Visual field theory and methods

The normal visual field

The normal visual field has been described as an island of vision in a sea of darkness.1 This island has a sharp central peak, corresponding to the fovea, with sloping sides. The sides are slightly steeper superiorly and nasally.2 The island of vision extends roughly 60° superiorly and nasally, 75° inferiorly, and 100° temporally (Fig. 8-1). The actual topography (sensitivity of various parts) of the island depends on the level of light adaptation of the retina.3 The peak is most sensitive when the retina is light adapted. The edges of the island have poor light sensitivity, so stimuli must be up to 3500 times (3.5 log) more intense to be perceived.4,5 If the retina is fully dark adapted, the cones of the fovea (center of the island) are less sensitive than the rods of the periphery.

Visual field testing is usually done in the photopic (light-adapted) or mesopic (partially light-adapted) state.Thus in the normal visual field examination, the fovea is the most sensitive point tested and represents the peak.

Fig. 8-1  (A) Isopter (kinetic) perimetry. Test object of fixed intensity is moved along several meridians toward fixation. Points where the object is first perceived are plotted in a circle. (B) Static perimetry. Stationary test object is increased in intensity from below threshold until perceived by the patient. Threshold values yield a graphic profile section.

(Modified from Aulhorn E, Harms H. In: Leydhecker W, editor: Glaucoma, Tutzing symposium, Basel, S Karger, 1967.)

Visual Acuity Versus Visual Field

Visual acuity measurement tests the resolving power of the retina for objects of distinct form. Static visual field measurement tests a more primitive retinal function – differential light sensitivity. Differential light sensitivity is the measure of the ability of the retina to distinguish a stimulus that is some degree brighter than the background illumination.

Terminology and Definitions

Fixation. That part of the visual field corresponding to the fovea centralis.Also, the ability of patients to keep their eyes directed at the center of the visual field apparatus. Patients with poor fixation move their eyes repeatedly and produce an unreliable visual field test result.

Central field. That portion of the visual field within 30° of fixation.

Bjerrum’s area (arcuate area). That portion of the central field extending from the blind spot and arcing above or below fixation in a broadening path to end at the horizontal raphe nasal to fixation. Bjerrum’s area usually is considered to be within the central 25° of the visual field.  This part of the visual field is quite susceptible to glaucomatous damage (see Fig. 10-9). Bjerrum’s area does not include non-specific peripheral depression that is commonly seen along the uppermost border of automated visual field charts. These defects may appear to arc because of the placement of test points, but they do not constitute a classic arcuate scotoma (Fig. 8-2; see also Fig. 10-2).

Peripheral field. That portion of the visual field from 30° to the far periphery. The shape of the normal peripheral field is governed­ by the shape and structures of the face.

Kinetic perimetry.Visual field test wherein the intensity and size of the stimulus are held constant while the stimulus location is moved.

Static perimetry. Visual field test wherein the position of the stimulus is held constant while the stimulus intensity is varied.

Isopter. The outline of a contiguous area of the visual field capable of perceiving a given stimulus.The isopter is most often used to define an area outlined by a given stimulus in kinetic perimetry.

Threshold. At a given retinal point, the intensity of a stimulus that is perceived 50% of the times it is presented.

Fluctuation. The variability in visual field measurement when tests are repeated over time.

Short-term fluctuation. The variability within a field during the time of its measurement. The test–retest interval is short: typically a few seconds or minutes.