Microperimetry’s expanding role
Advances have made the technology tops for assessing visual function.
By Samuel N. Markowitz, MD, FRCSC
Technological advances in computing and eye tracking have made possible the evolution of scanning laser ophthalmoscopes into today’s microperimeters.
A patient’s visual field is still assessed with technology developed 50 years ago. Automated perimetry, while still considered the gold standard for assessing the visual field, is a deficient way to test the perimetry from many vantage points. Those with poor fixation stability cannot be tested, and current fixation stability standards are quite lenient.
But microperimeters could change this situation thanks to their technological abilities — these can accurately assess retinal sensitivity (or scotoma characteristics) and associated oculomotor components. In addition, in patients who have lost central vision, microperimeters can accurately evaluate eccentric preferred retinal loci (PRLs) used for viewing and replacing macular function.1
Over the last few years, some ophthalmologists have chosen microperimeters as the new modality for testing clinical visual functions and assessing functional vision (see “U.S. microperimeter overview,” page 49). Here, I discuss why that is happening, and the technology’s growing numbers of applications in clinical practice.
TECHNICAL ABILITIES OF MICROPERIMETERS
Retinal sensitivity
Assessment of residual retinal sensitivity with exquisite topographic sensitivity provides information on retinal areas with the highest sensitivity as well as on scotomas, whether relative or absolute.
Estimates of retinal sensitivity with microperimeters accurately represent a specific point on the retina chosen for testing. Microperimeters project stimuli directly onto the retina, correlating exact points of retinal sensitivity with retinal pathology. Microperimeters use eye-tracking technology, which compensates and corrects for eye movements, allowing accurate testing — and retesting — of virtually the same retinal point. Retinal sensitivity estimates are within one-half a degree or less of retinal location.
One should notice, however, that the decibel scales of various microperimeters are not directly comparable or interchangeable with current automated perimeter scales. To compare decibel scales between instruments, ophthalmologists must take into account equivalency between threshold sensitivity outputs. A conversion factor of 15 dB can be used in most cases in which equivalency comparison between instruments has been considered.2
Preferred real estate
Microperimeters can evaluate topographical fixation areas on the retina. Known as preferred retinal loci (PRLs), these represent fixation attempts during viewing of targets, either central or eccentric. PRLs occur naturally in most cases in which macular visual function is compromised or lost and then exhibit dynamic performances. Such PRLs can develop on any part of the residual retina; can be single or multiple; are apparently related to performance of specific tasks connected to activities of daily living; and offer superior visual function unmatched by any other retinal locus (Figure).
Microperimetry output features
FRLs
Microperimeters can also assess functional retinal loci (FRLs), the retinal areas representing residual function. FRLs comprise the retinal area with the highest residual retinal sensitivity and the PRL area associated with it.3
After initial adaptation to loss of macular function, it seems that one single FRL is used by the person for eccentric viewing. Such an FRL has an established location and function on the retina, and the patient summons its services on a continuing basis. Recent research shows that established PRLs apparently exhibit various overlapping fixation stability patterns for different types of tasks within the area of an FRL.4
Fixation stability
This assessment is based on accurate recording of fixation locations during the length of any viewing attempt. Using microperimetry, physicians can obtain accurate estimates of fixation stability obtained from fixation locations using raw data provided by calculating a bicurve ellipse area (BCEA). A study found the mean BCEA was to measure 0.053 deg2 (SD 0.022) for healthy subjects and 6.76 deg2 (SD 8.36 with a range from 0.21 to 31.85 deg) in AMD cases.5
| Nidek MP-1 | Optos OCT SLO | CenterVue MAIA |
|---|---|---|
• Integrates advanced fundus imaging with computerized perimetry.1 • An eye-tracking system continuously registers eye positions (25 times per second) relative to an anatomical landmark and compensates for stimulus projection location. • Automatically calculates and displays fixation stability estimates. • Provides accurate retinal sensitivity estimates. • Follow-up function allows automatic retest of patients at the same locations and with the same conditions defined in any previous microperimetry exam. • Fixation data registration with color fundus photography allows perimetry and fixation results to be displayed on a color photograph, which allows comparison of visual function to retinal structures. • Auditory biofeedback feature allows users to train a new fixation location. |
