Abstract
purpose. The removal of the internal limiting membrane (ILM) for traction
maculopathy has recently been advocated. However, it is generally
believed that the ILM plays an important role in retinal function,
because it is the basal lamina of the Müller cells that are
involved in the generation of the electroretinogram (ERG) b-wave. To
date, there has been no objective assessment of retinal function on
removing the ILM. In this study, the changes of each component of the
focal macular electroretinograms (FMERGs) were investigated in eyes
before and after the ILM was removed in the macular area during surgery
for idiopathic macular holes (IMHs).
methods. FMERGs were elicited by a 15° stimulus centered on the fovea and
monitored by an infrared fundus camera. FMERGs were recorded from 49
eyes of 48 patients with IMHs before and 6 weeks after anatomically
successful macular hole surgery. Whether an eye had or did not have the
ILM removed was randomly determined. The ILM was removed in 30 eyes
(ILM-off group) and was not removed in 19 eyes (ILM-on group). Six
months after surgery, the same examination was performed in 27 eyes of
the ILM-off group and in 15 eyes of the ILM-on group. The amplitudes
and implicit times of the a- and b-waves and the mean amplitudes and
implicit times of the first three oscillatory potentials (OP1 to OP3)
were compared before and after surgery within and between the groups.
results. Visual acuity increased significantly after surgery in both groups. In
the ILM-on group, the amplitude of the a- and b-waves and the OPs
increased significantly 6 months after surgery (P =
0.0093, P = 0.0019, P = 0.0024,
respectively, paired t-test). In the ILM-off group, the
a-wave amplitude and mean OP amplitudes were significantly larger 6
months after surgery (P = 0.0077, P = 0.0030, respectively, paired t-test). The b-wave amplitude, however, did not change
significantly. The percentage increase in the b-wave amplitude 6 months
after surgery was significantly higher in the ILM-on group (44.0%)
than in the ILM-off group (15.0%; P = 0.037, t-test).
conclusions. The removal of the ILM had no adverse effect on visual acuity. However,
the selective delay of recovery of the FMERG b-wave 6 months after
surgery suggests an alteration of retinal physiology in the macular
region.
The functional role of the internal limiting membrane (ILM) has
not been determined. However, it is generally believed that the ILM
plays an important role in retinal function, because it is the basal
lamina of the Müller cells and is connected to the end feet of
the Müller cells. In addition, the anionic sites on the ILM may
act as a charge barrier between the retina and the vitreous
cavity.
1 2
In macular hole surgery, the surgical removal of the ILM in the macular
region has been performed often lately, because it appears to increase
the closure rate of macular holes.
3 4 5 6 7 The stripping of
the ILM for traction maculopathy has been endorsed because no
proliferative response and no adverse effect on subjective visual
function occurs after removal of the ILM.
8 9 We have also
been removing the ILM during surgery for idiopathic macular holes
(IMHs) and have not seen any clinical adverse effects after ILM removal
(
n = 76 eyes). However, there has not been a prospective
randomized controlled study with a large sample. The earlier results
were limited to the evaluation of subjective visual functions shortly
after surgery, and there has been no objective assessment using, for
example, the electroretinogram (ERG).
There are many factors to be considered before ILM removal is
undertaken. Particularly, attention should be focused on changes of the
b-wave of the macular ERG induced by the removal of the ILM, which has
been reported to include the end feet of the Müller
cells
5 that are involved in the generation of the b-wave.
In a recent report, a patient with a dominantly inherited Müller
cell disease, Müller cell sheen maculopathy, showed impaired
full-field b-waves.
10 11 This dystrophy is a diffuse
disease affecting the whole inner retina, whereas the removal of the
ILM during macular hole surgery is limited to the macular area. Thus,
it is necessary to examine the physiology of the macular area with
focal macular electroretinography (FMERG).
We have been conducting a layer-by-layer analysis in various types of
macular diseases with FMERGs using long-duration
stimuli.
12 13 14 15 16 17 18 This technique allows us to separate the on
and off responses and record the photopic a- and b-waves and the
oscillatory potentials (OPs), which are independent of the off
response. Using these procedures, we have studied the early and late
effects of removing the ILM of each component of the FMERG. This study
addresses two questions: first, whether the removal of the ILM has any
effect on the FMERGs and, second, whether the removal of the ILM during
macular surgery can be physiologically justified.
