WHAT: Optic neuritis. Optic Neuritis: the inflammation, degeneration, or demyelination of the optic nerve. WHY: Optic neuritis may be present in patients with sarcoidosis or Reiter's syndrome (rarely). HOW: Clinically, patients with optic neuritis present with sudden loss of central vision and pain on moving the eye. Opthalmoscopically there may be hyperemia of the optic disc and distention of the large retinal veins. At later stages the disk margins are blurred and may be slightly elevated simulating papilledema. REFS: Vaugn, D and Asbury, T: General Ophthamology, 8th edition, 1977.