New Technology Update: NIDEK
Tracking fixation function
places device in a category all its own.
ROCHELLE NATALONI, CONTRIBUTING EDITOR
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.
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
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,"
TRACKING THE EFFECT
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
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.
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.
Retinal Physician, Issue: January 2006