Cerebral Syncope: Loss of Consciousness Associated with Cerebral Vasoconstriction in the Absence of Systemic Hypotension

Abstract

Transcranial Doppler (TCD) ultrasonography done during head-upright tilt induced neurocardiogenic syncope has demonstrated that cerebral vasoconstriction occurs concomitant with (or precedes) loss of consciousness. This article demonstrates evidence that cerebral blood flow changes alone (vasoconstriction), in the absence of systemic hypotension, may result in syncope. Five patients (4 men, 1 woman; mean age 41 +/- 17 years) with recurrent unexplained syncope were evaluated by use of an upright tilt table test for 45 minutes with or without an infusion of low dose isoproterenol. TCDoppler ultrasonography was used to assess middle cerebral artery systolic velocity (Vs); diastolic velocity (Vd); mean velocity (Vm); and pulsatility index (PI = Vs = Vd/Vmean). Syncope occurred in five patients during the baseline tilt and in one patient during isoproterenol infusion. During tilt induced syncope, at an average mean arterial pressure of 89 +/- 16 mmHg, TCD sonography showed a 2% +/- 10% increase in systolic velocity; a 51% +/- 27% decrease in diastolic velocity; and a 131% +/- 87% increase in pulsatility index. One patient underwent continuous electroencephalographic recording during tilt, which demonstrated diffuse slow wave activity (indicating cerebral hypoxia) at the time of syncope concomitant with the aforementioned TCD changes in the absence of systemic hypotension. These findings reflect an increase in cerebrovascular resistance secondary to arteriolar vasoconstriction distal to the insonation point of the middle cerebral artery, that occurred concomitant with loss of consciousness and in the absence of systemic hypotension. We conclude that in some individuals abnormal baroreceptor responses triggered during orthostatic stress may result in a derangement of cerebral autoregulation leading to cerebral vasoconstriction with resultant cerebral hypoxia in the absence of systemic hypotension.

Neurovascular Deficits in Cocaine Abusers

“…The nondrug-using individuals in our sample fell within the normative range for both blood flow velocity and PI, thus supporting the absence of measurement error. Slowed cerebral blood flow velocity with increased pulsatility in certain patient populations indicate increased cerebrovascular resistance was due to vasoconstriction of small cortical vessels (Martin et al 1994;Biedert et al 1995;Cho et al 1997;Schondorf et al 1977;Grubb et al 1998). Such an interpretation of pulsatility does not hold in situations where blood pressure differed among groups (Czosnyka et al 1996).…”

Cerebral blood flow during orthostasis: role of arterial CO2

“…Altered cerebrovascular control unrelated to changes in PET CO 2 has been reported during LBNP following bed rest (44) and parabolic flight (38). In addition, a paradoxical cerebral vasoconstriction in healthy (5,6,22,38) and orthostatically intolerant (13,14) individuals has been reported during postural stress and LBNP. However, our previous work (38), as well as that of others (8,21), showed that cerebral autoregulation remains intact during HUT in healthy subjects, suggesting that decreases in perfusion pressure were unlikely to be the cause of the decrease in cerebral flow.…”

Synthetic consciousness: the distributed adaptive control perspective

“…The central role of the thalamus in the regulation of consciousness is further supported by the widespread thalamic neuronal cell death observed in vegetative state patients [111]. Another example, which illustrates the central role of the thalamo-cortical system as a core component of the neuronal substrate of consciousness, is that vasoconstriction of the middle cerebral artery, which provides blood to both cerebral cortex and the thalamus, can result in a transient loss of consciousness (syncope), associated with an increase of slow wave activity in the EEG indicative of functional inactivation of the cortex [112]. Also, TCD is accompanied by a characteristic downward shift of the power spectrum [108].…”