Ординатура / Офтальмология / Английские материалы / Electrophysiology of Vision_Lam_2005

Figure 3.3 Examples of transient pattern ERG responses. The macular dysfunction example shows reduced P50 as well as N95. The example of optic nerve dysfunction from demyelination demonstrates more preserved P50 with a greater selective loss of P95.

Compared to transient pattern ERG responses, steadystate responses have amplitudes that are similar to those of N95, and this makes steady-state responses suitable in assessing optic neuropathies such as glaucoma.

CLINICAL RECORDING OF PATTERN ERG

The pattern ERG response is small and usually less than 10 mV resulting in a high noise-to-signal ratio. Therefore, computer averaging of multiple recordings or sweeps is necessary to isolate this relatively small response. In addition, clarity of the pattern stimulus is critical, and best-corrected near visual acuity and consistent accurate fixation are needed to optimize this small response. Dilation of the pupils and contact-lens electrodes are not recommended because they cause blurring of the pattern stimulus. Instead, non-contact-lens type electrodes that contact the cornea and adjacent bulbar conjunctiva are utilized, and topical anesthesia to the cornea is not necessary. The Dawson–Trick–Litzkow fiber (DTL) electrode, the Arden gold foil electrode, and the Hawlina–Konec (HK) loop electrode are suitable, and proper placement of the electrode is critical in obtaining a high quality pattern ERG signal (see Chapter 1 for more information on ERG recording electrodes) (7–9). The DTL electrode should be placed with

the fiber in the lower fornix (Fig. 3.4). The distal end of the gold foil electrode should be placed vertically under the lower lid just below the center of the pupil with the foil curving downward over the lower lashes. No portion of the gold foil should touch the skin. The HK loop electrode should be hooked into the lower fornix, and the contact windows should be positioned on the bulbar conjunctiva. Skin electrodes may be used for those who are intolerant to fiber, foil, and loop electrodes, but they are likely to have high noise-to-signal ratio. The reference electrode may be a skin electrode placed at the ipsilateral lateral canthus. During monocular recording, the recording electrode of the occluded fellow eye may serve as the reference electrode. The ground electrode is typically placed on the forehead.

Figure 3.4 Dawson–Trick–Litzkow (DTL) and gold foil electrodes for pattern ERG recording. Clarity of the pattern stimulus is critical in pattern ERG, and non-contact-lens type electrodes such as the DTL electrode and the Arden gold foil electrode are suitable. The DTL electrode should be placed with the fiber in the fornix of the lower lid. The distal end of the gold foil electrode should be placed vertically under the lower lid just below the center of the pupil with the foil curving downward over the lower lashes. No portion of the gold foil should touch the skin.

The black and white checkerboard stimulus of the pattern ERG is similar but not identical to the stimulus of the pattern reversal VEP. Similar to pattern reversal VEP, the stimulus consists of equal number and size of alternating black and white squares. A fixation point is located at the corner of four checks located at the center of the stimulus. The luminance of the white squares should be at least 80 cd=m2. The pattern stimulus is defined by the visual angle subtended by the side length of a single check. To calculate the visual angle, the check side length is divided by the distance from the stimulus center to the tested eye. The result is the tangent of the visual angle subtended by each check, and the visual angle is obtained by taking the inverse tangent.

Several stimulus parameters are critical to the pattern ERG and differ from those of pattern reversal VEP. The pattern ERG amplitude increases with luminance contrast between the white and black checks, and a maximal contrast of as near 100% is desired (10–13). In pattern reversal VEP, the pattern contrast has little effect on the response for contrasts above about 50% (14). While the responses of both pattern ERG and pattern reversal VEP increase with greater stimulus field, the amplitude of pattern reversal VEP is more macular dependent (15,16). The pattern ERG correlates well with static perimetry with 6 and 10 stimulus fields but not with larger 20 stimulus field (17). The international standard for pattern ERG recommends a stimulus size between 10 and 16 , and the standard for pattern reversal VEP recommends a stimulus size of at least 15 .

A check size of 0.8 (48 min) is recommended for clinical pattern ERG, but results of studies into the optimal check size for pattern ERG are mixed and are in part related to the stimulus size (3,4,18). For most clinical recordings, transient pattern ERG responses are elicited with a stimulus reversal rate of six reversals per second or less, equivalent to a phase frequency of 3 Hz or less. For steady-state recordings, a reversal rate of 8 Hz demonstrates the best correlation to check size (i.e., spatial tuning) (10).

