What does actually low vision mean?
Low vision is a clinical condition in which the patient suffers from a pathology which reduces his visual acuity below normal standards and it is impossible to correct it with normal optic lenses.
Let’s see which are the changes of perception that can occur in low vision.
In low vision due to a central scotoma small objects cannot be seen.
So the patient should decentralize fixation in order to focus on the so called perceptive invariants of an object using intact retinal areas next to the damaged one and put the object closer to his eyes in order to obtain a magnification of its retinal projection. Of course we can achieve a proper magnification also using magnifying optical or electronic aids. Those aids are not so simple to get acquainted with, and so the patients usually need a training to get used to the correct reading distance and to the visual field obtained with the chosen aid.
The visual impaired patient: who is he?
The low vision patient is a difficult one. He knows he has an irreversible ocular pathology, he doesn’t believe it could exist a therapy, he has never considered possible to be rehabilitated. He is a pessimistic patient. But we can show him that it is possible to read with an optic aid or to improve his residual vision by far, giving him not only the promise of a better living but also the concrete possibility of realizing it with simple magnifying devices and exercises.
Usually the first need of a low vision patient is reading.
There are various pathogenetic mechanisms which can alter the reading process. There can be central scotomas, general loss of retinal sensitivity, contraction or amputations of the visual field.
Statistic evaluation are often not available or possible because of the great variability of symptoms and effects on vision of an ocular pathology. Even if the operator is an expert one, we know that the low vision patient usually cannot be precise after many tiring trials. Besides when residual vision by near goes under 18 points variability increases: what should be better? A Galileian aid or a magnifying device or a CCTV? Light should be normal or intense? Should the residual visual field be sufficient for an acceptable reading performance? What should be the eccentricity and direction of fixation? Should I use a fixation target to improve reading performances and what kind of IOL should be implanted if the patient has also a cataract?
As we can see only with the aid of virtual simulation we can easily answer to all the question and choose the best solution.
What does visual rehabilitation mean?
Visual impaire people need a training to get acquainted with the magnifying aid which allow them to read.
What is PRL?
PRL means” preferred retinal locus” and is the new locus on wich we must teach the patient to project fixation during visual rehabilitation.
What is the meaning of virtual visual rehabilitation?
Perimetry or visual field examination is the one which allow us to reproduce the patient’s visual perception. with This examination we can find scotomas and reduction of sensitivity.
Virtual maps start from static or kinetic perimetry.
The software elaborates the visual field maps in grey scales and allows the operator to project a reading string on the physiological foveal fixation area. If this area is cancelled by the scotoma, the operator can enlarge the reading string and decentralize it on a near free area that could become the new preferred retinal locus.
The operator becoming a virtual patient can choose the best magnifying system with a precise idea of what should be the perception of the real patient with that particular aid.
The prescription of a visual aid becomes easier and less tiring.
How to write a virtual visual map?
The first thing to do is to import the result of the visual field examination on virtual ipo using the “import visual field” key.
The software can request a minimal modification of the chosen image (to adapt dimensions and to centre it) for example if we import an Humprey examination of 30° the software will adapt it on the Goldman grid. This is particularly important if we import a microperimetric examination because the imagine should be rotated on its horizontal axe clicking on the “Mirror Y” key.
the image will be correctly centred clicking on the OK key.
At this point we can start writing the isopters on the map. On the HTML guide it is possible to find the equivalence in gray scale for each isopter.. It is possible to get the preview of the map in order to correct mistakes.
Now we can project a reading string on the visual field of the patient and we can see the way he perceives it. The patient cannot read because of a central scotoma which hides all the letters or part of them.. But we can enlarge the letters and decentralize them until we can read them.
It is important to remember that the median superior allignment of the letters is the one on which fixation is, and this part of the reading string is the one that must be decoded . this is the one that must be projected on the PRL in order to rehabilitate reading.
How much time is necessary to complete the compilation of a visual map?
From visual field import to the elaboration of virtual visual map we need almost 15 minutes.
How many trials are necessary to choose the proper magnifying aid?
We need only one trial: actually we already know through virtualipo which could be the best visual aid.
How many session of visual rehabilitation are necessary?
The number of rehabilitation sessions can vary from a minimum of three to a maximum of six.
Do the patients have a good compliance?
The patients who undergo this kind of visual training are satisfied in 90% of cases.