Documenta Ophthalmologica 81: 253-259, 1992. 9 1992 Kluwer Academic Publishers. Printed in the Netherlands.
New noncorneal HK*-loop electrode for clinical electroretinography MARKO
HAWLINA 1 & BLAZ KONEC 2
1University Eye Clinic and 2University Institute for Clinical Neurophysiology, Medical Centre Ljubljana, LjubIjana, Slovenia
Accepted 30 April 1992 Key words: Electrode, electroretinography, pattern electroretinogram Abstract. A new noncorneal electrode for clinical electroretinography was developed. It
consists of a thin wire forming a loop modeled to fit into the lower conjunctival sac. Electrical contact is made with the scleral conjunctiva through an exposed portion of otherwise insulated wire. The recorded pattern electroretinograms are in the same amplitude range as if recorded by the gold foil electrode, while the flash electroretinograms with the new electrode are of about two-thirds the amplitude of corneal electrodes. The new electrode is more durable and hence less expensive than gold foil electrodes and can likewise be used without topical anesthetic. Cleaning is easy and effective. The electrode rarely causes discomfort and produces stable responses for at least 2 hours. The electrode aims to match stability of skin electrodes with sensitivity of fragile foil and fiber electrodes.
Introduction
N o n c o r n e a l electrodes, such as gold-foil [1, 2] or D a w s o n , Trick, Litzkow ( D T L ) fiber [3] electrodes, yield high pattern electroretinograms ( E R G s ) and are also frequently used for flash E R G s . Their fragility, necessary for anesthetic-flee recording, u n f o r t u n a t e l y brings along instability and considerable variation [4], which m a y be due to change of electrode position in c o n t a c t with an eye [5]. Their cleaning and sterilization is delicate, and their application requires substantial o p e r a t o r skill and patient c o o p e r a t i o n , especially in children [6]. Because of these problems, the skin electrodes h a v e b e e n increasingly used for E R G recording and m a y provide the best o p t i o n for children [6]. Yet, although m o r e stable [6], the E R G s r e c o r d e d by skin electrodes have amplitudes that are only a b o u t one half to o n e third of those r e c o r d e d by the gold-foil or D T L electrodes. This article describes a n e w type of electrode that is less fragile than gold-foil or D T L electrodes but yields E R G s of c o m p a r a b l e amplitudes and h e n c e aims to m a t c h high responses with additional stability.
254 Materials and methods
The electrode, shown in Fig. 1 (No. 1) is formed from a thin stranded or monofilament noble metal (silver, gold or platinum) wire (2) (diameter range, 0.16-0.28mm), the ends of which (3 and 4) are soldered together
///
\\~\ \ \\
11 II I
\\\\ ~t
1
i .:2
III
3
4
7
8
B
\
14
12
13
I~ 1,
17
Fig. I. Perspective view of the electrode in two positions (A) with enlarged view of the area (B) showing electrically exposed windows. Lateral (C) and frontal (D) views after insertion into the lower conjunctival sac are shown. Full details of the legend are given in the text.
255 and insulated by a sleeve (5) to end with a pin (6). The whole length of the wire is insulated with Teflon (9 through 11) except in the central region, where three windows of 3 mm in length are formed on one side (7 and 8), as illustrated in Fig. lB. When silver wire is used, the exposed portions can be chlorided. The loop (2) in Fig. 1B is elastically deformable and selfsupporting in any position to which it is formed. Figure 1A illustrates the electrode in two different positions. The position illustrated in broken lines represents a flat loop of wire in a single plane. The position illustrated in solid lines is a configuration typical of that into which the electrode would be bent before use. When in use, the electrode is bent to a configuration that appears U-shaped in side view (Fig. 1C). The two limbs (12 and 13) together form a hooking member carrying the windows, which is inserted into the lower conjunctival sac (14), leaving the corneal surface (15) entirely free. Figure 1D shows how the loop shape of the electrode is able to avoid contact with the cornea, while the self-supporting loop configuration hooked into the fornix provides a stable, secure mounting for the electrode, especially when the tape (17) has been applied. The electrode is sterilized by immersion into a sterilizing solution. Since the electrode is heat resistant and Teflon lined, it may also be autoclaved. For this purpose, the complete electrode is disconnected from the socket (18) on the connecting wire (16). The electrode can be used several times, although we believe that with commercial production it could be inexpensive enough to be disposable. When in position, electrode resistance was always below 5 kOhm and did not change throughout recording. The reference electrode could be an identical electrode in the fellow eye, or a skin electrode on the ipsilateral temple, as used here. Topical anesthetic was rarely needed but is advisable in sensitive patients or young children.
