Computerised Static Perimetry (PEC) is a diagnostic test that allows a detailed qualification and quantification of any alteration in the visual field through the topographic study of retinal sensitivity.
No previous preparation is required to perform computerised static perimetry. The duration of the examination depends on the programme selected and the patient's cooperation, and can vary between 10 minutes and 1 hour.
The patient places the chin on the chin cup of the device and must keep the fixation, during the whole examination, on a central point that appears inside the dome of the perimeter. The head must remain still. Throughout the computerized static perimetry, light stimuli of different intensities and different sizes are presented in different places of the dome. Whenever the stimuli are identified, the patient has to press the button provided for that purpose. The test can be performed monocularly, i.e., one eye at a time, or binocularly, depending on the type of study to be performed.
It is important that the patient's refractive errors are corrected, through the use of corrective lenses for the existing ametropia, because if this is not done, there may be a general decrease in retinal sensitivity, which may lead to false results.
The visual field is defined as the area of space which encompasses all objects observed simultaneously by an eye in stable fixation. That is, the area in which we can identify more peripheral objects without looking for them, keeping our gaze on a fixed central point.
It can be assessed, the central and the peripheral. The central one comprises the most central 30 degrees, which corresponds to the area of greatest retinal sensitivity and which ensures greater visual discrimination. In the peripheral visual field, the area of perception is wider and extends to more than 150 degrees and has a lower retinal sensitivity. In a vision angle of 180 degrees, it is considered that the central 120 degrees are visualised by both eyes simultaneously, while the more peripheral 30 degrees, on each side, are visualised by only one eye.
Campimetry can be kinetic, static or computerised:
- Kinetic, when the light stimulus is moved along the meridians and the person perceives it moving.
- Static, when the light stimulus appears static.
- Computerised, in which the examination of the visual fields is standardised, without being so dependent on the subjectivity of the patient. The most commonly used technique is computerised static perimetry.
The term perimetry refers to the visual field examination performed by an arc-shaped or dome instrument, with the eye located at the centre of the curvature, allowing the light stimulus to remain at a constant distance from the eye.
In computerised static perimetry, the evaluation of the visual field is based on the determination of the retinal sensitivity thresholds, through the projection of lumen stimuli with different intensities (progressively lower), in locations defined by the computer, in accordance with the test programme chosen, a technique which, although more precise, always has its results dependent on external factors, such as the patient's collaboration - which may be affected by his/her physical and/or psychological state, experience (examinations carried out repeatedly end up training the patients) - and the duration of the examination.
The technician who explains and monitors the examination plays a fundamental role in obtaining a computerised static perimetry that is as reliable as possible, ensuring that the patient has understood the examination well and that his or her responses are coherent, guiding the patient in this direction.
There are some parameters that must be assessed and paid special attention to when analysing computerised static perimetry, such as the sensitivity threshold and reliability indices.
The sensitivity threshold defines the minimum intensity stimulus seen by the patient at each point of the visual field. The calculation of this threshold is presented in decibels, so, the higher the number presented in decibels, the lower the intensity of the stimulus observed. The sensitivity threshold determined for each point is influenced by several factors such as the size, duration and colour of the stimulus presented, as well as the perimeter dome illumination, which must be defined and not vary from examination to examination on the same patient. Generally, the stimulus used is white, on a white background. Furthermore, the age of the patient has to be taken into account. It is normal that the sensitivity threshold decreases with age. It is known that the sensitivity of the peripheral retinal points decreases more than the central retinal points.
Reliability indexes allow assessing the congruence in the patient's responses, thus analysing the quality of the test performed. There are ways of evaluating the credibility of the test by analysing the number of false-negatives, false-positives and fixation losses. These indexes are presented, along with the duration of the examination in the device's final report.
The number of false negatives represents the number of times that the patient does not identify a more intense stimulus in a place where the threshold has been previously determined, that is, in a place where a less intense light stimulus had been previously marked. Generally, tests with a false negative rate lower than 30% are considered reliable, however, in patients with visual field loss, this value can be higher.
The number of false positives represents the number of times the patient identifies the stimulus without any stimulus having been presented. In general, tests with a false positive rate of less than 15% are considered reliable.
The fact that computerised static perimetry is a computerised examination allows for great diversity in the choice of evaluation programme, making it easier to adapt the programme to the pathology in question. The equipment also allows us to compare previous exams and quantify the existing alterations. Thus, we have the capacity to classify the evolutive character of the pathology throughout time and guide the doctor in the choice of therapy and treatment to apply.
