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En face OCT imaging detects severity of IS/OS breaks in type 2 macular telangiectasia
Ocular imaging tool may also be used to monitor disease progression.
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For evaluating disease severity in type 2 idiopathic macular telangiectasia, en face optical coherence tomography of the retina is a valuable diagnostic technology, according to researchers.
“Type 2 idiopathic macular telangiectasia is the most frequent variety among macular telangiectasias of unknown cause,” principal study investigator Ferenc B. Sallo MD, PhD, told Ocular Surgery News. “Type 2 macular telangiectasia affects both genders and is a bilateral, potentially blinding eye disease with no effective treatment available.”
Furthermore, because of its subtle early signs and symptoms and the similarities between late disease and neovascular age-related macular degeneration, type 2 macular telangiectasia is probably underdiagnosed, he said.
Study
In a prospective study of 49 eyes of 28 patients, investigators in the MacTel Study Group looked at abnormalities of the photoreceptor inner segment/outer segment (IS/OS) junction layer using en face imaging of scans performed with a Topcon 3D-OCT1000 (Topcon Medical Systems).
Mean retinal sensitivity was 8.3 dB in right eyes and 8.7 dB in left eyes in retinal areas corresponding to an IS/OS break.
Automated microperimetry, which was correlated to the fundus, was performed using a Nidek MP1 microperimeter. Data analysis compared sensitivity thresholds within retinal areas corresponding to breaks in the IS/OS junction with test points outside that area.
Mean retinal sensitivity was 8.5 dB in retinal areas corresponding to an IS/OS break compared with 16.9 dB in retinal areas outside the lesion.
The study, published in Investigative Ophthalmology & Visual Science, demonstrated a clear loss of retinal sensitivity within the area of IS/OS breaks, on average more than 8 dB, Sallo said.
“This is significant both statistically and with respect to the repeatability of Nidek MP1 microperimetry. The magnitude of aggregate sensitivity loss measured correlated well with IS/OS lesion size,” he said.
Measuring progression
Type 2 macular telangiectasia was considered mainly vascular before OCT imaging became available, Sallo said.
“OCT brought a paradigm shift in our view of the etiology and pathogenesis of the disease. Supported by recently published information from autopsy eyes of participants enrolled in the Mac Tel Project suggesting that Müller cell abnormalities are associated, it is now viewed as both a vascular and neurodegenerative disease,” he said.
There is also a need for following disease progression in the natural history of the disease, as well as in therapeutic trials, according to Sallo.
“Vascular signs progress more slowly and can be challenging to quantify,” he said. “Our aim was to find a measure of disease severity that can be used to track progression as accurately as possible.”
En face imaging of the IS/OS junction layer allows a more accurate measurement of lesion area, despite the inaccuracies of scan grid placement in repeated scans, Sallo said.
“Structural patterns in the plane of the retina that would otherwise, in individual B-scans, go unnoticed may become visible. One example is the ring around the central lesion observed in type 2 macular telangiectasia,” he said.
Not compromising the accuracy of layer segmentation would increase the efficacy of en face OCT, Sallo said.
“While this can be challenging to perform in an automated fashion, it must be remembered that the retinal layers have individual shapes,” he said. “Unless these individual shapes are respected during segmentation, en face images produced may be unclear or even misleading. Real-time eye tracking is a major advantage in avoiding motion artifacts.”
The break in the IS/OS layer detected by en face OCT is a morphological feature that may correlate closely with retinal function in type 2 macular telangiectasia and be useful for detecting progression, Sallo said. En face OCT may also be an important outcome measurement in clinical trials of potential therapies for type 2 macular telangiectasia.
For suspected cases of macular telangiectasia, such as unclear vision despite good measured visual acuity, loss of retinal transparency around the central fovea and minimal vascular signs, Sallo strongly recommends OCT imaging.
“A deformation of the foveal contour — a thinning temporal of the central fovea — retinal empty spaces or an IS/OS break may be clues to the diagnosis,” he said.
Sallo and colleagues are working with a major OCT manufacturer to incorporate en face imaging technology in an OCT system. The technology will also likely be useful for imaging many other retinal diseases that are age-related, hereditary or inflammatory, Sallo said.
A follow-up study currently under peer review investigated the natural history of the IS/OS break in type 2 macular telangiectasia and the usefulness of en face imaging over time. – by Bob Kronemyer