Surgery was performed by three surgeons on 89 consecutive eyes
with IMHs from January 1998 through December 1999 in our institution,
and in all cases holes were closed with a single operation. The
operation time, manipulation of the macula, use of infusion during the
insertion of the intraocular lens, and amount of the removal of the
vitreous cortex after lens removal varied among the surgeons. To
minimize this surgical variation, we selected the operations performed
by a single surgeon (HT), who used a single operative technique. Of the
89 eyes, 49 eyes of 48 patients were subjected to vitrectomy combined
with lens removal by this surgeon, and the statistical analysis was
performed on these 49 eyes. In addition, the evaluation of visual
acuity (VA) after 6 months in this group was thought not to be
influenced by nuclear cataract, because phacoemulsification and
intraocular lens insertion were performed simultaneously.
Each eye was randomly placed into two groups prospectively according to
the day of the week that the surgery was performed. All the patients
underwent the same procedure including phacoemulsification, core
vitrectomy, removal of posterior hyaloid membrane, and intraocular lens
insertion. The management of the tissue around the macular hole varied
in the two groups. In the ILM-off group, the ILM was removed with ILM
forceps as a single piece of curled, glistening membrane in the area of
approximately 15° or slightly less than 15°. In the ILM-on group,
only scraping of the retinal surface around the macular holes with a
diamond-dusted eraser or brush-back flash needle was performed, and no
membranous structure was removed. After fluid air exchange, 0.6 ml of
pure perfluoropropane was injected into the vitreous cavity in both
groups.
Thirty eyes were placed in the ILM-off group and 19 eyes in the ILM-on
group. This distribution did not differ significantly from a chance
placement in the two groups (χ2 = 2.47, df = 1, P = 0.116). One patient had a
macular hole bilaterally, and the ILM was removed from the right eye
but not from the left eye.
FMERGs were elicited by a 15° stimulus positioned on the fovea and
monitored by an infrared fundus camera. FMERGs were recorded from the
49 eyes with IMH before and 6 weeks after macular hole surgery. Six
months after surgery, the same examination was performed on 27 eyes of
the ILM-off group and on 15 eyes of the ILM-on group. FMERG was not
performed at 6 months after surgery in another seven eyes. Five
patients were followed up by the referring ophthalmologists and did not
return or returned at a later time for examination, and two eyes were
observed for 5 months after the operation.
The amplitudes and implicit times of the a- and b-waves and the mean
amplitudes and implicit times of first three oscillatory potentials
(OP1 to OP3) were compared before and after surgery in both groups. The
changes in the amplitude and implicit time of a-, b-waves and OPs were
also compared between the two groups. The system and the assessment of
recording the FMERG under direct fundus observation have been described
in detail.
19 20 Briefly, an infrared television fundus
camera installed with the stimulus light, background illumination, and
fixation target, was used to monitor the exact locus of the stimulus on
the macula. The size, frequency, and intensity of the stimulus spot,
and the level of background illumination were adjustable. The
background light was projected into the eye from the fundus camera at
an angle of 45°. Additional background illumination outside the
central 45° produced homogeneous background illumination for nearly
the entire visual field.
The Burian–Allen bipolar contact lens electrode was used for the FMERG
recordings. This electrode allowed not only low noise recordings but
also permitted a clear view of the fundus that was displayed on a
television monitor. The luminance of white stimulus light and
background light was 29.46 candelas [cd]/m
2 and
2.89 cd/m
2, respectively. After the patient’s
pupils were fully dilated with a combination of 0.5% tropicamide and
0.5% phenylephrine hydrochloride, the FMERGs were recorded with 5-Hz
rectangular stimuli (100 msec light on and 100 msec light off). The
stimulus spot was centered on the fovea. A total of 512 responses was
averaged by a signal processor. A time constant of 0.03 seconds and a
100-Hz high-cut filter on the amplifier was used to record the photopic
a- and b-waves, and the time constant was reduced to 0.003 seconds for
recording the OPs. The amplitude of the a-wave was measured from the
baseline to the peak of the a-wave. The amplitude of the b-wave was
measured from the trough of the a-wave to the peak of the b-wave. The
amplitude of each OP wavelet was measured from a baseline, drawn as a
first-order approximation between the troughs of successive wavelets,
to its peak. The FMERGs elicited by this method have been shown to be
generated by the cone system, and the responses elicited by spot
stimuli 15° and smaller have been shown to be local
responses.
19 20
This research was conducted in accordance with the institutional
guidelines of Nagoya University and conformed with the tenets of the
World Medical Association Declaration of Helsinki. After providing
sufficient information on other treatment options including observation
alone and macular ERG examinations, informed consent was obtained from
each patient for the surgery.
The ILM is the basal lamina of the Müller cells, which are
involved in the generation of the b-wave of the ERG. The removal of the
ILM has been endorsed as an alternative to the meticulous epiretinal
membrane removal or to biochemical support in macular hole surgery.
