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CE Update [generalist | management/administration and training]
Eyes on the Lab
Paulus T.V.M. de Jong, MD, PhD1, FRCOphth, Bouke C.H., de Jong, MD2, P. Stoutenbeek, MD3, Kevin L. Peterson, MD4
Netherlands Ophthalmic Institute, Department, the Academic Medical Center, Amsterdam, and the Department of
Epidemiology and Biostatistics, Erasmus University, Rotterdam, the Netherlands; 2Division of Infectious Diseases and
Geographic Medicine, Stanford University, Stanford, CA; 3private practice in Winterswijk, the Netherlands; 4San Joaquin County
Public Health Services, Stockton, CA
1The
After reading this article, the reader should understand the effects of common visual problems related to performing or interpreting
laboratory tests and how to solve the problems.
Generalist 0104 exam questions and corresponding answer form are located after the “Your Lab Focus” section, p 535.
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Basic eye structure and function is
explained and related to how it
affects performance in the laboratory
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Age-related changes in the eye and
their effects on performance of
laboratory work are explained
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Different visual defects are explained,
and what can be done to correct
them are explained
A well functioning visual system is
an important asset for coping in daily
life. Humans can operate without any
visual information, as is exemplified by
the number of blind people functioning
well in professions not typically associated with the blind, including law, medicine, and others.1 However, most people
will be depressed and seriously handicapped initially when they lose important visual information, especially in the
acute phase. The visual system is said to
provide about three quarters of all sensory information to the human brain.2
The aim of this article is to highlight the
relevant physiologic visual functions in
the normal working population, with an
emphasis on those in laboratories. We
will review common disorders of these
functions associated with aging or disease and comment on results of a small
survey among laboratory workers.
Physiologic Properties of Vision
Giving a definition of vision or
seeing is not easy. Dorland3 has no
entry for the latter but defines vision as:
“The special sense by which objects in
the external environment are perceived
by means of light they give off or reflect, which stimulates the photoreceptors in the retina; called also sight.”
Webster’s Dictionary4 has as a single
entry for “see”: “To get knowledge or
an impression of, through the eyes and
the sense of sight.” The Oxford English
Dictionary5 writes for “see”: “To perceive (light, color, external objects and
their movements) with the eyes, or by
the sense of which the eye is the specific organ.” One definition of vision is:
“Observing with the eyes.” We would
define vision as: “Interpreting with our
brain images or phenomena perceived
with our eyes.”
There are 3 major steps in perception: detection, awareness, and identification or recognition.2 In order to see an
object, we use various visual cues such as
size, form, texture, and edge sharpness, as
well as its contrast with its surroundings.
The most important physiologic properties of the visual system, as well as the
changes in them, arise while aging or in
disease states. These properties are given
in [T1], together with a ranking of 1 to 5
with regard to their value in everyday life.
We will discuss the modalities in [T1]
and try to identify where these visual
properties might specifically interfere
with usual laboratory work.
Visual Acuity
Visual acuity (VA) is the reciprocal
of the smallest angle in minutes of arc
under which a person can discriminate a
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contour or can, in physical terms, resolve
spatial differences in luminance. In
essence, VA is a measure for and is the
counterpart to contrast sensitivity using
strong contrasts, in practice 1:10, the contrast or luminance ratio of normal black
print on white paper. Contrast can be defined as (L/L`–1) in which L is the luminance of an object and L` that of its
surroundings1 However, VA and contrast
sensitivity tests with various low contrasts
do a poor job of predicting how well a
person functions on a day-to-day basis.
High contrast tests do only a slightly better job, but nevertheless they are often
used in the form of Snellen acuity charts.
Snellen VA is determined with high contrast letters or symbols that in various
forms have been used in ophthalmology
for over 100 years to express someone’s
VA. A Snellen VA of 20/20 or 1.0 (in
decimal notation) thus points to the ability to discern 2 objects separated by 1
minute of arc. A new unit for VA is the
log MAR (Minimum Angle of Resolution) corresponding to minus the logarithm of the VA. The majority of the
working population has a Snellen VA of
1.0 or higher, sometimes even as high as
2.5. The distance should not really matter
when expressing VA because VA is defined by the angle of vision and thus distant and near VA should be the same.
There are situations, however, such as
impaired accommodation, or pupil diameters varying between 5 mm when looking nearby and 8 mm for far away, which
hamper near vision more than distant vi-
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Estimate of the Importance of Visual Modalities for Daily Functioning on a 5-Point scale.
T1
1. Very important for daily functioning
2. Of average importance
3. Of minor importance
4. Important for special occupations only
5. In general unimportant or hard to determine
The lower the number the more important the physiologic properties are or the more inconvenient the disturbances.
