A person with low vision is one who has impairment of visual functioning even after treatment of an operation and standard refractive correction and has a visual acuity of less than 6/18 to light perception or a visual field of less than 10 degree from the point of fixation but who uses or is potentially able to use vision of the planning and execution of a task.
• PARTIAL BLIND [6/24 TO 6/60]
• LOW VISION [<6/18 TO PL ]
• SUBNORMAL [ 6/18 TO PL]
WHO(World Health Organization) CLASSIFICATION DEFINES LOW VISION AS CORRESPONDING TO:
• VA of less than 6/18 but equal to or better than 3/60.
• Corresponding VA less to less than 20 degree in the better eye with best possible correction .
LOW VISION :
The best corrected VA in the better eye less than 6/18 and the vision field less than 20 degree from the point of fixation.
Refers to condition with decreased VA from blindness to partial sight.
THE BEST CORRECTED VISION ACUITY IN THE BETTER EYE LESS THAN 3/60 0R VISUAL FIELD LESS THAN 10 DEGREE FROM THE POINT OF FIXATION.
COMPLETE BLINDNESS :
It is defined as no usable vision with exception of light perception.
FUNCTIONAL EFFECT OF LOW VISION:
Loss of central vision.
Loss of peripheral vision.
Glare and contrast.
GRADES OF VISUAL IMPAIRMENT:
Category of visual Impairment (low vision) Visual acuity with best possible correction (Maximum less than the) Minimum equal to or better
6/6 (Normal) 6/18
1 6/18 (Visual impairment) 6/60
2 6/60 (severe visual impairment) 3/60- CF at 3m
3 3/60 – CF at 3 m (Partial blind) 1/60-CF at 1m
4 1/60 – CF at
1 m (Blind) Light perception(PL)
5 No PL (Total blind) ————–
MAIN CAUSES OF BLINDNESS IN 50+ POPULATIONS ARE AS FOLLOWS:
CATARACT – 62.6%
CORNEAL BLINDNESS- 0.9%
SURGICAL COMPLICATION- 1.2%
POSTERIOR SEGMENT DISORDERS- 4.7%
HOW TO PREVENT LOW VISION?
Try the 20-20-20 rule:
Every 20 minutes, look away about 20 feet in front of you for 20 seconds.This short exercise can help reduce eyestrain. Clean your hands and your contact lenses-properly. To avoid the risk of infection, always wash your hands thoroughly before putting in or taking out your contact lenses.
NOTE: low vision is vision loss that cannot be corrected with glsasses , contact lens and surgery. It isn’t blindness as limited sight remains.
TYPES OF VISION LOSS:
1.Peripheral field loss
2.Central field loss
3. Overall blurred vision
4. Hemianopic defect.
1.Peripheral vision loss: This groups refers to the patients who have lost their peripheral field of vision, totally or partially.
These defects are likely in the following pathologies :
Chorioretinitis, Colobomas, Glaucoma, Degenerative myopia, Optic atrophy(neurological), Retinitis pigmentosa, Retrobulbar neuritis etc.
The patients with peripheral loss of field will show some following sings;
• Mobility problems both indoors and outdoors.
• Slow dark adaptation
• Poor vision in dim light
• Poor near reading vision
• Glare and photophobia
2.Central vision loss: This group refers to patients who have total or partial loss of central field of vision.
These field defects are likely in the following disease: Chorioretinitis, diabetic maculopathy, hypertensive, hypertensive retinopathy, macular degeneration, degenerative myopia, optic atrophy(congenital), retrobulbar neuritis etc.
Such patients will show following sings:
• Partial perception of people and objects
• Cannot discriminate fine details for distance amd near
• Poor color vision
• Further reduction of visual acuity in bright light
3.Overall blurred vision: It means that the person has diminished visual acuity and is unable to perceive details.
Such patient will show following signs:
• Reduced visual acuity
• Reduced contrast
• Poor night vision
• Glare and photophobia
4.Hemianopic defects: It is a condition in which a person sees only one side-right or left- of the visual world of each eye.
The person may not be aware that the vision loss is happening in both eyes, not just one.
• Distorted sight
• Double vision
• Difficulty understanding what you are seeing
• Vision that appears dimmed
• Moving the body or head away from the affected side
• Visual hallucinations.