Optic nerve intraneural pressure trend under intraocular-intracranial pressure effects.

The imbalance between intraocular pressure (IOP) and intracranial pressure (ICP) is a recognised contributor to optic nerve (ON) injury. Clinically, the translaminar pressure difference is often simplistically interpreted as the direct subtraction of ON cerebrospinal fluid pressure (ON - CSFp) from IOP. While finite element analyses suggest that the transition between these two pressure compartments is neither abrupt nor uniform, instead forming a spatially graded finite transition zone, this concept remains unconfirmed from a fluid dynamics perspective. Given the inability to directly measure intraneural pressure in vivo, we modelled the ON as a continuous fluid medium. Using a mathematical approach, we visually illustrate how IOP and ON-CSFp interact to generate spatially varying intraneural hydrostatic pressure gradients. Our results show that these gradients assume a three-dimensional conical configuration. Elevation of IOP or reduction of ON-CSFp expands this conical gradient field, contributing to ON injury, whereas elevation of ICP produces an inverted conical gradient. The steepest gradients occur in the peripheral region posterior to the lamina cribrosa, rendering this area most vulnerable to damage. This study challenges the clinical oversimplification that pressure difference is equivalent to a mere subtraction of two values. It thereby improves clinicians' understanding of the biomechanical mechanisms underlying IOP and ICP imbalance-related ON injury. Importantly, it also highlights the often-overlooked phenomenon of intraneural pressure gradients, extending beyond the conventional focus on the translaminar pressure gradient alone.