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Quantification of Optic Nerve Axon Loss Associated With a Relative Afferent Pupillary Defect in the Monkey
John B. Kerrison, MD;
Kelvin Buchanan, DVM;
Michael L. Rosenberg, MD;
Robert Clark, PhD;
Kurt Andreason, MD;
Daniel V. Alfaro, MD;
Hans E. Grossniklaus, MD;
Lisa A. Kerrigan-Baumrind, MS;
Danielle F. Kerrigan, BS;
Neil R. Miller, MD;
Harry A. Quigley, MD
Arch Ophthalmol. 2001;119:1333-1341.
ABSTRACT
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Objective To quantify the amount of optic nerve axonal loss associated with the
presence of a mild relative afferent pupillary defect (RAPD) in an experimental
monkey model.
Methods The right macula of 5 rhesus monkeys (Macaca mulatta) was treated with concentrically enlarging diode laser burns until
an RAPD was detected using a transilluminator light and measured with neutral
density filters. Intervals between treatments were 3 to 7 days over a period
of 2 months. Pupillary responses to light stimulation were recorded with a
monocular infrared television pupillometer. Two months after detection of
an RAPD, 5 treated and 4 control monkeys underwent euthanasia and enucleation.
Histopathologic analysis and quantification of optic nerve axon counts using
an image analysis system were performed.
Results No RAPD was observed despite an estimated ganglion cell loss of up to
26%. A 0.6 log unit RAPD was present in 5 monkeys when the laser scar incorporated
the entire macula within the temporal vascular arcades. One eye had progressive
vitreomacular traction with worsening of the RAPD to 1.8 log units without
further laser treatment. Histopathologic evaluation disclosed complete loss
of the normal retinal architecture within the macula. The average fiber loss
for the 4 treated eyes with 0.6 log unit RAPDs compared with fellow eyes was
53.3% (95% confidence interval [CI], 45.0%-61.6%). The average difference
in axon counts between untreated pairs of optic nerves was 12.8% (95% CI,
10.0%-15.6%). Optic nerve axon loss between pairs of experimental and control
eyes was statistically significant (P<.001).
Conclusion In rhesus monkeys, an RAPD develops after an approximate unilateral
loss between 25% and 50% of retinal ganglion cells.
Clinical Relevance Owing to redundancy in the anterior visual pathways, unilateral retinal
ganglion cell loss may occur prior to the observation of an RAPD. The presence
of an RAPD measuring 0.6 log units implies that significant retinal ganglion
cell injury has occurred.
INTRODUCTION
ASSESSMENT OF the pupillary reaction to light is one of the few tests
of visual function that does not require a subjective patient response. Detection
of abnormalities in the pupillary light reflex is performed by alternately
illuminating each eye while comparing the velocity and amplitude of the pupillary
responses.1-2 Asymmetry in this
response is referred to as a relative afferent pupillary defect (RAPD) and
indicates either unilateral or bilateral asymmetric disease of the anterior
visual system.
An RAPD can be quantified by sequentially placing optical filters of
increasing density in front of the normal eye as a light source alternately
illuminates each eye.3 These filters logarithmically
reduce the light input into the normal eye until the pupillary responses are
symmetric. Using this technique, the severity of an RAPD can be quantified
as the density of the filter required to balance the response of each eye,
ranging from 0.3 to 3.0 log units. While an RAPD measuring 0.3 to 0.6 log
units might clinically be considered to be within the mild spectrum of disease,
it represents a 50% to 87% decrement in light input, respectively.
Although the severity of an RAPD does not correlate with reduction in
visual acuity, it does correlate with the visual field loss4-6
and the anatomic extent of retinal disease.7-9
Despite the fundamental clinical importance of the RAPD in assessment of visual
function, it is not known how much optic nerve damage is present when one
observes an RAPD. The present study quantified the amount of axon loss associated
with the presence of a 0.6 log unit RAPD in an experimental animal model (rhesus
monkey) of retinal nerve fiber loss produced by unilateral retinal laser photocoagulation.10
MATERIALS AND METHODS
EXPERIMENTAL MODEL AND PUPIL RECORDING
Rhesus monkeys (Macaca mulatta) were chosen
for use in this study because their eyes closely resemble the human eye in
foveal structure, pigmentation, and the pupillary response to light. They
were anesthetized and handled for treatment, photography, pupillography, and
euthanasia in accordance with standards established by the Association for
Research in Vision and Ophthalmology (Rockville, Md) resolution on Use of
Animals in Research and US Department of Defense guidelines. Based on 3 prior
unpublished observations (Dr Quigley, unpublished data, 1990), the number
of axons between monkey eyes can vary by 10%. We estimated the need to treat
between 4 and 5 monkeys to detect at least a 20% difference in axonal counts.
Prior to laser treatment, all animals underwent eye examination with a hand
light, clinical assessment of the pupillary response, measurement of intraocular
pressures, dilated fundus examination, and fundus photography using a Kowa
fundus camera (Kowa, Torrance, Calif). All monkeys participating in the study
had normal findings on eye examination prior to inclusion. Sedation consisted
of intramuscular injection of ketamine (10 mg/kg) using a 25-gauge needle.
