November 2016




Device focus

What cataract surgeons can learn from an OCT

by Michelle Dalton EyeWorld Contributing Writer


Glaucoma specialists weigh in on what cataract surgeons need to know about these devices

When it comes to identifying abnormalities in the eye and how those abnormalities progress over time, optical coherence tomography (OCT) is currently the most precise and sensitive of tests available. This technology has helped detect early disease where visual field testing could not, and glaucoma specialists recommend its use in some cataract patients as well. OCT provides an optical cross-section of the optic nerve and the structures around it, with objective, precise measurements of those structures that are not possible with other instruments that offer more of an estimate of tissue thickness. “If there’s a discrepancy in the visual acuity and cataract grade, or the view is quite poor, then I recommend obtaining a macular OCT,” said Sanjay Asrani, MD, professor of ophthalmology, head, Duke Glaucoma OCT Reading Center, and clinical director, Duke Eye Center of Cary, Duke University School of Medicine, Durham, North Carolina.

There are two commercially available OCTs—time-domain and spectral-domain—and a third (swept-source) that is not yet commercially available. Spectral-domain offers better resolution, but both commercially available technologies can help confirm a glaucoma diagnosis, said Doug Rhee, MD, chairman, Department of Ophthalmology and Visual Sciences, University Hospitals Cleveland Medical Center, visiting professor of ophthalmology, Case Western Reserve University School of Medicine, and director, Eye Institute, University Hospitals, Cleveland.

“I find OCT to be most useful to elucidate pre-perimetric, otherwise known as early, glaucoma, in which there is only structural change (optic nerve) but not yet functional (visual field) change,” he said.

Disease progression has routinely been determined by visual field; most of the randomized controlled clinical trials used optic nerve photographs evaluated by reading centers. But whether photographs or OCT are as good or better over time remains unknown. For that reason, Dr. Rhee obtains OCTs at initial diagnosis and then yearly to monitor progression.

OCT is used for quantitative analysis of the circumpapillary retinal nerve fiber layer (RNFL) thickness, as well as the ganglion cell layer segmentation analysis in the macular region, said Vikas Chopra, MD, medical director, University of California Los Angeles Doheny Eye Center, Pasadena, California, associate professor of clinical ophthalmology, David Geffen School of Medicine at UCLA, and associate medical director, Doheny Image Reading Center, Doheny Eye Institute. In his academic setting, OCT is also used to “determine correlation with any functional defects seen on visual field testing. Depending on the severity of the patient’s glaucoma, the OCT is performed between one and two times per year to develop a dataset that is used for longitudinal progression analysis.”

When to acquire an OCT

At Doheny Eye Institute, “practically every cataract or glaucoma patient gets a spectral-domain OCT scan as part of the initial evaluation,” Dr. Chopra said.

Patients undergoing evaluation for cataract surgery can benefit from having a macular OCT “to evaluate patients for any preexisting macular pathology, especially the presence of epiretinal membrane and/or macular edema if the patient has diabetes or evidence of diabetic retinopathy,” Dr. Chopra said.

OCT can help estimate visual field deficits, Dr. Asrani countered. In patients with “pure cataract,” and a confusing VA, OCT can be invaluable.

“For example, if the patient’s visual acuity is 20/100, but the cataract is only 2+, I would expect the VA to be at least 20/40 or 20/50,” he said. “If I get a macular OCT, it’s very useful for me to give the patient realistic expectations of visual acuity after the cataract surgery.” Dr. Rhee discourages basing a glaucoma treatment regimen on OCT alone, noting the visual field test, health of the optic nerve, and intraocular pressure are equally important. But it may be the perfect time to begin discussions about a combined cataract/glaucoma surgery and why that option may be better for the patient, especially in patients with advanced glaucoma, Dr. Asrani said. “These patients cannot tolerate pressure spikes after cataract surgery,” he said. Those patients would benefit from optic nerve OCTs.