The recording method for pattern ERG recording is described in detail elsewhere [7]. For averaged photopic flash ERGs, subjects with dilated pupils were fully light adapted under laboratory fluorescent tubes with illuminance of 400 lux. Scotopic single-flash ERGs and c-waves were recorded with dilated pupils after 20 minutes of dark adaptation in a room with a 25-W bulb under a Kodak A1 red filter. For averaged flash ERGs, a Medelec flash unit with 19-cm diameter at a distance of 25 cm from the patient was triggered at a frequency of 5 Hz by a Medelec ER 94/e Sensor. For scotopic ERGs, single flashes were triggered manually. The energy of the flashes was 0.72 J for photoptic ERGs and 0.18 J for scotoptic ERGs. Responses were differentially amplified by a Medelec ER94/a Sensor over a bandpass of 0.1-125 Hz and stored on an Apple IIe computer. The ERG c-wave was recorded by a Medelec Sensor over the bandpass 0.01-125 Hz and time base of 10 seconds after a green (565-nm) light-emitting diode pulse stimulus of 2000 ~W/cm 2 and 100-ms duration at a distance of 5 mm from the eye.
256 Results
The ERGs shown in this report were taken from recordings on three healthy subjects. Figure 2 shows an example of a typical pattern E R G recording carried out with three different electrodes. Photopic flash responses recorded by skin, Henkes-type contact lens (Miller & S6hne AG) and (HK)-loop electrodes are shown in Fig. 3. Scotopic single-flash responses are shown in Fig. 4. Stability of the signals from the HK-loop electrode was best illustrated by the E R G c-wave measurement. If the subject managed not to blink for 10 seconds after the light stimulation, a single-flash c-wave could be recorded (Fig. 5).
Pattern ERG, LE,dob 1958 Ag disc
Ag wire
i
20ms
I
I
I
I
I
I
I
I
Fig. 2. An example of a pattern E R G signal obtained by an HK-loop electrode made of silver wire, compared to that obtained by skin and gold-foil electrodes; all refer to skin electrode on ipsilateral temple. Note that the signal (128 sweeps, recorded twice) from the wire electrode (3rd trace) is clean and reproducible and of approximately double the amplitude of that obtained by a skin electrode (1st trace). It is about in the same range as that obtained with a gold-foil electrode (2nd trace).
257 Flash ERG, LE, dob 1961
Fig. 3. Averaged photopic flash ERG, recorded with three different electrodes: skin, HK-loop and Henkes-type corneal electrodes. Responses were averaged 64 times and recorded twice for reproducibility.
Fig. 4. Scotopic single flash, recorded twice. Flash energy was 0.18J. Note approximately 230-~LVb-wave amplitude and good reproducibility.
Discussion We have described a new type of noncorneal E R G electrode that, in a simple manner, solves some of the problems of E R G recording outlined in the literature. With a self-supporting but plastically deformable filamentary member, a looped and hooked configuration of the electrode enables the
258 c
i00
gV I _ _ I sec
Fig. 5. In a steady subject with a dilated pupil, a single-flashc-wave could be recorded. Sweep
time was 10 seconds with prestimulus delay to assess the baseline level. Light stimulus was light-emitting diode pulses of 2000 ixW/cm2 with a duration of 100 ms. electrical contact of the constant area to be placed firmly in the lower fornix of the eye, while the insulation provides shielding from unwanted body potentials. Insulation of the wire is essential for function of the electrode. The signals recorded by the equivalent electrode made of naked wire were continually overloaded by muscular noise, which rendered recording practically impossible. Results of the measurements with our electrode show that reproducible pattern E R G responses can be obtained that are of similar amplitude to those of the gold-foil electrode. Flash E R G amplitudes were approximately two thirds of those obtained by corneal electrodes and more than three times the amplitude obtained by skin electrodes. The HK-loop electrode is more stable and durable than gold-foil or D T L electrodes. Since it does not touch the cornea, the new electrode may also be used without topical anesthesia. Avoiding corneal contact makes them suitable also for eyes with damaged corneas and renders abrasions of intact cornea, as noted with the use of corneal [8] and gold-foil [9] electrodes, unlikely. Additional advantages of the electrode are easy insertion and adaptability to particular normal or abnormal anatomic configurations of the patient's eye and eyelid. It can likewise be adapted to individual size and shape for children of any age. The electrode is easily cleaned and can be effectively sterilized. The lack of stable lid retraction can be, when needed, provided by a simple Barraquer-type lid retractor that leaves the lower fornix deep enough for the electrode to be inserted. A recently completed normative study for pattern E R G s [10] showed an amplitude range recorded with the HK-loop electrode of 3.0-6.12 pN, with a coefficient of variation of 18.4% (n = 30).
