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Editors Selection IGR 15-1

Comment by Leopold Schmetterer on:

Cull and co-workers performed a study investigating longitudinal changes in optic nerve head (ONH) blood flow in experimental glaucoma in non-human primates. The data indicate a bi-phasic response of ONH blood flow after the IOP elevation. When retinal nerve fiber layer (RNFL) is only slightly reduced (< 10%) the authors observed an increase in ONH blood flow. With more advanced damage the authors report a progressive decline in ONH blood flow. A significant association was found between the reduction in RNFL and the decrease in ONH blood flow. The authors conclude that in the later stages of glaucoma the loss in ONH blood flow is closely associated with the loss of RNFL. This is a well-performed study that produced interesting and relevant results.

It is now generally accepted that glaucoma is associated with reduced ocular blood flow. It is, however, not established whether this is a cause of the disease or a consequence of the loss in retinal ganglion cells (RGCs). The present study indicates that in monkeys in which ocular hypertension is induced by laser treatment to the trabecular meshwork, the reduction in ONH blood flow is at least partially secondary to RNFL loss. One additional aspect needs, however, to be considered. Typically the loss of (RGCs) in glaucoma appears localized. If the decrease in ONH and retinal blood flow goes in parallel with the loss of RGCs it may well be that the remaining RGCs are exposed to ischemia unless there is a complex redistribution of blood flow.

Performing studies on the hemodynamic status in experimental animals has a number of significant advantages. Optical technologies such as laser speckle flowgraphy, employed in the present study, can more easily applied, because of the lack of eye movements. In addition, the authors reduced intraocular pressure (IOP) to values of 10 mmHg during all blood flow measurement periods. This is an important issue, because it ensures that the blood flow values are not caused by the increased IOP in this model.

One key issue can, however, not be answered in such type of studies. Is reduced ONH blood flow a risk factor for the development of glaucoma? To answer this question, a longitudinal study in humans is required. To date, however, no technique for the measurement of ONH blood flow in h umans seems to be reproducible enough to perform such a trial.

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