Intoxication with β,β'-iminodipropionitrile. A model of optic disc swelling

Human and experimental pathologic studies have suggested that optic disc swelling is due to enlargement of axons in the optic nerve head. β,β'-Iminodipropionitrile (IDPN), a toxin known to produce neurofibrillary swellings in proximal portions of motor and sensory axons, was used to induce axonal lesions in the visual system of guinea pigs and dogs. Funduscopic examination showed progressive optic disc swelling. The axonal swellings, containing maloriented skeins of neurofilaments, appeared initially in the lateral portions of the optic disc; eventually, the majority of axons in the optic disc and in the retinal nerve fiber layer were enlarged. The swellings developed proximal to the level of Bruch's membrane. In addition, myelinated axons in the optic disc and retinal nerve fiber layer developed swellings proximal to the first heminode, demarcating the junction between the unmyelinated/myelinated segments. The retinal ganglion cells remained normal, and there was little axonal degeneration in the distal optic nerve. When β,β'-iminodipropionitrile was discontinued, the axonal swellings gradually disappeared.