Abstract
An increasing number of studies indicate that the optic nerve head of the eye is sensitive not only to changes in intraocular pressure (IOP), but also to intracranial pressure (ICP). This study examines changes to optic nerve and retinal structure in a rat model in response to a range of IOP and ICP levels using optical coherence tomography. Furthermore, we examine the functional sequelae of these structural changes by quantifying the effect of pressure changes on the electroretinogram. IOP elevation (10–90 mmHg) induces progressive deformation of the optic nerve head and retinal surface (P < 0.05), compression of the retina (P < 0.05) and bipolar cell (b-wave), and retinal ganglion cell (scotopic threshold response) dysfunction (P < 0.05). Simultaneously altering ICP (−5 to 30 mmHg) modifies these IOP-induced responses, with lower ICP (−5 mmHg) exacerbating and higher ICP (15–30 mmHg) ameliorating structural and functional deficits. Thus, the balance between IOP and ICP (optic nerve pressure gradient, ONPG = IOP − ICP) plays an important role in optic nerve integrity. Structural and functional parameters exhibit a two-phase relationship to ONPG, with structural changes being more sensitive to ONPG modification (threshold = −0.6 to 11.3 mmHg) compared with functional changes (threshold = 49.7–54.6 mmHg). These findings have implications for diseases including glaucoma, intracranial hypertension, and long-term exposure to microgravity.
Highlights
The optic nerve head is an area which is susceptible to pressure stress
Fortune et al (2011) demonstrated posterior displacement of the optic nerve head (ONH) surface and outward bowing of peripapillary tissue (10° around the ONH) when intraocular pressure (IOP) was raised to 50 mmHg
Consistent with previous studies (Fortune et al 2009, 2011) we show that tissue deformation with IOP elevation was greatest at the ONH (200 lm) and peripapillary retina (400 lm, ~10°) and progressively less with increased retinal eccentricity
Summary
The optic nerve head is an area which is susceptible to pressure stress. The optic nerve is susceptible to pressure, because at this location the sclera thins to form sieve-like layers of collagen allowing retinal ganglion cell axons to exit the eye. As such, these areas along with the peripapillary sclera suffer the highest levels of stress and strain (Downs et al 2008) and are prone to deformation. What has not yet been established is how these biomechanical changes relate to retinal function over a range of IOP and ICP levels Elucidating this relationship will have important implications for conditions involving abnormal IOP and/or ICP. These include glaucoma (elevated IOP, perhaps decreased ICP), traumatic brain injury (elevated ICP), and long-term exposure to microgravity-associated visual impairment (elevated ICP, elevated IOP) (Mader et al 2011; Woo et al 2012)
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