Nancy Groves

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Physicians are increasingly relying on OCT to determine progression. It shows changes sooner and is an objective measure.


	
	Introduced nearly 30 years ago as a noninvasive technique for viewing the retina, 

 has established a definitive niche in the diagnosis and management of glaucoma.
	
	“We are relying more and more on OCT to try to determine progression because, one it shows changes sooner than visual fields, and two, it is an objective measure, rather than a visual field, which is a semi-objective measure,” said 

Duke Eye Center, and professor of ophthalmology, Duke University, Durham, NC

.
	
	There are a number of steps clinicians can take to obtain the most accurate data and draw appropriate conclusions from the scans, Dr. Asrani explained.
	
	Just as with visual fields, an important step is following a reliability checklist: make sure that the signal strength is good and that there is no tracking error; measure the slope of the 

retinal nerve fiber layer (RNFL) thickness

 over time to see if there is any unusual pattern of tissue loss; search for artifacts due to pathology; and determine if the RNFL loss matches the macular loss.
	
	Dr. Asrani cautions clinicians against overreliance on the technology. “Do not accept the software interpretation of stability of progression,” he said. “Confirm by examining the raw image. It takes time, but we owe it to our patients.”
	
	


	
	The parameters for structural change include focal loss in the RNFL profile and the average thickness in the superotemporal or inferotemporal sectors. If the average thickness change is greater than 10 Î¼m, then the progression is more likely to be real rather than an artifact. The other key parameter is total macular thickness. This is useful in all stages of glaucoma, but especially for advanced cases and in high myopes, Dr. Asrani said.
	
	“In those two conditions, the other parameters are not easy to use for detection of progression,” he added. Macular features to watch for are a change in an arcuate shape and whether it corresponds to the loss on the RNFL. For example, imaging may show RNFL thinning over time and a drop in average thickness of the sector; however, the global average thickness may not be changing.

“That is because there are certain areas where the RNFL appears to be slightly thicker, but that is averaged out and therefore the global average doesn’t change while the sectoral average does,” Dr. Asrani explained. “Try to confirm changes in the RNFL with macular thickness to be sure that the change is due to glaucoma,” he added. “If you are not sure, repeat the measurement in 4 months to confirm the changes.”
	
	

 he shared is to use caution with coexisting pathologies and be extremely careful about the effects of vitreous on the retinal tissues. The epiretinal membrane can create a hyper-reflective band along the internal limiting membrane, making it seem that the RNFL is thicker than it actually is. Dr. Asrani pointed out that, in many cases, the vitreous traction pulls on the RNFL, causing it to become thicker.
	
	“That gives you false reassurance that the patient’s glaucoma is not that bad because the nerve fiber layer appears normal in thickness,” he said. “But what happens is as the vitreous relaxes its traction on the nerve fiber layer, the nerve fiber layer thins out, and that appears like glaucoma progression. But when the nerve fiber layer changes do not match up with the macular changes, there is a high chance of an artifact.”
	
	Other misinterpretation artifacts can also arise, and clinicians should consider whether a pattern of change is nonglaucomatous. For example, if RNFL thickness decreases dramatically from one visit to another, it may be due to non-arteritic ischemic optic neuropathy having developed in the interim, which can be identified if the thickness loss is only in one sector.
	
	Misinterpretation is also possible when the RNFL is hollow because the axonal bundles have degenerated, leaving their sheaths intact and making the RNFL thickness seem normal. But when the structures collapse, the thickness drops dramatically.
	
	However, this is not a “real” change, because the axonal loss was pre-existing, which is a very subtle difference to detect. 

 requires extra vigilance in interpreting OCT scans, Dr. Asrani noted.
	
	“Because the RNFL is swollen, it appears thicker than it actually is, giving you a false sense of reassurance that the glaucoma is not bad,” he concluded. “As the inflammation is treated, the thickness of the RNFL significantly decreases, resulting in thinning that appears as if the glaucoma is getting worse. OCT must be used with extreme caution in patients with uveitis.”

Intravenous prostaglandin E1 infusion for acute central retinal artery occlusion


	P: 919/684-8656 E: sanjay.asrani@duke.com
	

This article was adapted from Dr. Asrani’s presentation at the 2019 meeting of the American Glaucoma Society. Dr. Asrani receives lecture honoraria from Heidelberg Engineering.

Articles

Technology makes great tool, requires extra vigilance with co-existing disease

OCT finds niche in glaucoma diagnosis, management

OCT imaging can prove to be a valuable tool for physicians going forward. The primary questions continue to be what each scan measures and how it can be used.


	
	Ophthalmology has come a long way since the first 

 images of the optic nerve and retina ex vivo were shown in 1991 by Huang et al.

, and ganglion cells are routine.
	
	Literature on OCT has also increased, with more than 2,500 publications in the field of ophthalmology by 2015. But for the average clinician, both the new technological capabilities and the abundance of reading material could be obscuring the core questions: What does each OCT scan measure? How can it be used clinically? How can multiple scans be integrated, and are there any special considerations?
	
	

, has some answers, starting with the ONH.
	
	


	
	A relatively new parameter is the BMO-MRW, the minimal rim width at Bruch’s membrane opening. This measurement does not depend on a reference plane because it measures the minimal distance between the internal limiting membrane and the termination of Bruch’s membrane, Dr. Shen explained.
	
	“It measures a neuroretinal rim with an anatomically accurate border, which is the Bruch’s membrane opening,” she said.
	
