Visual acuity

Practical medical physiology visual acuity
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11- Visual acuity
Introduction:
Visual acuity (sharpness of vision) is the shortest distance by which two lines
can be separated and still be perceived as two lines.
The curvature of the optic lens alters to adapt the focal length to suit the
varying distance of the entering light rays and produce a clear, focused image.
Visual acuity measurement is a good screening tool because normal visual
acuity requires that all levels of the visual system function properly. The
optical system of the eye must project a sharp image of the outside world onto
the retina. The retina must then be able to translate this image into neural
impulses. Finally, the neural impulses must travel to the brain, where they are
analysed and recognized. Therefore, a wide array of different visual disorders
(but not all) can affect visual acuity, however visual acuity (tested by chart test)
is used primarily to detect refractive errors while other visual disorders is
recognised by other sophisticated techniques.
Snellen s test type, is a series of letters of different sizes, the top letter is
visible to the normal eye at 60 m., and the subsequent lines at 36, 24, 18, 12, 9
and 6 m. respectively. The letters are in lines, each line has types of same size.
The top row of types contains large letters and succeeding rows contain
gradually smaller and smaller letters, figure (11-1).
Figure 11-1: Snellen chartPractical medical physiology visual acuity
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V = d / D
V = Visual acuity.
d = distance of from the type (6 meters).
D = distance at which the eye should be capable to read it clearly.
Emmetropia means normal refractive status of the eye
Ametropia means refractive error might be caused by aberrations in the shape
of the eyeball, the shape of the cornea, and reduced flexibility of the lens.
Types of ametropia:-
Hyperopia (farsightedness):
The eye ball is shorter than normal one; the rays of light from distant object
are brought to focus behind the retina. The defect can be corrected by using
glasses with convex lenses.
Myopia (nearsightedness):
The antero-posterior diameter of the eye ball is too long; the light rays from
distant object fall in front of retina. Myopia is said to be genetic in origin. In
young adult humans the extensive close work involved in activities such as
studying accelerates the development of myopia. This defect can be corrected
by glasses with biconcave lenses, figure (11-2).
Astigmatism
Occur when the cornea is irregularly curved preventing light rays from focusing
properly on the retina. As a result, vision becomes blurred at any distance
figure (11-3).
Presbyopia
Presbyopia is the gradual loss of eyes ability to focus on nearby objects
because the lens loses its ability to accommodate for near objects. It s a
natural, often annoying part of aging. Presbyopia usually becomes noticeable
in early to mid-40s and continues to worsen until around age 65.
Objective: To examine the visual acuity of eyes.
Subjects and instruments:
1- Subjects.
2- Snellen s letter chart. 22
Methods:
1- The subject is placed at a distance of six meters from the test types (chart).
2- Each eye is tested separately.Practical medical physiology visual acuity
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3- Snellen visual acuity is expressed as 6 (the distance at which the chart is
read) over the number corresponding to the lowest line read. This indicates
the distance at which someone with normal vision should be able to read that
line, i.e. Snellen visual acuity of 6/60 indicates that at 6 metres patients can
only see letters they should be able to read 60 metres away.
4- If the person can’t see the biggest letter at 6 meters bring him to 1 meter
and visual acuity will be 1/60. If the person see nothing at 1 meter; visual
acuity is tested as:
a- Counting finger (CF).
b- Hand movement (HM).
c- Perception of light (PL).
5- If the subject wears glasses, test eyes with and without them.
Figure 11-2: hyperopia and myopia
Figure 11-3 stigmatism