The monkeys were then placed in primate restraint chairs as a means of stabilizing
the head.
Following baseline pupillometric recording from all animals, the right
eye was dilated with 2.5% phenylephrine and 1% tropicamide. The treatment
protocol was initiated with an approximately 800-µm laser lesion centered
on the nasal aspect of the foveal depression in the papillomacular bundle
of the right eye. The laser consisted of an Oculight Diode Laser (Iris Medical
Instruments Inc, Mountain View, Calif) administered through an indirect ophthalmoscope
delivery system. The laser emission was approximately 810 nm in the infrared
part of the optical spectrum. The spot size was 500 µm with duration
ranging from 400 to 600 milliseconds and power ranging from 340 to 810 mW.
To ensure damage to the ganglion cell and nerve fiber layers, treatments were
repeated and intense. Between 2 and 5 days following each treatment, the eyes
were examined clinically for the presence of an RAPD. If an RAPD was not present,
the eyes were dilated and further laser emissions were administered to the
prior treatment area followed by concentric expansion around the lesion. Once
the nasal aspect of the lesion reached the optic nerve, the lesion was expanded
temporally.
Fundus photography was performed periodically. Images were digitized
and a photomontage created (Adobe Photoshop 4.0; Adobe Systems Inc, San Jose,
Calif). A map of retinal ganglion cell isodensities from monkey eyes published
by Perry and Cowey11 was projected onto each
retinal photomontage (Figure 1).
This template was referenced to the horizontal diameter of the optic nerve
determined in the pupil/optic nerve head histopathologic sections. The area
of scarring within each isodensity contour line was outlined and measured
using computer software that allows one to capture, display, analyze, and
measure images (Scion Image; Scion Corp, Frederick, Md). This area was multiplied
by the ganglion cell density for that isocontour. The total estimated ganglion
cell loss was the sum of the estimated ganglion cell loss for each area.
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Figure 1. Photomontage of the lesion prior
to (A) and after (B) the development of a relative afferent pupillar defect
(RAPD) in monkey 89-135/R99-41 with overlay of a template of retinal ganglion
cell density isocontours by Perry and Cowey.11
Prior to the development of an RAPD, the size of the lesion was 5.82 mm2 with a predicted percent ganglion cell loss of 8.9% to 11.1% (Table 2). After the development of an RAPD,
the size of the lesion was 37.66 mm2 with a predicted percent ganglion
cell loss of 29.8% to 37.2%. The actual percent ganglion cell loss was 43.2%.
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Table 2. Estimate of Retinal Area Lesioned at an Interval Prior to
Relative Afferent Pupillary Defect (RAPD) in 3 Monkeys and After Development
of RAPD in 5 Monkeys
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Clinical testing for the presence of an RAPD was performed under ketamine
sedation in darkness using a transilluminator light (Welch-Allyn, Skaneateles
Falls, NY), illuminating each eye separately while assessing the pupillary
response for asymmetry. The transilluminator used a 3.5 V halogen bulb. Following
this, the light was alternated back and forth at varying intervals between
1 and 3 seconds. If asymmetry in the pupillary responses was observed, neutral
density filters of increasing density were placed in front of the untreated
eye while the light was once again alternated back and forth. The density
of the filter was recorded when the pupillary responses were symmetric. Pupil
recordings were performed in treated monkeys with a monocular infrared television
pupillometer (Eye Scan Inc, Burlington, Mass). The source of light photostimulation
was a handheld mini-Ganzfeld photostimulator (LKC Technologies Inc, Gaithersburg,
Md). The photostimulator provided a square-wave stimulation of 13 candela
(cd)/mm2 as a 2-second pulse or continuous mode, the signal for
which was output to the pupillometer.
The recording protocol consisted of illuminating 1 eye while recording
the response from the fellow eye in the darkened room. The mini-Ganzfeld photostimulator
was used to stimulate an eye for 2 seconds at 3- to 5-second intervals. The
photostimulator output signaled to the computer the onset and cessation of
the stimulus. Each trial consisted of 6 to 9 stimulations. After a trial,
several minutes were allowed to elapse in darkness before conducting another
trial. Each eye underwent 3 to 6 trials. The constriction amplitudes were
averaged for each eye at examinations performed at the initial observation
of an RAPD and 2 weeks later.
EUTHANASIA, HISTOPATHOLOGIC ANALYSIS, AND AXON COUNTING
For euthanasia, monkeys were administered intramuscular ketamine (10
mg/kg) in the caudal thigh muscle using a 25-gauge needle, followed by intravenous
pentobarbital sodium via the lateral saphenous vein using a 23-gauge needle.
Perfusion fixation was performed after exsanguination through an incision
in the femoral artery. An abdominal incision was performed with isolation
of the descending aorta approximately 3 inches prior to the bifurcation. A
14-gauge cannula was inserted into the aorta, and a preplaced 4-0 silk suture
was tied. After clearing the line with heparinized isotonic sodium chloride
solution, infusion with 1% procaine/isotonic sodium chloride solution was
performed until the blood began to clear. Then, infusion with approximately
2 liters of 4% paraformaldehyde/2% glutformaldehyde in 0.1mM phosphate buffer
(pH 7.4) was performed. The eyes and optic nerves were harvested along with
other organs.
