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Ординатура / Офтальмология / Английские материалы / Glaucoma An Open Window to Neurodegeneration and Neuroprotection_Nucci, Cerulli, Osborne_2008.pdf
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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 neuro-retinal rim, a key anatomical component of the optic nerve (Jonas and Budde, 2000). Although we cannot speculate about the possible implications of the present data for the pathophysiology of glaucomatous optic neuropathy, the finding of an abnormal flicker-evoked flow regulation in the superficial layers of the optic nerve recorded in both OHT and EOAG eyes clearly merits further investigation.

In conclusion, this LDF study demonstrates a decreased blood flow response to flicker stimulation measured at the neuro-retinal rim of the optic disk in both OHT and EOAG patients. This decrease, which in EOAG is associated with early loss of nerve fiber layer, could be due to a decrease in the activity of the M-cellular pathway in response to flicker. Furthermore, the relation of blood flow changes with visual function data suggests that neural activity and vascular response can be independently altered early in the disease process.

Conclusions and futures directions

Evidence from experimental and clinical studies has been accumulated so far indicating that retinal and optic nerve head microcirculation is modulated by visually evoked neural activity. It has been shown, indeed, that functional hyperemia measured at the neuro-retinal rim of the optic nerve is tightly coupled with retinal activity, mostly generated in inner retinal layers. The human optic nerve may represent an open ‘‘window’’ to the brain where neurovascular and neurometabolic coupling can be noninvasively investigated by optical techniques such as LDF and activity-induced changes in intrinsic optic signals. As recently discussed (Girouard and Iadecola, 2006), the functional hyperemia involves the coordinated interaction of neurons, glia, and vascular cells. Recent evidence in the central nervous system suggests that various vaso-active agents are implicated in functional hyperemia. These have

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been reviewed in details in a recent paper (Iadecola and Nedergaard, 2007). They include adenosine, nitric oxide, and arachidonic acid metabolites.

OHT and glaucoma may disrupt the neurovascular coupling, indicating that novel clinical protocols aimed at evaluating both visually induced functional hyperemia and activity changes are potentially valuable either for early detection of neuro-ophthalmic disorders or for better understanding of their pathophysiology.

Clearly, much remains to be investigated in this area. The mediators and modulators of the neurovascular and neurometabolic coupling need to be fully elucidated. In normal subjects, the full set of visual stimuli that, being parametrically modulated in their characteristics (i.e. space, time, chromaticity, adaptation state), are able to reveal the neurovascular coupling at the optic nerve needs to be established. Finally, more systematic clinical studies in different ophthalmic disorders will reveal the role of the current proposed approaches in the diagnosis and management of optic nerve diseases.

Acknowledgements

This study was supported by research grant from Fondazione Cassa di Risparmio, in Bologna (to C.E.R. and B.F.) and Fondi di Ateneo, ex 60% (to B.F.).

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