Physiologic properties of vision
Importance
Distant visual acuity
1
Near visual acuity
1
Intact central visual field within 30 degrees
1
Single vision in a 1-eyed person
1
Single vision binocular
2
Depth perception monocular
2*
Ametropia (correctable with glasses or contact lenses)
3
Intact accommodation
3
Color discrimination
4
Vision in twilight
4
Stereoscopic vision or stereopsis
4**
Disturbances in vision due to aging or disease
Visual agnosia
1
Metamorphopsia
2
Oscillopsia
2
Presbyopia
3
Photophobia
3
Glare
3
Hemeralopia
4
Anisometropia and aniseikonia
4
Asthenopia
5
Dyschromatopsia
5
* Monocular
depth perception depends on many different cues.12 Among them are static ones, such as the size of retinal image that is inversely proportional to the distance. Thus the
longer the distance, the a) more perspective convergence b) finer the surface structures, c) more crowding of nearby images, d) more partial covering of images by neighboring ones, e)
smaller the shade and light angle effects, e) more blurring and blue haze due to atmospheric light dispersion, f) higher and closer to the horizon the image, and g) more reduced
influence of accommodative convergence. Dynamic cues for monocular depth perception include movement parallax, optic flow (the continuous and systematic changes in retinal image)
due to movement of an image in against the texture of the surface, and radial movement (objects becoming smaller or larger, respectively, by moving away from or toward the observer;
important in estimating time to collision in traffic or parachute diving). Monocular depth perception is, of course, quite important in daily life but difficult to measure in an overall way.
Because it will be rare for someone with good visual acuity to have no monocular depth perception, this was arbitrarily ranked as 2.
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**The median stereopsis for the population lies at about 40 seconds of arc. This means that with an interpupillary distance of 65 mm, one can estimate a difference in depth of 3 mm at 1
m distance, 2 m at 25 m, and nothing at 300 m.12
sion. Given the relative importance of
near vision in many modern professions,
it has become customary to specify requirements for near vision separately. The
relative importance of near VA is also
reflected in the AMA Guides to the eval-
uation of permanent impairment.6 In
these guides, impaired near vision was
considered to be twice as important as
impaired far vision during the time between 1925 and 1941. In 1955 this rule
was reestablished. Possibly during the
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interim, a war-time change of rules was
instituted in order to obtain more recruits
or snipers.
With regard to minimum standards
for VA, a rule of thumb is that a person
with VA of 0.3 in the better eye, if neces-
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in that way. The cones in the retinal center are necessary for obtaining a good VA
as well as intact color vision. That is why
macular degeneration, in which the retinal cone receptors degenerate, is nearly
always associated with VA loss and often
with decreased color vision. The rods,
predominant in the peripheral retina, do
not contribute to color perception but are
important for our VF and for seeing
under mesopic or scotopic circumstances
such as twilight or moonlight.
[F1] Anatomic Lesson by Rembrandt. See Figs 5, 7, and 8 for comparison with visual distortions
due to various eye disorders.
sary with normal reading addition, can
read normal small print on paper and
can perform administrative and routine
laboratory functions. Pathologists can
compensate a loss of VA by using higher
magnification while performing
microscopy, provided they have a sufficient width of their visual field (VF; see
below) without too many small paracentral VF defects or scotomata. In performing an autopsy or while dissecting subtle
tissue structures, however, one might
miss cues with a suboptimal VA [F1].
Similarly, a technician embedding and
sectioning samples for electron
microscopy might be challenged by a
VA lower than 0.3, at the very least requiring more time in order to perform as
well. When assessing if someone can
perform a task, one should also keep in
mind that in many countries people depend on the ability to drive a car in order
to get to work, and this implies meeting
the legal requirement of a VA of 0.5.
The necessity to reach the work-place,
of course, holds for all aspects of vision
(eg, VA, VF, absence of diplopia) that
are discussed here. An irony behind
some of our last remarks is that there is
not much hard evidence for these statements. One would have to compare car
drivers, lab technicians, or pathologists
with good and poor vision according to
well-defined end points in their daily
performance; this is often not possible,
and in the case of drivers on public
roads, this would probably be illegal.
In order to obtain a good VA, the
visual system from the tear film on the
cornea, through the optic media and the
retina, to the brain areas past the primary
visual cortex, must function well. The
optic media are those clear parts of the
eye through which the light must pass in
order to reach the retina. The most common turbidities are in the lens (cataract)
and vitreous body (floaters) that may create glare and irritating obfuscations while
performing light microscopy. For good
VA, one needs a more-or-less normal central part of the retina, the macula (lutea),
and its center, the foveola. In [F2] the
relation between VA, density of cones,
and degrees of excentricity from the foveola is given in a graph projected onto the
macula. Many amblyopic (also called
lazy eyed) subjects fixate outside the
foveola and, thus, cannot achieve full VA
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Issues Related to Refraction
Not wearing appropriate corrective
lenses is the main cause of decreased VA
in the Western population over age 55.
