Biography: Rett is chief of optometry for the Greater Boston VA Healthcare system, where he teaches students and residents of optometry.
February 10, 2020
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BLOG: What is outer retinal tubulation?

Biography: Rett is chief of optometry for the Greater Boston VA Healthcare system, where he teaches students and residents of optometry.
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Last month we talked about the “new” nomenclature for the bands of the outer retina as identified on OCT – and how we should all adopt this terminology starting now. This month, I thought we could continue the trend and talk about another OCT-specific posterior retinal finding: outer retinal tubulations.

Raise your hands if you’ve heard of these. First of all, I love the title. In 2009, Dr. Sandrine Zweifel and her team in New York coined this term as the title of a paper they published describing the lesions. If you’re a regular reader to this column, you’ve heard me describe my dislike of “eponymization” of medical terms (ie, please call it granulomatosis with polyangiitis and not Wegener’s disease, hepatolenticular degeneration and not Wilson’s disease). We have enough to memorize, and the eponyms are not helping; a more descriptive term would benefit everyone (except Wegener and Wilson).

Unless you’re going to name something after me (which I am totally fine with, by the way), then I say no one gets eponyms. Anyway – I digress – I just want to say that outer retinal tubulations is a perfect term in my opinion, although Zweifel doughnuts could totally work.

SD-OCT of an ORT. Note the hyperreflectivity of the shell, the relative hyporeflectivity of the inside of the tube and the location within the ONL. Source: Doug Rett, OD, FAAO
SD-OCT of an ORT. Note the hyperreflectivity of the shell, the relative hyporeflectivity of the inside of the tube and the location within the ONL.
Source: Doug Rett, OD, FAAO

Outer retinal tubulations (ORTs) are “ovoid hyporeflective spaces with hyperreflective borders on conventional B-scan OCT” (Zweifel et al.). On a cross-sectional OCT, we see them as circles (doughnuts), but an en-face OCT reveals them as tubes, some straight and some with large, branching networks. They’re found in multiple retinal degenerative conditions, but the most common is age-related macular degeneration and usually severe cases when the retinal pigment epithelium (RPE) is compromised. The pathogenesis of ORT is murky, but these tubes seem to form when photoreceptors lose interdigitation of the RPE.

En-face OCT (curved C-scan) of the ORT in the same patient. This is a (relatively) straight, single tube, but ORTs can also make extensive branching networks. Source: Robert Dunphy, OD, FAAO
En-face OCT (curved C-scan) of the ORT in the same patient. This is a (relatively) straight, single tube, but ORTs can also make extensive branching networks.
Source: Robert Dunphy, OD, FAAO

Zweifel postulates that after the photoreceptors detach from the RPE, there is a disruption of tight junctions in adjacent photoreceptors, leading to an “outward folding of the photoreceptor layer until opposite sides of this fold establish contact and form new lateral connections through tight junctions.” Essentially, the damaged photoreceptors start rolling up in a tube to protect themselves. ORTs typically occur in areas of normal retinal thickness (not areas that are completely atrophic retina), so perhaps they are a misguided attempt at repair.

But what are they made of? What makes it hyporeflective on the inside but hyperreflective on the outside? What’s on the inside of the tube?

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In 2015 Dr. Karen Schaal published a paper that detailed histological examination of 40 eyes from 40 donors with neovascular AMD, preserved less than 4 hours after their death. She described it as “the largest number of advanced AMD eyes examined by histology to date specifically for photoreceptor degeneration.” On the inside of the ORT, her group found cone bodies in varying states of degeneration, mostly without their outer segments. On the outside of the ORT, they found mostly external limiting membrane, with some translocated mitochondria from the inner segments of cones. This accounts for the hyperreflective ORT shell as the mitochondria in the ellipsoid zone (of a healthy retina) are quite reflective on OCT.

The ORT literature also mentions how it’s important to differentiate ORTs from intraretinal cysts or subretinal fluid (SRF). Both cysts and SRF could be the sign of active exudative macular degeneration, and if an ORT is mistaken for either one, the patient could be unnecessarily treated with anti-VEGF injections.

So be diligent when checking for ORTs: Look for the hyperreflective shell of the ORT, which would not be present in cystoid macular edema (CME) cysts or SRF, and look at the location of the lesion; ORTs are considered to be always in the outer nuclear layer (ONL), whereas SRF and CME cysts are below and above the ONL, respectively. Anti-VEGF therapy has not been shown to have any effect on ORTs, which makes sense now that we know it’s a general destructive sign rather than a sign of active exudative disease (Filho et al.).

I think there are always things to discover when examining patients. Keep staring at your OCTs and when you find something you’re not sure of, look it up! You just might learn something interesting.

References:

Zweifel SA, et al. Arch Ophthalmol. 2009;doi:10.1001/archophthalmol.2009.326.

Schaal KB, et al. Retina. 2015;doi:10.1097/IAE.0000000000000471.

Filho RGG, et al. Int J Retin Vitr. 2016;doi:org/10.1186/s40942-016-0029-8.

Litts KM, et al. JAMA Ophthalmol. 2015;doi:10.1001/jamaophthalmol.2015.126