The Pathogenesis of Papilledema: Review of the Literature and a New Hypothesis.

Since the first description of swelling of the optic nerve head in patients with increased intracranial pressure, our understanding of its pathogenesis has undergone significant changes. Early theories postulated that the swelling was caused by excessive extracellular fluid, but these views were disproved when electron microscopy showed that the swelling arose from dilated optic nerve axons, and autoradiography demonstrated blocked axonal transport at the posterior lamina cribrosa. This led to the currently prevailing view that the axonal swelling is caused by the damming back of axoplasm. However, this theory cannot account for the extent of swelling, its rate of development, and the variety of morphological changes in papilledema. It also cannot explain the differing patterns of swelling in papilledema and acute glaucoma despite identically located blockages of axonal transport. We conducted a biomechanical analysis, in which we calculated the stresses induced in a cylindrical nerve by external compression and the effect of these stresses on the nerve's axons and the axoplasm within them. We propose a new theory in which the axial gradient of tissue pressure causes displacement of axoplasm from the extraocular to the intraocular segment of the nerve, accounting for the intraocular axonal swelling. In addition, a sharply localized axial shear stress disrupts the axonal cytoskeleton to block axonal transport. Although the pressure gradient and the shear stress are both caused by the external compression of the nerve, they differ in their relative magnitudes across the nerve cross-section. The proposed hypothesis resolves the difficulties with the damming back hypothesis.