Of 7,983 subjects aged 55 years and over
from the Rotterdam Study, 20% needed
no glasses to achieve their best visual
acuity and, thus, are called emmetropic,
assuming that they did not need to use
their accommodation. The remainder was
ametropic (refractive error present), 20%
of the whole cohort was over 0.5 diopter
myopic (near sighted; minus lenses necessary) and the remaining 60% was over
0.5 diopter hypermetropic (farsighted;
plus lenses needed). Myopia up to 4
diopters may be handy in daily lab work
because those individuals will typically
not require reading glasses, assuming, of
course, they do not have astigmatism, the
condition in which the asymmetric curvature of the cornea requires glasses with
what is called “cylinder.” Whereas myopic subjects cannot bring distant objects
into sharp focus, young hypermetropic
subjects have to use their accommodation in order to see sharply at a distance.
The necessity to accommodate comes at
a cost, however, and may lead to complaints of fatigue, squinting,or headache.
As accommodative power wanes with
age-related stiffening of the lens in people over 40, bifocals or reading glasses
may become necessary. Wearing glasses
while performing laboratory work generally creates no problems and even has a
protective effect against particles or
squirting fluid. However, glasses may be
hampering during microscopy, while fogging may occur going from a cold room
such as a mortuary or walk-in refrigerator into a humid, warm environment.
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People with astigmatism over 1.5 to 2
diopters who are troubled by glasses during light microscopy may wish to try
hard or toric soft contact lenses. For
some people, these contacts may be annoying during microscopy because the
border of the lens distorts the retinal
image or because they do not blink
enough while concentrating on their task.
Moreover, one should keep in mind that
contact lenses, in particular soft ones, are
the main cause of serious corneal ulcers
nowadays. Given the potential chemical
and biological contaminants in the lab
environment, the utmost hygiene is indicated, and contacts should be left out if
there is any redness of the eye.
It is common to speak of high anisometropia when the difference in refractive error between both eyes of a subject
is over 2 diopters. In general, this creates
no problems, and with microscopy one
can usually adjust the oculars individually. This is best done with about 20×
magnification to decrease depth of focus.
However, greater than 4 diopters
anisometropia can create unequal sizes of
the images on both retinas of a subject, so
called aniseikonia, and the brain cannot
fuse both images into a single image.
This disturbing phenomenon can be corrected by wearing contacts instead of
glasses or, in case this is not feasible, by
prescribing a corrective lens for only the
least ametropic eye.
Lab work nowadays is hardly feasible without the use of computers. Most
people read a book or the newspaper at
40 to 50 cm distance and look at a computer at 70 to 80 cm. This means that the
reading glasses one uses for computer
work should be 1 diopter weaker than
those used for reading printed material.
As one requires a stronger reading addition with advancing age, keeping old
reading glasses for looking at the computer may work quite well.
Accommodation is the ability of the
eye to change its focus and, thus, to see
sharply both near and far. The
accommodative power is about 13
diopters at age 10 years and decreases by
about 1.5 diopter for each 5 years. Thus
at age 40, only about 5 diopters accommodative power is left, which typically
[F2] Image of macula lutea of the right eye as seen on direct ophthalmoscopy. Black blur on the
right is the temporal border of the optic disc. The “a” in the word "water" marks exactly the center
of the foveola. The white concentric circles denote the foveola, the fovea, the parafovea and the
perifovea, from the center outwards respectively with their outer radius from the center of the
foveola 175 m (35' of arc), 925 m (3.1E), 1.425 mm (4.75E), and 2.925 mm (9.75E). Five
micrometers equal one minute of arc. The outer circle is more or less the boundary of the macula
lutea. Examples of the maximum visual acuity that can be obtained at any point on the retina
eccentric to the foveola are given only in the upper right quadrant by the white curve, but this holds
for the whole macula. The steep drop of number of cones per mm2 in relation to eccentricity to the
foveola is similarly shown in the lower left quadrant. The minimum central visual field necessary to
read is indicated by the white ellipse around the text "the water w. (From Trauzettel-Klosinski20)
will not allow an emmetropic person to
focus nearer than 20 cm (100/5) away.
After this age, subjects typically need
progressively stronger reading glasses, a
phenomenon called presbyopia. When an
older lab worker needs to look repeatedly
at various distances, such as when alternatively reading, computer data entry, and
perhaps helping visitors at a desk, this
may become cumbersome when it calls
for different glasses. Multifocal or trifocal
glasses or monovision (1 eye wears contacts or a glass for near work, the second
eye for distance) might be a solution.
Visual Field
The monocular VF is that solid
angle that an immobile eye in an immobile head can oversee. The functional
VF is the field covered by rotating head
and eye. The VF is of great importance
to prevent a subject bumping into obstacles on the sidewalk or the road, but
also in the office or at home. Loss of
the peripheral VF beyond the central
10E to 15E is called tunnel or barrel
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vision. One can experience what a person with this disorder can see by looking
through a slightly opened fist in front of
each eye. Thus, one can imagine that a
subject with barrel vision might not
want to walk with a jar of picric acid or
radioisotopes in a lab where people tend
to move chairs or cabinets.