Acknowledgments The HK-loop electrode forms the subject of British and US patent applications 9008901.2 and 07/686,235. *HK refers to the initials of the authors. T h e electrode is commercially available. Information can be obtained from the authors.
259 References 1. Borda RP, Gilliam RM, Coats AC. Gold-coated Mylar T M (GCM) electrode for electroretinography. Doc Ophthalmol Proc Ser 1977; 15: 339-43. 2. Arden GB, Carter RM, Hogg C, Siegel IM, Margolis S. A gold foil electrode: Extending horizons for clinical electroretinography. Invest Ophthalmol Vis Sci 1979; 18: 421-6. 3. Dawson WW, Trick GL, Litzkow CA. Improved electrode for electroretinography. Invest Ophthalmol Vis Sci 1979; 18: 988-91. 4. Holopigian K, Snow J, Seiple W, Siegel I. Variability of the pattern electroretinogram. Doc Ophthalmol 1988; 70: 104-16. 5. McAllan A, Sinn J, Aylward GW. The effect of gold foil electrode position on the electroretinogram in human subjects. Vision Res 1989; 29: 1085-87. 6. Coupland SG, Janaky M. ERG electrode in pediatric patients: Comparison of DTL fiber, PVA-gel and non corneal skin electrodes. Doc Ophthalmol 1989; 71: 427-33. 7. Hawlina M, Strucl M, Stirn-Kranjc B, Finderle Z, Brecelj J. Pattern electroretinogram recorded by skin electrodes in early ocular hypertension and glaucoma. Doc Ophthalmol 1989; 73: 183-91. 8. Dawson WW, Zimmerman TJ, Houde WL. A method for more comfortable electroetinography. Arch Ophthalmol 1974; 91: 1-2. 9. Aylward GW, McClellan KA, Thomas R, Billson FA. Transient corneal changes associated with the use of gold foil electrodes. Br J Ophthalmol 1989; 73: 980-84. 10. Hawlina M. Pattern electroretinogram with the new 'HK-loop' electrode [Abstract]. Presented at the 30th symposium ISCEV, Vienna, 1992.
Address for correspondence: Marko Hawlina, MD, University Eye Clinic, 61000 Ljubljana, Slovenia Tel: +38 61 113-066; Fax: +38 61 329-981
New noncorneal HK*-loop electrode for clinical electroretinography MARKO
HAWLINA 1 & BLAZ KONEC 2
1University Eye Clinic and 2University Institute for Clinical Neurophysiology, Medical Centre Ljubljana, LjubIjana, Slovenia
Accepted 30 April 1992 Key words: Electrode, electroretinography, pattern electroretinogram Abstract. A new noncorneal electrode for clinical electroretinography was developed. It
consists of a thin wire forming a loop modeled to fit into the lower conjunctival sac. Electrical contact is made with the scleral conjunctiva through an exposed portion of otherwise insulated wire. The recorded pattern electroretinograms are in the same amplitude range as if recorded by the gold foil electrode, while the flash electroretinograms with the new electrode are of about two-thirds the amplitude of corneal electrodes. The new electrode is more durable and hence less expensive than gold foil electrodes and can likewise be used without topical anesthetic. Cleaning is easy and effective. The electrode rarely causes discomfort and produces stable responses for at least 2 hours. The electrode aims to match stability of skin electrodes with sensitivity of fragile foil and fiber electrodes.
Introduction
N o n c o r n e a l electrodes, such as gold-foil [1, 2] or D a w s o n , Trick, Litzkow ( D T L ) fiber [3] electrodes, yield high pattern electroretinograms ( E R G s ) and are also frequently used for flash E R G s . Their fragility, necessary for anesthetic-flee recording, u n f o r t u n a t e l y brings along instability and considerable variation [4], which m a y be due to change of electrode position in c o n t a c t with an eye [5]. Their cleaning and sterilization is delicate, and their application requires substantial o p e r a t o r skill and patient c o o p e r a t i o n , especially in children [6]. Because of these problems, the skin electrodes h a v e b e e n increasingly used for E R G recording and m a y provide the best o p t i o n for children [6]. Yet, although m o r e stable [6], the E R G s r e c o r d e d by skin electrodes have amplitudes that are only a b o u t one half to o n e third of those r e c o r d e d by the gold-foil or D T L electrodes. This article describes a n e w type of electrode that is less fragile than gold-foil or D T L electrodes but yields E R G s of c o m p a r a b l e amplitudes and h e n c e aims to m a t c h high responses with additional stability.
