Visually evoked cerebral blood flow velocity changes in different states of brain dysfunction

By assessment of metabolically induced cerebral blood flow velocity changes, transcranial Doppler sonography offers the opportunity to evaluate vasoneuronal coupling in different states of brain activation and in critically ill patients.

With simultaneous transcranial Doppler monitoring of the posterior cerebral artery (PCA) and the middle cerebral artery (MCA), 27 control subjects, 11 patients under general anesthesia, 5 patients in the vegetative state, and 12 patients with aneurysmal subarachnoid hemorrhage were stimulated with a 10-Hz flashlight for 30 seconds. Ten cycles of stimulation were averaged, and a specific flow response (SFR) was computed as the normalized ratio of PCA/MCA mean flow velocity.

Maximal SFR was 14.2% in control subjects. Eye closure significantly reduced maximal SFR (11.6% versus 15.4%, P<.01). In subarachnoid hemorrhage, SFR was markedly decreased in the early phase (4.8%, P<.01) but became normal later on. Four of 5 patients with abolished SFR suffered delayed ischemia due to vasospasm. Of 7 patients with preserved SFR, 5 had vasospasm but none had delayed ischemia. No SFR was observed in patients under general anesthesia or in the vegetative state.

Although reflecting fast and local neuronal activity patterns, metabolically induced blood flow response is highly dependent on stimulus-directed attention. In subarachnoid hemorrhage, decreased metabolic flow response suggests severe depression of vasoneuronal coupling, and abolished SFR might indicate increased vulnerability to vasospasm and a higher risk for delayed ischemia.