During pattern ERG recording, the room light should be at low or medium luminance level, and the luminance of

the screen beyond the checkerboard stimulus is not critical (19). Simultaneous binocular or sequential monocular recordings are typically performed. Interocular pattern ERG asymmetry is relatively low in a normal subject. Collection of age-matched normative data by each facility is recommended. The maturation and age-related changes of pattern ERG are discussed in Chapter 6.

REPORTING PATTERN ERG RESULTS

Report of pattern ERG results should include waveforms with values of amplitudes and latencies for P50 and N95. The report should also include normative data from each individual facility. Stimulus parameters such as check sizes should be stated as well as whether international standard was followed. If the response is abnormal, an interpretation of the N95 to P50 amplitude ratio is helpful. For steadystate pattern ERG, amplitude and phase shift derived from Fourier analysis should be available.

REFERENCES

1.Bach M, Hawlina M, Holder GE, Marmor MF, Meigen T, Vaegan, Miyake Y. Standard for pattern electroretinography. Doc Ophthalmol 2000; 101:11–18.

2.Riggs LA, Johnson LP, Schick AM. Electrical responses of the human eye to moving stimulus pattern. Science 1964; 144:567–568.

3.Berminger T, Schuurmans RP. Spatial tuning of the pattern ERG across temporal frequency. Doc Ophthalmol 1985; 61: 17–25.

4.Schuurmans RP, Berminger T. Luminance and contrast responses recorded in man and cat. Doc Ophthalmol 1985; 59:187–197.

5.Viswanathan S, Frishman LJ, Robson JG. The uniform field and pattern ERG in macaques with experimental glaucoma:

removal of spiking activity. Invest Ophthalmol Vis Sci 2000; 41:2797–2810.

6.Harrison JM, O’Connor PS, Young RS, Kincaid M, Bentley R. The pattern ERG in man following surgical resection of the optic nerve. Invest Ophthalmol Vis Sci 1987; 28:492–499.

7.Arden GB, Carter RM, Hogg CR, Siegel IM, Margolis S. A gold foil electrode: extending the horizons for clinical electroretinography. Invest Ophthalmol Vis Sci 1979; 18:421–426.

8.Dawson WW, Trick GL, Litzkow CA. Improved electrode for electroretinography. Invest Ophthalmol Vis Sci 1979; 18:988–991.

9.Hawlina M, Konee B. New noncorneal HK-loop electrode for clinical electroretinography. Doc Ophthalmol 1992; 81: 253–259.

10.Hess RF, Baker C Jr. Human pattern-evoked electroretinogram. J Neurophysiol 1984; 51:939–951.

11.Korth M, Rix R, Sembritzki O. Spatial contrast transfer function of the pattern-evoked electroretinogram. Invest Ophthalmol Vis Sci 1985; 26:303–308.

12.Thompson D, Drasdo N. The effect of stimulus contrast on the latency and amplitude of the pattern electroretinogram. Vision Res 1989; 29:309–313.

13.Zapt HR, Bach M. The contrast characteristic of the pattern electroretinogram depends on temporal frequency. Graefes Arch Clin Exp Ophthalmol 1999; 237:93–99.

14.Tetsuka S, Katsumi O, Mehta M, Tetsuka H, Hirose T. Effect of stimulus contrast on simultaneous steady-state pattern reversal electroretinogram and visual-evoked potential. Ophthalmic Res 1992; 24:110–118.

15.Aylward GW, Billson V, Billson FA. The wide-angle pattern electroretinogram: relation between pattern electroretinogram

amplitude and stimulus area using large stimuli. Doc Ophthalmol 1989; 73:275–283.

16.Sakaue H, Katsumi O, Mehta M, Hirose T. Simultaneous pattern reversal ERG and VER recordings: effect of stimulus field and central scotoma. Invest Ophthalmol Vis Sci 1990; 31:506–511.

17.Junghardt A, Wildberger H, Torok B. Pattern electroretinogram, visual evoked potential and psychophysical functions in maculopathy. Doc Ophthalmol 1995; 90:229–245.

18.Bach M, Holder GE. Check size tuning of the pattern electroretinogram: a reappraisal. Doc Ophthalmol 1997; 92:193–202.

19.Bach M, Schumacher M. The influence of ambient room lighting on the pattern electroretinogram (PERG). Doc Ophthalmol 2002; 105:281–289.

106 Chapter 4

Table 4.1 Summary of International Standard and Recommendations for Clinical Electro-oculogram (EOG)

Table 4.1 (Continued )