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J. P. Shulman, G. Mandel, M. J. Kupersmith; Differentiating Extra-Cellular Edema From Axonal Thickening in Papilledema Using OCT and SLP. Invest. Ophthalmol. Vis. Sci. 2009;50(13):932.
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Ocular coherence tomography (OCT) and scanning laser polarimetry (SLP) effectively show retinal nerve fiber layer (RNFL) loss in glaucoma and optic neuritis. Since SLP principally depends on birefringent structures in the retina, it should be insensitive to extracellular edema. Previous studies have utilized either method to study swollen optic disks from a variety of etiologies. We investigated subjects with papilledema due to intracranial hypertension, as an in vivo model for edema of the RNFL.
We prospectively studied 18 eyes of 9 subjects, with symptoms of intracranial hypertension, bilateral clinically apparent papilledema and no visual acuity loss or optic atrophy. Each subject had complete clinical evaluation, threshold perimetry using SITA 24-2, and RFNL evaluation with OCT3 and enhanced corneal compensation GDx. RNFL was judged thicker (by OCT) or had increased retardance (by SLP) when greater than the upper 95th percentile of the provided age matched controls. We calculated a percent (%) thickness ratio: Study Eye Thickness/Control Eye Thickness, for OCT and SLP.
Global RNFL averages were thicker in 10 eyes of 7 subjects by OCT and in 4 eyes in 3 subjects by SLP. Compared to our controls for OCT (114 µ as 95th percentile), 13 eyes in 8 subjects were thickened. Using the Zeiss controls for SLP (72.7 µ as 95th percentile), 4 eyes in 3 subjects had increased retardance. RNLF swelling (better demonstrated by OCT, Table) was more prevalent in superior and inferior quadrants. For the 13 eyes with OCT swelling, the mean for OCT%-SLP% was 78 (range 14-235) and provided a relative measure of edema. There was no correlation between the degree of swelling and visual field or visual acuity.
Intracranial hypertension associated RNFL swelling occurs in part due to extracellular edema that is better demonstrated by OCT. SLP appears to better reflect changes within the axons (i.e. axonal dilation) that comprise the optic nerve.
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