OCT devices increasingly finding use outside retinal practice

Optical coherence tomography has changed the way retina specialists see the back of the eye and, in turn, the way they treat conditions that affect the retina, vitreous and other structures in the posterior segment. Now that the technology is being incorporated into use in other parts of the eye, ophthalmologists may find themselves rethinking how they treat eye conditions such as glaucoma, uveitis and cataract.

The addition of Fourier-domain principles has resulted in spectral-domain OCT with significantly faster scanning speeds compared with time-domain OCT, and the addition of more powerful light sources has yielded greater image resolution. Those additions have allowed better depiction of ophthalmic structures and have generated interest in using OCT to scan other parts of the eye.

Recent research has suggested that OCT can be used to grade cataracts and anterior uveitis. Glaucoma specialists have started to adopt OCT because it can potentially quantify change of the optic nerve over time. Additional enhancements in OCT hardware and software are further improving imaging capability and, as a result, functionality.

What is driving this cross adoption of OCT technology? According to Alexander C. Walsh, MD, of the Doheny Eye Institute, just as retina specialists came to appreciate the availability of objective data in the diagnosis of retinal conditions, subspecialists practicing in other parts of the eye are realizing that OCT images capture data on anatomical structures that can be referenced and compared with images taken at the next exam.

Alexander C. Walsh, MD, says that OCT provides quantitative analyses that will become critical in future eye examinations. Image: George DeLoache Photography

“I have patients with OCTs over the course of 5 or 6 years, and I can go back and see the status of the retina over 5 and 6 years in a way that I can’t do with my exams,” Dr. Walsh said. “I think the longitudinal capabilities of OCT in terms of quantitative analysis are just now becoming evident to a lot of people and will hopefully dominate the future of the clinical exam.”

Glaucoma

Because it directly images the retinal nerve fiber layer, optic nerve and macula, glaucoma practice was an early adopter of OCT technology. According to Joel S. Schuman, MD, of the University of Pittsburgh School of Medicine and a co-inventor of OCT, imaging helps the clinician appreciate fine architectural changes that might be missed on clinical exam or with other testing modalities.

“In particular, the retinal nerve fiber layer is well-suited to evaluation by OCT,” Dr. Schuman, an OSN Glaucoma Board Member, said.

The evolution of OCT technology from time-domain to spectral-domain has the potential to enhance the usefulness of imaging in glaucoma care. The elimination of the moving reference mirror has allowed faster scanning speeds that result in images that are less sensitive to eye movement and less prone to artifact degradation. The addition of wider bandwidth light sources has further improved axial and transverse resolution, which yields images reassembled from a larger data set of scans.

That larger data pool of images from various points of the eye — instead of centered on the macula as in time-domain devices — allows for creation of three-dimensional volumetric measurements. The clinician can look beyond the circumpapillary retinal nerve fiber layer and also at subtle changes in the fiber and tissue layers surrounding the optic nerve that may be damaged in glaucoma.

“You can look for characteristic nerve fiber layer abnormalities that are seen in glaucoma and these will show up as arcuate abnormalities — deviations from normal — on the deviation map of the spectral-domain OCT,” Dr. Schuman said.

A second benefit of the larger data pool available with spectral-domain as compared with time-domain OCT imaging is the increased potential for reproducibility. “That gives the promise to better track progression to detect change over time, and that is really a big advance in terms of changes from time-domain OCT,” Dr. Schuman said.

Joel S. Schuman

The introduction of spectral-domain OCT and, overall, the rapid adoption of imaging into retinal practice spurred market interest in software upgrades, also of benefit in glaucoma treatment. According to Dr. Schuman, technological improvements with spectral-domain technology allowed for creation of statistical software that had additional capability to quantitatively assess patients.

But however promising the potential of spectral-domain OCT is, the use of diagnostic imaging remains somewhat controversial in glaucoma. Although the technology was invented nearly 2 decades ago, the first time-domain device was released in 2002. Spectral-domain technology, and with it the ability to chart disease or treatment progression longitudinally, was not commercially available until 2007, and so the scientific literature is somewhat limited in verification studies. Specifically, the correlation between changes seen on OCT exams and actual disease has not been definitively established.

Screening for glaucoma remains controversial because current OCT devices are not capable of establishing a three-dimensional view of the angle. In addition, there remain unanswered questions about whether subtle glaucomatous changes can be recognized on OCT exam and how they correlate to disease, Dr. Walsh said.