Visual fields are examined with
perimeters, formerly mostly manually
handled with moving (kinetic) stimuli
of various intensities and sizes as in the
Goldmann perimeter. In the majority of
perimeters, these stimuli are offered to
a single eye of a subject while that eye
is fixating a central point in the perimeter bowl. Nowadays, mostly automated
static perimeters are used in which on 1
point in the visual field the luminance
of the test object is raised until the
stimulus is seen. The light sensitivity
thus measured on different locations in
the retina may vary and in the foveola
up till 10.000 times or 4 log units. Just
as in audiometry 0.1 log unit is 1 decibel (dB) and, thus, VF loss is quantita-
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[F3] Dependency of visual acuity (VA) on luminance levels. The abscissa indicates the luminance in cd/m2, the ordinate gives the visual acuity in
decimals. Note the steep drop in VA between 2 and 100 cd/m2. On top the transition between scotopic cone vision and photopic rod vision is indicated
together with the corresponding luminance strata. The indicated objects that may be seen by a normal person give an idea about the tremendous range
of sensitivity of the human visual system. (Courtesy J.J. Vos)
tively expressed both in dB units and in
extent of the scotoma expressed in degrees. Although some laws require a
120° VF for driving a car, in daily practice, most people can function well with
a central field of 30E. For VF angles up
to 30E 1 degree corresponds to 1 cm at
57 cm (arms length) distance. The
width of someone’s VF in cm can be
calculated with: VF angle × cm
distance/57. Thus a person with a central
VF remnant of 15E has at 10 meters a VF
width of 15 × 1,000/ 57 = 2.63 m. Subjects with a VF smaller than 5E to 10E
often automatically compensate by making scanning movements with their eyes
and head. For the VF to be intact, we
need (apart from a normal optic nerve
and higher visual pathways) the peripheral retina where mostly the rod photoreceptors are situated. Although the VA
derived from the peripheral retina is poor
[F2], the VF is indispensable in activities
of daily life such as driving. Thus, the
World Health Organization considers
someone to be blind, even with full VA,
when the remaining central VF is smaller
than 10 degrees.7 The same peripheral
©
retina with the rods is necessary for twilight vision.
Night and Twilight Vision
Visual acuity very much depends on
ambient lighting. This is most clearly
expressed in [F3]. Luminance levels are
usually divided in photopic, mesopic,
and scotopic ranges and at a luminance
below 100 cd/m2 the VA drops fast. In
mesopic conditions, people cannot read
any more. In the lab this means that people working in a dark room will have a
problem reading small print, particularly
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[F4] Dark adaptation curve for normal subjects. After looking in a white light for 7 minutes (bleaching the visual pigments in the retina), a test light with
increasing intensity is flashed every 30 to 60 seconds till observed. The hatched area is the normal curve with a break point around 10–3cd/m2 where
cone adaptation ends and rod adaptation takes over. Objects to be seen at various luminance levels are mentioned at the right. (Courtesy J.J.Vos)
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if they have defective night vision as
often happens with age or in patients
with retinitis pigmentosa. Older people
not only need more time to adapt to
mesopic conditions [F4], but they also do
not reach the same level of sensitivity. In
other words, their dark adaptation curve
ends too high. These persons not only
have trouble in dark rooms but often find
driving through tunnels difficult if the
change from sunshine to tunnel darkness
is too abrupt. Another common disorder
in which subjects will need much longer
dark adaptation times than their healthy
peers is age-related maculopathy in
which the macular area of the retina degenerates.
Single Vision.
Single vision (the opposite of
diplopia, or double vision) is quite important for daily functioning. Most adults
with acute diplopia (eg, due to diabetic
ocular muscle paresis) can only read the
ophthalmologist’s office chart with a
patch on 1 eye. Imagine driving and not
knowing which oncoming car is real or
being in the lab and puzzling over which
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of the 2 test tubes should be filled. Small
children cope with double vision by suppressing 1 visual image or by amblyopia.
After age 7 to 9 years, amblyopia cannot
be treated effectively; its prevalence is
about 4% in the population. The suppression mechanism, however, is not effective
in adults. In stereoscopic vision, the small
differences in the images of both eyes
due to the interpupillary distance of about
60 mm are used to advantage. Nearly
everyone acutely losing VA in 1 eye will
need up to 6 months adaptation time before ordinary actions, such as pouring tea
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[F5] Picture of Rembrandt's anatomical lesson as seen by a person with monocular diplopia due to
cataract.
in a teacup and gently and smoothly
replacing it in a saucer. Finer tasks may
take even longer adaptation times. Also
misjudging the curb, the last step of a
staircase, or the garage door at the far
side of the driver are notorious. Also
monocular diplopia may occur! Individuals with only 1 eye can experience
diplopia eg, due to uncorrected astigmatism, cataract [F5] or corneal deformities. Because a 2-eyed subject can
wear an occluder in case of diplopia,
double vision in a monocular case is
much more debilitating than in someone with 2 eyes.
Color Discrimination.
In the US, 8% to 10% of males have
an X-linked, inherited color vision defect.
