New Technology Update: NIDEK MP-1 MicroPerimeter
Tracking fixation function places device in a category all its own.
ROCHELLE NATALONI, CONTRIBUTING EDITOR

Figure 1

A sophisticated retina treatment minus effective evaluation equals an unmet need. The MP-1 MicroPerimeter manufactured by NIDEK solves that equation, according to retinal specialists who say the device does not replace outmoded technology, but rather facilitates acquisition of patient data that simply could not be gathered before. "Microperimetry gives us a way to map scotomas in and around the macula and to very sensitively track changes," says Greg Rosenthal, MD. "What we gain is a significantly higher value piece of data that really can't be acquired any other way," he adds.

Dr. Rosenthal is in private practice at Vision Associates in Toledo, Ohio, where he has been using the MP-1 for about a year. He points out that central visual acuity (VA) is a highly complex affair and Snellen VA testing is simply not sophisticated enough to provide more than a crude measurement of visual function. "There are plenty of people who see well on the Snellen chart, but have significant central visual difficulty," says Dr. Rosenthal. Moreover, he adds that Snellen measurements alone are not sensitive enough to measure subtle changes in VA. As you know, there are other ways of measuring central visual functioning, such as reading speed, and evaluation of quality of life indicators. These alone are not adequate for a complete understanding of central visual function.

MICROPERIMETRY PROVIDES ACCURACY

"Microperimetry very cost effectively and sensitively measures the subjective visual functioning at every point in the macula, while Snellen takes more of a global function perspective of what the person is able to see on a very arbitrary eye chart," Dr. Rosenthal says. Focal electroretinography (ERG) also provides a different type of data. "While focal ERG measures the electrical activity at each individual point along the macula, it says nothing about how the patient is actually able to function, so there is not necessarily a direct correlation between focal ERG and actual subjective patient performance," he explains.

Figure 2

Standard perimetry is not as precise a method of evaluating retinal disease because central vision loss can make fixating on a target nearly impossible. "Standard computerized visual field testing has algorithms for testing the central macula, but they require that the patient fixate on a target or participate in some other method for maintaining steady fixation, and they do not test nearly as many points through the macula [as the MP-1]. Standard perimetry is crude by comparison," notes Dr. Rosenthal. Microperimetry, on the other hand, uses automatic fixation tracking technology to eliminate the need for patient fixation. "Essentially, the MP-1 has taken the patient compliance portion of the information gathering process out of the picture, and to that extent it is much less subjective," says Dr. Rosenthal.

Eugene de Juan, Jr., MD, uses the NIDEK MP-1 MicroPerimeter routinely in his practice. "The MP-1 allows an assessment of central macular function � basically foveal function � in a very objective way by addressing patient fixation patterns. This is the only instrument that objectively does that," he says. "Just by measuring how the patient is tracking to the target, the machine can get a very objective evaluation of macular cone function," he adds. Dr. de Juan is professor of ophthalmology at the University of California San Francisco.

ELIMINATING SUBJECTIVE VARIABLES

Figure 3. The MP-1 can track both the location and quality of a patient's ability to fixate. Figure 1 shows the interpolated view of a patient with a newly diagnosed macular hole. The blue dots are where the patient is fixating. Fixation is central and stable. This is a strong indication that the patient will respond to treatment. Figure 2 shows a close-up of the fixation analysis of the same exam. Figure 3 shows a patient with fixation that has moved eccentric and is unstable. Figure 4 is a close-up of the fixation analysis of the same exam. Figure 5 displays both macula function and fixation analysis overlaid on a color fundus photograph for anatomic correlation.

In diseases such as macular degeneration with choroidal neovascularization or diabetic retinopathy with macular edema, the patient's visual function, as recorded on a Snellen visual acuity chart, can be skewed by the patient's anxiety, among other things, says Dr. de Juan. "Those variables greatly affect the patient's subjective response to Snellen visual acuity. The MP-1's fixation tracking function essentially eliminates that variable and makes for a much more objective and reliable measurement, he says. "We've done studies that show that fixation is a better predictor than visual acuity for positive responses in patients who have poor vision," adds Dr. de Juan. "Some patients with poor vision and AMD have reversible damage and the MP-1 tends to pick that up, and some patients with poor vision already have irreversible damage. I think it is extremely useful to segregate the patients into those who are likely to respond to treatment and those patients who are unlikely to respond � especially in terms of those patients with poor vision," he says.

TRACKING THE EFFECT OF TREATMENTS

Because microperimetry measures function at each individual point through the macula, it is extremely effective at following response rates to various treatments. "With this device you can very precisely map out the visual dysfunction as a correlation of lesion size and location, and also as a function of edema or sub-retinal fluid," says Dr. Rosenthal.

"The MP-1 allows us to map the scotoma and identify areas of preferred retinal loci," says Bert M. Glaser, MD, founder and medical director of the National Retina Institute, in Baltimore Md. The device, he says, has significant applicability for wet macular degeneration. "Now that we have better tools to reduce leakage, we'll be able to see what the impact of those tools is by visualizing the actual macular field. It's going to provide an important comparison point among treatments to be able to decide which of these treatments is working better than the other," he notes.

Another area in which Dr. Glaser is using the MP-1 to his patients' advantage is in vision rehabilitation for low- vision patients. The device is equipped with a function that allows occupational therapists to train patients to relocate their fixation. "This recently introduced module enables us to actually project letters on the retina and have the patient read those letters, and by doing this we can train the patients to use those areas of healthier retina better than they had been," he says. In some instances, these patients had not been using these healthy areas at all, he points out. "It's been absolutely amazing. After several hours of training, we have seen patients who are reading 60 words per minute who had previously been reading 6 or 7 words per minute," he says.

MP-1 VS SLO

Figure 4

The MP-1 is often compared to the scanning laser ophthalmoscope (SLO); however, Dr. Glaser says that there are some significant differences. "The problem with the scanning laser ophthalmoscope is that it's not preprogrammed to shine any size and intensity light on the retina. You have to do it manually, and more importantly the larger problem is if the patient moves you have to throw that data point away because when you're dealing with the macula rather than peripheral visual fields every little eye movement impacts the accuracy. This makes for a very tedious exam and one that cannot easily be left to a technician," says Dr. Glaser. That problem was addressed with the MP-1's tracking fixation system, he points out. "It makes a world of difference and makes it so much better and more reproducible than the SLO."

It all comes down to accuracy, says Dr. Glaser. "If you're not precisely tracking central fixation, you don't really know if the results are meaningful or not. The information provided by the MP-1 is therefore more reliable in my opinion than a manual scanning laser ophthalmoscope. I don't have a clinical trial to prove it but I've done both tests myself many times and the time factor even becomes an issue," he notes.

Figure 5

Diagnosis, treatment, and follow-up of patients with macular diseases is made easier and more precise with the NIDEK MP-1 MicroPerimeter. "When retinal specialists see how easy it is to use and how useful it really is, use of the MP-1 will become much more widespread," concludes Dr. de Juan.