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S. Nath, S. J. Bass, J. Sherman, M. Aswani; The Retinal Nerve Fiber Layer (RNFL) Measured by GDxVCC in Retinitis Pigmentosa (RP) Is Thicker Than Age-Matched Controls. Invest. Ophthalmol. Vis. Sci. 2007;48(13):2601.
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© ARVO (1962-2015); The Authors (2016-present)
To determine the thickness of the RNFL measured by the GDxVCC in patients with RP having good visual acuity
Ten eyes of 5 consecutive patients with various forms of RP and visual acuity of 20/30 or better in each eye were tested. Patients had moderate to advanced disease. Six eyes had visual field constriction ranging from 5 degrees to 20 degrees. Four eyes had mean deviations on Humphrey 24-2 threshold visual field testing ranging from -12.35 dB to -24.4 dB. The GDxVCC was used to measure the superior average (SA), inferior average (IA), nasal average (NA), and temporal average (TA) RNFL thickness. The ages ranged from 27 years to 58 years. The same parameters in 16 eyes of 8 age-matched controls were also measured and compared.
For the study group, the SA was 87.1 microns (u), SD=6.79, IA was 82.9u, SD=9.56, NA was 27.1u, SD=8.69 and TA was 25.9u, SD=10.93. In the normal group, the SA RNFL average was 66.70u, SD=8.74, IA was 63.0u, SD=10.75, NA was 44.4u, SD=7.57 and the TA was 34.93u, SD=9.48. The averages between the 2 groups were compared using a t-test for independent groups. SA and IA in the RP group was thicker than the normal group (t=6.27, df=24, p=.001 and t=4.78, df=24, p=.001, respectively). NA and TA in the RP group was thinner than the normal group (t=-5.37, df=24, p=.001 and t=-2.23, df=24, P=.035, respectively).
The superior and inferior RNFL was found to be thicker than normal in RP patients with good VA, whereas the nasal and temporal RNFL thickness was thinner than the normal group. Therefore, it is not likely that the increase in thickness is due to artifact or diffuse RPE thinning. In early LHON, the RNFL is also thickened (Barboni P at el, ARVO 2004) and this is believed to be due to an increase in the number of mitochondria. The reason(s) for a thicker superior and inferior RNFL in RP are not known. Of the four cylindrical structures known to be responsible for birefringence, i.e.cell membranes around nerve axons, microtubules, neurofilaments and mitochondria (Zhou Q and Knighton RW, 1997), it is unclear which of these (or other) structures are responsible for the increased birefringence in RP.
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