D4 Campimetry: Visual Field and Differential Sensitivity

Topic

The Humphrey or Octopus perimeter requires the patient to maintain their gaze on a central fixation point throughout the test without moving it, while the device presents a brief light stimulus—one at a time—at various locations within the visual field surrounding that point. The patient presses a button whenever they detect the light stimulus, without needing to look directly at it; the goal of the perimeter is precisely to measure how well the patient detects a light stimulus in various parts of the visual field without looking at it head-on.

At each location in the visual field, the perimeter does not simply present the light stimulus once at a fixed intensity; instead, it automatically adjusts the intensity from one presentation to the next. If the patient detects the stimulus, the perimeter reduces the intensity for the subsequent presentation at that same location; if the patient fails to detect it, the perimeter increases the intensity. This process continues until the lowest intensity that the patient can still detect at that specific location in the visual field is identified. This lowest detectable intensity at a given location is known as the differential sensitivity of that location.

The differential sensitivity of each location in the visual field is expressed in decibels, but on a scale that is the inverse of those used in other models within this group. On the perimeter's decibel scale, a higher value does not indicate a more intense light stimulus, but rather a weaker, more attenuated one. Consequently, high differential sensitivity—expressed as a high number of decibels—means that the location can still detect a very weak light stimulus, whereas low differential sensitivity—expressed as a low number of decibels—means that the location can only detect a much more intense light stimulus. By repeating this measurement of differential sensitivity at dozens of distinct locations across the visual field, the perimeter generates a comprehensive sensitivity map; this map reveals something that no single isolated measurement could show: whether differential sensitivity declines uniformly across the entire visual field or specifically within a cluster of adjacent locations forming a recognizable pattern. This is because each point in the visual field relies on a specific set of fibers along the visual pathway—extending from the optic nerve to the visual cortex—and the pattern of reduced sensitivity on the map reflects exactly which part of the visual pathway is damaged.

For this reason, the sensitivity map is the standard tool for diagnosing and monitoring glaucoma, neuropathies, and lesions along the visual pathway. In glaucoma, for instance, the map accurately reproduces the characteristic pattern of loss—starting in the peripheral visual field and progressing toward the center—associated with the disease. Furthermore, comparing a patient's sensitivity maps over multiple visits allows clinicians to determine whether the pattern of reduced sensitivity is progressing or remaining stable under treatment.

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