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Grodno State Medical University
VISUAL FUNCTIONS.
CLINIC REFRACTION.
Pavel Ch. Zavadski
Assistant lecturer
Of the Department of Ophthalmology
VISUAL FUNCTIONS
VISUAL ACUITY
Visual acuity (VA) is the easiest to perform and most important test of
visual function. Spatial visual acuity is the ability to distinguish separate elements
of a target and identify it as a whole. It is quantified by the minimum angle of
separation (subtended at the nodal point of the eye) between two objects that
allow them to be perceived as separate.
Early astronomers found two stars resolvable as separate if the distance
between the stars subtend an angle of at least 1 minute of arc
The standard test is the Snellen chart, consisting of rows of letters of
decreasing size. Each row is numbered with the distance in meters at which each
letter width subtends 1 minute of arc at the eye. Acuity is recorded as the reading
distance (e.g. 6 meters) over the row number, of the smallest letter seen. If this is
the 6 meter line, then VA is 6/6; if it is the 60 meter line then VA is 6/60.
VISUAL ACUITY
VISUAL ACUITY
Peripheral vision is a part of vision that occurs outside the very
center of gaze.
There is a broad set of non-central points in the field of view that
is included in the notion of peripheral vision.
"Far peripheral" vision exists at the edges of the field of view,
"mid-peripheral" vision exists in the middle of the field of view, and "nearperipheral", sometimes referred to as "para-central" vision, exists adjacent
to the center of gaze.
PERIFERAL VISION
Each field can be represented as a series of contours or isopters,
demonstrating the ability to resolve a target of given size and brightness.
The field is not flat; towards the center the eye is able to detect much
smaller objects than at the periphery. This produces a ‘hill of vision’ in
which objects which are resolved in finest detail are at the peak of the hill
(at the fovea).
On the temporal side of the field is the blind spot. This corresponds to the
optic nerve head where there is an absence of photoreceptors. The visual
field may be tested in various ways.
PERIFERAL VISION
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PERIFERAL VISION
These machines permit more accurate plotting of the visual field. They
measure:
•The kinetic visual field in which the patient indicates when he first sees a
light of a specific size and brightness brought in from the periphery. This is
rather like the moving pinhead of the confrontation test.
•The static visual field in which the patient indicates when he first sees a
stationary light of increasing brightness. These techniques are particularly
useful in chronic ocular and neurological conditions to monitor changes in
the visual field (e.g. in glaucoma).
PERIFERAL VISION
COLOUR VISION
Colour vision testing is sometimes useful in the clinical evaluation of
hereditary fundus dystrophies, where impairment may be present prior to the
development of visual acuity and visual field changes.
Colour vision is a function of three populations of retinal cones each with
its specific sensitivity: blue (tritan) at 414-424nm, green (deuteran) 522-539 nm
and red (protan) at 549-570nm.
A normal person requires all these primary colours to match those
within the spectrum. Any given cone pigment may be deficient (e.g. protanomaly
- red weakness) or entirely absent (e.g. protanopia - red blindness). Trichromats
possess all three types of cones (although not necessarily functioning perfectly)
while absence of one or two types of cones renders an individual a dichromat or a
monochromat respectively.
Most individuals with congenital colour defects are anomalous
trichromats and use abnormal proportions of the three primary colours to match
those in the light spectrum. Those with red-green dellciency caused by
abnormality of red-sensitive cones are protanomalous, those with abnormality of
green-sensitive cones are deuteranomalous and those with blue-green dellciency
caused by abnormality of blue-sensitive cones are tritanomalous.
BINOCULAR VISION
Binocular vision is the coordinated use of the two eyes in order to
produce a single unified image in three dimensions. It is not present at birth.
Three factors are required for its development: reasonably clear vision in both
eyes, precise co-ordination between the eyes for all directions of gaze, ability
of the visual areas of brain to promote fusion of the two slightly dissimilar
objects.
