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Editors Selection IGR 13-2

Anatomical Structures: RNFL in Parkinson's Disease

Comment by Chris Leung on:

Findings from studies investigating retinal changes measured by optical coherence tomography (OCT) in Parkinson's disease have been inconsistent. Stemplewitz and colleagues revisited the issue and examined retinal nerve fiber layer (RNFL) and macular thickness measurements obtained with spectral-domain OCT (Cirrus HD-OCT) and scanning laser polarimetry (SLP) (GDx VCC) in patients with Parkinson's disease and healthy controls. While no significant differences in average/quadrant RNFL thicknesses measured by OCT were found between the groups, the average/inferior RNFL thicknesses and NFI (nerve fiber index) measured by SLP were smaller in patients with Parkinson's disease. SLP measures RNFL birefringence/retardance values around the optic disc and converts these values into RNFL thicknesses using a fixed conversion factor. Although the discrepancies in findings between OCT and SLP could be explained by the fact that reduction in RNFL retardance may occur before RNFL thinning (which has been demonstrated in animal models of optic nerve injury - Fortune B, et al. Invest Ophthalmol Vis Sci 2008;49:4444-4452; Fortune B, et al. Invest Ophthalmol Vis Sci 2013;54:5653-5661), it is worth noting that the difference in the SLP average RNFL thickness between the patients and controls were minute (57.1 ± 0.3µm versus 55.8 ± 0.5µm, p = 0.04). Further, patients with Parkinson's disease were significantly older (64.3 ± 12.3 years versus 56.9 ± 13.0 years). Age-related RNFL loss would complicate the interpretation of RNFL measurements. Investigating the rate of change of RNFL thickness in longitudinal studies would be a more informative approach to address whether the RNFL may serve as a biomarker for neurodegenerative diseases.

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