Objective visual field determination in forensic ophthalmology with an optimized 4-channel multifocal VEP perimetry system: a case report of a patient with retinitis pigmentosa

Abstract

We present the case of a 59-year-old male patient with progressive vision impairment and consecutive visual field narrowing ("tunnel view") for 7 years and a known retinitis pigmentosa for 5 years. The remaining Goldmann perimetric visual field at time reported was less than 5°. A request for blindness-related social benefits was rejected because an ophthalmologic expert assessment suggested malingering. This prompted us to assess an objective determination of the visual field using multifocal VEPs. Objective visual field recordings were performed with a four-channel multifocal VEP-perimeter using 58 stimulus fields (pattern reversal dartboard stimulus configuration). The correlated signal data were processed using an off-line method. At each field, the recording from the channel with the maximal signal-to-noise ratio (SNR) was retained, thus resulting in an SNR optimized virtual recording. Analysis of VEP signals was performed for each single field and concentric rings and compared to an average response measured in five healthy subjects. Substantial VEP responses could be identified in three fields within the innermost ring (eccentricity, 1.7°) for both eyes, although SNR was generally low. More eccentric stimuli did not elicit reliable VEP responses. The mfVEP recording was correlated with perimetric visual field data. The current SNR optimization by using the channel with the largest SNR provides a good method to extract useful data from recordings and may be appropriate for the use in forensic ophthalmology.

Steady-state multifocal visual evoked potential (ssmfVEP) using dartboard stimulation as a possible tool for objective visual field assessment

“…We used the electrode configuration as suggested by the manufacturers of the equipment. Mounting four electrodes around the inion and the evaluation of six channels has proven to be useful in the detection of the conventional long-term mfVEP measurements [2,6,13]. Similarly, the present data showed that maximal SNRs can be found in any one of the six channels.…”

“…VEPs were recorded using four different electrodes placed on the subject's skull using a cross-shaped electrode fixation device (positioned relative to the inion: 3.5 cm above, 3 cm below, and 4 cm to the left and right) [6]. Signals from four electrode pairs were differentially amplified.…”

“…The method may be an objective electrophysiological tool for detection of visual field losses [2][3][4][5][6][7]. Recent studies have revealed the presence of topographical associations between losses in mfVEP and structural damages in patients with optic nerve disorders [4,8,9,10].…”

Objective perimetry using a four-channel multifocal VEP system: correlation with conventional perimetry and thickness of the retinal nerve fibre layer

“…Correct visual fixation was monitored with a video camera system. VEPs were recorded with four different electrodes placed on the subject's skull using a cross-shaped electrode fixation device (electrode distance from inion: above, 3.5 cm; below, 3 cm; horizontal left and right, 4 cm) 9. The joint mounting of the electrodes ensured an equal distribution of their pressure and an improved electrode–skin contact.…”

A new method for quantifying mfVEP signal intensity in multiple sclerosis

“…However, there is no consensus on where to set the boundaries of these two windows. Examples of windows boundaries used are: SW = 45-150 ms and NW = 325-430 ms [10]; SW = 0-200 ms and NW = 300-500 ms [13]; SW = 50-150 ms and NW = 230-330 ms [14]; and SW = 37-260 ms and NW = 667-890 ms [12].…”