Multifocal ERG

Reference: PVM-MU-ERG

This exam allows the realization of a map of local ERG responses (mfERG).
The visual stimulator generates a matrix of 16 to 217 zones which are stimulated with independent sequences of flashes. The global ERG response is recorded from a unique electrode.
Local responses are obtained by computing the inverse correlation between this global ERG response and the stimulation sequence.

During the exam, the program displays in real time the local responses with an automated identification of the N1, P1 and N2 peaks of each response. The quality of the recorded signal is evaluated by computing the noise level and displaying the signal evolution since the beginning of the recording (base line).

In the example hereby, the base line appears very unstable at the beginning of the exam, which is frequently due to eye movements, eye blinks or electrode instability. After about 1 minute, the baseline becomes stable and the noise level indicates that the quality of result is sufficient for a valid interpretation.

The program allows to display 2D and 3D interpolated maps of the amplitude and implicit time of the N1, P1 and N2 peaks of the local responses.

The zone analysis allows the comparison of responses within given zones. Responses can be grouped as a function of eccentricity (ring) or by quadrants or even by zones defined by the operator.

The program performs a comparison of analysis results within given zones with a normal subjects data base from several clinical centers.

Quantitative measurements of amplitude and implicit times outside the normal range are immediately outlined with a color code (green = normal, red = abnormal, pink = borderline). In addition, the program performs a comparison between central and peripheric responses which improves significantly the detection of central alterations (when the periphery is found normal) and a comparison of the P1 to N1 amplitudes to distinguish alterations of photoreceptors and alterations of the inner layers of the retina.

The analysis of 2nd order kernel is used to analyze temporal interaction between the responses of successive stimuli. It is available on all procedures. However, the default tests are using a slow stimulation rate of 17 Hz which is designed to obtain a response similar to the "classic" photopic ERG waveform by minimizing temporal interactions between stimuli responses. As a result of this reduced temporal interaction, the amplitude of the 2nd kernel is very small.

For this reason, 2nd order kernel responses are obtained by using specific procedures with a faster stimulation rate of 40 Hz minimum.

This analysis allows the superposition of the map of local mfERG responses over the image of the eye fundus of the patient. The image of the eye fundus is imported as an image file either through the computer network or by USB key, CDROM etc. The operator identifies the position of the fovea and papilla to obtain a precise superposition.

This analysis evaluates the evolution of the patient results over several successive exams. The program automatically searches within the result data base all the results from the patient. It computes a map showing the rate of change of the amplitudes of the local responses over the last exams. The results are displayed in a graphic showing in red color the areas where amplitudes are deteriorating and in green color where they are improving.

An accurate correction of refractive errors is important to obtain results of "good" quality.

Metrovision recommends the use of its set of " large field " eye glasses which avoid masking artifacts of the peripheral visual field.

The patient's fixation is stimulated by the presentation in the center of the fixation area of a small pattern which changes orientation in a pseudo random way. The patient is asked to press a button every time the pattern orientation changes. The high resolution video camera also allows an accurate control of fixation by the operator.

As an option, Metrovision proposes an automated fixation control which uses the image of the eye to reject the responses in case of movement.

Multifocal ERG is clinically useful in a large number of macular or paramacular pathologies as it can point out local alterations or normal islands of vision which cannot be detected by standard ganzfeld ERG exams.