Laser Doppler flowmetry and optic nerve head blood flow

Ischemic disorders of the optic nerve head constitute an important cause of visual loss. The optic nerve head is supplied by two main sources of blood flow: the superficial layers by the central retinal artery and the deeper layers by the posterior ciliary arteries. This study was conducted in rhesus monkey eyes to obtain a better understanding of which part of the optic nerve head circulation is measured by laser Doppler flowmetry.

By means of a fundus camera-based laser Doppler flowmetry technique to measure blood flow in the optic nerve head tissue, laser Doppler flowmetry measurements were taken at baseline and then after experimental occlusion of central retinal artery (12 eyes), posterior ciliary arteries (nine eyes), and combined occlusion of central retinal artery and posterior ciliary arteries (nine eyes). Optic nerve head, choroidal, and retinal circulations were investigated by fluorescein fundus angiography after the various arterial occlusions.

Average laser Doppler flowmetry flow during central retinal artery occlusion alone was significantly decreased (P<.001) by 39%+/-21% (mean +/- 95% confidence interval) compared with normal baseline. Combined occlusion of central retinal artery and posterior ciliary arteries reduced laser Doppler flowmetry flow even more markedly by 57%+/-27% (P<.0005), but the difference between this flow reduction and that with central retinal artery occlusion alone was not significant (P>.20). After posterior ciliary artery occlusion alone, however, measurements showed a nonsignificant increase in laser Doppler flowmetry flow of 17%+/-37%.

The findings of this study suggest that the standard laser Doppler flowmetry technique is predominantly sensitive to blood flow changes in the superficial layers of the optic nerve head and less sensitive to those in the prelaminar and deeper regions, and their relative proportions are not known. In this laser Doppler flowmetry technique, the weaker Doppler signal from the deep layers cannot be separated from the dominant signal from the superficial layers to exclusively study the circulation in the deep layers; the latter circulation is of interest in optic nerve head ischemic disorders, including glaucoma. Emerging new optical modalities of the laser Doppler flowmetry technique may help in selectively measuring blood flow in the deeper layers.