	This is different from the ONH parameters measured by the 

 Currently, automated MRW measurements are only available on the Spectralis OCT (Heidelberg Engineering). Related to MRW are the minimal rim area (BMO-MRA) and the minimal distance band, Dr. Shen said.
	
	There are other ONH parameters that do not depend on a reference plane. They are not commercially available at this time but have been well described in research work by several investigators. It is likely that these ONH parameters will become available to clinicians soon.
	
	A study by Gmeiner et al.

showed that BMOMRW measurements, along with RNFL thickness, are useful in differentiating preperimetric glaucoma from healthy controls.

BMO-MRW, BMO-MRA, and RNFL have shown comparable diagnostic power and significant correlation to Humphrey visual field mean deviation in eyes with mild to moderate glaucoma, although one study showed a weaker correlation to the visual field with BMO-MRW than with RNFL in

, Dr. Shen said.
	
	In advanced glaucoma, a study by Belghith et al.

 showed that structural loss was best detected by ganglion cell scan rather than MRW or RNFL, while a paper by Lavinsky et al.

 found that MRW could still be useful in eyes with advanced RNFL loss.
	
	When interpreting MRW measurements, Dr. Shen recommended looking closely for focal thinning on the scans, even when using automated software.
	
	Her research group has found that focal thinning of BMO-MRW is associated with paracentral visual field loss.

 The RNFL thickness measurements, which are familiar to many ophthalmologists, are useful in preperimetric, mild, and moderate glaucoma for both diagnosis and structure-function correlation.
	
	“We need to be aware of the floor effect in advanced glaucoma when the average RNFL thickness reaches 50-60 Î¼m,” Dr. Shen cautioned. “At this point, RNFL thickness may not change even if the patient is still getting worse.”
	
	In these cases, patients can be monitored with ganglion cell measurements of the macula instead of peripapillary RNFL. 

-inner plexiform layer (GCIPL) scan measures the cells and their axons in the macula, where more than half of all retinal ganglion cells are found.
	
	Studies have suggested that this scan is useful to detect progression in advanced glaucoma and may have better diagnostic power than RNFL in eyes with paracentral visual field loss.
	
	Ganglion cell scans have moderate diagnostic capability, comparable to that of RNFL, in preperimetric glaucoma, suggested by findings from a study by Rao et al.

Dr. Shen also noted that theses scans may be useful in diagnosing early glaucoma in myopic eyes

 and have good structure-function correlation with 10-2 visual field testing.


	
	She added that she often obtains both RNFL and GCIPL scans and reviews them together.

 sometimes yields better results than using only one technique.
	
	Dr. Shen cited research that combining BMO-MRW and RNFL showed slightly better diagnostic power for preperimetric glaucoma than ether parameter alone. Wide-field RNFL thickness mapping, which includes both peripapillary and macular regions, has the potential to offer better sensitivity in detecting preperimetric and early glaucoma than RNFL and GCIPL.


	
	Despite all of the advances in OCT technology, Dr. Shen cautioned that imaging can only be complementary to the clinician’s assessment of the

 and should never take its place.
	
	Dr. Shen used the example of a patient with multiple disc hemorrhages in the inferior optic nerve, while the OCT of the optic nerve head (BMO-MRW), peripapillary RNFL, and GCIPL were only demonstrating the existing structural damage to the superior part of the optic nerve and macula.
	
	“As we are getting better images of optic nerve head, RNFL, and ganglion cells and learning to integrate and to see the big picture, we should not forget about the details which brought us here in the first place,” Dr. Shen said.

Neuroprotective implant for treatment of glaucoma in phase II trial


	P: 617/573-3569 E: lucy_shen@meei.harvard.edu
	

This article is based on Dr. Shen’s presentation at the American Glaucoma Society 2019 annual meeting in San Francisco. Dr. Shen is a consultant for Topcon and has received research support from the company.

1. Huang D, Swanson EA, Lin CP, et al. Optical coherence tomography. 

2. Chen TC, Hoguet A, Junk AK, et al. Spectral-domain OCT: helping the clinician diagnose glaucoma: A report by the American Academy of Ophthalmology. 

3. Gmeiner JM, Schrems WA, Mardin CY, Laemmer R, Kruse FE, Schrems-Hosel LM. Comparison of Bruch’s membrane opening minimum rim width and peripapillary retinal nerve fiber layer thickness in early glaucoma assessment. 

4. Belghith A, Medeiros FA, Bowd C, et al. Structural change can be detected in advanced glaucoma eyes. 

5. Lavinsky F, Wu M, Schulman JS, et al. Can macula and optic nerve head parameters detect glaucoma progression in eyes with advanced circumpapillary retinal nerve fiber layer damage? 

6. Taniguchi EV, Woodward AM, Magnelli P, et al. Thin minimal rim width at Bruch’s membrane opening is associated with glaucomatous paracentral visual field loss. 

7. Nakanishi H, Akagi T, Hangai M, et al. Sensitivity and specificity for detecting early glaucoma in eyes with high myopia from normative database of macular ganglion cell complex thickness obtained from normal non-myopic or highly myopic Asian eyes. 

8. Rao HL, Addepalli UK, Chaudhary S, et al. Ability of different scanning protocols of spectral domain optical coherence tomography to diagnose preperimetric glaucoma. 

9. Raza AS, Cho J, de Moraes CG, et al. Retinal ganglion cell layer thickness and local visual field sensitivity in glaucoma. 

Despite advances, imaging can only be complementary to clinician’s assessment

Nancy Groves

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