About 5% of the defects are the common
green (also called deutan) range. The red
range deficiency is called a protan defect.
In females, the prevalence is about 0.3%.8
The cone and rod photoreceptors share
the same chromophore 11-cis-retinal but
differ in protein component, the opsin.
The genetics are far more complicated
than previously thought, in that the red
and green opsin genes are next to each
other and are identical for about 98% of
the structure. Most subjects with a red
deficiency lack the red opsin gene, but
have 2 green ones. The opposite holds for
the subjects with a green deficiency.8
Several reports on subjective problems in
daily life of subjects with varying degrees
of color vision impairment leave no doubt
that deficient color perception may create
difficulties. Examples are selecting
clothes or accessories, fruits or vegetables, and recognizing sunburn or the
color of cooked meat.9 One can appreciate the handicap of partially color blind
people by comparing detection of a bird
in a tree through a black and white view
finder on a video camera versus through a
colored one. In general, most persons
with a red-green deficiency perform well
in daily life. They are banned from some
occupations, sometimes more on historical than realistic grounds. During World
War I it was necessary for a pilot about to
land to discern the difference between a
green grass field and a brown freshly
ploughed area. But that does not justify
the present day regulation that disqualifies pilots who have some red-green deficiency. Nowadays, there is a development
©
to try to adapt the job to a person. With
modern means, this is often possible for
workers with a color anomaly. Thus, it
should not be a problem to come to an
international regulation that, for
instance, 10 years from now all red
(alarm) and green (safe) lights in new
planes and boats should be changed to
orange and blue or violet. This would
solve problems for about 8% of the male
population who are now excluded. An
example of adjusting the job to the electrician is the replacement in Europe of
green and red electrical wires with
brown and blue ones.
In the lab and for physicians, defective color vision may be a handicap.
Physicians with a severe congenital color
vision deficiency had more trouble properly reading color blocks on a blood glucose test stick.11 Green fluorescent
microscopy might be another example.
“Color weak” surgeons sometimes have
more difficulty telling arterial from venous bleeding. Similarly, pediatricians
judging the color of a baby or ophthalmologists detecting a shallow retinal detachment by subtle changes in red may
be challenged by moderate degrees of
color blindness. A periodic acid–Schiff
or Gram stain may create problems for a
protanomalous person in the lab.
Disturbances in Vision That
May Influence the Practice of
Laboratory Medicine
The most prevalent pathologic
change in the visual system for daily
work probably is amblyopia, which
often hampers stereo vision. Most visual functions eg, VA, contrast sensitivity, and retinal sensitivity for light
gradually wane with aging. A small
survey by questionnaire among 50 subjects working in a laboratory setting
revealed that up to 14% had some difficulties with their visual tasks. The
most common complaint was poorer
vision in dim light. This can be
expected from data showing up to 10
fold retinal sensitivity loss in aging and
some diseases. However, none of the
laboratory professionals surveyed felt
that this influenced their overall performance in the lab.
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[F6] Decrease in visual acuity with increasing age. Each dot represents a study of visual acuity at various ages (Courtesy J.J. Vos and Verriest.21).
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We have discussed the physiologic
properties of vision earlier in this article. There is often insufficient knowledge about the gradual decline of these
properties and their function in daily
life, as well as about the many possibilities for subjects with visual handicaps to
perform tasks with low vision aids or in
special facilities. The visual changes,
mainly due to aging, will be briefly discussed together with the results of the
survey among lab personnel. It is not
feasible to discuss all kinds of
ophthalmic diseases in this article. However, it is hoped that the reader may find
some help for estimating the functional
consequences of an eye disorder could
hamper working in the lab.
image. These defective stimuli may lead
to lack of maturation and/or abnormal
development of the monocular and binocular visual system. Even after early (from
age 6 months) and proper treatment, only
one third of the amblyopic children will
later achieve some binocular vision.
Given the prevalence of amblyopia, one
may expect that 2% to 4% of all lab
workers will have 1 eye with lower VA
and poor stereopsis. This does not in itself mean that they cannot do their jobs
well, apart from tasks that require good
stereo vision. It does require special care,
eg, shatterproof glasses where there is a
Amblyopia
Amblyopia is a defect in monocular
vision caused by insufficient visual stimuli during early development. In general,
the defect results from the failure of the
image to be focused on the retina, as in
congenital cataract or strong hypermetropia. Amblyopia can also be caused by
asymmetric images with regard to size or
location on the retina of each eye, so that
the brain cannot fuse them into a single
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[F7] Anatomic lesson of Rembrandt as probably seen by a subject with metamorphopsia due to
exudative age-related maculopathy.
[F8] Central scotoma in same picture as [F7] due to a (disciform) scar in fovea. The central scar is
a common endstage of retinal hereditary or inflammatory (eg, toxoplamosis) retinal degenerations.
Note blurred peripheral picture because this image falls outside central 5 degrees around the
foveola [F2].
risk of flying particles, given that their
overall VA is dependent on their better
eye and because the better eye of an amblyopic person seems to be more often
involved in injuries than the poorer one.
