Results i. Mean 30Hz latency for the UGA group (10 cases – OD only)

1. The Royal Liverpool and . Broadgreen University Hospitals NHS Trust St Pauls Eye Unit Clinical NHS Engineering Royal Liverpool University Hospital ERG tests are performed awake using the LKC UTAS 2000# system with full field Ganzfeld stimulation bowl and lower lid skin electrodes or gold foil corneal electrodes. UGA these are performed with JET corneal electrodes and the LKC Mini-Ganzfeld stimulator. Figure 1: Awake, the subject is brought up to the Ganzfeld bowl Fgure 2: UGA (under-general-anaesthesia) the Mini-Ganzfeld stimulator is held over the eye being tested Electro-retinogram (ERG) photopic flash ERG and 30Hz flicker delays under general anaesthesia M.C.Brown, R.P.Hagan, M.A.Thomas, A.M.Mackay Dept. of Clinical Engineering, Royal Liverpool University Hospital These data were examined as follows: i. 30Hz response latencies were compared for the whole UGA group with values from an independent group of awake subjects, ii. photopic flash B-wave latency was compared in 4 children who had tests both UGA and awake. iii. 8 adult volunteers had flash and flicker tests with skin electrodes to compare latencies when using the full-size and the Mini-Ganzfeld stimulator. • Summary • A recording of the electrical response from the retina to a flash of light is the electro-retinogram (ERG) • The time taken for the retina to respond to a flash can indicate the health of the eye • Under general anaesthesia (UGA) we found increased response times which we have now shown were due to the anaesthetic, and not the different equipment, or recording techniques used Results i. Mean 30Hz latency for the UGA group (10 cases – OD only) was 33.2ms (SD 3.35), and for the awake group (70 cases) with skin electrodes 26.8ms (SD 2.46) (p<0.05). ii. Of the four cases tested in both conditions: B-wave latency in ms iii. The type of flash stimulator used had no systematic effect on latencies, although amplitudes were typically smaller using the Mini-Ganzfeld. Purpose of the study To review photopic flash and 30Hz flicker ERG latency results in subjects who have undergone ERG tests under general anaesthesia (UGA) and those who undertook the tests whilst awake, in order to see if a difference in latency exists and to consider the possible influences of the recording apparatus, the mode of stimulation, and the anaesthetic on the latencies. Introduction We have noticed that ERG tests in photopic background conditions under general anaesthesia often have considerably increased latencies compared to awake tests. This study tests the validity of this observation and considers the possible reasons. case awake UGA delay 1 30.5 32.1 1.6 2 26.6 35.6 9.0 3 28.9 34.4 5.5 4 25.2 30.0 4.8 Discussion and observations 1. Photopic B-wave latency is a useful diagnostic measure because of its relatively narrow normal range. However, these results suggest that it cannot be used as reliably UGA because of the observed delay due to the anaesthetic. This delay may also be dose dependent. 2. There may be room for improvement in the reliability of measuring photopic B-wave latency. Using skin electrodes the traces are often small and noisy, making accurate cursoring difficult. 3. In general all response amplitudes were lower using the Mini-Ganzfeld (observed 40% - 80%) (see Fig.3) even though the photometer measurements showed higher flash intensity from the Mini unit. Also we found that the amplitudes were greatly affected by minor changes in the relative position of the stimulator and the eye. We attributed this to the construction, as the small integrating bowl is some distance from the eye. 4. Flicker latency is usually measured to the first peak. However other algorithms exist where the phase of the 30Hz (fundamental) response is determined and presented as a ‘time to peak’ In ‘peaky’ responses the presence of higher harmonics can produce discrepancies of several milliseconds between the two methods. Fig. 3 illustrates where the marked peak latency is different from the result from the LKC algorithm. 5. The LKC Mini-Ganzfeld stimulator has the flash tube close to the eye. Without sufficient care with electrode position, and with the relative positions and routes of the stimulator and measuring leads there can be a significant stimulator flash artefact which may even affect the calculation of latency. Figure 3: Shows (i) typical response amplitude difference between stimulators (ii) difference in the calculated latency (implicit time) when cursor values are the same at 28.0ms (iii) presence of flash artefact on Mini-Ganzfeld responses Methods Diagnostic ERG tests are sometimes performed UGA using halogenated agents. From pre-existing records of such procedures, photopic flash latency was measured to the peak of the B-wave, and 30Hz latency was taken from the LKC flicker latency calculation. Conclusions Mean photopic flash and mean 30Hz ERG latencies were longer in our cases tested under general (inhalational) anaesthesia. The choice of the full size or hand held Mini-Ganzfeld stimulator did not affect latency. #LKC Technologies Inc. Professional Drive, Gaithersburg, MD 20879 www.LKC.com Funded in part by the RLBUH NHS Trust, R&D Support Fund 2003