• Integrates spectral domain optical coherence tomography (OCT) and confocal scanning laser ophthalmoscopy (SLO).1 • Generates SLO and OCT images through the same optics and displays them simultaneously on the computer screen. • Users can map location of a PRL with a Fixation Test tool, which uses SLO/tracking to document where the patient fixates over a five-, 10-, 15- or 20-second time frame. • Automatically calculates and displays fixation stability estimates. • Accurately relates function to structure. • The correlation between measures of retinal sensitivity and structural abnormalities allows accurate topographic identification of disease and its extent. |
• Integrates a high frequency eye tracker and a line confocal SLO.1 • Eye tracking system continuously registers eye positions relative to anatomical landmarks and compensates for stimulus projection location. The eye tracker compensates for eye movements during testing and ensures point-to-point correspondence between the stimulus and the measured retinal location during the test and on subsequent tests. • Automatically calculates and displays fixation stability estimates. Microperimetry testing runs simultaneously with SLO imaging and can cover up to a 36-degree field of view with the newest 1.6.3 software version. • Macular integrity assessment module. • Instrument software, with normative database and statistical analysis module, is designed to identify the normal, age-related decrease in sensitivity and differentiate it from the pathologic changes associated with AMD and other retinal diseases. • Automatically calculates and displays a macular integrity index estimate. Auditory biofeedback feature allows users to train a new fixation location. |
Therapeutic benefits
Microperimeters can provide therapeutic benefits with biofeedback modules aimed at training refixation precision of eye movements. The eye follows an audio-visual biofeedback-training pattern until the eye reaches a more desirable fixation location.
CLINICAL APPLICATIONS
Monitoring glaucoma
Glaucoma patients are likely the largest target group who could benefit from microperimetry. Microperimetric assessment of retinal sensitivity was found to be at least equivalent to or superior to standard automated perimetry (SAP) at detecting early visual function loss.6
In cases with established glaucoma, records of scotoma sizes produced with the SAP are about half the size of the same scotoma recorded with microperimetry instruments.7
In the presence of macular-function loss, SAP produces scotoma records with the displaced scotoma location, as fixation during testing is performed using the PRL, for which SAP cannot account.8
Retina cases
Reduced retinal sensitivity is positively correlated to areas with pathologic changes, such as drusen or retinal pigment epithelial layer changes.9 Also, in patients with AMD, one study showed a significant positive relationship between outer segment layer thickness values on OCT and reduced retinal sensitivity.10 Highest correlations were reported between reduced retinal sensitivity and retinal pigment epithelium, photoreceptor layers disruptions and macular edema.11 Another study showed a significant relationship between photoreceptor layer thickness values on OCT and reduced retinal sensitivity in patients with Stargardt disease.12 Positive correlation between changes in retinal sensitivity and disease progression was documented in geographic atrophy after treatments for retinal neovascularization.13,14
Low vision rehabilitation
Microperimetry allows detection and characterization of PRLs in cases with macular function loss. In AMD cases, the majority of PRLs occur on the retina’s upper and right quadrants, which correspond to the inferior and left parts of the visual field.15 In Stargardt disease and other macular dystrophies, most PRLs tend to be localized on the upper retina.16 Accurate PRL data facilitates rehabilitation of residual vision with prisms and eye movement training.17
Fixation stability is evolving as a reliable outcome measure with predictable attributes to visual acuity. This is due to a proven and strong relationship between fixation stability estimates and visual acuity measurements.18 Also, biofeedback modules attached to microperimeters can be used for PRL rehabilitation. PRLs might be useless in about one-quarter of low-vision patients because they develop on an unfavorable location on the retina. Relocating the PRLs to a favorable location significantly improves visual functions.19
CONCLUSION
Going mainstream
Microperimeters offer advanced technology to diagnose and improve visual functions. For general ophthalmology, many indicators point to microperimeters taking the lead role played by SAP in clinical practice while providing additional information previously available only in labs. For vision-rehabilitation practitioners, the advent of microperimeters portends the introduction of modern rehabilitation concepts in most low-vision clinical practices. The future is here, but there is probably more to come. OM
REFERENCES
1. Markowitz SN, Reyes SV. Microperimetry and clinical practice: an evidence based review. Can J Ophthalmol. 2013;48:350-357.
