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Physiology and Pharmacology: Benedetto Falsini, Charles E. Riva, and Eric Logean Flicker-Evoked Changes in Human Optic Nerve Blood Flow: Relationship with Retinal Neural Activity Invest. Ophthalmol. Vis. Sci. 2002; 43: 2309-2316 [Abstract] [Full text] [PDF]

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Flicker-evoked changes in human optic nerve blood flow: relationship with retinal neural activity

Sohan S. Hayreh   (23 October 2002)

Author Response: Flicker-evoked changes in human optic nerve blood flow: relationship with retinal n

Benedetto Falsini, Charles E. Riva, Eric Logean   (23 October 2002)

Flicker-evoked changes in human optic nerve blood flow: relationship with retinal neural activity 23 October 2002
Sohan S. Hayreh, Professor Emeritus University of Iowa Send letter to journal: Re: Flicker-evoked changes in human optic nerve blood flow: relationship with retinal neural activity sohan-hayreh{at}uiowa.edu Sohan S. Hayreh	I was interested to read the paper entitled: "Flicker-evoked changes in human optic nerve blood flow: relationship with retinal neural activity" by Falsini, Riva and Logean.1 The authors derived their data on "optic nerve blood flow" in this study from the laser Doppler flowmetry, and they cite two studies (published in 19912 and 19923) by Riva and colleagues in support of their claim that laser Doppler flowmetry measures "optic nerve blood flow" ("Fonh" - onh = optic nerve head). In 1999 Riva and colleagues published a study,4 the primary objective of which was "to obtain a better understanding of which part of the optic nerve head circulation is measured by laser Doppler flowmetry" using the same technique as cited in their studies in 19912 and 1992.3 That systematic experimental study in rhesus monkeys clearly showed that "laser Doppler flowmetry technique is predominantly sensitive to blood flow changes in the superficial layers (supplied by the retinal circulation) of the optic nerve head and less sensitive to those in the prelaminar and deeper regions (supplied by the posterior ciliary artery circulation)." This is because when posterior arteries were cut in that study, laser Doppler flowmetry showed no decrease in its measurements. In all the circulatory optic nerve head disorders, e.g., anterior ischemic optic neuropathy and glaucomatous optic neuropathy, it is the area supplied by the posterior ciliary artery circulation which is relevant and NOT the retinal circulation. In spite of the above findings,4 the same author in the current paper1 in 2002 continues to claim that laser Doppler flowmetry measured optic nerve blood flow ("Fonh"). Since laser Doppler flowmetry measures mostly the retinal blood flow (NOT the optic nerve head blood flow), an accurate title for this paper1 would be: "Flicker-evoked changes in human retinal blood flow (Fret): relation with retinal neural activity;" it is unwarranted to claim that the study provided information on "Fonh." This would also be in tune with the authors' conclusion that: "the results of this study provide direct evidence that, in humans, the retinal vascular and neural changes elicited by flicker stimulation are quantitatively associated." Sohan Singh Hayreh, MD, PhD, DSc, FRCS, FRCOphth Professor Emeritus of Ophthalmology & Director Ocular Vascular Clinic References 1. Falsini B, Riva CE, Logean E. Flicker-evoked changes in human optic nerve blood flow: relationship with retinal neural activity. Invest Ophthalmol Vis Sci. 2002;43:2309-2316. 2. Petrig BL, Riva CE. Near-infrared retinal laser Doppler velocimetry and flowmetry: new delivery and detection techniques. Appl Opt. 1991;30:2073-2078. 3. Riva CE, Harino S, Petrig BL, Shonat RD. Laser Doppler flowmetry in the optic nerve. Exp Eye Res. 1992;55:499-506. 4. Petrig BL, Riva CE, Hayreh SS. Laser Doppler flowmetry and optic nerve head blood flow. Am J Ophthalmol. 1999;127:413-425.
Author Response: Flicker-evoked changes in human optic nerve blood flow: relationship with retinal n 23 October 2002
Benedetto Falsini Istituto di Oftalmologia, Universita Cattolica del S. Cuore, Rome, Italy, Charles E. Riva, Eric Logean Send letter to journal: Re: Author Response: Flicker-evoked changes in human optic nerve blood flow: relationship with retinal n MD0571{at}mclink.it Benedetto Falsini, et al.	We would like to thank Prof. Hayreh for the thoughtful comments on our recently published article investigating the relationship between human optic nerve blood flow and neural retinal activity changes elicited by flicker stimulation.1 In response to his criticisms, we would like to point out that: 1. Our laser Doppler flowmetry (LDF) measurements were obtained by directing the probing laser beam at the temporal site of the neuroretinal rim of the optic disc. Regardless of which layer of the optic nerve head circulation was in fact sampled by the LDF technique, our data may be considered as reflecting the blood flow changes (elicited by a visual stimulus) in a limited area of the microcirculatory district belonging to the neuroretinal rim, a key anatomical component of the optic nerve (see, for example, Jonas and Budde2). 2. We agree with Prof. Hayreh that in humans, as in monkeys,3 LDF is probably predominantly sensitive to blood flow changes in the most superficial layers (supplied by the retinal circulation) of the optic nerve head. However, this is only marginally relevant to the purposes of our study, aimed at evaluating the relationship between vaso- and neural activity changes in the human eye, and by no means affects the validity of our results. 3. We did not speculate in our paper about the possible implications of our data for the understanding of human disorders involving the optic nerve (i.e., anterior ischemic or glaucomatous optic neuropathy) and/or the retina. We merely discussed the possible physiological mechanisms underlying the neurovascular coupling in the human eye. Further studies are needed to determine the clinical importance of this coupling. In conclusion, we feel that the term "optic nerve" employed in our article may correctly reflect the anatomical region where measurements have been performed, even though the circulatory system (i.e., retinal versus posterior ciliary artery circulation) sampled by the LDF technique still needs to be clarified in humans. Benedetto Falsini1 Charles E. Riva2 Eric Logean2 1Istituto di Oftalmologia, Università Cattolica del S. Cuore, Rome, Italy. 2Institut de Recherche en Ophtalmologie, Sion, Switzerland References 1. Falsini B, Riva CE, Logean E. Flicker-evoked changes in human optic nerve blood flow: relationship with retinal neural activity. Invest Ophthalmol Vis Sci. 2002;43:2309-2316. 2. Jonas JB, Budde WM. Diagnosis and pathogenesis of glaucomatous optic neuropathy: morphological aspects. Progress in Retinal and Eye Research. 2000;19(1):1-40. 3. Petrig BL, Riva CE, Hayreh SS. Laser Doppler flowmetry and optic nerve head blood flow. Am J Ophthalmol. 1999;127:413-425.