“An instrument that can measure perimetry, can measure angle geometry, can measure the pressure in the eye, can measure the pupillary reactions, can measure the nerve layer thickness and the optic nerve topography — I think all those things together give us many points that we would need, potentially, to screen for glaucoma,” Dr. Walsh said.

Although Dr. Schuman believes in the utility of OCT to track glaucoma progression, “I believe we owe it to our community and our patients to validate that hypothesis, and so we need to make sure that the progression we are measuring is real progression that represents a true change in the status of the patient,” he said.

Global blindness

Ophthalmologists are finding uses for OCT in other parts of the eye because that it is a minimally invasive modality potentially capable of in vivo biopsy. Where that potential will lead is uncertain, but some envision that OCT may dramatically affect how care is delivered to underserved populations.

According to Dr. Walsh, OCT has potential to deliver ophthalmic care in developing nations because it is easy to operate. Currently, he said, global blindness disproportionately affects developing nations that are underserved by ophthalmologists. In sub-Saharan Africa, for instance, there are about 500 ophthalmologists, or about one per 1 million people, and similar disparity exists in places such as India and rural China.

Even in the U.S., the number of patients per ophthalmologist is growing, particularly in the elderly population. As the ophthalmic community looks for answers to increased demand and need in the face of a diminishing supply of physicians, OCT may offer a way to improve efficiency at a relatively low cost.

“The major advantage of OCT to me is that it is really, truly non-mydriatic and can potentially be operated by an individual without ancillary personnel being involved. With that, you can begin to touch hundreds of millions of people around the world,” Dr. Walsh said. “The one doctor-one patient paradigm was great in the past but is not going to scale to the entire world and may need to be re-thought.”

An intriguing concept

Teleophthalmology, the idea of an ophthalmologist remotely diagnosing a patient’s condition, has been hailed as a potential way to extend care to currently underserved populations. In diseases such as retinopathy of prematurity, on-site screening with remote interpretation has been used as a way of extending expertise on an infrequently occurring disease, thereby reducing cost at the local center while also insulating the center against a missed diagnosis and potential litigation.

As far as a screening method for the major diseases causing global blindness, however, teleophthalmology may have limited applicability. According to Dr. Walsh, transmission of large images requires high-speed Internet access and a person available to receive images so that patients do not get lost to follow-up. These two factors add cost and logistical concerns that might limit its adoption.

“I believe that point-of-care diagnostics is the future for this. You get an immediate result,” Dr. Walsh said. “With the new OCT light sources that are now available and are relatively inexpensive, there may now be a way to do this in a way that we never could before, even a year ago.”

One intriguing possibility that Dr. Walsh is now working on is a scanning device that combines the diagnostic tests used in various parts of the eye with a hand-held device, much like a pair of binoculars, to test for visual acuity, refraction, biomicroscopy, extraocular motility and alignment, perimetry, keratometry, gonioscopy, pupillometry, Amsler grid, reading speed, color testing, stereo acuity and exophthalmometry.

Dr. Walsh said that he has designed the schematics of such a device and is looking to secure funding for building a prototype. The device, he said, is designed to use components that already exist and that have been tested in either clinical or research OCT devices. Even if the concept does not come to fruition, Dr. Walsh sees benefit in challenging ophthalmologists to reconsider how the eye is examined, with the end goal of improving diagnostic acumen to benefit patients.

“I’d like to get rid of slit-lamp biomicroscopes. I think they have served us well for 100 years, but it’s an archaic technology that doesn’t do a whole lot of the things we need to do in modern medicine,” Dr. Walsh said.

An evolving technology

In the decade since OCT first became available on the market, it has become entrenched as an essential diagnostic and management tool. The success that it has enjoyed has prompted market interest in expanding utility to other parts of the eye.

“OCT has higher resolution than any of the anterior segment modalities,” David Huang, MD, PhD, of the Doheny Eye Institute, said. “I think eventually any function that a topography, ultrasound or slit scanning can do, OCT can do better.”

David Huang

If OCT imaging is still in its infancy in the posterior segment, it is even more nascent in anterior segment imaging. Several of the Fourier-domain devices currently on the market have an anterior segment application, with some requiring add-on lenses to view the cornea and surrounding structures. However, software is limited, and so is actual clinical application, to say little of verification and correlation to clinical exam.

