Microperimetry

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Article initiated by:

Maria Filipa Cunha Madeira

All contributors:

Leo A. Kim, MD, PhD, Maria Filipa Cunha Madeira

Assigned editor:

Brittni A. Scruggs, MD, PhD

Review:

Assigned status Update Pending

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Contributing Editors:	add

Description/Overview

Microperimetry is a visual field test that incorporates perimetry and retinal imaging. It allows for the direct mapping of the stimulus in the region of interest, thereby correlating functional information (visual field testing) with structural/anatomical data (retinal imaging). Other designations for microperimetry include fundus-related perimetry or macular perimetry.

Indications

Microperimeters are the method of choice for residual visual function assessment. They have been used for the study of retinal macular disorders (age-related macular degeneration [AMD], diabetic macular edema (DME), macular dystrophies). Low vision training also benefits from microperimetry. The identification of eccentric fixation locations, know as preferred retinal loci (PRL), and fixation-stability estimates at those sites allows the practitioner to use the best residual visual function available for rehabilitation.

Procedure

As in conventional perimetry, visual field testing starts by determining the patient’s light sensitivity by finding the minimal light intensity stimulus that the patient can recognize when spots of light stimulate specific locations of the retina. Each software has a standart exam, that can be adapted to the patient (variations of diameter area and number of measurement points). Of note, care is needed when comparing results from different devices. Maximum luminance may vary between devices and the Decibel scale is relative to that value.

Constant tracking of the eye during microperimetry allows for real-time correction of retinal movement and evaluation of fixation. Data regarding PRL may be presented in two ways: as the percentage of fixation points inside circles centered by retinography or by mathematical calculation of the area and orientation of the best elipse that describes such points, the bivariate contour elipse area (BCEA). The first method can be used to produce a clinical classification, as suggested by Fuji et al. (table 1 and 2). Whilst not providing a classification, the second technique has the advantage of giving a more accurate and reproducible fixation stability measurement.

Table 1 - Classification of location of fixation, by Fuji et al.

Classification of location of fixation	Percentage of preferred fixation points located within 2º-diameter fovea centered circle
Predominantly central fixation	>50%
Poor central fixation	25-50%
Predominantly eccentric fixation	<25%

Table 2 - Classification of fixation stability, by Fuji et al.

Classification of fixation stability	Percentage of preferred fixation points located within 2º- and 4º-diameter circles centered in the gravitational center of all fixation points
Stable fixation	>75% within 2°-diameter circle
Relatively unstable fixation	< 75% within 2°-diameter circle, but > 75% within 4°-diameter circle
Unstable fixation	< 75% within 4°-diameter circle

The retina may be imaged through an integrated color fundus camera (e.g., MP-3) or scanning laser ophthalmoscopy (SLO) (e.g., MAIA and Optos OCT-SLO), depending on the choice of device.

After completion of measurements, the perimetry grid is superimposed on the retinal image facilitating evaluation of a specific region of interest. Frequently, the Decibel scale is color coded according to normative studies and summarized indices that describe the mean macular sensitivity data, adjusted for age and/or gender normative data (e.g., Macular Integrity Index from MAIA) are displayed to aid with interpretation.

Devices

The first microperimeter (SLO101) was manufactured by Rodenstock Instruments (Munich, Germany) in 1982. Fundus mapping was based on SLO technology and a modulated helium-neon laser (633nm) was used to manually generate background and stimulus illumination in the 33x21º central field. It lacked an eye-tracking capability and the stimulus were presented in a semi-automated fashion.

MP-3 and MP-3 type S (scotopic) microperimeters, from NIDEK. [Reproduced with authorization]

ICare MAIA confocal microperimeter, from CenterVue [Reproduced with authorization]

To date, there are three microperimeters available on the market: the MP-3 by Nidek Technologies (Padua, Italy), the OCT-SLO from Optos (Marlborough, USA) and the MAIA microperimeter from CenterVue (Padua, Italy). Albeit different technical specificities, their functions are similar. Relevant particularities include that Optos OCT-SLO offers the possibility of superimposing functional deficits with cross-sectional OCT retinal images (besides en face images) and MP-3 and MAIA allow for scotopic microperimetry, performed under scotopic luminance conditions.

Benefits

When compared to conventional perimetry, microperimetry offers the following advantages:

• Real-time detection and correction of testing in a given retinal location (stimuli are directly projected onto the retina with eye-tracking feedback, allowing for testing-retesting).
• Direct correlation between fundus morphology and light sensibility, by presenting the light stimulus in a particular region of interest (e.g., small scotoma created by a macular scar).
• Continuous eye-tracking makes it the preferred exam when foveal function or fixation stability is compromised (it does not assume a stable foveal fixation, like conventional perimetry).
• Biofeedback for training in low vision patients (e.g., relocation of a PRL to a TRL [trained retinal loci] predetermined by the practitioner to improve functional visual outcomes).

Clinical applications of microperimetry include, but are not limited to:

• Atrophic and neovascular AMD: functional assessment of vision quality; identification of central scotoma; diminished macular sensitivity in AMD correlates with disease severity and progression; these alterations are also observed at a microstructural level (morphofunctional correlation), with worst macular sensitivity in areas of retinal pigment epithelium-drusen complex, pigment epithelium detachment, subretinal fluid and geographic atrophy; anti-VEGF therapy efficacy assessment.
• DME: diminished macular sensitivity in DME correlates with degree of macular edema; evaluation of different laser modalities on macular function.
• Glaucoma: although standart automated perimetry remains the gold standart for monitoring glaucoma, detection of nerve fiber layer defects and eccentric fixation in advanced glaucoma can be enhanced by microperimetry.
• Low-vision patients: rehabilitation with microperimetry biofeedback in patients with central scotoma has demonstrated to improve fixation stability, visual function and quality of life.
• Any other retinal pathologies affecting macular structure and/or function: central serous corioretinopathy, hydroxychloroquine maculopathy, rod-cone alterations, macular hole and epiretinal membrane…

Limitations

As with most diagnostic tests, patient cooperation is necessary, especially since it relies on patients answers to identify light sensibility (e.g., it is subject to false negative/positive answers). Because it is a relatively recent technology it is not yet in widespread clinical use. In a study of their own experience, Romaniuk et al. pinpoint long examination time in selected strategies (fatigue effect) and cost as microperimetry’s drawbacks.