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Editors Selection IGR 8-2

Basic science: Axon degeneration

Comment by James Lindsey on:

15259 Assessment of axonal degeneration along the human visual pathway using diffusion trace analysis, Ueki S; Fujii Y; Matsuzawa H et al., American Journal of Ophthalmology, 2006; 142: 591-596

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An important challenge for the management of glaucoma patients is to determine whether there has been progression of glaucomatous damage. It is plausible that better care can be provided with more sensitive detection of optic nerve damage than is used in current clinical practice. To this end, Ueki et al. (1028) have examined whether a new technical advance in magnetic resonance imaging (MRI) using diffusion tensor imaging (DTI) will facilitate detection of optic nerve damage. As an initial investigation, they studied ten patients with unilateral chronic optic neuropathies including traumatic optic neuropathy, compressive optic neuropathy, and retrobulbar optic neuritis. In each patient the extent of damage was severe. The results were provided for each patient as well as for all of the patients combined. The parameter studied, called trace, typically varies according to tissue water content. For statistical analyses, the results from all the patients were considered together and comparisons were made between the normal nerve or tract and the corresponding affected nerve or tract. Briefly, they found that trace was significantly higher in the affected optic nerves, in the ipsilateral uncrossed fibers at the chiasm, and in the crossed fibers posterior to the chiasm. Results in the optic tracts and optic radiations were not significantly different.

These results, while promising, should be considered to be preliminary. In each case, the amount of vision loss, and presumably axon loss, was severe. An important strength of this study is that it was conducted using a 3-tesla scanner, a type of MRI scanner that is becoming more widely available in recent years to typical clinical usage. Substantial work needs to be done to determine the accuracy and sensitivity of this approach for the assessment of optic nerve axon loss. Also, the impact of other diseases that induce local tissue changes that could influence the measurements needs to be done. Finally, there should be careful comparisons with existing methods as well as evaluation in experimental models where direct histological assessments of optic nerve damage can be made.

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