a

The above indices are taken from the visual field you have just studied. What do you understand by:
a. Central 24-2 as compared with Central 24-1?
b. Fixation losses?
c. False positive and negative errors, and what are the
    normal values?

Answer a
Central 24-2 refers to the test pattern. In central 24-2, 
the central 24 degrees of fixation is tested and on either
side of meridian. Whereas central 24-1 tests the central 
24 degrees as well as test on the meridian.
Fixation loss refers to positive response to blind spot stimulation.
This is caused by patient moving his/her eye. The normal value
should be less than 20%. Higher value invalidates the test.
False positive occurs when there is positive response but no stimuli. 
This occurs in patients who are" trigger happy". The normal value
should be less than 33%.
False negative refers to negative response with brighter than threshold 
stimuli. This occurs in patients who are not concentrating. Again the normal value should be 33%. 
 

What is the difference between threshold and suprathreshold tests?

Answer 
In threshold test, stimuli intensity increases in 4dB steps until threshold
is crossed ie when the patient see stimuli. Threshold is recrossed with
stimuli intensity decreasing in 2 dB steps. It produces quantitative 
information, the level of sensitivity at the test point and therefore a reliable
baseline for future reference. However, it takes longer than a suprathreshold test.
Suprathreshold test is used as a screening test. It presents stimuli at 6 dB higher than expected threshold. Each tested point is recorded as normal or 
abnormal. It can determine with reasonable certainty if glaucomatous visual field exist. It does not provide quantitative information and therefore not reliable as a baseline for future comparison.

 
 

The above picture is taken from the perimetry you studied earlier. What do you understand by:
a. total deviation
b. pattern deviation?

Answer 
Total deviation refers to age-corrected nromal dB minus
the raw threshold value for example if the age corrected 
normal is 4dB and the point tested is 8dB. The value of
that point is -4dB ie. -4dB less than what is normal at that point.
Pattern deviation is teh total deviation adjusted upwards or 
downards topick up localized defects. This helps to remove 
the loss caused by loss of  sensitivity. 
 

Is it possible to have abnormal total deviation but normal pattern deviation? What could be the cause?

Answer
Yes. It indicates generalized loss of sensitivity as seen in cataract.
 
 

What do you understand by the above:
a. MD
b. PSD?

Answer 
MD stands for mean deviation and refers to average deviaiton of each 
point from age-corrected normal. Here a value of -0.17 dB means that
on average each point has a 0.17 dB lower threshold than normal

PSD stands for pattern standard deviation and refers to standard deviation

of each point from age-corrected normal. It measures the variablity of the 
visual field. The higher the value, the more irregular the field and this is 
seen in glaucomatous field defect.