However, the effects of ILM removal on retinal function remain unknown.
An ultrastructural study has shown that the prefibrous tissue attaches
not only on the back surface of vitreous cortex but also extends around
and inside the macular hole.
3 One of the key factors in
closing IMHs is the elimination of the traction around the macular hole
as completely as possible.
3 Thus, it has been suggested
that removing the ILM may eliminate almost all traction and lead to a
higher probability of macular hole closures.
3 4 5 6 7 Another
effect of removing the ILM may be the induction of sufficient surgical
trauma to induce gliosis that helps keep the macular hole
closed.
21
It is interesting to note that specimens obtained during macular hole
surgery or surgery for other macular disease such as vitreomacular
traction syndrome were found to include the ILM inadvertently
removed.
22 23 24 25 In reports of the epiretinal membranes
removed during vitrectomy, the eyes in which the specimen included the
ILM showed lower postoperative VA.
25 Unfortunately, there
are no studies on the natural history of the retina after the loss of
the ILM.
22 Eckardt et al.
5 reported
that good anatomic results were achieved with the intentional removal
of the ILM during macular hole surgery. However, they also reported an
important observation that canals leading from the inner to the outer
surface of the ILM contained Müller cell processes with clear
signs of necrosis or degeneration. The pathogenic significance of these
findings remains unknown; however, they suggest possible damage to the
Müller cells.
Because the electrical potential changes in the Müller cells
contribute to the b-wave of the ERG, it was reasoned that defects of
the ILM in the macular region should affect the macular ERGs. Thus, we
recorded FMERGs using a 15° stimulus that is approximately the same
size as the removed ILM during macular hole surgery. However, it is
possible that a small area of the ILM was still attached in the
recorded area.
In photopic ERGs, the best evaluation of the b-wave is performed by
separating the on- and off-components with long-duration stimuli. This
is because the positive deflection recorded by brief stimuli is a
combination of the b- and d-waves. Thus, in this study, the positive
deflection reflects the pure b-wave without contamination by the
d-wave.
The delay in the implicit time of the b-waves in the ILM-off group was
found early after surgery. The delayed implicit times recovered to
preoperative times 6 months after surgery, which suggests that the
surgical effects of removing the ILM on inner retinal function are
temporary. In peeling the ILM, great care was taken to avoid causing
retinal injury that would lead to macular edema. The ILM was peeled off
as a single membrane in the ILM-off group without difficulty by using
ILM forceps. Some specimens were identified as the ILM histologically;
however, most membranes were determined to be the ILM from the texture
of glistening membrane. In the surgery of eyes with epiretinal
membranes, a retinal whitening is sometimes noted during surgery after
the removal of epiretinal membrane with the ILM. In this study, no
fundus change such as a whitish edematous appearance was found in the
macular region after removing the ILM in eyes with macular holes in
this study. In some eyes, a very small whitish point was noted at the
pick-up point of the ILM by the forceps; however, no difference was
detected in the appearance of the retina between the area with and
without ILM at the time of surgery. No patient in either group was
found to have macular edema by fluorescein angiography and/or optical
coherence tomography performed on the same day as the FMERG recording.
Early after surgery, we also demonstrated that the amplitude of the
b-wave had not increased significantly in the ILM-off group, and even
after 6 months, it was depressed and at the same level as before
surgery. In contrast, the amplitude of the b-wave in the ILM-on group
increased significantly both early after surgery and at 6 months after
surgery, compared with that before surgery. The increase of the
amplitude of the a-wave and the mean of the OPs 6 months after surgery
in both groups suggested a selective delayed or incomplete recovery in
the amplitude of the b-wave.
Before surgery, the logMAR VA in the ILM-off group was 0.91
(20/160 − 1), whereas that in the ILM-on group was 0.77 (20/125 +
1). This difference was not statistically significant. The relatively
poorer VA before surgery in the ILM-off group is probably related to a
longer duration and/or larger size of IMH in this group. However, the
amplitude and implicit time of all components in the FMERGs were
approximately the same or even relatively worse for the OPs in the
ILM-on group before surgery, even though the differences were not
significant. This would mean the retinal function in the central 15°
was approximately the same in both groups. According to the
preoperative FMERG data, we believe a small difference in VA could not
explain the decreased b-wave amplitude recovery in the ILM-off versus
ILM-on groups.
Histopathologically, the ILM regenerates very little in the defective
area, whereas the rim of the defective area has some regenerative
capacity.
26 The limited recovery of the b-wave may not
depend on a defect of the ILM but may be due to the recovery of the
Müller cells.
