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Editors Selection IGR 15-4
Clinical examination methods: Retinal venous flow
Comment by Murray Johnstone on:
67231 Phase and amplitude of spontaneous retinal vein pulsations: An extended constant inflow and variable outflow model, Levine DN; Bebie H, Microvascular Research, 2016; 106: 67-79
Patients with glaucoma have both a lower amplitude and reduced presence of spontaneous venous pulsations. The amplitude of venous oscillations is thought to be proportional to the amplitude of the transmural driving forces and to venous capacitance. Pulse amplitudes increase when intraocular pressure (IOP) increases but other parameters may also contribute to venous pulsations.
Parameters including intralaminar resistance, alterations in cerebrospinal fluid pressure (CSVP), CSFP oscillations and phase differences may also be important factors. In this study, Levin et al. explore the hypothesis that in chronic glaucoma the above parameters and resultant combination of forces may explain a reduction in amplitude of retinal vein oscillations.
The phase of vein and IOP oscillations predicted by CIVO has been unclear and controversial
A constant venous inflow and variable outflow (CIVO) theory has previously been used to predict spontaneous retinal vein pulsations. However, the phase of vein and IOP oscillations predicted by CIVO has been unclear and controversial. In an attempt to clarify relationships, the authors have extended the CIVO model to a larger domain that includes not only the intraocular but also the retrobulbar portion of the veins and the intervening resistance in the lamina cribrosa. This latter model posits that intraocular venous pulsations vary depending on the relative difference between means of IOP and CRVP, differences in amplitudes and differences in phase of IOP and CRVP oscillations. The authors' modeling demonstrates changes in intraocular venous pulse amplitudes that vary in response to changes in the new parameters they introduce into their model.
Model imitations include a paucity of available data related to both mean and oscillations of CSFP surrounding the optic nerve, particularly while subjects are upright. The paper nonetheless extends the conceptual framework that needs to be considered in relation to CRV pulsations. If future technologies can better define CSFP parameters in vivo, the author's model might prove to be useful for interpreting clinical finding in glaucoma patients.
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