Aging
The eye continues to mature after birth
and most visual functions are only fully
developed around age 30, when they start
declining again.13 This has best been inves-
©
tigated for VA, contrast sensitivity, glare,
color vision, dark adaptation, and extent of
the VF and should be understood when
considering operating motor vehicles as
well as for face recognition.14 We will discuss these aspects respectively.
Visual acuity [F6] gradually reaches its
peak with a mean acuity of 1.6 around age
15 years and gradually decreases to 0.3 by
age 90; senescent decline in VA may start
after age 40.13 There is controversy
whether this deterioration occurs due to
central or a retinal neuronal process dysfunction or optic changes in the eye itself,
and a pragmatic solution is to look first for
the latter.13 The obvious ocular causes are
diminishment of pupillary diameter in
older age and increasing lens opacities. The
size of the lens’ central nucleus, 2.6 mm
thick and 6.8 mm in diameter, allows light
through an 8-mm-wide pupil to pass differently through the lens margin. It has been
shown that the pupillary diameter decreases
by about 40% from age 20 to age 90,
which causes a reduction in light reaching
the retina by a factor of 2. The reduction in
lens transparency over the years adds another factor of 2, so that there is a net 4fold loss in luminance at the retina.
Depending on individual differences, this
may even go up to a factor of 10 or 1 log
unit, and this means that elderly people
need much more light to obtain a good VA
and to see details. More light decreases the
pupillary diameter, again reducing the retinal luminance but improving depth of
focus and point spread function of the eye.
The intensity of the light source should be
an initial question to older people who
complain that they can no longer read.
Contrast sensitivity is comparable to
VA. The difference is in measuring Snellen
acuity when maximum contrast is used
while testing spatial resolution, while in
sensitivity tests spatial resolution is fixed,
and a range of lower contrasts is tested.
Contrast sensitivity also reaches its peak at
age 30 and decreases by about 10% per
decade.13 This decrease is not due to variation in retinal illumination as described
above, but possibly due to neuronal
processes. Thus, in slight optic neuritis, eg,
due to multiple sclerosis, contrast sensitivity
is reduced earlier than VA. An important
age-related pathologic process resulting in
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Shortened Questionnaire About Visual Functions for Laboratory Staff
Yes
No
?
12. As far as I know I have good/
no good binocular stereo vision/
don't know
44
2
2
13. I can judge distances well/
poorly/ don't know
45
2
3
14a. The visual field is that solid angle
(or part of the environment) that a subject
can see with one immobile, nonrotating eye
without turning the head.
As far as I know I have no/a visual field defect
NA
NOA
2
50
14b. As far as I know I have a visual field
defect in my right eye/left eye/both eyes
14c. The defects are within/
outside the central 30 degrees
15. I have as far as I know normal color
vision yes/no
I have most difficulty in discriminating red colors/
green colors/cannot specify
16. We need relatively fast dark adaptation to see
sufficiently in twilight.
I never felt/felt seeing less in twilight than my
comrades/peers. From age (decade strata) on.
48
2
36
10
4
Questions related to laboratory work
17. Years experience in laboratory work:
stratified in < 5 y and in climbing decades.
<5y
5 < 15
15 < 25
25 < 35
35 < 45
> 45
528
9
14
13
12
1
1
18. Do you estimate that you experience
more problems than your peers in one of the
following items? If so, could you fill in to which
previous questions your problems seem to be related?
If you never perform these tasks, please write n.a.
after the boxes.
A. Performing autopsies
B. Examining tissue specimens
C. Dissecting tissue fragments <1 cm3
D. Preparing tissues for cutting, mounting, etc
E. Using the (ultra)microtome
F. Mixing chemicals or preparing special stains with
delicate colors eg, PAS
G. Examining slides under 40×, 50-100×, 110-200×,
210-400× magnification
H. Examining slides with fluorescent staining
I. Working in a room with dim light such as may be
necessary for question H
7
3
6
3
2
43
37
38
35
26
9
5
11
19
1
1
1
3
2
18
29
1
3
3
47
42
3
2
1
43
2
4
?, answered: do not know
NA, not applicable
NOA, no answer
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T2
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severe reduction of contrast sensitivity is
age-related macular degeneration.
Glare is a subjectively irritating perception of bright or poorly directed light
and can be divided into discomfort glare
and disability glare. Only in the latter case
does it obstruct vision, but in both cases it
hampers the performance of a task. The
most common cause of glare is an early
cataract. [F5]. Examples of glare encountered daily are light around a computer or
television screen placed in front of a window, a traffic light without a shield to keep
away the light from the sky around it, sunlight scattered by a dirty windshield, or the
lights of oncoming traffic at night. Glare
may also be disturbing with light reflected
off water or shiny surfaces. In the lab, glare
sources might include the backlighting of a
microscope or a lightbox for looking at
gels. Apart from subjects with early
cataracts, those with a retinal degenerative
disease often complain about glare from
surfaces or fluorescent light tubes.