2. Springer C, Bültmann S, Völcker HE, Rohrschneider K. Fundus perimetry with the Micro Perimeter 1 in normal individuals: comparison with conventional threshold perimetry. Ophthalmology. 2005;112:848-858.
3. Shima N, Markowitz SN, Reyes SV. The concept of a functional retinal locus in age-related macular degeneration. Can J Ophthalmol. 2010;45:62-66.
4. Markowitz SN, Reyes SV, Shima N. Functional retinal locus rather than multiple PRLs? Invest Ophthalmol Vis Sci. 2011;52:1191; author reply 1191.
5. Tarita-Nistor L, González EG, Markowitz SN, Steinbach MJ. Fixation characteristics of patients with macular degeneration recorded with the MP-1 microperimeter. Retina. 2008;28:125–133.
6. Lima VC, Prata TS, De Moraes CG. A comparison between microperimetry and standard achromatic of the central visual field in eyes with glaucomatous paracentral visual-field defects. Br J Ophthalmol. 2010;94:64-67.
7. Lee K, Markowitz SN. Scotoma size reduction as an adaptive strategy in age-related macular degeneration. Can J Ophthalmol. 2010;45:393-398.
8. Markowitz SN, Aleykina N. The relationship between scotoma displacement and preferred retinal loci in low-vision patients with age-related macular degeneration. Can. J. Ophthalmol. 2010:45:58-61.
9. Midena E, Vujosevic S, Convento E, et al. Microperimetry and fundus autofluorescence in patients with early age-related macular degeneration. Br J Ophthalmol. 2007;91:1499-1503.
10. Acton JH, Smith RT, Hood DC, Greenstein VC. The relationship between retinal layer thickness and the visual field in early age-related macular degeneration. Invest Ophthalmol Vis Sci. 2012;53:7618-7624.
11. Sulzbacher F, Kiss C, Kaider A, et al. Correlation of SD-OCT features and retinal sensitivity in neovascular age-related macular degeneration. Invest Ophthalmol Vis Sci. 2012;53:6448-6455.
12. Testa F, Rossi S, Sodi A, et al. Correlation between photoreceptor layer integrity and visual function in patients with Stargardt disease: implications for gene therapy. Invest Ophthalmol Vis Sci. 2012;53:4409-4415.
13. Meleth AD, Mettu P, Agron E, et al. Changes in retinal sensitivity in geographic atrophy progression as measured by microperimetry. Invest Ophthalmol Vis Sci. 2011;52:1119-1126.
14. Parravano M, Oddone F, Tedeschi M, et al. Retinal functional changes measured by microperimetry in neovascular age-related macular degeneration treated with ranibizumab: 24-month results. Retina. 2010;30:1017-1024.
15. Fletcher DC, Schuchard RA. Preferred retinal loci relationship to macular scotomas in low-vision population. Ophthalmology. 1997;104:632-638.
16. Messias A, Reinhard J, Velasco e Cruz AA, Dietz K, MacKeben M, Trauzettel-Klosinski S. Eccentric fixation in Stargardt’s disease assessed by Tübingen perimetry. Invest Ophthalmol Vis Sci. 2007;48:5815-5822.
17. Markowitz SN, Reyes SV, Sheng Li. The use of prisms for vision rehabilitation after macular function loss: an evidence based review. Acta Ophthalmologica. 2013;91:207-211.
18. Tarita-Nistor L, González EG, Mandelcorn MS, Lillakas L, Steinbach MJ. Fixation stability, fixation location, and visual acuity after successful macular hole surgery. Invest Ophthalmol Vis Sci. 2009;50:84-89.
19. Tarita-Nistor L, González EG, Markowitz SN, Steinbach MJ. Plasticity of fixation in patients with central vision loss. Vis Neurosci. 2009;26:487-494.
About the Author | |
| Dr. Markowitz is professor of ophthalmology and vision sciences and director, Low Vision Service, (University Health Network Hospitals), Department of Ophthalmology and Vision Sciences, University of Toronto, Toronto, Ontario, Canada. Contact him at snm1@rogers.com. The author reports no relevant financial disclosures. |
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