Sweep Visual Evoked Potentials (sweep VEP) is a method for objectively assessing visual acuity using electrophysiology. Unlike traditional behavioral acuity tests such as Teller cards or the Snellen chart, which rely on subjective responses, the Sweep VEP method offers an objective option for when behavioral methods are not viable. These can be used to estimate acuity for anyone with unexplained or functional vision loss, or patients with cortical visual impairment (CVI). Diagnosys offers two variations of the Sweep VEP: the pediatric Glasgow and the Freiburg method, discussed below.
How does a Sweep VEP work?
Sweep VEP tests are conducted in a similar manner to pattern VEP tests, by presenting checkerboard patterns on a monitor to a patient with occipital scalp electrodes. The exception is children aged 0 to 3 years for whom a one-channel recording is used.
To present the stimulus, a specialized monitor displays black and white checkerboard patterns that become progressively smaller with each step of the test. The checkerboard size is referred to as the spatial frequency. The goal is to find the spatial frequency limit (SF limit) or checkerboard size which can no longer be detected – as evidenced by a lack of VEP signal.
To determine the SF limit, three-channel VEPs utilize Laplacian transform mathematics to isolate the signal by removing the noise common to all three occipital channels. The process by which the Glasgow and Freiburg methods determine the visual threshold differs, however in both cases, this signal is used to determine whether or not there is a detectable VEP response.
Both Glasgow and Freiburg methods report the final visual acuity estimate in the familiar Snellen 20-foot equivalent as well as a 97.5 % confidence interval. The Snellen equivalent can be used as the final acuity estimation; while the 97.5% confidence interval is a more conservative estimate, reflecting the natural variability in test repeatability. This conservative estimate is particularly important in clinical and legal contexts, where a higher level of confidence is required.
Glasgow Sweep VEP
The Glasgow Sweep VEP is optimized for children and patients who have difficulty maintaining fixation. To accommodate short attention spans and challenges with focusing, the Glasgow method features a button-box that allows the technician to manually start and pause the recording. This setup enables the technician to capture data while the child is looking at the screen and pause when needed to refocus their attention. Additionally, noise-making toys are included to help draw the child’s gaze toward the visual stimulus on the screen.
Another feature that makes the Glasgow Sweep VEP ideal for children is the speed of testing, made possible by real-time signal analysis and an efficient thresholding algorithm. Although the method includes 27 steps, not all steps are tested for every patient. The technician can begin the test at a step that roughly corresponds to the patient’s projected acuity. The thresholding algorithm then automatically adjusts the checkerboard size until the final threshold is reached. The spatial frequency limit, which determines visual acuity, is based on the spatial frequency of the final step identified by the algorithm. Real-time signal analysis further accelerates the test by allowing the system to immediately proceed to the next step as soon as a sufficient signal is detected. This method can assess a range of visual acuities ranging from 20/20 to 20/2000.
Freiburg Sweep VEP
The Freiburg Sweep VEP is sometimes regarded as a more accurate method of assessing visual acuities in the range of 20/20 to 20/400. This method consists of six steps, each presenting a checkerboard stimulus with progressively smaller check sizes. The outcome of each step is evaluated as either significant or not significant based on specific criteria. The significant points are then used to draw a line of best fit, which intersects with the X-axis at zero magnitude (µV); this intersection is considered the threshold for visual acuity. Due to this precise approach, the Freiburg method can be more accurate, provided that the test is conducted properly and the patient maintains steady fixation.
Summary
The selection of the appropriate sweep VEP method depends on your patient population. The Glasgow Sweep VEP is particularly well-suited for children and patients with difficulty maintaining fixation, offering speed and flexibility through real-time signal analysis and manual control features. While the Freiburg Sweep VEP may provide a more precise visual acuity assessment, especially in adults, by utilizing a detailed analysis of significant data points to determine the threshold. Both methods offer reliable, objective measurements of visual acuity and can be invaluable in cases where traditional behavioral tests are not feasible.