Color discrimination seems to
decrease with aging. This is primarily in the
blue-violet range and thought to be mainly
due to yellowing of the lens. This is why
evaluators of the purity of diamonds are not
usually over 35 years old (unless they have
had a successful cataract extraction) and
that painters have been known to be
shocked at their paintings’ colors after a
cataract extraction. It is unlikely that this
age-related form of reduced color vision
will create many problems in the laboratory.
Dark adaptation is the property of the
visual system that allows sensitivity to light
to increase as luminance decreases [F4].
With aging, the absolute threshold of light
perception, as measured with a dark adaptation curve, rises. Even when corrected for
smaller pupil diameter and increased lens
opacity, as done in an experimental setting,
the retinal sensitivity under both photopic
and scotopic circumstances decreases with
aging.16 The former decreases roughly 0.04
log units per decade, the latter by 0.08 log u.
In daily work a subject does not usually
have the pupil and lens corrections, that
have been used experimentally. On average, a 60-year-old subject will have a dark
adaptation level that is 3 times or nearly 0.5
log unit higher than a young person and
this could be bothersome while driving to
work as well as in mesopic circumstances, such as a room used for fluorescent microscopy. In older subjects
sensitivity loss may be tenfold or 1 log
unit. Moreover dark adaptation usually is
measured at locations 10E to 12E eccentric to the foveola. We found, however,
that subjects with pathologically lowered
dark adaptation levels due to a disorder
called retinitis pigmentosa also need a
factor 10 more light to discern contrasts
in a microscope. This shows that not
only excentric but also foveal sensitivity
is reduced in these subjects.
The visual field sensitivity decreases at
a rate of about 0.7 dB per decade up to
about age 50 years.17,18 After that period,
the VF loss rate amounts to 2 dB per
decade,18 although some authors found this
only occurred after age 70 years. The extent
of the VF decreases while aging.
Older drivers, especially those over
age 60 years, recognize highway signs at
65% to 75% of the distance as compared
with those under 25 years.19 There are, of
course, many more (visual) reasons why
driving ability is diminished in the elderly,
although they meet the official requirement for driving licensing of VA > 0.5 in
the better eye. There is, however, a general consensus that most older drivers
compensate for their deficits. Many avoid
driving at night or in bad weather and
may drive slower under certain circumstances. It is clear that a person depending
on a personal car to commute to work has
less freedom in this matter.
Face recognition is another declining
capacity above age 60, partly independent
from VA.14,15 It may be that slowing of
cerebral processes makes the complicated
task of face recognition more time consuming. Although not life threatening, unless
one is not cordial enough when greeting
some thug’s friend, it is often mentioned as
socially embarrassing. It seems that contrast sensitivity plays a relatively greater
role in face recognition than VA. Subjects
with age-related maculopathy often complain about not recognizing friends. In the
early stages of age-related maculopathy
prolonged readaptation after a blinding
light source is one of the first complaints.
In the late, exudative stage, metamorphopsia or distortion of the image is quite dis-
©
turbing [F7], while at the final stage with a
central scar in the fovea a central scotoma
remains [F8].
Reactions to Questionnaire on Visual
Performance in Laboratory Work
In order to try to gain an impression
about the extent to which VA, accommodation, stereo vision, judging distances,
VF extent, color vision, and dark adaptation might influence laboratory work, a
questionnaire was designed and circulated to individual pathologists and chairpersons of departments with a request to
distribute them. The first 11 questions
were meant for demographic data and to
estimate the respondent’s visual capacities and refraction. Of the 50 respondents, 33 were male, 5 older than 55 years
of age, 1 had no driver’s license, 25
needed glasses for driving of which 24
were myopic, and 12 respondents were
presbyopic. The answers to the remaining questions are given in [T2].
Initially, the majority of the respondents seemed to have little difficulty in their
laboratory work. Fourteen percent felt that
they had more visual problems than their
peers in their work, but only 1 of these 7
respondents mentioned to which aspect of
vision this was related: congenital cataract.
One male respondent felt that he performed
on 7 of 9 items worse than his peers, without mentioning why. Two subjects with
color vision problems mentioned no problems with their laboratory work. Surprisingly, most of the problems (20% of the
responders) were mentioned with regard to
twilight vision. None of them, however,
indicated that it impaired their functioning
in the laboratory.
As no reliable information was
obtained how many questionnaires had
been handed out, the response rate was unknown. The sampling was not random.
Thus, interpretation of this small set of
mostly anonymously obtained historic data
should be approached with caution. Among
the many confounders that could be mentioned would be that people either do not
know, or do not admit, their deficiencies. In
a recent study , it was found that 5.6% of
6250 Caucasian subjects aged 55 and over
from a population study had VF loss in at
least 1 eye. Of course, the age range was
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Glossary
530
Accommodation: The ability of the eye to change its focus and, thus, to clearly see objects both near and far.
Age-related maculopathy: Degeneration of the macular area of the retina.
