FDT/Matrix and Proper Screening Techniques Flashcards
about 20% of patients do not respond well to FDT/Matrix. at UEC, they should use what instead?
HFA using C40 screening program
according to the selective loss theory, glaucoma causes selective early loss of what type of nerve fibers?
large diameter
according to the selective loss theory, what cell pathway may be selectively damaged in glaucoma?
M-cell pathway
why FDT may detect glaucoma early according to the reduced redundancy theory
there is such a large amount of redundancy (overlapping receptive fields) that a small subset of all ganglion cells should be stimulated to detect early VF loss
what is important about the C24-2-5 Matrix test?
it has a smaller stimuli (5x5) than C20/N30 (10x10) but we get more artifactous misses on screening so DONT USE IT
how is the Matrix stimuli location different than FDT
Matrix stimuli are offset from the midlines to prevent “spill” of normal VF over the midline in neurological VF loss
describe the FDT stimulus spatial and temporal properties
- low spatial frequency sinusoidal grating (0.25 cycle/degree)
- rapid counterphase shift-high temporal frequency (25 Hz)
how many stimuli are on the FDT C20-5 versus FDT N30?
C20-5 has 17
N30 has 19
why does the frequency doubling illusion occur
My cells do not respond linearly so you have the illusion of twice as many bars as are actually present
about ___% of ON is M cells
15
what % of M cells have nonlinear responses
15 to 25%
what % of all ganglion cells respond with the frequency doubling illusion
3 to 5%
when do you retest FDT?
if there are any misses, after a brief break
how can the troxler phenomenon affect FDT?
particularly on 2nd eye tested, pt may report “dimming out, black out”
important things to remind patient to help with troxler phenomenon problems
remind patient to blink any time they need and best to blink as they hit button
how can you help with retinal rivalry that also contribute to the 2nd eye artifact on FDT?
occlude the untested eye with a patch
what types of misses are significant in FDT?
any miss of any stimulus at any contrast level should be considered significant
how does the sensitivity of the automated perimetry increase logarithmically?
with the number of test points
but limited by time
sensitivity and specificity values for HFA C40 test
sensitivity: greater or equal to 85%
specificity: 95%
HFA C40 criterion for VF defect
- any one miss in central 20
- two adjacent misses outside of central 20
common causes of general depression
- blur (wrong trial lens)
- media opacity
- small pupil (<3mm)
- fatigue
- age
- wrong age keyed into perimetry (lower age than actual)
how does the HFA stimulate a testing distance of infinity
bowl, 30cm from patients eye, and a thick plus lens in front of patient’s eye stimulates infinity viewing
for trial lens on automated screening, what is the rule on astigmatism correction?
if equal or greater to 1 D, use full cylinder power
for trial lens on automated screening, what do you use for a patient that is dilated and why?
assume full cycloplegia and use full add of +3.00 for HFA
for trial lens on automated screening, what is best to use if the patient has greater than + or - 8D
contact lens vertexed to the cornea
for the HFA, many of the lens rim artifacts are found:
inferior and temporal to BS
most commonly, missed points temporal to the BS are from the
lens rim artifact (not due to disease)
additional changes intern must make to the VF section in exam writer
- correct screening VF run
- describe location and p value of misses
- describe reliability of the test for each eye
- scan in VF print out any time there is any miss
