship between blood pressure and cortical activity . noradrenaline is involved in the control of cortical arousal and attentional processes , as well as in the genesis of the cnv [ 59 , 61 ] . catecholamines are also of great influence in the regulation of blood pressure . bearing this in mind , one could speculate on the neurochemical level about a specific role of noradrenaline linking cardiovascular and cortical activation . it is generally assumed that in healthy individuals processes of autoregulation keep the cerebral blood flow constant within a wide range of arterial pressure . in order to ensure stable perfusion , autoregulation is considered to be a protective mechanism which prevents brain ischemia during blood pressure decrease , and guards against capillary damage and edema formation during periods of elevated blood pressure . under normal conditions the limits within which cerebral blood flow is assumed to be constant are approximately between a mean arterial pressure ( map ) of 60 and 150  mmhg [ 5 , 63 , 64 ] . if blood pressure is outside of this range , cerebral blood flow rises or falls with respective increases or decreases in blood pressure . slight reductions of blood pressure below the lower limit of autoregulation can be compensated by an increase of the extraction coefficient of oxygen from the blood . further reductions are accompanied by symptoms such as pallor and dizziness and ultimately lead to irreversible brain damage . map in individuals with constitutional hypotension usually does not decrease beyond the assumed lower map limit of autoregulation ( 60  mmhg ) . in light of this , low blood pressure in these subjects should be compensated , and cerebral blood flow should not be affected . they recorded blood flow velocities by means of transcranial doppler sonography in both middle cerebral arteries ( mca ) in hypotensive individuals , defined according to the who , and in normotensive control subjects . contrary to the current doctrine , mca blood flow at rest was found to be substantially bilaterally reduced in hypotensives ( c.f . 2mean flow velocities in the left and right middle cerebral arteries under resting conditions in hypotensive ( n  =  40 ) and control subjects ( n  =  40 ) ; bars represent standard errors   mean flow velocities in the left and right middle cerebral arteries under resting conditions in hypotensive ( n  =  40 ) and control subjects ( n  =  40 ) ; bars represent standard errors   the perfusion territory of the mca includes subcortical areas , large fractions of the frontal and parietal lobes , as well as the temporal lobes . thus , the latter finding demonstrated that , despite autoregulation , blood pressure in subjects with constitutional hypotension is not sufficient in maintaining the perfusion of a large part of the brain at the level of normotensive individuals [ see also 67 ] . in explaining this unexpected result , it must be noted that the determination of the lower limit of autoregulation was exclusively based on the experimental manipulation of blood pressure employing pharmacological methods , as well as head up - tilt [ 6871 ] . however , such a transient reduction of blood pressure is most certainly not comparable to conditions of chronic hypotension . additionally , in all of these studies global cerebral blood flow was assessed by means of the oxygen difference method according to lennox and gibbs , which may be less sensitive to perfusion changes than doppler sonography . nevertheless , more recent findings suggest values for the lower map limit of autoregulation to be considerably higher than 60  mmhg [ 7375 ] . in a critical review of the literature , waldemar et al . , for instance , reported an inter - individual range between 53 and 103  mmhg . in accordance with duschek and schandry s   data , the doctrine of stable cerebral perfusion down to the limit of a map of 60  mmhg can no longer be supported by the current state of research . a further important aspect of cerebral hemodynamics concerns the continuous adjustment of brain perfusion to current requirements . due to the close coupling of neural activity and brain metabolism , neurovascular coupling is based on the contraction and dilation of small resistance vessels , resulting from the changing metabolic demands of neuron populations in the vicinity . with regards to hypotension - related cognitive deficits , the extent of blood flow adjustment to mental activity was also tested in the duschek and schandry   study . subjects were presented with a simple attentional task ( motor reactions to visual stimuli cued by acoustic signals ) . during task execution blood flow velocities in the mca of both hemispheres were recorded by means of functional transcranial doppler sonography ( for technical details see ) . as expected , mental activity was accompanied by a substantial increase of mca perfusion . in control subjects , however , this increase was approximately 70% stronger than in hypotensives ( c.f . it would seem that blood pressure in the latter group was not sufficient to enable adjustment of brain perfusion to cognitive demands , as seen in normotensives . moreover , hypotensive participants showed prolonged reaction times , and a positive correlation was found between reaction speed and the extent of blood flow increase . 3changes of blood flow velocities in the left and right mca during the execution of an attentional task in hypotensives ( n  =  40 ) and controls ( n  =  40 ) . the subjects had to press a key in response to a visual imperative stimulus which was announced by a cuing tone . in both mca the rise in flow velocity which occurred during the anticipation of the imperative stimulus was substantially less pronounced in hypotensives . a second flow velocity maximum visible after the motor reaction was also slightly reduced in hypotensives   changes of blood flow velocities in the left and right mca during the execution of an attentional task in hypotensives ( n  =  40 ) and controls ( n  =  40 ) . the subjects had to press a key in response to a visual imperative stimulus which was announced by a cuing tone . in both mca the rise in flow velocity which occurred during the anticipation of the imperative stimulus was substantially less pronounced in hypotensives . a second flow velocity maximum visible after the motor reaction was also slightly reduced in hypotensives there is strong evidence suggesting that chronically low blood pressure is accompanied by diminished cognitive performance , primarily involving attention and memory [ 26 , 29 , 38 ] . recent research has provided an insight into the psychophysiological mechanisms of action underlying these deficits : eeg studies have demonstrated that the weaker cognitive performance is associated with reduced cortical activity [ 22 , 29 , 47 ] . in addition to diminished tonic brain perfusion , reduced adjustment of brain perfusion to cognitive demands was documented . it stands to reason that , as a consequence of this situation , a diminished metabolic supply of the brain tissue accounts for the cognitive deficits . the cognitive deficits can widely affect the every day life of hypotensive persons , and more attention should therefore be allocated to this topic within basic and clinical research , as well as in clinical practice [ 1 , 17 ] . empirical knowledge concerning the effectiveness of antihypotensive therapy continues to be scant . a variety of treatment strategies such as physical training , increase of liquid intake and pharmacological measures have been suggested . in two clinical trials the administration of sympathomimetics was found to result in a reduction of subjective hypotensive symptoms [ 20 , 21 ] . recent data of our own group suggest that cognitive performance may also be enhanced by pharmacological blood pressure elevation . based on a placebo controlled design , the vasopressor agent midodrine was shown to raise cerebral blood perfusion as well as performance on a test assessing selective attention   in hypotensive subjects , defined according to the who . further research would appear worthwhile and should aim to establish precise guidelines which enable chronic hypotension to be efficiently and effectively dealt with in practice .
this review article includes a systematic evaluation of the empirical data concerning deficits in mental ability , brain perfusion , and cerebral functioning due to chronically low blood pressure . 
 a number of studies have provided strong evidence for reduced cognitive performance in hypotension , particularly in the domains of attention and memory . 
 eeg studies have demonstrated that the hypotension - related poorer mental ability is also reflected in diminished cortical activity . 
 contrary to convention , more recent research has suggested a deficient regulation of cerebral blood flow in persons with low blood pressure . 
 in addition to reduced tonic brain perfusion , studies demonstrated insufficient adjustment of blood flow to cognitive requirements . 
 altogether , these findings suggest that more attention should be allocated to chronic hypotension in both research and clinical practice .