Microperimetry (MP) was developed in the 1980s as a tool to measure retinal sensitivity at targeted points in the macula. Early systems were difficult to use, and data could not reliably be reproduced. The introduction of systems with eye-tracking features has resulted in a resurgence of interest in these systems for both clinical and research applications.

The challenge that MP was designed to address was the difficulty of assessing visual function in patients with retinal degenerative diseases.¹ Visual acuity is often not the best indicator of disease progression, particularly when a retina specialist is assessing retinal diseases in which visual acuity does not reflect the onset or the severity of the disease.² Microperimetry is a procedure that allows retina specialists to correlate macular pathology and corresponding functional abnormality.³

There are several commercially available MP devices. The key differences among them are the range of stimulus intensity, eye-tracker frequency, background illumination, and imaging features. The available options include the MP-3 (Nidek), iCare MAIA (macular integrity assessment; iCare), and the OptosOCT/SLO (Optos).⁴

APPLICATIONS OF MICROPERIMETRY

Fixation Testing

Fixation stability has been associated with visual acuity.⁵ It is reduced in eyes with macular pathology, and this can explain how patients with severe disease maintain good visual acuity.⁶ Using a fundus image to identify and track land marks, the portion of the retina the patient uses to see the stimuli is identified. The resulting scatter plot can then be analyzed to score fixation stability (Figure 1).

Currently, 2 methods are in use to report a patient’s fixation stability. The first measures the patient’s percentage of fixation points that fall within a circle that is 2 degrees or 4 degrees in diameter.⁷ Fixation is stable if 75% or more of the points land within a 2-degree circle; fixation is relatively unstable if less than 75% of the fixation points land within a 2-degree circle and 75% lands within the 4-degree circle; and fixation is unstable when fewer than 75% of points land within a 4-degree circle.⁷ This method is advantageous because of its clinical thresholds, but it has been criticized because some feel the 75% is too low.⁸

The second method is called the bivariate contour ellipse area. This method calculates the area and orientation of an ellipse that includes a prespecified percentage of the fixation points. A lower bivariate contour ellipse area indicates better fixation stability.⁶

Using MP with one of the above-described methods, one can find the area or areas of the retina that patients are using to function, known as the preferred retinal loci (PRL). Patients with macular disease use one or multiple of these PRL to compensate for loss of central function.^5,8 This information is useful when trying to develop a vision rehabilitation program.

Diabetic Retinopathy

Microperimetry has been used to evaluate retinal function across a wide array of questions in diabetic retinopathy. Studies have indicated that there is a decrease in retinal sensitivity in patients with diabetes before any structural changes are visible, suggesting underlying neuronal damage.⁹ Retinal sensitivity has been found to be altered by varying levels of glucose control. Loss of retinal sensitivity was found in patients with higher HbA1c levels.⁹

Diabetic macular edema clinical studies have used retinal sensitivity on MP as a way to stage and measure response to therapy.¹⁰ Fixation characteristics have been used, along with visual acuity improvement and reduction in central macular thickness on optical coherence tomography, to measure treatment response to intravitreal anti-VEGF therapy.¹¹

Age-Related Macular Degeneration

Microperimetry has been demonstrated to be a useful tool to help stage and measure the functional impact of age-related macular degeneration (AMD) on patients and can help retina specialists monitor disease progression. In studies evaluating the functional impact of AMD in early to intermediate disease, MP was found to be the most sensitive measure of disease severity compared to other traditional testing.¹² Microperimetry has also been helpful in following the impairment that more advancing disease can cause. Retinal sensitivity and fixation stability have been shown to deteriorate as the stage of AMD advances.¹³ Visual acuity, fixation stability, and PRL distance from the normal fovea have all been shown to worsen as the time from diagnosis of AMD increases. In combination with spectral-domain OCT, MP has been used to study the relationship between retinal morphology and function. Loss of the normal retinal architecture, as documented on OCT, results in measurable loss of function over the affected areas. ^14,15 In patients with geographic atrophy, one can measure a reduction in retinal sensitivity surrounding the areas of retinal pigment epithelium loss, giving us a better idea of the patient’s true functional impairment.^16,17 There is a rapid loss of retinal sensitivity at the margin of areas of geographic atrophy. Retinal sensitivity and fixation stability are currently being used as outcome measures in clinical trials for both surgical and medical treatment of AMD.^17,18 Mean retinal sensitivity was found to improve as retinal thickness deceased and worsened as retinal thickness increased on OCT in a trial evaluating anti-VEGF treatment of patients with wet AMD.^18,19

Vision Rehabilitation

Microperimetry currently is being used to help accurately measure a patient’s residual functional vision. To better design a rehabilitation program for each patient, the vision rehabilitation specialist first needs to understand the extent to which the patient’s disease is impacting visual function. This will help in teaching and coaching the patient how to quickly find their PRL and to help position this area of the retina around the scotomas to allow for the best visual function.⁵

CONCLUSION

Microperimetry can play an important role in understanding the functional vision status of the patient with retinal degeneration. Visual acuity alone is not always an accurate indicator of the true impact of the disease on the patient’s visual function. The ability to reliably measure and identify patients’ preferred retina loci, retinal sensitivity, and fixation stability gives retina specialists an opportunity to use this technology in both clinical and research settings. RP

REFERENCES

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