To investigate the effect of ILM removal on retinal function, eyes with
idiopathic macular holes are quite suitable, because the alterations
are uniform and uncomplicated compared with eyes with epiretinal
membranes or with diabetic macular edema. Whether the ILM should be
removed during the surgery for IMH or other tractional maculopathies
must be considered for each eye. There may be certain clinical
advantages of removing ILM for the successful closure of IMH.
Although this was not a purely randomized, controlled study, the
results demonstrated a limited and delayed recovery of the b-wave
amplitude of the FMERG in the ILM-off group in a relatively short
period of 6 months after surgery. These findings suggest some
dysfunction or physiological changes in the Müller cells. Further
long-term follow-up with electrophysiological and clinical examinations
of eyes with the ILM removed will demonstrate the final condition of
the Müller cells.
Supported by Grants-in-Aid 12470361 and 11470363 from the Ministry of Education, Science and Culture, Tokyo, Japan.
Submitted for publication May 1, 2000; revised August 21, 2000; accepted September 6, 2000.
Commercial relationships policy: N.
Corresponding author: Hiroko Terasaki, Department of Ophthalmology, Nagoya University School of Medicine, 65 Tsuruma-cho, Showa-ku, Nagoya 466-8550, Japan.
[email protected]
Table 1. Preoperative Clinical Findings
Table 1. Preoperative Clinical Findings
| ILM-Off Group | ILM-On Group |
| Number of eyes | | |
| Before and 6 weeks after surgery | 30 | 19 |
| Six months after surgery | 27 | 15 |
| Age (y) | 66.1 ± 1.3 | 67.2 ± 2.3 |
| Visual acuity | | |
| LogMAR | 0.91 ± 0.06 | 0.77 ± 0.07 |
| Snellen | 20/160−1 | 20/125+1 |
| Size of macular hole (disc diameters) | 0.29 ± 0.02 | 0.25 ± 0.02 |
| Operation time (min)* | 60.3 ± 2.0 | 61.4 ± 2.4 |
| Duration of gas tamponade (d) | 15.4 ± 0.8 | 15.3 ± 0.9 |
Table 2. Changes of Best Corrected VA and FMERGs before and after Vitrectomy
Table 2. Changes of Best Corrected VA and FMERGs before and after Vitrectomy
| ILM-Off Group | | | ILM-On Group | | |
| Before | 6 Weeks | 6 Months | Before | 6 Weeks | 6 Months |
| Visual acuity | | | | | | |
| LogMAR | 0.91 ± 0.06 | 0.51 ± 0.05 | 0.31 ± 0.04 | 0.77 ± 0.07 | 0.45 ± 0.07 | 0.27 ± 0.04 |
| Snellen | 20/160−1 | 20/60−1 | 20/40−1 | 20/125+1 | 20/60+1 | 20/40+1 |
| P | | <0.0001 | <0.0001* | | <0.0001 | <0.0001* |
| a-Wave | | | | | | |
| Amplitude (μV) | 1.17 ± 0.06 | 1.32 ± 0.09 | 1.44 ± 0.09 | 1.14 ± 0.10 | 1.35 ± 0.09 | 1.53 ± 0.12 |
| P | | | 0.0077 | | | 0.0093 |
| Implicit time (msec) | 20.4 ± 0.29 | 21.3 ± 0.45 | 20.8 ± 0.31 | 20.0 ± 0.34 | 20.5 ± 0.32 | 20.6 ± 0.34 |
| P | | 0.0202 | | | | |
| b-Wave | | | | | | |
| Amplitude (μV) | 3.04 ± 0.21 | 3.15 ± 0.20 | 3.56 ± 0.25 | 2.94 ± 0.18 | 3.24 ± 0.19 | 4.00 ± 0.32 |
| P | | | | | 0.0429 | 0.0019 |
| Implicit time (msec) | 44.8 ± 0.45 | 46.2 ± 0.50 | 45.5 ± 0.49 | 44.9 ± 0.73 | 44.7 ± 0.70 | 45.2 ± 0.55 |
| P | | 0.0016 | | | | |
| Oscillatory potentials | | | | | | |
| Mean amplitude (μV) | 0.71 ± 0.06 | 0.79 ± 0.08 | 0.93 ± 0.08 | 0.56 ± 0.04 | 0.79 ± 0.09 | 0.96 ± 0.12 |
| P | | | 0.0030 | | 0.0004 | 0.0024 |
| Mean implicit time (msec) | 31.5 ± 0.28 | 31.7 ± 0.37 | 32.0 ± 0.31 | 31.0 ± 0.40 | 31.0 ± 0.27 | 31.3 ± 0.29 |
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