254 Materials and methods
The electrode, shown in Fig. 1 (No. 1) is formed from a thin stranded or monofilament noble metal (silver, gold or platinum) wire (2) (diameter range, 0.16-0.28mm), the ends of which (3 and 4) are soldered together
///
\\~\ \ \\
11 II I
\\\\ ~t
1
i .:2
III
3
4
7
8
B
\
14
12
13
I~ 1,
17
Fig. I. Perspective view of the electrode in two positions (A) with enlarged view of the area (B) showing electrically exposed windows. Lateral (C) and frontal (D) views after insertion into the lower conjunctival sac are shown. Full details of the legend are given in the text.
255 and insulated by a sleeve (5) to end with a pin (6). The whole length of the wire is insulated with Teflon (9 through 11) except in the central region, where three windows of 3 mm in length are formed on one side (7 and 8), as illustrated in Fig. lB. When silver wire is used, the exposed portions can be chlorided. The loop (2) in Fig. 1B is elastically deformable and selfsupporting in any position to which it is formed. Figure 1A illustrates the electrode in two different positions. The position illustrated in broken lines represents a flat loop of wire in a single plane. The position illustrated in solid lines is a configuration typical of that into which the electrode would be bent before use. When in use, the electrode is bent to a configuration that appears U-shaped in side view (Fig. 1C). The two limbs (12 and 13) together form a hooking member carrying the windows, which is inserted into the lower conjunctival sac (14), leaving the corneal surface (15) entirely free. Figure 1D shows how the loop shape of the electrode is able to avoid contact with the cornea, while the self-supporting loop configuration hooked into the fornix provides a stable, secure mounting for the electrode, especially when the tape (17) has been applied. The electrode is sterilized by immersion into a sterilizing solution. Since the electrode is heat resistant and Teflon lined, it may also be autoclaved. For this purpose, the complete electrode is disconnected from the socket (18) on the connecting wire (16). The electrode can be used several times, although we believe that with commercial production it could be inexpensive enough to be disposable. When in position, electrode resistance was always below 5 kOhm and did not change throughout recording. The reference electrode could be an identical electrode in the fellow eye, or a skin electrode on the ipsilateral temple, as used here. Topical anesthetic was rarely needed but is advisable in sensitive patients or young children.
The recording method for pattern ERG recording is described in detail elsewhere [7]. For averaged photopic flash ERGs, subjects with dilated pupils were fully light adapted under laboratory fluorescent tubes with illuminance of 400 lux. Scotopic single-flash ERGs and c-waves were recorded with dilated pupils after 20 minutes of dark adaptation in a room with a 25-W bulb under a Kodak A1 red filter. For averaged flash ERGs, a Medelec flash unit with 19-cm diameter at a distance of 25 cm from the patient was triggered at a frequency of 5 Hz by a Medelec ER 94/e Sensor. For scotopic ERGs, single flashes were triggered manually. The energy of the flashes was 0.72 J for photoptic ERGs and 0.18 J for scotoptic ERGs. Responses were differentially amplified by a Medelec ER94/a Sensor over a bandpass of 0.1-125 Hz and stored on an Apple IIe computer. The ERG c-wave was recorded by a Medelec Sensor over the bandpass 0.01-125 Hz and time base of 10 seconds after a green (565-nm) light-emitting diode pulse stimulus of 2000 ~W/cm 2 and 100-ms duration at a distance of 5 mm from the eye.
256 Results
The ERGs shown in this report were taken from recordings on three healthy subjects. Figure 2 shows an example of a typical pattern E R G recording carried out with three different electrodes. Photopic flash responses recorded by skin, Henkes-type contact lens (Miller & S6hne AG) and (HK)-loop electrodes are shown in Fig. 3. Scotopic single-flash responses are shown in Fig. 4. Stability of the signals from the HK-loop electrode was best illustrated by the E R G c-wave measurement. If the subject managed not to blink for 10 seconds after the light stimulation, a single-flash c-wave could be recorded (Fig. 5).
Pattern ERG, LE,dob 1958 Ag disc
Ag wire
i
20ms
I
I
I
I
I
I
I
I
Fig. 2. An example of a pattern E R G signal obtained by an HK-loop electrode made of silver wire, compared to that obtained by skin and gold-foil electrodes; all refer to skin electrode on ipsilateral temple. Note that the signal (128 sweeps, recorded twice) from the wire electrode (3rd trace) is clean and reproducible and of approximately double the amplitude of that obtained by a skin electrode (1st trace). It is about in the same range as that obtained with a gold-foil electrode (2nd trace).
