Comparison of visual field sensitivities between the Medmont automated perimeter and the Humphrey field analyser

Background: Two commonly used perimeters in Australia and in many parts of Asia are the Humphrey field analyser II (HFA) and the Medmont automated perimeter (MAP). Each device maps the incremental light threshold of the visual field and describes the sensitivity at each point in 'decibels' (dB); however, these values are not interchangeable between devices. This study was designed to compare directly the sensitivity values of HFA and MAP visual fields. Methods: Sixty-three subjects who had suspected glaucoma, ocular hypertension or glaucoma, or were normal controls were recruited selectively. One eye from each patient was tested with the MAP and HFA in random order on the same day. Corresponding points between the two tests were identified and their sensitivities were compared. Results: Sensitivities between MAP and HFA were strongly correlated (r2 = 0.45; P < 0.0001), with the relationship between them being described by the linear equation: MAP = 0.75*(HFA) - 0.87. On average, across the entire field MAP sensitivities were 7.4 dB (standard deviation 4.6 dB) lower than HFA. However, this relationship was modified by eccentricity and field sensitivity loss. Conclusion: Visual field sensitivities for MAP and HFA may be related by a linear relationship. Theoretical and clinical predictions that this difference may be on average approximately 5 dB have been confirmed.