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“Hypovision” is a term used to describe the bilateral, irreversible
central and/or peripheral functional impairment that cannot be resolved
by means of standard optical correction and which leads to different
degrees of visual disability. It has a central retinal component associated
with a macular lesion that mainly affects visual acuity and thus causes
difficulties in near activities (reading, performing fine work, recognising
facial physiognomies, etc.), and a peripheral component associated with
an impairment of the retina and/or extramacular fibres that causes an
alteration in the visual field and thus reduces the autonomy of movement
of affected subjects.
From
a clinical point of view, it is held that visual disability begins when
visual acuity decreases to less than 3/10 and the reduction in the visual
field exceeds 60%. Hypovision is considered mild, moderate or severe
on the basis of the severity of the functional impairment (Tab.
1), and the sum of the scores attributed to peripheral
and central hypovision defines the degree and score of global hypovision.
With the support of the UIC, the Italian Hypovision Study Group (GISI)
has developed a new classification of perimetric defects for pension
purposes, and presented it to the Italian Parliament in the form of
a White Paper.
It
is worth pointing out here that the current legislation almost exclusively
recognises only central and paracentral perimetric defects, whereas
the new classification proposes three levels of severity (mild, moderate
and severe) concerning both the central and peripheral deficits revealed
by means of the indices of computerised perimetry or its equivalent
(Zingirian M, Gandolfo E, 1998).
Hypovision
and age-related macular degeneration
Age-related
macular degeneration (AMD) is currently the most frequent cause of legally
recognised blindness among the people aged more than 60 years in the
Western world. No precise estimates are available concerning the prevalence
of hypovision due to this disease, but the table
2 shows the distribution of AMD by level of visual acuity
as revealed by an analysis of the 1989 UIC lists made by Milan’s National
Research Council.
Visual
rehabilitation and AMD
AMD
causes central hypovision, which means people who are no longer capable
of using the fovea for fixation purposes and who therefore have to learn
how to “look” using a less sensitive part of the retina.
The
position of the new fovea is defined by the size of the scotoma and,
in order to avoid the need to use excessive magnifications and minimise
the angle of eccentric fixation, it must be immediately outside the
area of the scotoma because the greater the angle of eccentric fixation,
the greater the magnification needed to allow the perception of images
and the shorter the reading distance.
In
AMD patients, the new fovea may be decentralised laterally (in the case
of small scotomas of 1-2o) or vertically above or below larger scotomas.
The preferential localisation of the new fixation site should be stimulated,
which can be done by using the scotometry programme of a scanning laser
ophthalmoscope (SLO) or by establishing connections between the healthy
retinal areas (revealed by means of fluoroangiography) and the visual
field, in order to control the decentralisation, verify the concordance
of the two examinations and establish whether it is necessary to correct
the process.
The
rehabilitation of these patients needs to consider other aspects, such
as the ability to follow the direction of fixation, ensure stability
of fixation during movement, and allow searching and finding by means
of explorations involving both head and eye movements. Given that they
are elderly, it is also necessary to bear in mind the presence of head
or hand tremors, attention and memory capacities, cognitive skills and
the characteristics of learning processes.
After
having completed the clinical evaluation (near and distant visual acuity,
with correction of the refractive defect; the size and location of the
scotoma; contrast and chromatic sensitivity), the rehabilitation programme
must consider:
a)
the pertinence of the needs expressed by the patients (the need to read
correspondence is pertinent, whereas the need to drive is not);
b)
the qualitative and quantitative prognosis concerning the possible reduction
of the visual disability, which must be formulated in such a way as
to satisfy the pertinent requirements of the patient;
c)
once having prescribed the magnifying aid most appropropriate to each
individual patient (also bearing in mind his/her real economic possibilities!),
it is necessary to organise a suitable follow up in order to evaluate
the stability of the results.
Computer
programmes that store and process the data concerning the functional
characteristics of subjects with hypovision have been on the market
for some years, and are capable of evaluating the clinical evolution
of the condition and indicating the best prescribable magnifying aid.
One
of the most innovative programmes (Essilor) is based on a video technology
that makes it possible to perform perimetric, microperimetric and scotometric
examinations, and evaluate the preferential fixation.
It
is also capable of evaluating the degree of magnification necessary,
the exploitable reading field and the characteristics of the ocular
saccades in order to recommend the most suitable rehabilitation treatment.
Finally, instruments for improving the visual performances of the hypo-sighted
have been developed. These are based on monitoring the bioelectrical
activity of the retina and the occipital cortical areas by means of
PERG and VEP, and allow an objective quantification of treatment-related
recovery. They make use of a sound feedback that informs patients about
the efficacy of their visual apparatus in such a way as to optimise
treatment results. It has been estimated that, up to the end of 1999,
only 1% of the patients with hypovision living in Lombardy had attended
a visual rehabilitation centre.
Magnifying
aids
Magnifying
lenses
These
are the simplest and most convenient aids and, in the case of mild hypovision,
make it possible to read texts with normal-sized characters. It is advisable
to use lenses with the lowest useful dioptic power insofar as this allows
the use of lenses with a larger diameter. It needs to be borne in mind
that the best performances of all lenses are reached under good lighting
conditions which, by reducing relative scotomas, allow the use of lower
degrees of magnification.
