Transcript of Perimetry visual field Akram Rismanchian MD Farabi Hospital.
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- Perimetry visual field Akram Rismanchian MD Farabi
Hospital
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- The field of vision is defined as the area that is perceived
simultaneously by a fixating eye. The limits of the normal field of
vision are 60 into the superior field, 75 into the inferior field,
110 temporally, and 60 nasally
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- THE NORMAL VISUAL FIELD. Traquair, in his classic thesis,
described an island of vision in the sea of darkness. The island
represents the perceived field of vision, and the sea of darkness
is the surrounding areas that are not seen. In the light-adapted
state, the island of vision has a steep central peak that
corresponds to the fovea, the area of greatest retinal
sensitivity.
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- The single filed printout
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- There are several different packets of information present on
the printout, including Basic identification of patient and test
Indicators of test reliability Raw unprocessed threshold
sensitivity measurements
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- Deviations of measured sensitivity from age normal ( total
deviation ) Deviation from normal after adjustment for the patients
overall sensitivity( pattern deviation ) Overall indices of
normally ( global indices ) Plain language analysis ( the glaucoma
hemi field test )
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- STRATEGY Fastpac _ Full Threshold Sita _ Swap
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- Reliability parameters fixation loss fixation loss false
positive response errors false positive response errors false
negative response error
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- Gaze stability The accuracy of the test result can be strongly
affected by the strength or weakness of the patients ability to
stay focused on the fixation target during test The Heiji Krakau
blind spot method can be used to monitor and record a patients
fixation loss ( FL ) rate during the test
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- Perimeters equipped with a gaze monitor show fixation errors on
the screen during the test so that perimetrist can make adjustments
or instruct the patients when necessary A tracing appears at the
bottom of the printout
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- False positive error rates The perimetric false false positive
(FP) error rate in the non SITA strategies is the frequency with
which the patient presses during FP catch trials, which are pauses
during which no stimulus is presented The letter X is printed twice
next to the FP ratio when it exceeds 33%
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- The SITA strategies do not make explicit FP catch trials but
calculate FP rates from responses at unexpected times during the
test m
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- False negative error rates The perimetric false negative (F N)
error rates the frequency with which the patient fails to press the
response button when a visible stimulus is presented
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- For tests run using the full threshold and Fastpac
strategies.the false negative rate (FN) is expressed the printout
as the ratio of the number of times patients failed to respond to a
stimulus 9 db more intense than the previously determined threshold
estimate divided by the number of such FN catch trials As with
false positive responses an XX appears next FN rate on the printout
when it exceeds 33%
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- For tests using the SITA strategies the result is displayed as
a percentage instead of ratio The recorded parameter more closely
represents the percentage of time the patient fails respond to
visible stimuli because of inconsistency or inattentiveness because
the strategy dose not consider any failure to respond during the
test that might be caused by disease
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- Test labeled as having low reliability Filed were excluded if
the fixation loss rate was greater than 20% or if either the false
positive rate or the false positive rate the false negative rate
exceed 33%( in sita stratgy false +ve =8% false ve =12% )
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- Raw test results a) Threshold sensitivity values Decibel
threshold sensitivity values are displayed as a ( sensitivity value
table ), except that the sensitivity value at the fovea is listed
over to the side along with the reliability parameters b) Gray
scale
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- Total deviation ( from normal values for age) The total
deviation display has two parts : a table of numeric values, which
represents the actual decibel deviation from age normal, and below
that, a probability plot showing symbols that indicate the
statistical significance of each measured deviation
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- Deviations of less than 5 db may be note worthy near the center
of the filed and that even 10 db deviations near the edge of the
visual field may well within the normal range
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- Pattern deviation ( localized loss ) The function of the
pattern deviation plot is to expose localized defects that may be
masked by either a generalized depression or an elevation of the
hill of vision This is accomplished by making an adjustment of the
threshold values according to the general height ( GH) of the
visual filed
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- Thus, of the 51 points that are finally considered, the seventh
highest sensitivity value relative to age normal is taken to
represent the overall general height of hill of vision
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- A key to the probability symbols is shown near the bottom of
the printout. The symbols increase in darkness as the deviation
become more significant
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- Marking a test location with the symbol for p