Assessment of the ophthalmic artery as a collateral to the cerebral circulation. A comparison of transorbital Doppler ultrasonography and regional cerebral blood flow measurements.

The role of the ophthalmic artery (OA) as a collateral to the cerebral circulation in carotid occlusive disease is somewhat controversial. The aim of this study was to assess the importance of this collateral by comparing the results of transorbital Doppler ultrasonography and regional cerebral blood flow measurements using SPECT and Xenon-133 inhalation in 41 patients with unilateral high-grade internal carotid artery occlusive disease. Both measurements were performed under basal conditions and after the intravenous administration of 1 g acetazolamide to test cerebral vasoreactivity. Based on the direction of OA flow, the patients could be divided into three groups. In Group 1 (n = 16) with anterograde OA flow before and after acetazolamide, baseline rCBF values did not differ significantly between the two sides, or from those of the controls. The side-to-side asymmetry of the response to acetazolamide was also normal. In Group 2 (n = 11) where the OA flow was anterograde or "0 flow" before, but became retrograde after acetazolamide, rCBF was significantly reduced on the symptomatic compared to the non-symptomatic side under basal conditions. However, the increase in rCBF after acetazolamide was within normal limits. In Group 3 (n = 14) the OA flow was reversed both under basal conditions and after the vasodilatory stimulus. Baseline rCBF was significantly lower (p < 0.05) on the symptomatic side compared to the non-symptomatic, and the asymmetry became even greater (p < 0.001) after acetazolamide. Our findings demonstrate that the OA may be an important collateral pathway in patients with ICA occlusive disease, and contribute to the cerebral perfusion reserve in the case of compromised blood supply.

Simultaneous assessment of vasoreactivity using transcranial Doppler ultrasound and cerebral blood flow in healthy subjects.

Blood flow velocities in both middle cerebral arteries and regional cerebral blood flow in their perfusion territories were measured simultaneously in 36 healthy subjects. In 20 subjects, the measurements were first performed under basal conditions and then repeated 15-20 min after an intravenous injection of 1 g of acetazolamide. Reproducibility of simultaneous blood flow and velocity measurements was tested by examining 16 subjects on two occasions under basal conditions with an interval of 20 min. Relatively good reproducibility was found for repeated measurements of velocity and blood flow, being best when side-to-side asymmetry was assessed. The increase in blood flow velocities after acetazolamide was symmetrical (right side, 34.2%; and left side, 35.5%), and the velocity increase was significantly correlated with basal values. The increase in cerebral blood flow was also symmetrical (right side, 29.8%; left side, 30.1%) but not correlated with basal flow values. No significant relationship was found between velocity increase and increase in regional cerebral blood flow. This finding is probably not only due to methodological inaccuracies but may suggest that acetazolamide has an effect on the diameter of the middle cerebral artery or on the magnitude of this artery's perfusion territory. This study supports the use of acetazolamide for assessing cerebral vasoreactivity following the definition of lower limits for velocity and flow increase and for asymmetry of the response.