Magnifying
lenses may be used manually or mounted on spectacles. The former make
it easy to perform various activities. They must be placed over the
text to be read and then raised with the convex side facing the patient,
and may contain their own light source, be equipped with a support or
have the form of a ruler.
They
have the advantage of being cheap. There are both aspheric and aplanar
lenses: the first can reach a dioptic power of 20 D (corresponding to
a magnification of 5x) and the second a dioptic power of 40 D (corresponding
to a magnification of 10x).
However,
in the latter case, the size of the visual field, as well as the distances
between the eye and the lens and between the lens and the page, are
greatly reduced and thus cause visual fatigue when used for prolonged
periods of time. The lenses mounted on spectacles are used by raising
the text towards the eyepiece until the correct distance has been reached,
keeping the convex part of the lens facing the page. There is the possibility
of incorporating the optical correction of the patient (spherical and
cylindrical), as well as a coloured filter if necessary.
Before
prescribing this aid, it is worth checking for the presence of head
tremor, because this may compromise the use. Spectacle-mounted magnifying
lenses offer a larger visual field, are lighter (given their small size),
give rise to few aberrations, and leave the hands free. The appropriateness
of bilateral application depends on the functional condition of the
two eyes and the necessary spheric addition. In the case of additions
of up to 14 D, it is advisable to use bilateral correction and prisms
with a nasal base (e.g. for an addition of 10 D, a prism of 10 D).
Monocular
use is preferable in the case of functionally different eyes and additions
of more than 14 D; the addition can be as much as 40 D, but the distance
of application is extremely limited. Fibre optic enlargers have recently
become available, which have a magnification power of 2-3x. They do
not need to be focussed, can be kept tilted and are extremely manageable,
but they cost more than traditional magnifying lenses. Finally, there
are also comfortable and cheap mirror-based magnifiers, and still experimental
liquid crystal versions.
Telescopic
systems
Usable
for both near and distant vision, the vast majority of telescopic systems
are monocular although can also be used binocularly. The simplest is
the Galilean system, which consists of two coaxially mounted lenses
(a negative eyepiece positioned in front of the eye and a positive objective
at the front) separated by a distance equal to the sum of their focal
lengths.
The
posterior focus of the positive lens coincides with the anterior focus
of the negative lens in such a way that the rays parallel on entry also
exit in parallel, thus producing direct images. In the case of Keplerian
systems, the fact that both the eyepiece and objective are positive
means that the images are upturned and need to be righted using a prism.
Keplerian
telescopes have greater magnification power than their Galilean counterparts,
which are suitable up to about 2.5x: at higher magnifications, the quality
of the image worsens and the usable visual field becomes very small
and not very practical. Keplerian systems are used for patients with
a 10-degree tubular visual field whereas, if the visual field is larger,
it is worth using a Galilean telescope despite its lower level of magnification
because it encompasses a visual field of up to 30 degrees. Keplerian
telescopes are heavier and more cumbersome but, for the same visual
field, allow a near work distance that is twice that of Galilean telescopes.
The
correction is applied binocularly for distant sight but, for close vision,
it is better to adopt a monocular correction after having selected the
functionally better eye and applied an opaque lens over the other. The
distance magnification is fixed (1.8x) and the aid needs to be used
at a fixed distance; in order to obtain the correction for near vision,
it is sufficient to apply a positive lens. In the case of Galilean systems,
these are pressure mounted or screwed onto a threaded ring; Keplerian
systems can be equipped with a zoom lens for both distant and near focussing.
The
majority of telescopic systems are mounted on spectacles, but hand-held
models are also available.
Videomagnifiers
These
are devices designed to aid reading and writing. They work by means
of an optical electronic process that projects the images onto a video
screen at the necessary magnification (from 3x to 60x), and can be used
by the hypo-sighted whose visus is so poor that they cannot use other
enlargement systems. Almost all of the models allow the contrast of
the text and the brightness of the backgound to be adjusted, the transition
from a negative to a positive imagement, and the selection of a green
background in order to make reading more relaxing. The size and weight
of these devices mean that they are stably installed in the home.
A
number of special functions have been recently introduced in order to
improve their use, including electronic diaphragms, lines for underlining
the lines to read, and anti-dazzle and anti-reflective screens. In some
models, the distance between the work surface and the reading head is
sufficient to allow the use of a typewriter, the reading of very thick
volumes, and the execution of manual work such as sewing or model making.
The
new electronic technologies have allowed the development of systems
with palm-sized videocameras, which are extremely versatile, portable
and can be connected to normal televisions. Some models have pushbutton
or pedal-controlled videocameras.
Finally,
there are also some small and truly portable systems consisting of a
manual microcamera and an enlarging screen just a little larger than
a notebook, which can be powered by battery as well as via the mains
supply. It is now possible to find software that can magnify PC programmes
and thus allow the normal use of a computer, which can also be equipped
with a voice processor.
(traduzione
dell’autore)
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