LEC 4: REPRESENTATION OF SENSITIVITY & SELECTION OF STIMULUS PARAMETERS Flashcards
True or false- Probability analysis is the most important component diagnostically?
True
5 visual field indices?
1.Mean deviation
2.Pattern Standard Deviation
3.Short-term fluctuation
4.Corrected Pattern Standard Deviation
5.Visual Field Index
What do visual field incides do?
These indices reduce all the numbers down into a single entity- collapse measured values into 1 value. This 1 value represents what is going on around the entire measured field. They are called data reduction statistics. They reduce the probability analysis data down into 1 single entity.
Which indices are shown in the results of visual field indices?
MD (mean deviation), PSD (pattern standard deviation) and VFI (visual field index).
Px with controlled hypertension, asides from the visual field, what should the optometrist look at?
Ocular hypertension, structural abnormality- optic nerve, looking for cup within optic nerve head and retinal nerve fiber thickness.
2 structural effects of glaucoma?
Glaucoma increases cupping of optic nerve head and reduces retinal nerve fiber layer thickness.
Damage to superior visual field- what section in the optic nerve head is examined?
Inferior optic nerve head
Between the probability map of total and pattern deviation- which map shows focal loss and which map shows generalized loss?
Total deviation representing generalized loss and focal loss and pattern deviation shows only focal loss.
What is the most common cause for generalized loss in probability map?
Cataract
What is mean deviation?
*Weighted mean of the Total Deviation values, (which are the difference between the measured value and the average normal value for the age of the patient).
*Signifies overall severity of loss (diffuse + focal)
What is the theoretical value if mean deviation was normal, effect of MD value as field becomes worse?
If normal theoretically- the value would be 0dB.
As field becomes worse, value becomes more negative.
MD (Mean deviation) is weighted towards central deviation more, why?
Central deviations are diagnostically more important than those at the edges and the Px is likely to generate these values more accurately than those at the edges. THAT IS WHY IT IS WEIGHTED
MD (Mean deviation) shows focal and general loss
Both focal and general loss
Describe the math’s behind calculating MD?
Add up all the total deviations and divide by the number of locations.
Before you add the numbers- more weight applied to central deviations.
Mean deviation is what in terms of octopus perimeter? +ve values in this octopus perimeter corresponds to?
Mean defect
+Ve value is abnormal.
Octopus perimeter uses unweighted or weighted mean for MD?
Unweighted mean of the difference between the average normal value for the age of the patient and the measured value.
Relationship between standard deviation and mean?
Wider the SD the less representative the mean is
What is patterned standard deviation (PSD)?
The Standard Deviation (mean) of the values in the Total Deviation plot.
PSD is indicative of?
Indication of the non-uniformity. Demonstrates whether the Total Deviation values are all approximately the same (PSD ≈ 1.0dB) or are different from one another
Relation between focal loss and PSD?
As focal loss increases, PSD becomes increasingly more +ve
PSD Value greater than 12dB?
Greater than 12 dB suggests visual field loss is due to focal loss becoming more diffused.
What does mean and standard deviation represent?
Mean: Statical uniformity
Standard deviation: gives the non- uniformity of mean.
Wider the SD the less reprehensive the mean is.
What is short term fluctuation index?
This incides is no longer used.
It is the weighted SD of repeated measurements of sensitivity at different locations. It measures how inconsistent a Px was with responses.
3 factors that increase fluctuations/ variability in threshold?
- (Relative) defect depth- Deeper the defect less consistent Px responses.
- Eccentricity- Variability gets worse in the periphery compared to the center.
- Overall severity of field loss
