WHAT IS VISUAL ACUITY?
Visual acuity is the examination that checks how well you see the details of a letter or symbols from a specific distance.
Visual acuity can also be referred to your ability to discern the shapes and details of the things you see.
According to WHO ( world health organization) Visual acuity can be classified into four (4) types, namely:-
Mild- visual acuity worse than 6/12 – 6/18.
Moderate- Visual acuity worse than 6/18 – 6/60.
Severe- Visual acuity worse than 6/60 – 3/60.
Blindness- Visual acuity worse than 3/60
A distant visual acuity is usually tested by snellen test types.
PRINCIPLES
The fact that two distant point can be visible are separate only when they subtend an angle of 1 minute at the nodal point of the eye, forms the basis of the snellen test type.
It consists of a series of black capital letter on a white board, arranged in lines, its progressively diminishing in size. The lines comprising the letters have such a breadth that they will subtend an angle of one minute at the nodal point.
Each letter of the chart is so designed that it fits in a square,
NEAR VISUAL ACUITY
Visual
acuity for near vision is tested by asking the patient to read the near vision
chart, kept at a distance of 35cm in good illumination, with each eye
separately. In near vision chart, a series of different size of printer types
are arranged in increasing order and marked accordingly.
TYPES OF VISUAL ACUTY TEST
I.
Snellen
Chart
III. Cardiff Card
II.
E Chart
IV. Pin
Hole
REQUIREMENTS/EQUIPEMENTS FOR VISUAL ACQUITY TEST
• A
comfortable space that is at least 20feets (6metres).
• Visual
acuity chart
• A pointer
• A Pen light
• Pen and
patient chart
• An occlude
• Trial frame
WHO CAN PERFORM THIS EXAMINATION?
• An
ophthalmologist
• An
optometrist
• An optician
• A
technician
• An
ophthalmic Nurse
• A CHEW
• A trained
health provider
VISUAL ACQUITY PRACTICAL PROCEEDURE
• Identify
and welcome the patient.
• Observe the
patient and ensure the patient is comfortable.
• Position
the patient 20feets from and illuminated snellen chart.
• Have the
patient cover the left eye with an accluder or with the palm.
• Ask the
patient to read the letters from left to right, up to the last line.
• Record the
corresponding acuity fraction as the number of letters missed e.g: 6/12-1.
• Record the
acuity value for each eye separately.
• If the
patient cannot read the largest letter at 6 metres, he should be asked to walk
towards the types. For example if he sees the top letter(60) from a distance of
3metres then VA =3/60 where the 3 is the distance (meters) and the 60 is the
letters.
• If not
possible-VA=CF (count fingers) at 1metre • If no possible-VA=HM (hand
movement) 6m-1m.
• If not
possible-VA=PL (perception of light).
• If not
possible-VA= N PL (No perception of light) this is total blindness.
To be continued….