Using OCT in practice

Dr. Asrani likes to use OCT images to manage patient expectations. If there’s an epiretinal membrane or macular hole, he can use the macular images to explain why simultaneous retina/cataract surgery may be warranted, or why prolonged nonsteroidal anti-inflammatory drug therapy after surgery is going to be warranted. “Whatever we can do to manage our cataract patients’ expectations before surgery will be substantially more valuable than trying to reset expectations after the surgery,” he said. “If the cataract is too dense to see the macula, you need that OCT to determine what else—if anything—may be going on that impacts vision.” In early stages of primary open-angle glaucoma, OCT is a useful way of monitoring for progression “since optic disc and RNFL loss can precede development of glaucoma-related visual field defects by many years,” Dr. Chopra said. “Furthermore, in patients with ‘suspicious’ optic discs, OCT RNFL and ganglion cell analysis can provide evidence to support clinical impression. OCT allows a clinician to truly distinguish ‘glaucoma suspects’ from pre-perimetric glaucoma.” If cataract surgeons suspect glaucoma, obtaining an OCT of the RNFL with near normal results may help rule out the need for simultaneous surgery. “If the glaucoma has not yet started to affect the superior pole of the optic nerve, we can—at the minimum—tell patients they have glaucoma and will need to be managed for that disorder after the cataract surgery,” Dr. Asrani said.

Cataract surgeons need to recognize artifacts and when to discard the OCT as a result, Dr. Rhee advised.

“For example, if time-domain OCT is not centered on the optic nerve, there will be all kinds of changes [artifacts] that are attributed to glaucoma disease progression, but are really because the OCT has been misaimed,” he said. Training the techs to recognize a misaligned OCT is relatively easy—there will be a circle on the optic nerve. If it’s not centered, then the technician needs to retake the test. Spectral-domain has eliminated most of these types of artifacts, but they still present on occasion.

For cataract surgeons who prefer to bring in glaucoma colleagues to interpret the OCT images, Dr. Asrani recommends sending both the report and the raw images. “When a cataract surgeon sees significant cupping in the optic nerve, or the cupping is toward the upper part of the optic nerve (that is, toward the superior aspect of the optic nerve), those patients would benefit from a glaucoma consultation,” he said. “Superior cupping tends to correlate with loss of vision in the lower part of the field. If the cupping is extensive, that may affect visual acuity as well.” Unless cataract surgeons are skilled at observing the extent of cupping, “it’s not reasonable to assume the acuity is down because of the cataract alone. That may lead to a very disappointed patient when the visual acuity does not improve after cataract surgery,” Dr. Asrani said.

Dr. Asrani added, however, that if the OCT cannot confirm any pathology, “you can’t charge for them; there is no reimbursement if there is no pathology.” Sometimes, however, absorbing the cost of the test is worthwhile if it means the surgeon is more confident about the patient’s pathology.

Pearls for use

One of the potential pitfalls of OCT (and any technology for that matter) is that “it can be challenging to use for the long-term progression analysis since the hardware and software are always updated and are often not ‘backward compatible,’” Dr. Chopra said. As an example, Dr. Chopra noted patients with primary open-angle glaucoma scanned on time-domain devices 10 years ago have a different dataset than that provided by the current generation spectral-domain OCT devices. “Before long, the even newer swept-source OCT devices may provide datasets that are not fully comparable to the datasets from the current devices. For this reason, many glaucoma specialists still consider fundus/optic disc stereo photographs the ‘gold standard’ for long-term evaluation of glaucoma,” Dr. Chopra said.

A key pearl for successful use of OCT in cataract patients is to make sure the OCT is free from artifacts, Dr. Chopra said. “Many reports of the RNFL or the nerve fiber layer do show the raw image on the same page. You can see if the OCT software has correctly identified the nerve fiber layer or if it has misidentified the boundary before you look at the report. Most people go straight to the red or the green, with green serving as normal and red as highly abnormal,” he said. “But did the software do a good job? Is there any other pathology on the surface of the nerve fiber layer that is causing a warped appearance? Maybe severe myopic changes—or maybe epiretinal membranes on the nerve fiber layer—can give a misinterpretation by the software.” Another pearl is to instill preservative-free artificial tears in patients’ eyes before performing the scan, “especially in patients with ocular surface disease, which is highly prevalent in the glaucoma population,” Dr. Chopra said. Although a majority of the time the OCT device can provide high quality scans in undilated eyes, there will be some patients who need to have a well-dilated pupil to obtain a good scan.

With the detailed imaging OCT can provide, cataract surgeons should not hesitate to use the diagnostic tool in cases of suspected concurrent glaucoma, when the macula cannot be viewed, or to confirm any other retina abnormality that may impact visual acuity outcomes after surgery.

Editors’ note: Dr. Asrani has financial interests with Heidelberg Engineering (Heidelberg, Germany). Drs. Chopra and Rhee have no financial interests related to their comments.

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