Metrovision - Examples of Multifocal ERG exams  

MULTIFOCAL ERG

CLINICAL EXAMPLES

J. Charlier in collaboration with
Pr Jean-Claude Hache, Dr Sabine Defoort-Dhellemmes, Dr Xavier Zanlonghi,
Mme Danielle Basset

 

 

 

INTRODUCTION

The following examples are provided to illustrate the interpretation of multifocal ERG exams on the Vision Monitor System.

The first step in the interpretation of results is to assess their quality. Results of poor quality may lead to erroneous conclusions. Therefore quality should be evaluated before any conclusion is drawn on alterations of responses. 
Results' quality is mainly dependent upon the the amount of electric noise present in the recorded signals and the accuracy of the patient's fixation.
These items can be evaluated by the number of indicators. It is also important to use these indicators during the examination and correct possible problems before the examination is completed.

The second step in the interpretation is the analysis of responses and the detection of abnormalities. For this purpose, the Vision Monitor program compares the results to a normal data base and points out those parameters which differ significantly from the normal range.

All the following results have been obtained with identical test conditions which correspond to the test conditions of the normal data base :

NORMAL SUBJECT

ERG multifocal normal

Evaluation of recording quality

The noise level of the recorded signal is within normal limits :
 2.2 mV is lower than the "normal" limit of 5 mV

Background : the noise level corresponds to the electrical activity recorded in the absence of stimulation throughout the exam.
An abnormally large noise level may result from bad electrode contact, from eye blinks or from eye movements.    

 

Fixation is of good quality:
- the control of attention indicates less than 20 percent errors

- the foveolar peak is clearly visible


Background : the control of attention is achieved by presenting a small line in the center of the fixation point. The orientation of this line is changed in a pseudo random manner and the patient is asked to press a response button every time the orientation is changed.
This provides an excellent stimulus for fixation. It is also used as a final control of the quality of fixation: an error score of 5/53 indicates that there were 53 orientation changes during the exam and 5 of them were not detected by the patient.

 

 

2D and 3D maps of local responses
The waveform of local responses is very similar to photopic ERG responses.
The 2D and 3D maps show a significant peak of response amplitude at the fovea which is due to the higher density of cones in this location.
The blind spot is not frequently visible as an absolute deficit but most frequently as a relative deficit.
This is due to the large size of the stimulus in the blind spot are, about 4 times the stimulus area of the Goldmann size V stimulus.

Zone analysis

 

All indicators are  « green » : no abnormality has been detected when comparing the result to the normal data base.

Background : in the ring analysis, the program computes the average response of the 5 following areas: : fovea,
2-5 degrees, 5-10 degrees, 10-15 degrees, > 15 degrees.
For each averaged response, the program performs two different analysis :
- the « RMS » analysis determines the energy of the signal (within a time window right after the stimulation) and the energy of the noise (within a time window where the stimulus response becomes negligeable)
- the « waveform » analysis: this analysis performs an automated identification of the waveform and detects the N1, P1 and N2 peaks). It also determines automatically the amplitudes and the implicit times of these peaks.  

RETINITIS PIGMENTOSA

(result from Lille University Medical Center)

multifocal ERG in retinitis pigmentosa

Evaluation of recording quality

The noise level is within normal limits :
 2.8 mV is lower than the « normal » limit of 5 mV

The quality of fixation is good:
- the control of attention indicates an error rate lower than 20 percent.

- the foveolar peak is clearly visible

 

Zone analysis

 

The response from the fovea (<2 degrees) is normal : for this area, indicators are green for the measurements of the signal RMS value, for the amplitudes of N1, P1 and N2 as well as for the implicit times of these peaks.

All the amplitudes of peripheral responses are significantly lower than normal (all indicators are red) and, for eccentricities larger than 5 degrees, the implicit times are larger than normal.

The ratio of amplitudes P1/ N1 (somehow similar to the B/A ratio used in ganzfeld ERG) remains normal, which characterizes a defect of the photoreceptor layer.

