Functional laser Doppler flowmetry of the optic nerve: physiological aspects and clinical applications

The present paper reviews the methodology and clinical results of recording, by laser Doppler flowmetry, the hemodynamic response of the optic nerve head elicited by visual stimulation. The basic mechanism underlying this novel technique (which is called here functional laser Doppler flowmetry (FLDF)) is the coupling between visually evoked neural activity and vascular activity within the neural tissue of the optic nerve (neurovascular coupling). The blood flow responses elicited by various visual stimuli (luminance and chromatic flicker, focal and pattern stimulation) have been characterized in humans by FLDF. These responses are similar to those assessed by electrophysiological methods (flicker and pattern electroretinography) evoked by the same stimuli. In addition, a significant correlation has been demonstrated between the hemodynamic responses and the neural activity induced electrical signals arising from the inner retina, providing evidence in support of the presence of a neurovascular coupling in humans. The application of FLDF in patients with ocular hypertension and early glaucoma demonstrates that the visually evoked hyperemic responses are significantly depressed even when neural retinal activity may be still relatively preserved, suggesting that abnormal optic nerve head autoregulation in response to visual stimuli may be altered early in the disease process. FLDF may open new avenues of investigation in the field of glaucoma and other neuro-ophthalmic disorders, providing new pathophysiological data and outcome measures for potential neuro-protective treatments.