Perimetry

Perimetry is the systematic measurement of visual field function. The two most widely used types of perimetry are Goldmann's kinetic perimetry and threshold static automated perimetry. With Goldmann or "kinetic" perimetry, a trained perimetrist moves the stimulus; the brightness of the stimulus remains constant. The limits of the visual field are assigned to lights of different sizes and brightness.

With automated threshold static perimetry, a computer program is selected. The most widely used tests the central 30 ° of the visual field using a grid spaced six degrees. This is accomplished by keeping the size and location of a target constant and varying the brightness until the dimmest target that the patient can see at each of the test locations is found. These visual sensitivity maps, produced by any of these methods, are very important in the diagnosis of diseases of the visual system. Different patterns of visual loss are found with diseases of the eye, central nervous system of the optic nerve.

 Types of perimetry

There are four types of perimetry:

•  Kinetic Kinetic perimetry is a two-dimensional assessment of the boundary of the hill of vision. It involves the presentation of a moving stimulus of known luminance or intensity from a non-seeing area to a seeing area until it is perceived. The stimulus is moved at a steady speed along various meridia (clock hours) and the point of perception is recorded on a chart. By joining these points along different meridians an isopter is plotted for that stimulus intensity. Using stimuli of different intensities a contour map of the visual field with several different isopters can be plotted. Kinetic perimetry can be performed by simple confrontation or by means of a perimeter such as the Goldmann.
•  Static A method of assessing fields in which the location of a stimulus remains fixed at a certain location within the field, with the intensity increased until it is seen by the subject (or decreased until it is no longer seen). In other words, the target intensity is increased (or decreased) until threshold is reached. The most frequently used automated perimeter is the Humphrey Field Analyzer (HFA); others include the Henson, Dicon and Octopus. Automated static perimetry now constitutes the method used for the great majority of visual fields monitoring in the care of patients with glaucoma.
•  Suprathreshold Suprathreshold perimetry involves testing with stimuli of luminance above the expected normal threshold levels for an age-matched population to assess whether these are detected; in other words, testing to check that a subject can see stimuli that would be seen by a normal person of the same age. It enables testing to be carried out rapidly to indicate whether function is grossly normal or not. However, it is not highly quantitative, and so is usually reserved for screening. 
•  Threshold Threshold perimetry is used for detailed assessment of the hill of vision by plotting the threshold luminance value in various locations in the visual field and comparing the results with age-matched ‘normal’ values. In Humphrey perimetry (see below), a stimulus of higher than expected intensity is presented; if seen, the intensity is decreased in 4 dB steps until it is no longer seen (‘staircasing’). The stimulus is then increased again in 2 dB steps until seen once more. If the stimulus is not seen initially, its intensity is increased in 4 dB steps until seen, then decreased in 2 dB steps until no longer seen. Essentially, the threshold is crossed in one direction with large increments, then crossed again to ‘fine-tune’ the result with smaller increments. Threshold testing is quantitatively detailed and is therefore used for monitoring glaucomatous fields.