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In this presentation I will discuss the differences between how the eyes function and how the
person functions. Visual functioning can be approached from
different points of view. Each point of view will show us a different aspect. What is meant
by speaking of aspects? When I show you this picture do you see three objects here or could
it be that you are just looking at three aspects of an unfamiliar object.
Turning the object around to look at it from different points of view will discover that
what seemed a different object may just be a different aspects of the same object.
To apply this to a practical situation consider a patient with AMD who comes to the office
to make an appointment. The front desk will think about when to schedule her, the doctor
will think about which treatment to select, the Office Manager may worry whether the insurance
will pay, the daughter will worry about whether Mother can still drive.
These considerations may seem to have little in common, yet they represent several different
aspects of a single clinical case. Each aspect reveals not only something about the subject,
but also about the point of view of the beholder. When we talk about vision and visual functioning
I have found it useful to consider four main aspects.
First we may consider various changes at the tissue levels such as scaring, atrophy or
loss here we need a Pathologist to look at the structure of the organ. However the structural
changes do not tell us how well the eye actually functions. We need to broaden our view to
include functional changes; we need the clinician to measure aspects such as facial acuity,
visual field and contrast sensitivity. Yet, telling us how the eye functions does
not tell us how the person functions so we need to widen our perspective again to consider
tasks such as reading mobility activities of daily living. Here we need various low
vision professionals to work with the patient. Beyond that we need to look at the person
in a societal context. Do these changes impact on the person’s participation in society
causing a reduced quality of live? How can we be sure of a contented patient since that
is the end goal of all of our interventions and particularly vision rehabilitation?
One thing that we can already see at this point is that comprehensive care can not be
the job of one person, it needs a team of professionals and the patient must be part
of that team. I will use this schema in many of my slides.
It is useful to draw a line in the middle on the organ side we describe how the eyes
function and we speak of visual functions. On the person's side we describe how the person
functions especially in regard to vision related activities daily living and we speak of functional
vision. Let us start with the patient's point of view.
The patient comes in with a very practical complaint, "Doctor I can not read". Her primary
concern is on how she can function on her quality of life and on daily living skills.
For her the details on how her eyes function are of secondary interest. Compare that with
the doctor's point of view. He immediately translates the patient's complaint to a statement
about the eyes, the patient has lost three lines. His professional interest is on how
the eyes function, for him how the person functions is of secondary interest.
Let us look a little further. When we think of Eye Doctors the emphasis tends to be on
letter chart acuity. That emphasis is often so strong that we refer to visual acuity as
vision. Her vision is 20/40. When we shift to the patient's side the emphasis associated
are activities of daily living. In this context we need to take into account many more aspects
of vision, acuity fields, contrast color movement, etc.
When we realize that we must make the earlier statement more accurate by saying that the
visual acuity is 20/40. One of the things that the eye doctor must learn is that normal
letter chart acuity does not mean that there are no vision problems. On the patient's side
we are not done when we normalized her eyes we must also provide coping skills and tools.
Not only do the left and right side differ in their emphasis they also differ in their
objectives. When measuring how well the eyes function the objective is to learn more about
the underlying condition. When measuring how well the person functions,
the objective is to learn more about the societal consequences. Rehabilitation provides a link
between these two aspects. The complexity of the problems on the left
and on the right is also different. On the left where we deal with how the eyes function
there is a fairly straightforward pass from the object seen through the optics of the
eye to the retinal image and on to the optic nerves signal. On the right where we deal
with how the person functions we must deal with the interaction of visual demands and
visual resources. It is only the balance between these two that determines functional vision.
Traditional low vision care addresses mainly the optical aspects. When we want to provide
comprehensive vision rehabilitation however, we need to consider much more. Vision substitution
skills which offer non-visual skills such as brail, long-cane or speech output can be
important to enhance the resources. We also need to consider the task demands. For example,
when we provide a student with a magnifier we practice vision enhancement. When we provide
large print we modify the task. When we provide talking books we practice vision substitution. Beyond
these factors we also must consider the environment. The human environment can be either supportive
or prejudiced. The physical environment can provide barriers or facilitators.
Finally we must consider the individual scoping skills, which may effect how well all of our
interventions are received and adopted. In summary, on the right the number of variables
and their interactions are much more complex than on the left.
To summarize what this means for our assessment in vision rehabilitation, we can assess three
main aspects, the organ, the person and the societal context. On the organ function we
must deal with the parameters of visual functions. For the person we must consider task performance
and functional vision. in a societal context we must consider vision-related quality of
life. What do we consider under each of these headings?
Under visual functions we look at ocular functions such as visual acuity, visual field contrast
sensitivity and so on. Under task performance, we look at activities such as reading, orientation
and mobility and activities of daily living. Finally under quality of life, we look at
attributes such as social skills and the ability to make and keep friendships. it should be
clear that these groups are very different and therefore should be evaluated separately.
The ways in which we evaluate each aspect are also very different. For organ functions
we can exact threshold measurements one parameter at a time. When looking at the activities
and abilities of the person we must always consider the interaction of multiple factors
and ask for sustainable performance. Quality of life, finally, is a subjective experience,
which is hard to measure numerically. Satisfaction is an important goal.
When planning rehabilitation we must realize that one activity can span several aspects.
Let us take reading as an example. Print size and reading speed, words per minute reflect
organ function. Reading endurance, how many hours a day someone can read reflects an ability
of the person. Reading enjoyment, finally, is a component of quality of life. Because
these are different aspects, we need to assess them separately and with different tools.
We can not predict reading enjoyment for measuring print size and reading speed.
Here are three familiar eye tests, which measure different parameters. Visual acuity, contrast
sensitivity and dark adaptation. Since we use separate tests, we may think of them as
separate unrelated parameters. They each determine a separate performance threshold, which is
defined as 50% above guessing. Real life situations however, always involve multiple parameters.
Detail varies, as does contrast, as does elimination so we can not get away with measuring one
parameter at a time for the more we ask for sustainable performance threshold performance
is not good enough for actual tasks. Let us look at more detail at some of the
difference between assessing the functioning of the eye and assessing the functioning of
the person. On the left and example of the threshold measurement is letter chart acuity.
We generally define threshold performance as 50% above guessing and pose no time limit.
We have defined the reference standard as the ability to recognize a 1m letter at 1
meter. The requirement for reading on the right are a different matter, we require near
100% correct performance and a reasonable reading speed. When reading, we generally
bring 1m print to about 40 centimeters which is two and a half times above threshold. That
difference is not negligible. We mention the difference between static,
artificial environment and a dynamic one. This is esthetic criterion most countries
set for a driver’s license, this reflects actual driving performance, the difference
is obvious. When measuring how the eyes function we measure
one parameter at a time. We know that size, contrast and lighting all effect visibility.
When we vary the size while keeping contrast and lighting constant we have a letter chart
and measure visual acuity. When we vary the contrast while keeping the other parameters
constant, we measure contrast sensitivity. When we vary the lighting, we measure dark
adaptation. Each of these separate measurements gives us a single number.
On the right, where we deal with multiple interacting parameters a single number does
not suffice we need at least a two dimensional display. This diagram shows the interaction
of contrast and detail and how it is captured by the contrast sensitivity curve. It shows
that visual acuity and contrast are not independent variables, better contrast results in better
acuity. In summary, because of all of these differences,
determining how the person functions is not just an extension of how the eyes function.
The different aspects require different approaches and the efforts of different members of the
vision rehabilitation team.