Advances in Automated Perimetry

Visual field testing is an important tool for detecting and monitoring progression of glaucoma, and standard threshold automated perimetry (SAP) is the cornerstone of visual field testing. SAP determines the minimum luminescence needed to detect a static white light stimulus in the visual field. However, SAP is a long test (15-20 minutes per eye), depends heavily on patient reliability, and requires careful monitoring by the technician. As a result, it is difficult to obtain meaningful data in many patients or monitor disease progression in multiple tests. The ideal visual field test would be fast, cheap, easy to use, relevant to the whole population, and of course highly sensitive and specific.

Advances in automated perimetry have reduced testing times and increased accuracy in visual field testing. One of the major such advances has been the introduction of new software algorithms. Several algorithms are commercially available, such as the Swedish Threshold Interactive Algorithm (SITA), SITA Fast, and the FastPAC. Perhaps the most popular algorithm used in SAP is the SITA. SITA is a strategy designed to reduce testing time substantially without decreasing test quality. In SITA, a probability model of the threshold value is constructed from available knowledge of normal and glaucomatous visual fields. This model is then continuously modified and updated throughout the test as the patient responds to stimuli, generating an estimate of the threshold at each point. SITA significantly reduces the testing time (5-8 minutes) without affecting diagnostic accuracy. SITA testing also yields greater sensitivity and reproducibility and less inter-test variability when compared with standard full threshold testing. A recent modification of SITA, the SITA Fast algorithm, may further reduce testing time to 4-5 minutes. The test accuracy of the SITA Fast protocol is similar to that for the FastPac (Carl Zeiss Ophthalmic Systems, Humphrey Division, Dublin, CA), which is slightly less sensitive in detecting glaucomatous visual field loss compared with SITA and standard full-threshold algorithms.

In addition to new algorithms, visual field testing is becoming more sophisticated with the development of new perimetry technologies. Since standard threshold perimetry uses a static achromatic stimulus, it is thought to non-selectively invoke both major groups of ganglion cells of the optic nerve: 1) the parasol ganglion cells of the magnocellular visual pathway subserving motion perception, low spatial resolution, high contrast sensitivity, and stereopsis; and 2) the midget ganglion cells of the parvocellular visual pathway subserving central visual acuity, color perception, low contrast sensitivity, high spatial resolution, static stereopsis, pattern recognition, and shape. There is considerable overlap in the receptive fields of these cell types; therefore, a nonselective, white-on-white stimulus cannot detect the earliest loss of retinal ganglion cells, and standard threshold perimetry therefore may not detect visual field loss until the optic nerve is significantly damaged. New technologies are aimed at detecting glaucoma at an early stage and recognizing slight disease progression. Four technologies - frequency doubling technology perimetry (FDT), short wavelength automated perimetry (SWAP), high-pass resolution perimetry (HPRP), and motion automated perimetry (MAP) - are particularly interesting. Each of these techniques attempts to dissect the visual pathway by targeting specific ganglion cell types, theoretically leading to earlier detection of subtle deficits and enhancing diagnostic accuracy.

FDT and SWAP are already being used by many ophthalmologists. FDT targets a subset of the magnocellular pathway by using a stimulus of alternating black and white stripes. This stimulus measures spatial contrast sensitivity and has the advantage of being a fast (2-4 minute) screening tool. FDT is not as affected by blur and pupil size as standard threshold automated perimetry, and also may have reduced intra-test and inter-test variability compared to standard perimetry. However, since the technology is new, few longitudinal studies have examined its role in glaucoma patients and controls. Interestingly, a recent longitudinal study comparing FDT with standard perimetry (Am J Ophthalmol. 2004 May;137(5):863-71) suggested that FDT abnormalities can precede standard perimetry visual field loss by as much as 4 years.

SWAP, or blue-yellow perimetry, targets a subset of the midget cells in the parvocellular visual pathway by using a 440 nm blue light stimulus. Threshold is therefore measured by light sensitivity, which is thought to be an indicator for early glaucomatous damage. The test is significantly longer (14-18 minutes) than FDT, and is sensitive to blur and media opacities. However, there is excellent evidence that SWAP can detect visual field loss earlier than standard automated perimetry. Current research is aimed at developing new algorithms designed to reduce testing time and thereby make SWAP a more practical test.