The three advantages of binocular vision are:
1) an enlarged field of vision with compensation of each blind spot,
2) an ability to appreciate depth with stereoscopic vision,
3) a combined binocular visual acuity is better than the uniocular visual acuity.
COLOUR VISION
Ishihara test is used mainly to screen for congenital
protan and deuteran defects. It consists of a test plate
followed by 16 plates each with a matrix of dots arranged
to show a central shape or number which the subject is
asked to identify. A colour deficient person will only be
able to identify some of the figures. Inability to identify the
test plate (provided visual acuity is sufficient) indicates
malingering.
Hardy-Rand-Rittler is similar to Ishihara but more
sensitive since it can detect all three congenital defects.
City University test consists of ten plates each
containing a central colour and four peripheral colours.
The subject selects one of the peripheral colours which
most closely matches the central colour.
DARK AND LIGHT ADAPTATION
Dark adaptation is the phenomenon by which the visual system
(pupil, retina and occipital cortex) adapts to decreased illumination.
This test is particularly useful in the investigation of patients
complaining of night-blindness (nyctalopia). The retina is exposed to an
intense light for a time sufficient to bleach 15% or more of the rhodopsin in
the retina. Following this normal rods are Insensitive to light and cones
respond only to very bright stimuli. Subsequent recovery of light sensitivity
can be monitored by placing the subject in the dark and periodically
presenting spots of light of varying intensity in the visual field and asking the
subject if they are perceived.
Technique of Goldmann-Weekes adaptometry
a. The subject is exposed to an intense light that
bleaches the photoreceptors and then suddenly
placed in the dark.
b. The threshold at which the subject just
perceives the light is plotted.
c. The flashes are repeated at regular intervals:
the sensitivity of the eye to light gradually
increases.
CLINICAL OPTICS
Light can be defined as that part of the electro-magnetic spectrum to
which the eye is sensitive. The visible part of the spectrum lies in the
waveband of 390 nm to 760 nm.
For the eye to generate accurate visual information light must be
correctly focused on the retina. The focus must be adjustable to allow equally
clear vision of near and distant objects.
The cornea, or actually the air/tear interface is responsible for twothirds and the crystalline lens for one-third of the focusing power of the eye.
These two refracting elements in the eye converge the rays of light
because:
1) The cornea has a higher refractive index than air; the lens has a
higher refractive index than the aqueous and vitreous humours that surround
it. The velocity of light is reduced in a dense medium so that light is refracted
towards the normal. When passing from the air to the cornea or aqueous
to lens the rays therefore converge.
2) The refracting surfaces of the cornea and lens are spherically
convex.
CLINICAL OPTICS
When parallel rays of light from a distant object are brought to
focus on the retina with the eye at rest (i.e. not accommodating) the
refractive state of the eye is known as emmetropia. Such an individual can
see sharply in the distance.
CLINICAL OPTICS
In ametropia, parallel rays of light are not brought to a focus on
the retina in an eye at rest. A change in refraction is required to achieve
sharp vision.
Ametropia may be divided into:
1) Myopia (short sightedness); the optical power of the eye is too
high (usually due to an elongated globe) and parallel rays of light are
brought to a focus in front of the retina
2) Hypermetropia (long sightedness); the optical power is too low
(usually because the eye is too short) and parallel rays of light converge
towards a point behind the retina.
3) Astigmatism; the optical power of the cornea in different
planes is not equal. Parallel rays of light passing through these different
planes are brought to different points of focus.
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CLINICAL OPTICS
All three types of ametropia can be corrected by wearing spectacle
lenses.
These diverge the rays in myopia, converge the rays in
hypermetropia and correct for the non-spherical shape of the cornea in
astigmatism It should be noted that in hypermetropia, accommodative effort
will bring distant objects into focus by increasing the power of the lens.
This will use up the accommodative reserve for near objects.
Thank You
For Your Attention