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    Frisen grades. a. Grade 0. Notice there is no halo of obscuration of the peripapillary nerve fiber layer. b. Grade 1. There is a C-shaped halo of retinal nerve fiber layer edema obscuring the peripapillary retina. c. Grade 2. The halo is now circumferential; there is no temporal gap and no vessel obscuration. d. Grade 3. Major vessels are obscured by edema as they leave the disc (arrow). e. Grade 4. Major vessels are now obscured by edema on the optic disc. f. Grade 5. All vessels are at least partially obscured by edema.