TOPIC 10: PRESCRIBING MAGNIFIERS Flashcards
How to calculate relative size magnification and relative distance magnification
relative size magnification: = (π§ππ° π¨ππ£πππ π¬π’π³π )/(π¨π₯π π¨ππ£πππ π¬π’π³π)
relative distance magnification= (π¨π₯π π―π’ππ°π’π§π ππ’π¬πππ§ππ )/(π§ππ° π―π’ππ°π’π§π ππ’π¬πππ§ππ)
What are the 3 steps to determine how much magnification a patient needs?
- identify the visual task and target acuity for the task
common acuity equivalents:
Bus numbers: 6/12
Medicine label: N4 (6/7.5)
Newspaper: N8 (6/15)
Remember to take into account ACUITY RESERVE when determining the target acuity:
Survival/ spot reading: 1.3 : 1
Fluent reading: 2 : 1
- calculate the amount of magnification needed to achieve the target VA
If you have measured VA in Snellen notation,
Magnification required = current Snellen denominator at 6m / required Snellen denominator at 6m
If using logMAR notation, the formula to calculate the amount of magnification to prescribe is:
Magnification required = 1.25 number of steps
Near chart Magnification required = current NVA / Required NVA
- select the LVA for the task
Telescopes are the only LVA for distance task, various LVAs are available for near
What are the possible reasons why the patient doesnβt achieve the target VA with the LV aid?
For distance:
1. Low light due to loss by reflection due to multiple lenses inside the telescope (some light is lost at each optical interface)
2. Inaccurate focusing as the patient may have difficulty adjusting the focus of the telescope correctly
3. Hand tremor. Patient may have pre-existing condition that affects coordination, such as Parkinsonβs disease.
4. βIncorrectβ acuity measurement. If acuity was measured using Snellen chart, the unequal jump in acuity between lines limits how precisely we have measured the patientβs acuity
For near:
-distance VA is measured using a letter chart whereas NVA is measured using a word chart (the task is different)
-Words may be easier as the patient can guess the word from context
-Words may be more difficult due to 1) crowding effect from closely spaced letters.
Near acuity may also be much worse if the patient has a 2) central scotoma that obscures some letters, or, 3) central media opacity (the effect on vision is more pronounced when the pupil constrict at near).
What kind of correction (SVD / SVN) should be used with what type of LV aid? (telescopic, handheld, stand, spec mounted, brightfield and bar)
-Telescopic should be DISTANCE
Telescope can be adjusted and compensate for spherical ametropia
However, focusing for high ametropia can increase or decrease magnification β depends on the type of telescope and refractive error
Cylinder Power ~1.50 unlikely to influence VA
Vertex distance less and FOV greater without Rx
-Hand held should be DISTANCE
-Stand magnifier should be NEAR
-Spec-mounted plus lens REPLACES ametropic correction but takes it into account
E.g. patient is +4.00D hyperope, using a 3x magnifier.
3x magnifier is actually a +12D lens. The patient borrows +4.00 from the magnifier to correct his hyperopia, leaving +8.00D for magnification.
+8D lens only gives (8/4)=2x magnification.
-Bright field and Bar magnifiers should be NEAR
How to calculate total magnification
The final magnification is calculated by multiplying the magnification amount of both methods together.
E.g. magnification obtained by changing print size from 6mm to 15mm, and viewing distance from 30cm to 10cm
Mag = new object size = 15 = 2.5x
old object size 6
Mag = old viewing distance = 30 = 3x
new viewing distance 10
total magnification = 2.5 x 3 = 7.5x
how to calculate estimated near VA from distance VA?
distance VA denominator/3
e.g 36/3 = 12
hence near estimated VA is N12