Dr. Huang, who co-invented OCT as part of his doctoral research, is currently working on several software applications that could expand OCT in the anterior segment. His research includes corneal power calculation, IOL power formula for post-LASIK cataract surgery and pachymetry mapping for keratoconus screening and diagnosis.

Another application that he is exploring is grading the angle opening at the Schwalbe’s line (termination of corneal endothelium) with high-resolution OCT images. This will aid the adaption of Fourier-domain OCT in narrow-angle glaucoma diagnosis.

OCT is sometimes viewed as inferior to ultrasound in anterior segment applications because the latter can penetrate deeper beyond the iris and sclera. However, in most anterior segment applications, such deep penetration is not necessary, and some ophthalmologists prefer OCT for its higher resolution and noncontact nature.

For instance, the fine resolution capable with OCT may be able to precisely depict the tear meniscus for diagnosis of dry eye. The ease of use may also be a benefit to surgical planning in refractive procedures: determining whether there is adequate corneal thickness to cut a flap, screening for possible keratoconus by detecting focal or asymmetric thinning and gauging residual stromal thickness before LASIK re-treatments. Dr. Huang said he also uses OCT in pathologic cases for measuring the opacity depth and epithelial thickness in planning transepithelial phototherapeutic keratectomy procedures.

“I most often use it to plan LASIK because I think the corneal thickness map is more accurate than the Orbscan map I have,” Dr. Huang said. “In fact, I routinely scan the cornea 1 week post-LASIK to see how deep my microkeratome is cutting.”

The future potential of OCT is being investigated now. Glaucoma researchers are looking to the technology as a way to screen patients for narrow angles and to visualize ganglion cell loss in glaucoma, among other potential applications. One of the criticisms of OCT use in glaucoma is that the device cannot adequately image finer structures involved in aqueous outflow, but early studies have suggested that anatomical features such as Schlemm’s canal can be imaged with spectral-domain OCT, which may introduce a key marker for tracking therapeutics or for surgical planning.

Key software improvements will inevitably expand the usefulness of current technology. Likewise, a new package of OCT hardware applications could further expand the imaging capabilities in the eye. Intraoperative OCT, swept-source OCT and Doppler OCT each represent technological advances that may significantly improve diagnostic capabilities and, as a result, treatment effectiveness in ophthalmology. – by Bryan Bechtel

How has the introduction of spectral-domain technology changed how ophthalmologists use OCT?

References:

• Chen TC. Spectral domain optical coherence tomography in glaucoma: qualitative and quantitative analysis of the optic nerve head and retinal nerve fiber layer (an AOS thesis). Trans Am Ophthalmol Soc. 2009;107:254-281.
• Ide T, Wang J, Tao A, et al. Intraoperative use of three-dimensional spectral-domain optical coherence tomography. Ophthalmic Surg Lasers Imaging. 2010;41(2):250-254.
• Kagemann L, Wollstein G, Ishikawa H, et al. Identification and assessment of Schlemm’s canal by spectral domain optical coherence tomography [published online ahead of print March 17, 2010]. Invest Ophthalmol Vis Sci. doi:10.1167/iovs.09-4559.
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• Tan O, Chopra V, Lu AT, et al. Detection of macular ganglion cell loss in glaucoma by Fourier-domain optical coherence tomography. Ophthalmology. 2009;116(12):2305-2314.
• Wykoff CC, Berrocal AM, Schefler AC, Uhlhorn SR, Ruggeri M, Hess D. Intraoperative OCT of a full-thickness macular hole before and after internal limiting membrane peeling. Ophthalmic Surg Lasers Imaging. 2010;41(1):7-11.
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• David Huang, MD, PhD, can be reached at Doheny Eye Institute, 1450 San Pablo St., DEI 5702, Los Angeles, CA 99033; e-mail: dhuang@usc.edu.
• Joel S. Schuman, MD, can be reached at UPMC Eye Center, 203 Lothrop St., Suite 816, Pittsburgh, PA 15213; 412-647-2205; fax: 412-647-5119; e-mail: schumanjs@upmc.edu.
• Alexander C. Walsh, MD, can be reached at Doheny Eye Institute, 1450 San Pablo St., DEI 4300, Los Angeles, CA 90033; 323-442-6397; e-mail: awalsh@doheny.org.