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Part 5 - Analysis of visual fields data
Dr Robert Cubbidge looks at the analysis of the various data produced by modern fields machines. Module C19275, one general CET point for optometrists and dispensing opticians
Analysis of suprathreshold static perimetry
The visual field plot from suprathreshold perimetry usually consists of a series of dots or circles indicating the stimulus locations tested during the visual field examination and a numerical value showing the threshold level of the patient from which the suprathreshold level was set. Dots or open circles indicate that the patient observed the suprathreshold stimulus, whereas a closed circle indicates that the patient was unable to detect the stimulus. Some perimeters quantify the visual field loss by assessing its depth using brighter stimuli. The depth of the scotoma may therefore be identified by a decibel value or a greyscale symbol.
When single stimulus suprathreshold is carried out, the perimeter also makes measures of fixation losses, false negative and positive responses which give the examiner an indication of the reliability of the results.
Analysis of full threshold static perimetry
The advantage of full threshold automated static perimetry over suprathreshold static perimetry is the greater information it yields about the tiireshold across the visual field which enables comparison of measured data with age-matched normal data contained within the perimeter software. A number of sophisticated software packages exist to aid the practitioner in the evaluation of the final visual field outcome; each specific for a given perimeter manufacturer, the commonest being Field View (Dicon), Statpac (Humphrey) and Peri-Trend (Octopus). Basic analysis software is also contained in the Henson perimeters for full threshold evaluation. Data analysis is only possible for stimulus and grid configurations where the perimeter has age-matched normal data. It should be remembered that although these software packages provide diagnostic information, they should be considered as an aid to diagnosis, requiring clinical evaluation and decision making by the practitioner. Visual field data is provided for evaluation in a variety of ways, either by a printout or electronically via a personal computer.
Numeric data
The simplest form of data presentation is a representation of threshold values in decibels arranged in the spatial locations of the test grid. Some threshold algorithms estimate the threshold twice at...