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Editors Selection IGR 8-1

Structure and function: Split-bundles

Comment by Felipe Medeiros on:

20003 Retinal nerve fiber layer split bundles are true anatomic variants, Kaliner E; Cohen MJ; Miron H et al., Ophthalmology, 2007; 114: 2259-2264

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Split-bundles are an interesting pattern of retinal nerve fiber layer (RNFL) distribution that is often seen during evaluation of this structure with imaging instruments or with red-free or color photographs. In the spilt-bundle pattern, the superior, or less often, the inferior RNFL bundle, is split into two smaller bundles giving the impression of a triple or quadruple hump to the RNFL thickness modulation profile around the optic disc. This pattern has always intrigued clinicians and researchers in this area, as there was no clear evidence as to whether they represented a true anatomic variant or just an artefact of imaging methods. Kaliner et al. (1194) conducted a histologic study to clarify this issue. They evaluated 14 eyes of 13 human subjects using a histologic technique called the 'umbrella technique', which allows continuous evaluation of the peripapillary RNFL at various distances from the optic nerve head. Five of the 14 eyes (36%) showed split-bundles, three in the superior bundle and two in the inferior bundle.

RNFL split-bundles are true anatomic variants

The study by Kaliner et al. seems to provide evidence that split-bundles are true anatomic variants, although no correlation was provided with other clinical information, such as assessment of the RNFL by photography or imaging techniques. The limited number of subjects in the study precluded evaluation of any relationship between split-bundles and demographic factors. Also, there still remains the question as to why this variant is seen in some but not in all patients. What factors are associated with split-bundles? Is there any particular ocular anatomical variation that is associated with split-bundles? Is it related to the number of nerve fibers in the eye or to any other measure such as optic disc size or relative position or ocular structures? Finally, the presence of split-bundles can sometimes generate the impression of a localized loss of RNFL between the bundles and clinicians should be aware of this variant when evaluating and inter-preting RNFL assessment obtained by imaging devices.

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