Clinicopathological Correlation of Outer Retinal Tubulation in Age-Related Macular Degeneration

Outer retinal tubulation (ORT) on spectral-domain optical coherence tomography (SD-OCT) is a hyporeflective area surrounded by a hyperreflective band in the outer nuclear layer.¹ It comprises interconnecting tubes containing degenerate photoreceptors, almost exclusively cones, and Müller cells.² In longitudinal studies, ORTs are dynamically involuting structures that may portend a poor visual outcome.³ We present clinical and postmortem imaging and histological findings of ORT in a patient with advanced age-related macular degeneration. We test our hypothesis⁴ that the reflective border includes translocated mitochondria within degenerating inner segments (ISs) and the external limiting membrane (ELM).

A woman in her mid-90s presented with bilateral vitelliform lesions. Two years later, the vitelliform lesion in her right eye began shrinking, with complete absence 3 years after presentation. On her last evaluation, 4 years after presentation, she had central geographic atrophy (GA) bilaterally, with ORT in her right eye noted on SD-OCT. She died of stomach cancer 8 months after the last evaluation.

Eyes were recovered by eye bank personnel 8 hours 55 minutes after death, opened anteriorly with an encircling cut at the limbus, and preserved by immersion into 10% neutral buffered formalin. Following overnight shipping, formalin was replaced with buffered 1% paraformaldehyde and 2.5% glutaraldehyde. Eye tracking software (Spectralis; Heidelberg Engineering) was used to align in vivo and ex vivo B-scans from preserved globes. Tissue was postfixed with osmium tannic acid paraphenylenediamine. Macula-wide, 0.8-µm-thick sections at locations matching SD-OCT B-scans were stained with toluidine blue and imaged as described.⁴ Sections at silver-gold thickness were viewed by transmission electron microscopy.

An ORT found in the inferior macula by clinical SD-OCT was also visible by ex vivo SD-OCT and histological analysis (Figure 1). Comparison of in vivo (Figure 1A and B) and ex vivo (Figure 1C and D) SD-OCT reveals edema due to the interval between death and preservation. The ORT is nasal to the temporal border of GA formed by the collapsed central vitelliform lesion (Figure 1E). The ORT luminal wall contains cone photoreceptors, with ISs and without outer segments (ie, at the mature phase⁴). Transmission electron microscopy reveals translocated mitochondria internal to the ELM (Figure 2A and B). At a level superior to that in Figure 1E, a glancing section through the ORT wall reveals cone cell bodies (Figure 2C). A section at a level inferior to Figure 1E shows that the ORT opens up to and is continuous with the GA border (Figure 2D).

To our knowledge, this is the first clinicopathologic correlation of SD-OCT and histological analysis from the same patient with age-related macular degeneration. By histological analysis and transmission electron microscopy, the reflective border of ORT can be accounted for by a combination of the ELM and IS mitochondria that are translocated to the same level as the ELM by shrinkage of the IS, supporting the proposal that mitochondria are independent reflectivity sources in SD-OCT.⁴ This mature-phase⁴ ORT lacked outer segments but was still visible, indicating that photoreceptors do not require outer segments to contribute reflectivity to SD-OCT. In healthy eyes, hyperreflective band 2 aligns with IS ellipsoids, which are mitochondria rich, implicating these organelles as reflectivity sources.⁵ Mitochondria may also contribute to reflectivity in synaptic layers and basolateral retinal pigment epithelium, where they are also abundant.

Histological analysis demonstrates the microscopic continuity of the ELM at the ORT with the ELM at the GA border. By en face OCT, curvilinear ORT can be seen to line the inner border of atrophy.¹ A defining feature of ORT is a free edge to scroll.⁴ A connection of ORT to nonatrophic areas could conduct trophic support to persisting photoreceptors⁴ via luminal fluid and trapped cells, as postulated.⁶

Corresponding Author: Christine A. Curcio, PhD, Department of Ophthalmology, University of Alabama School of Medicine, 1670 University Blvd, Room 360, Birmingham, AL 35294 (curcio@uab.edu).

Published Online: March 5, 2015. doi:10.1001/jamaophthalmol.2015.126.

Author Contributions: Dr Curcio had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.

Study concept and design: Litts, Dellatorre, Freund, Curcio.

Acquisition, analysis, or interpretation of data: Litts, Messinger, Yannuzzi, Freund, Curcio.

Drafting of the manuscript: All authors.

Critical revision of the manuscript for important intellectual content: Litts, Dellatorre, Yannuzzi, Freund, Curcio.

Obtained funding: Yannuzzi, Freund.

Administrative, technical, or material support: Messinger, Dellatorre, Yannuzzi, Curcio.

Study supervision: Yannuzzi, Freund, Curcio.

Conflict of Interest Disclosures: All authors have completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest. Dr Freund reported being a consultant for Heidelberg Engineering, Genentech, Bayer HealthCare, Regeneron, Optos, and Thrombogenics. No other disclosures were reported.

Funding/Support: This work was supported by grant R01EY06109 from the National Institutes of Health and by grants from The Macula Foundation; the Vision Science Graduate Program, University of Alabama at Birmingham; the Eyesight Foundation of Alabama; and Research to Prevent Blindness to the Department of Ophthalmology, University of Alabama at Birmingham.

Role of the Funder/Sponsor: The funders had no role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; and decision to submit the manuscript for publication.