257 Flash ERG, LE, dob 1961
Fig. 3. Averaged photopic flash ERG, recorded with three different electrodes: skin, HK-loop and Henkes-type corneal electrodes. Responses were averaged 64 times and recorded twice for reproducibility.
Fig. 4. Scotopic single flash, recorded twice. Flash energy was 0.18J. Note approximately 230-~LVb-wave amplitude and good reproducibility.
Discussion We have described a new type of noncorneal E R G electrode that, in a simple manner, solves some of the problems of E R G recording outlined in the literature. With a self-supporting but plastically deformable filamentary member, a looped and hooked configuration of the electrode enables the
258 c
i00
gV I _ _ I sec
Fig. 5. In a steady subject with a dilated pupil, a single-flashc-wave could be recorded. Sweep
time was 10 seconds with prestimulus delay to assess the baseline level. Light stimulus was light-emitting diode pulses of 2000 ixW/cm2 with a duration of 100 ms. electrical contact of the constant area to be placed firmly in the lower fornix of the eye, while the insulation provides shielding from unwanted body potentials. Insulation of the wire is essential for function of the electrode. The signals recorded by the equivalent electrode made of naked wire were continually overloaded by muscular noise, which rendered recording practically impossible. Results of the measurements with our electrode show that reproducible pattern E R G responses can be obtained that are of similar amplitude to those of the gold-foil electrode. Flash E R G amplitudes were approximately two thirds of those obtained by corneal electrodes and more than three times the amplitude obtained by skin electrodes. The HK-loop electrode is more stable and durable than gold-foil or D T L electrodes. Since it does not touch the cornea, the new electrode may also be used without topical anesthesia. Avoiding corneal contact makes them suitable also for eyes with damaged corneas and renders abrasions of intact cornea, as noted with the use of corneal [8] and gold-foil [9] electrodes, unlikely. Additional advantages of the electrode are easy insertion and adaptability to particular normal or abnormal anatomic configurations of the patient's eye and eyelid. It can likewise be adapted to individual size and shape for children of any age. The electrode is easily cleaned and can be effectively sterilized. The lack of stable lid retraction can be, when needed, provided by a simple Barraquer-type lid retractor that leaves the lower fornix deep enough for the electrode to be inserted. A recently completed normative study for pattern E R G s [10] showed an amplitude range recorded with the HK-loop electrode of 3.0-6.12 pN, with a coefficient of variation of 18.4% (n = 30).
Acknowledgments The HK-loop electrode forms the subject of British and US patent applications 9008901.2 and 07/686,235. *HK refers to the initials of the authors. T h e electrode is commercially available. Information can be obtained from the authors.
259 References 1. Borda RP, Gilliam RM, Coats AC. Gold-coated Mylar T M (GCM) electrode for electroretinography. Doc Ophthalmol Proc Ser 1977; 15: 339-43. 2. Arden GB, Carter RM, Hogg C, Siegel IM, Margolis S. A gold foil electrode: Extending horizons for clinical electroretinography. Invest Ophthalmol Vis Sci 1979; 18: 421-6. 3. Dawson WW, Trick GL, Litzkow CA. Improved electrode for electroretinography. Invest Ophthalmol Vis Sci 1979; 18: 988-91. 4. Holopigian K, Snow J, Seiple W, Siegel I. Variability of the pattern electroretinogram. Doc Ophthalmol 1988; 70: 104-16. 5. McAllan A, Sinn J, Aylward GW. The effect of gold foil electrode position on the electroretinogram in human subjects. Vision Res 1989; 29: 1085-87. 6. Coupland SG, Janaky M. ERG electrode in pediatric patients: Comparison of DTL fiber, PVA-gel and non corneal skin electrodes. Doc Ophthalmol 1989; 71: 427-33. 7. Hawlina M, Strucl M, Stirn-Kranjc B, Finderle Z, Brecelj J. Pattern electroretinogram recorded by skin electrodes in early ocular hypertension and glaucoma. Doc Ophthalmol 1989; 73: 183-91. 8. Dawson WW, Zimmerman TJ, Houde WL. A method for more comfortable electroetinography. Arch Ophthalmol 1974; 91: 1-2. 9. Aylward GW, McClellan KA, Thomas R, Billson FA. Transient corneal changes associated with the use of gold foil electrodes. Br J Ophthalmol 1989; 73: 980-84. 10. Hawlina M. Pattern electroretinogram with the new 'HK-loop' electrode [Abstract]. Presented at the 30th symposium ISCEV, Vienna, 1992.
Address for correspondence: Marko Hawlina, MD, University Eye Clinic, 61000 Ljubljana, Slovenia Tel: +38 61 113-066; Fax: +38 61 329-981
