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S. J. Bass, J. Sherman, W. He; Clock Sector Analysis of Retinal Nerve Fiber Layer (RNFL) Thickness in Retinitis Pigmentosa (RP) Subjects With Good Visual Acuity Using Optical Coherence Tomography (OCT). Invest. Ophthalmol. Vis. Sci. 2008;49(13):906. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
To determine RNFL thickness in subjects with Retinitis Pigmentosa (RP) having good visual acuity using the clock sector thickness measurements obtained by the StratusR OCT.
Eleven eyes of 11 consecutive RP subjects with visual acuity of 20/30 or better, no visible optic atrophy and no other ocular disease, were tested. All subjects had well documented RP for several years. Ages ranged from 21 to 64 years. Nine eyes of nine normal subjects (N) in the same age range were also tested. Subjects were scanned using the Fast RNFL Scan (3.4) protocol where thickness values are determined in circular scans of 3.46 mm in diameter around the optic disc and displayed in 12 "clock" sectors and in quadrants. Sectors were assigned anatomically, where S=superior, N=nasal, I=inferior and T=temporal and for sectors in between e.g. where SSN=superior superior nasal. Mean RNFL thickness between the RP and N group was compared for all 12 sectors using a t-test for independent groups.
The RNFL in the RP group was significantly thicker than normal in three clock sectors: a) SSN (t=2.66,df=18,p=.01) b) IIT (t=2.65,df=18,p=.01) c) TSS (t=2.12,df=18,p=.04). Mean RNFL thickness difference in the S clock sector approached but did not reach statistical significance (t=1.8,df-18, p=.08).
In this study, the RNFL in RP with good visual acuity and no optic atrophy is significantly thicker than in normals in three clock sectors. This is not a diffuse effect and therefore not likely due to artifact or diffuse retinal thinning. These findings are in general agreement with reported increases in superior and inferior RNFL thickness measured by the GDxVCC (Nath, Bass, Sherman, ARVO 2007). However, these results differ from RP patients with optic atrophy, where RNFL thinning using OCT was reported (Walia S, et al, IOVS 2007). RNFL thickening has also been reported in Leber Hereditary Optic Neuropathy (LHON), using both OCT and GDxVCC imaging, where the RNFL thickens immediately prior to acute onset, persists for months and then decreases markedly over years (Sadun A, et al, Trans Am Ophthal Soc, 2006). An up-regulation of mitochondria is the prevailing hypothesis in LHON. In RP, the time course of RNFL thickening appears to be much longer and the reason for this thickening remains obscure at present. A plausible explanation may enhance our understanding of the pathophysiology of RP.
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