Notes:
One may conclude that there is no significant response from the periphery because the noise is within normal limits   (the RMS noise indicators are « green »)  and the signal RMS value is not larger than the noise (in the histogram "AVERAGE RMS AMPLITUDES" , the colored rectangles that represent the signal are masked by the black rectangles that represent the noise).

  The amplitude ratios (s/sp, N/Np, P/Pp) have not been interpreted because the measurements from the periphery are lower than normal.

STARGARDT

(result from Lille University Medical Center)

Evaluation of recording quality

The noise level is within normal limits  :
 3.2 mV is lower than the normal limit of 5 mV.

 

 

.

Quality of fixation :
There is no significant foveolar peak (due to the presence of a central scotoma) and the patient's visual acuity was too low to allow the use of the attention control.  However, the stability of the base line indicates that fixation was stable. Furthermore, the operator did verify the stability of fixation throughout the exam thanks to the real time video monitor.
Finally, the 2D and 3D maps clearly show a central scotoma with fixation on the border of the scotoma.

 

Zone analysis

 

 

 

The ring analysis confirms the alteration of central responses :  the amplitudes of N1 and P1 are signficantly reduced for the rings with eccentricities lower than 5 degrees.

 

PLAQUENIL INTOXICATION

(result from Lille University Medical Center)

 

ERG multifocal plaquenil

Evaluation of recording quality

The noise level is within normal limits :
 2.2 mV is lower than the normal limit of 5 mV

 

Fixation is good:
- the attention control indicates an error rate lower than 20 percent

- the fovea peak is clearly visible

 

Zone analysis

 

The amplitudes of peaks N1, P1 and N2 are normal in the periphery (over 15 degrees of eccentricity).

However, these amplitudes are significantly reduced for the central zones. The histogram of amplitudes shows that these alterations are more extensive for the perifoveolar area (2 – 5 degrees).

The P1/N1 ratio is within normal limits which characterizes a defect of the photoreceptor layer.

EARLY STAGE OF PLAQUENIL INTOXICATION

(result from Lille University Medical Center)

ERG multifocal plaquenil

Evaluation of recording quality

The noise level is within normal limits :
 2.5mV is lower than the normal limit of 5 mV

 

Fixation is good :
- the control of attention indicates less than 20 percent errors

- the fovea peak is clearly visible

 

Zone analysis

 

The amplitude of N1 and P1 peaks is reduced for the 2-5 and 5-10 degrees zones.

 

The periphery over 10 degrees and the fovea are normal.

 

The P1/N1 ratio is within normal limits indicting an alteration of the photoreceptor layer.

 

BIRDSHOT CHORIORETINOPATHY

(result from Lille University Medical Center)

ERG multifocal birdshot

 

Evaluation of recording quality

The noise level is within normal limits :
3.5 mV is lower than the normal limit of 5 mV

 

Fixation:
- the control of attention could not be used because the visual acuity was too low

- the fovea peak was not visible because of the alteration of the central field.
- the only usable informations were the stability of the base line and the report from the operator watching the video. Both were OK.

 

Zone analysis

 

 

 

The amplitudes and implicit times of the N1 peaks are within normal limits for all locations.

 

The responses from the more eccentric zones are normal.

 

The P1 amplitude is significantly reduced for all central zones.

 

The P1/N1 ratio shows an alteration of the inner layers of the retina which is more important in the center but spreads out to the periphery as far as 15 degrees of eccentricity.

CYSTOID MACULAR OEDEMA

(result from Dr Zanlonghi, Nantes)

 

Evaluation of recording quality

The noise level is within normal limits :
 2.6 mV is lower than the normal limit of 5 mV

Quality of fixation :

Le control of attention could not be used on this patient.

The fovea peak is not clearly visible. However the stability of the base line indices that fixation was stable.

 

 

Zone analysis

 

The ring analysis shows a significative reduction of the P1 amplitude for the fovea and perifovea areas. The reduced value of the P1/N1 ratio is characteristic of an alteration of inner layers of the retina.

 

The visual field exam shows deficits that correspond to the result of the multifocal ERG. :

© Copyright - Metrovision ---------------- Legal notice ----------------
Top