TOPIC 9: LOW VISION AND CLINICAL ROUTINE Flashcards
What VA is considered mild, moderate, severe vision impairment, and blindness?
Mild:better than 6/18
Moderate: worse than 6/18, better than 6/60
Severe: worse than 6/60, better than 3/60 or 6/120
Blindness: worse than 3/60 or 6/120
What are the major causes of blindness?
Refractive error 43%
Cataract 33%
Glaucoma 2%
Why do we need the following information during a LV consult?
a. Onset of visual impairment
b. Stability of vision
a. Onset of visual impairment
determine whether vision loss is congenital vs acquired), earlier onset may also mean that they are more adapted to the poor vision
b. Stability of vision
if vision is unstable, prescribing an LVA with fixed magnification may not last very long as vision might deteriorate soon after
Why is Snellen VA chart not ideal for LV VA measurement
relatively few letters in large sizes
different task at different sizes (number of letters per row changes)
large and variable size difference between adjacent rows
What is acuity reserve? What is the acuity reserve needed for fluent and survival reading?
This is the Ratio of size of letters to be read to the smallest letters patient can see
fluent: 2:1
survival: 1.3:1
What is contrast reserve? What is the contrast reserve needed for fluent and survival reading?
This is the Ratio of contrast of text letters to be read to the faintest letters patient can see
fluent: 10:1
survival: 3:1
What field of view is needed for fluent and survival reading?
The field of view is determined by either the eye condition (e.g. presence of scotoma, tunnel vision), or by the characteristics of the magnifier (diameter of magnifier, amount of magnification)
fluent: 4-6
survival: 1
Why is it necessary to perform refraction at a distance where at least 4 lines of letters on the VA chart can be seen?
a. How to compensate the result for infinity if you refracted at <6 m?
how to convert logmar to snellen?
This gives you some ‘buffer’ rows of letters to check if there is improvement/worsening of acuity when power is changed.
-half the distance and then convert back to 6m (snellen)
-The ETDRS chart is designed to be used at 4m. If you halve the viewing distance, add 0.3 to score.
To convert 1.6 log MAR to Snellen: [(10^1.6) x 6] = 238…… (~6/240)
To convert 6/240 Snellen to log MAR: log [1/ (6/240)] = 1.6 log MAR
How to determine minimum noticeable difference in power for subjective refraction?
divide denominator of snellen VA at 6m by 60.
Conventional methods such as +1.00D fog, duochrome and binocular balancing will not work with visually impaired patients.
instead ask the client to compare his old Rx with the new Rx to decide whether his old glasses need to be updated.
Why does pinhole not always improve vision? what if pinhole worsens vision?
pinhole only improves vision if the cause for reduced vision is refractive error.
in cases of eye diseases, Pin hole will not improve vision.
If the pinhole worsens vision, this can indicate macular disease, central lens opacities or other causes of reduced vision
How many lines of improvement should the low vision patient experience to justify updating their spectacle prescription?
Only change to new Rx if there is at least 2 lines improvement in VA compared to old Rx.
Why do we need the following information during a LV consult?
c. Knowledge of condition & prognosis
d. Past, current and future treatment for the condition
c. Knowledge of condition & prognosis
Different diseases cause different type of vision loss. eg AMD = central scotoma, Glaucoma = good VA but constricted visual field
d. Past, current and future treatment for the condition
if treatment is ongoing, may not be a good idea to prescribe fixed magnification LVA as vision could be unstable.
Why do we need the following information during a LV consult?
e. Occupation / visual task
f. Past and present LV aids used
g. General health & medications used
e. Occupation / visual task
different occupations have different visual demand/tasks)
-Student, office worker etc
f. Past and present LV aids used
Don’t re-prescribe a LVA that had been tried (and failed) in the past. very depressing ! Explore other options instead
g. General health & medications used
any trembling hands? (you don’t want to prescribe handheld magnifier to someone who cannot hold it steadily!), does the client needs to read his/her medication label (he may need higher magnification to read small print on medication labels)
why do we need to manage px expectations when we give LVA? goal of LVA?
Low vision aids (LVAs) are TASK-SPECIFIC. Hence it is crucial to identify the task precisely and manage the client’s expectations, and ensure they understand that
- they need to put in effort to use the LVAs
- goal of LVAs aim to maximise their remaining vision to remove or minimise ‘activity limitation’. LVAs cannot restore lost vision.
how to modify your prescription for infinity if you refracted at a shorter working distance (<6m)
formula for compensation needed: 1/(m)distance
if you refracted at 2m, add -0.50D to the prescription…
