Purpose
To compare optical coherence tomography (OCT) and histology of outer retinal
tubulation (ORT) secondary to advanced age-related macular degeneration (AMD) in
patients and in post-mortem specimens, with particular attention to the basis of the
hyper-reflective border of ORT.
Method
A private referral practice (imaging) and an academic research laboratory
(histology) collaborated on two retrospective case series. High-resolution OCT raster
scans of 43 eyes (34 patients) manifesting ORT secondary to advanced AMD were compared
to high-resolution histological sections through the fovea and superior perifovea of
donor eyes (13 atrophic AMD and 40 neovascular AMD) preserved ≤4 hours after
death.
Results
ORT seen on OCT corresponded to histologic findings of tubular structures
comprised largely of cones lacking outer segments (OS) and lacking inner segments (IS).
Four phases of cone degeneration were histologically distinguishable in ORT lumenal
walls, nascent, mature, degenerate, and end-stage (IS and OS; IS only; no IS; no
photoreceptors and only Müller cells forming external limiting membrane, ELM,
respectively). Mitochondria, which are normally long and bundled within IS ellipsoids,
were small and scattered within shrunken IS and cell bodies of surviving cones. A
lumenal border was delimited by an ELM. ORT observed in closed and open configurations
were distinguishable from cysts and photoreceptor islands on both OCT and histology.
Hyper-reflective lumenal material seen on OCT represents trapped retinal pigment
epithelium (RPE) and non-RPE cells.
Conclusions
The defining OCT features of ORT are location in the outer nuclear layer (ONL),
a hyper-reflective band differentiating it from cysts, and RPE that is either dysmorphic
or absent. ORT histologic and OCT findings corresponded in regard to composition,
location, shape, and stages of formation. The reflectivity of ORT lumenal walls on OCT
apparently does not require an OS or an IS/OS junction, indicating an independent
reflectivity source, possibly mitochondria, in the IS.