How OCT innovations are changing glaucoma care

Optical coherence tomography (OCT) is swiftly improving the diagnosis of glaucoma, according to Joel S. Schuman, MD, who helped invent the technology.

“It’s the most rapidly adopted ophthalmic technology in history,” said Dr. Schuman, professor and chairman, Department of Ophthalmology, NYU Langone Medical Center, NY.

Already, OCT studies have yielded key insights into the relationships of retinal nerve fiber layer thinning and vessel density to visual field loss, Dr. Schuman said.

Delivering the Drs. Henry and Frederick Sutro Memorial Lecture at the 6th Annual Glaucoma 360 New Horizons Forum, Dr. Schuman recounted his role in the development of OCT and outlined some of its major findings.

He defined OCT as a form of optical biopsy-“the in situ imaging of tissue microstructure with a resolution approaching that of histology, but in real time, without the need for tissue excision and processing.”

OCT uses near-infrared light in a Michelson-type interferometer. The underlying principle that made OCT possible is time gating, Dr. Schuman said, a system for isolating a portion of a time record for further viewing and analysis.

“Time gating removes unwanted scattered light and partially recovers the image,” Dr. Schuman explained. The low coherence allows high resolution.

During his fellowship, Dr. Schuman was working in the laser lab at Massachusetts Eye and Ear and became interested in the possibility of using optical coherence domain reflectometry (OCDR) which other researchers were using to measure the thickness of the cornea for refractive surgery. Dr. Schuman became interested in using it to measure the thickness of the retina in glaucoma and macular disease.

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His supervisor at Massachusetts Eye and Ear was not interested, but allowed him to take the idea to Massachusetts Institute of Technology (MIT), where he began to collaborate with David Huang, then an MD-PhD student.

They cut calf eyes in half and looked at the back halves under OCDR to see if there was a signal. When they found that there was, Dr. Huang came up with the idea of moving the OCDR beam transversally to create a B-scan that could be interpolated with the original A-scan, creating a tomogram.

First prototype

That led to a 1991 article in Science that first publicized the idea to the scientific community. By 1994, the team had built a prototype at the New England Eye Center. Carl Zeiss Meditec created a commercial system in 1996, and finally found a large market with its third version in 2002.

The global market for OCT reached $795.4 million in 2014. Forecasters say it will grow at a compound annual growth rate of 7.9% to reach nearly $1.2 billion in 2019, Dr. Schuman said.

Much of the research resulted from funding by the National Institutes of Health (NIH), he noted.

“It’s an example of how the NIH can make a difference in the everyday lives of people and make lives better,” Dr. Schuman said.

Currently, spectral-domain OCT-which measures wavelengths of back-reflected light-is replacing time-domain OCT, which measures time-of-flight light, because it is 50 to 1,000 times faster, he said.

For spectral-domain OCT, the entire signal from a broadband source is recorded at all wavelengths in parallel by a spectrometer. Spacial information is derived through a Fourier transform.

One of the key discoveries is that nerve fiber layer can continue thinning until it reaches a tipping point around 75% of normal when visual field loss begins.

“It’s not until you reach the tipping point that you start to see visual field abnormalities,” he said.

Also, Dr. Schuman and his colleagues found that after this tipping point, further nerve fiber layer thinning may no longer be detected, even though the patient’s visual field continues to decline. This can create a false sense of security that the patient’s disease itself has stabilized when it has not, he warned.

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“We need to be very careful,” he said.

The ganglion cell complex focal loss volume has proved the best predictor of visual field conversion, he said, citing a study of 513 glaucoma suspect eyes followed for an average of 52 months.

Eighty-two percent of visual field conversation was preceded by abnormal OCT findings. There was a lag of about 2 years between these abnormalities and visual field conversion.

OCT and glaucoma

OCT angiography is also uncovering important information about glaucoma. One recent discovery is that the macular superficial vascular complex low-perfusion area matches ganglion cell complex thinning and visual field defects, Dr. Schuman said.

Glaucoma mostly affects the superficial vascular complex, he said.

In measurements of vessel density, worse hemisphere superficial vascular complex vessel density provides the best diagnostic accuracy, Dr. Schuman said.

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In one recent study, worse hemisphere superficial vessel density had a 97% sensitivity at a 95% specificity compared with a 70% sensitivity for overall superficial vascular complex, 70% for overall all-plexus, 77% for overall ganglion cell complex thickness, and 87% for worse hemisphere ganglion cell complex thickness.

Also, recent research has found that peripapillary retinal perfusion defects match nerve fiber layer thinning and visual field defects. Glaucoma mostly affects the radial peripapillary capillary plexus, Dr. Schuman said.

Measurement of the radial peripapillary capillary plexus vessel density achieved a 90% sensitivity at a 95% specificity, compared with 87.5% for superficial vascular complex, vessel density 80% for all-plexus vessel density, and 87.5% for nerve fiber layer thickness.

Such discoveries are only the tip of an iceberg of knowledge gradually being brought to light by OCT, Dr. Schuman said.