Amblyopia: Impairment of vision without detectable organic lesion in the eye.
Ametropic: Having a discrepancy between the size and refractive powers of the eye, so that images are not focused properly on
the retina, resulting in hypertropia, myopia, or astigmatism.
Aniseikonia: A condition in which the ocular image of an object seen by one eye is different in size and shape from the image
seen by the other eye.
Anisometropia: Having a difference in the refractive power of two eyes.
Astigmatism: A condition resulting from unequal curvature of the refractive surface of the eye: a point of light cannot be brought
to a point focus on the retina, but is spread over a more or less diffuse area.
Central field: Field of vision that is clear and unobstructed.
Diopter: A unit of refractive power of lenses, which is the reciprocal of the focal length.
Diplopia: The perception of 2 images in a single object. Sometimes called double vision.
Emmetropic: A state of correlation between the refractive system of the eye and the axial length of the eyeball so that rays of
light enter the eye parallel to the optic axis, focusing exactly on the retina.
Fixation: Direction of the eye to an object so its image falls on the fovea centralis, the depression in the center of the macula
where vision is most acute.
Foveola retinae: A small depression in the floor of the fovea centris (in the macula lutea, the area of clearest vision) devoid of
rod cells but containing rod-like elongated cones.
Hypermetropic (hyperopia): The error of refraction that causes rays of light entering the eye parallel to the optic axis to focus
behind the retina. The eyeball is too short from front to back to accommodate the refraction. The condition is popularly called farsightedness.
Luminance: The light-emitting intensity of a given surface viewed from a given direction.
Macular degeneration: Degeneration of the lutea, including the retinal cone receptors.
Mesopic: Descriptive of vision at intermediate light levels, such as at twilight.
Monovision: Different lenses (external or contact lenses) are worn for each eye: one for near vision, one for distance.
Myopic: Defect in which the focal point for light rays from a distant object is in front of the retina when the accommodative muscles are relaxed.
Optic media: The clear parts of the eye through which light passes to hit the retina.
Photopic: Pertaining to vision in the light, in which an eye is said to become light-adapted.
Presbyopia: Hyperopia and impairment of vision caused by advancing years or old age. It is dependent on the diminution of the
power of accommodation from loss of elasticity of the crystalline lens, causing the near point of distinct vision to be farther from
the eye.
Protanomalous: A form of dichromasy characterized by retention of a sensory mechanism for only blue and yellow color, with diminished ability to perceive red, green and their derivatives. In addition, the condition causes a loss of luminance and shift of brightness and
hue curves toward the short-wave end of the spectrum. Protonopia is an X-linked genetic trait that affects about 1% of males.
Retinal cone: A visual cell that serves light and color vision and visual acuity.
Retinal rods: Rod-like cells in the retina specialized for night and peripheral vision. Their density is greatest around the fovea
and decreases toward the retinal periphery.
Retinitis pigmentosa: A group of diseases — often hereditary — that are marked by progressive loss of retinal response, retinal
atrophy, attenuation of retinal vessels, and clumping of the pigment, with contraction of the field of vision.
Scotoma: An area of lost or depressed vision within the visual field, surrounded by an area of less depressed or normal vision.
Scotopic: Night vision or adaptation of vision to dark conditions.
Single vision: A single image formed of an object seen with both eyes.
Snellen acuity charts: Use of Snellen test types, in which the person looks at alternate red and green letters, with the sound eye
covered with a red glass. If the green letters are read, the acuity can be determined. Named for Hermann Snellen, a Dutch ophthalmologist, 1834-1908.
Toric: Shaped like a torus, a generally doughnut-shaped cylinder with the radius at one end larger than the other.
Visual acuity: Clarity of vision, measured as the reciprocal of the smallest angle in minutes of arc that permits the person to discriminate a contour.
Visual field: The entire area that can be seen while the eye is fixed on a target in the direct line of vision.
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different from the present study group
but it remains possible that 1 in 3 people with VF defects were not aware of
them. This has also be seen with color
vision deficits.9 Whether people are not
aware of their deficits because they
never really compared their performance with their peers, or whether they
simply do not wish to acknowledge
them, is not known. There will also
often be socio-economic differences
between countries. While in the Netherlands, employers are obliged to offer a
certain percentage of jobs to physically
impaired employees, in countries with
less protection against employment discrimination, people may tend to hide
their problems to avoid dismissal or
other adverse treatment at the
workplace. We should consider these
findings to be preliminary, given the
limitations of the current study.
In conclusion, of the many difficulties that may arise in the visual system,
few were reported to cause problems
among the 50 laboratory workers who
answered our survey.
Acknowledgements
Thanks to JJ Vos, PhD, and D Baer,
MD for comments. Thanks to AA Put for
his excellent art work in the modifications
of the anatomical lesson and in other illustrations. Rembrandt’s Anatomical Lesson
courtesy of Royal Cabinet of Paintings,
Mauritshuis, The Hague, The Netherlands.
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