PC - Clinical Assessment of Contrast Sensitivity - Week 5 Flashcards
True or false
It is not possible to have normal visual acuity but reduced contrast sensitivity.
False, some patients have normal VA but reduced contrast sensitivity.
Contrast sensitivity often used in assessing patients with what condition?
Cataracts.
At which spatial frequencies does spherical refractive error cause a loss in?
High spatial frequencies.
What conditions cause a loss at medium to low spatial frequencies? What is a consequence of this?
AMD, macular oedema, and cataracts.
VA may still be good, resulting in a hidden loss.
A notch defect is a result of what two errors from what kind of lens?
A rare defect that is the result of astigmatic refractive error and monocular diplopia from bifocal contact lens.
Can assessment of contrast sensitivity differentiate between diseases it can be used to screen for?
No, it only gives an indication there may be pathology.
Name four reasons why contrast sensitivity should be assessed.
- CS cannot always be predicted from other measures such as VA and visual field tests
- Screening for early ocular disorders, isnt able to provide differential diagnosis
- To monitor visual function
- To preduct vision-related ability
Consider a contrast sensitivity function. What 5 factors affect the shape of the function? Will the shape of the CSF always remain the same if test conditions are the same?
-Luminance
-Temporal characteristics
-Target size
-Grating motion
-Grating shape
Shape remains the same if test conditions arent changed, a change indicates there is vision dysfunction.
What is a disadvantage of measuring contrast sensitivity functions, and are they used clinically?
Very time consuming to measure the the whole function.
They are useful in research, but not practical clinically.
Define the Pelli Robson method for measuring contrast sensitivity.
Describe the working distances and instructions to patients.
Define the final measurement and how it is made.
A series of equally sized letters are presented with gradually decreasing contrasts.
The patient is asked to read at 1 metre, RE then LE.
They read down the chart until they are no longer able to.
Final measurement is based on the last correct triplet, recorded in dB.
On a Pelli Robson chart, what is a normal result for <50? What about >50?
<50 - 1.80 log units
>50 - 1.65 log units
Describe the Melbourne Edge test, the working distance used, and the final measurement.
Circles are presented on a chart with different contrast halves (half moons).
The contrast progressively decreases.
Patient is presented the chart at 40cm and asked the orientation of the edge.
The last correct answer is recorded in dB.
What is the normal result for patients under 65 on a Melbourne Edge Test?
What about over 65?
<65 - 19-22dB
>65 - 16-18dB
Define 2 abnormal results on a Melbourne Edge Test.
Define a severe impairment measurement for a Melbourne Edge Test.
Abormal
-<15dB - significant CS loss, needing differential diagnosis
-Change in 2 or more dB between tests
Severe Impairment
-<10dB indicating difficulties with everyday tasks
Describe the Australian Vision Chart method, the working distance, and how the final measurement is recorded.
Two logMAR charts are presented side by side. One is high contrast, the other is low.
Patients are presented the chart at 40-60cm, and asked to read to limit.
The line difference between the two charts is recorded.
