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Foresee PHP™

Figure 1 : Foresee PHP Device
Figure 1 : Foresee PHP Device

The Foresee Preferential Hyperacuity Perimeter (Foresee PHP™) system is indicated as a diagnostic aid for detection and monitoring of the progression of Age-related Macular Degeneration (AMD) including, but not limited to, the detection of Choroidal Neovascularization (CNV).

The FORESEE PHP is a non-invasive visual field analyzer for monitoring Age-Related Macular Degeneration (AMD) and early detection of conversion to Choroidal Neovascularization (wet AMD or CNV).

Clinically validated in clinical trials,1, 2, 3 the Foresee PHP technology provides an excellent accuracy in CNV detection – 82% sensitivity3

Foresee PHP Benefits

The key benefits of the Foresee PHP system are:

  • Structured tutorial- reduced technician dependency.
  • Technician free test operation
  • Easy to use by the patient
  • Test history presentation- enables long term tracking of AMD
  • Rapid testing – approximately 5 minutes per eye
  • Ergonomically improved solutions (chin rest, hand switch)

Foresee PHP Features

  • The key features of the Foresee PHP system are:
  • Full macular field testing of 14 X 14 degrees
  • Automated analysis
  • Visual field mapping
  • Statistically significant correlation with stereo-color fundus photography and fluorescein angiography
  • Software upgrade capability for future applications

Foresee PHP Technology

Foresee PHP technology is based on the well-studied visual phenomenon of hyperacuity. Hyperacuity (also termed "Vernier acuity") is defined as the ability to perceive a minute difference in the relative spatial localization of two or more visual stimuli (see Figure 2). The hyperacuity threshold may be as low as 3 to 6 seconds of arc, which is approximately 10 times better than the resolution ability (around 30-60 seconds of arc) in the fovea. The hyperacuity stimuli are highly resistant to retinal image degradation, and thus suitable for assessing retinal function in patients with opaque media as well.

Figure 2 : Hyperacuity Diagram
Figure 2 : Hyperacuity Diagram

When a dot-deviation signal is projected to a healthy retina, it stimulates a collinear set of retinal receptive fields. Specialized neuronal circuits in the visual cortex process this information, leading to the perception of a straight dot-deviation signal (see Figure 3, page 1). When retinal pigment epithelium (RPE) elevation occurs, both in the case of when drusen are present as in intermediate AMD, and often to a more significant degree when CNV develops, a geometric shift in photoreceptor location might occur. As a result, an object image that stimulates certain photoreceptor fields when presented to a normal retina stimulates different ones when RPE is elevated. Therefore, linear signals might be perceived as distorted or misaligned (see Figure 3).

Note:

Figure 3 : Normal Retina vs. AMD with Photoreceptor Elevation
Figure 3 : Normal Retina vs. AMD with
Photoreceptor Elevation

Figure 3 illustrates a simplified scheme of photoreceptor architecture. In the normal retina illustrated on the left, the two dots in the projected line activate photoreceptor fields 2 and 6. This data is processed by the visual cortex and perceived as a straight line.
However when the same line is projected on a retina with an AMD lesion, as illustrated in the diagram on the right, the geometrical shift of the photoreceptors caused by retinal pigment epithelium [RPE] elevation, causes activation of photoreceptors 2 and 7, instead of 2 and 6. Consequently, one of the dots is perceived as if it shifted to another location in the visual field and the line appears distorted.

How the Foresee PHP Test Works

a patient marks pathological deviation on the Foresee PHP during a test.
Figure 4: a patient taking the test

  1. The Foresee PHP test consists of a series of dot deviation signals (white dots on a black background for maximum contrast) flashed at predefined intervals, across the central 14° of the patient’s macular visual field (figure 4).
  2. Projection of a dot deviation signal on a retinal lesion can cause the patient to perceive distortion, scotoma, and/or blurring of part of the line.
  3. The patient is asked to report any abnormalities in the dot-deviation signal by touching the screen at the location of the abnormality with the stylus. Existing visual field defects can be identified should the dot-deviation signal be positioned on a lesion.
  4. During the course of the test, artificial distortions, that imitate distortions similar to those seen by patients with AMD related macular lesions, are flashed. These artificial distortions act as both a quantifying measure and a reliability measure. The patient’s responses to these distortions are evaluated to identify AMD lesions and to judge test reliability.
  5. The Foresee PHP system consists of a screen-based application that runs on the Foresee PHP device. The patient’s responses are recorded and analyzed by a pre-designed algorithm.
  6. Following the analysis, the resulting report indicates whether progression of AMD is suspected or not.

References:

  1. Alster, Y. & group, P. Evaluation of test-retest variability for the preferential hyperacuity perimeter (PHP) in AMD patients. ARVO abstracts (2004).
  2. Loewenstein, A. & group, P. r. Differentiating Intermediate AMD from Recent Onset CNV with the PHP. ARVO abstracts (2003).
  3. Loewenstein, A. et al. Macular perimetry for diagnosing neovascular AMD. The Annual Vitreous & Retinal Society, San Francisco, California (2002).