The loss of vision due to heredodegenerative diseases of the receptors traditionally has been followed in the clinic with behaviorally measured visual fields and/or electrophysiologically measured electroretinograms. The behavioral techniques, such as standard automated perimetry (SAP), have the advantage of providing topographical information about the disease process. With the introduction of optical coherence tomography (OCT), it became possible to obtain topographical information about the anatomical/structural damage to the receptors as well.
A number of studies have shown a correspondence between a loss of light sensitivity with SAP and a decrease in receptor layer thickness on OCT scans. Earlier studies with time domain OCT showed good quantitative agreement between the extent of local field loss and the thickness of the outer nuclear layer (ONL) [1
]. With the improved resolution of frequency domain (fd) OCT, it became possible to measure the thickness of the outer segment (OS) layer. Rangaswamy et al. [5
], for example, found that the OS thickness decreased approximately linearly with local field loss, becoming non-detectable in regions where visual field sensitivity loss was worse than −10 dB. A study of the transition zone from regions of normal to abnormal vision reported that the earliest sign of damage in RP was a thinning of the OS layer, followed by a decrease in ONL thickness, and then a disappearance of the OS layer [6
]. The disappearance of the OS layer occurs, by definition, at the point where the so-called inner segment (IS)/OS border is no longer visible.
What is called the IS/OS border is a prominent and clearly visible landmark on OCT scans. Whether this is in fact the IS/OS border or the ellipsoids at the distal ends of the IS is open to debate [7
]. In any case, there are many reports of this IS/OS border being disrupted in patients with diseases of the outer retina. (See for example [8
].) Of particular relevance here is the finding of Fischer et al. [11
] that the distance from the center of the fovea to the loss of the IS/OS signal correlated with distance to the edge of the visual field as measured with Goldmann perimetry.
Taken together, these studies suggest that the point at which the IS/OS border disappears should correspond to a visual field sensitivity loss of −10 dB and may provide a structural marker for the edge of the visual field. Here we test this hypothesis. In addition, we explore the use of this OCT marker as a possible measure of disease progression.