Figure 1.

Retinal structure and degeneration in inherited retinal diseases (IRDs). The retina has a laminar structure consisting of distinct cell types (A). The neural retina consists of the ganglion cell layer (GCL, containing the cell bodies of retinal ganglion cells), inner plexiform layer (IPL), inner nuclear layer (INL - bipolar cell bodies), outer plexiform layer (OPL), outer nuclear layer (ONL - photoreceptor cell bodies), inner segments (IS) and outer segments (OS). The retinal pigment epithelium (RPE) supports the metabolism of overlying photoreceptors, is attached to the Bruch’s membrane/choroid and forms outer blood-retinal barrier. A representative spectral domain optical coherence tomography (OCT) of the retina is shown which demonstrates normal retinal layers. (B) Early Stage IRD, such as retinitis pigmentosa, is typically characterised by dysfunction and degeneration of rod, which can be seen as peripheral outer retinal thinning on the OCT (note that parafoveal architecture is relatively preserved). (C) Retinal degeneration progresses to Mid Stage 1 where cone function (day light vision) remains relatively intact while rod function (night vision) is severely impaired. The OCT shows widespread disruption of the ellipsoid line which represents IS/OS junctions. RPE thinning can also be seen. (D) Mid Stage 2 sees cone degeneration with shortened OS and loss of rods. (E) In Late Stage (or end stage) IRD, there is complete loss of photoreceptors while inner retinal layers remain relatively preserved. OCT shows complete outer retinal atrophy with areas of RPE hypertrophy which correspond to bone spicules seen clinically.