Perimetry knows a wide range of "image types": grayscale, defect map, indices, cumulativ curve... They are always based on 1 of 3 potential basic values. The basic value is decisive for the meaning of a visual field representation.
Threshold values are the untreated measured values of the examination (raw data). The importance is more traditional: In the 1970 decade, threshold values were the only representation option for the young computer perimetry
Threshold values depend on many factors:
- Position of the measured point (normal value is lower in the periphery than in the center, nasal lower than temporal)
- Age of patient
- Instrument type
- Stimulus size
- Background luminance and other parameters
- Strategy (measurement algorithm)
- Damage
- individual variation
- Measurement error
Defect depth values are the deviation of the measured values from the age-corrected normal value for the instrument type at the specific measurement point. In case of double measurement, the average measured value is taken. The formula is:
measured value – normal value
Therefore, defect depth values are already corrected for measurement position, age, instrument, parameter and strategy. The values only depend on:
- Damage
- individual variation (usually less than 1 db shift)
- Measurement error
Pattern deviation is derived from total deviation (defect depth) by shifting all values with a shift value. The formula is:
total deviation (defect depth) - shift value
The shift value is calculated in a way to get the better values in the field being around zero damage pattern is better visible by the shifting, because the diffuse damage is removed from total damage.
NOTE: Pattern deviation values are mostly lower than total deviation values. It is possible to underestimate the defect depth and the damage
NOTE: When you compare visual fields in a follow-up: the shift value is different for each examination. Pattern deviation values may differ even when the actual defect depth is equal - and vice versa. Therefore, the PeriData Trend Analysis does not use pattern deviation values.
NOTE: There is no generally recognised calculation method. Therefore, different instrument types use different calculation methods. PeriData uses the percentile 20 as standard reference value for all devices, i.e. the corrected deviation of percentile 20 is 0dB.