Following enucleation, globes were placed in fixative after slits were
made in the pars plana. The globes were opened by removal of the superior
and inferior caps. Pupil/optic nerve sections incorporating the area of laser
treatment were prepared and stained with hematoxylin-eosin. Cross sections
of the optic nerves were also obtained. A 1-mm thick section of optic nerve
was obtained within 3 mm of the posterior surface of the globe and the superior
and nasal quadrants marked with single and double razor blade cuts, respectively.
These sections were rinsed in cacodylate buffer (pH 7.4), postfixed in 2%
osmium tetroxide in cacodylate buffer, dehydrated in alcohol, and embedded
in epoxy resin. Cross sections 1-µm thick were cut with an ultramicrotome,
mounted on glass slides, and stained with toluidine blue, which allows one
to distinguish residual, degenerating myelin bundles from normal ones. Neural
bundle areas, from normal profiles, were measured by planimetry on enlarged
photographs of each nerve cross section, and each nerve was divided into 16
segments of approximately equal area. Four random 50 x 50 µm areas
from each of 16 segments were examined using an image analysis system (Zeiss
VIDAS; Carl Zeiss Inc, Thornwood, NY) to determine absolute axon number and
fiber diameter. To determine fiber diameter, an algorithm was used by which
each myelinated nerve fiber had its geometric center identified and 32 radii
drawn to the inner edge of the myelin sheath. The smallest radius was multiplied
by 2 to determine the smallest diameter. Fiber diameters were sorted into
bins separated by 0.1 µm. Optic nerve axon counts were performed on
all specimens and the difference between eyes determined. The difference in
axon counts between pairs of eyes from treated monkeys was compared with the
difference in axon counts between pairs of eyes from untreated animals using
the 2-tailed t test with unequal variance.
RESULTS
Five monkeys received concentrically enlarging diode laser treatments
to the macula until an RAPD was detected (Table 1). The amount of treatment varied from 390 to 810 mW delivered
between 6 and 11 treatment sessions over the course of 3 months. The number
of spots per session varied from 1 to 798, with the total number of spots
varying from 2262 to 3378 spots per eye. Four eyes experienced hemorrhaging
during laser treatment. Two eyes developed small choroidal hemorrhages that
resolved. Two eyes had choroidal hemorrhages with extension into the vitreous
following laser-induced breaks in the Bruch membrane. The vitreous hemorrhages
cleared over 2 weeks. In 1 eye, vitreomacular traction became apparent on
subsequent examinations.
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Table 1. Summary of Treatment*
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Prior to each successive treatment, the monkeys were examined for the
presence of an RAPD. When the treatment area, based on the retinal photomontage,
measured an average of 9.43 mm2 (3 monkeys: range, 5.82-14.65 mm2) at the nasal aspect of the foveal depression within the maculopapillary
bundle, the pupils of treated and untreated monkeys were examined by an observer
(M.L.R.) who was masked to treatment status of the monkey and which eye was
treated (Table 2). The observer
did not detect an RAPD in any of the monkeys at this stage. Estimation of
the retinal ganglion cell death at this time, based on the optic nerve axon
counts of untreated eyes in the present study and the projection of monkey
retinal ganglion cell isodensity maps11 onto
the retinal photomontage, resulted in a mean estimated percent ganglion cell
loss of 16.5% (range, 11.1%-26.0%) (Table
2).
When the lasered area incorporated the entire area within the arcades,
a 0.6 log unit RAPD was detected in the treated eye of 5 monkeys (Table 1). All monkeys were euthanized 2
months after the detection of an RAPD. In 4 monkeys, to destroy any possible
remaining ganglion cells within the laser scar, a final laser treatment session
was applied only within the previously treated area 1 month prior to euthanasia.
At the time of euthanasia, the size of the RAPD remained 0.6 log units in
all 4 animals. The treated eye of the fifth monkey, which had experienced
a vitreous hemorrhage, had no further photocoagulation. This eye developed
worsening vitreomacular traction, and a 1.8 log unit RAPD was detected prior
to euthanasia.
Prior to euthanasia, the lesioned area in all the monkeys incorporated
the entire macula within the temporal arcades (Figure 1). Based on reconstruction of the retinal photomontage,
the lesions measured an average of 39.38 mm2 in 5 monkeys (range,
28.36-47.90 mm2) (Table 2).
Pupillography was performed on 5 lesioned monkeys after initial observation
of an RAPD and again 2 weeks later. Technically consistent recordings were
obtained after stimulation with the mini-Ganzfeld photostimulator. The amplitude
of pupillary constriction was decreased an average of 30% in treated eyes
in comparison with untreated eyes (Table
3; Figure 2).
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Table 3. Results of Pupillography With Mini-Ganzfeld Stimulus Performed
After Initial Observation of an RAPD (Examination 1) and 2 Weeks Later (Examination
2)*
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Figure 2. Pupil recording from monkey 89-140/R99-42
(examination 1) after detection of a 0.6 log unit relative afferent pupillary
defect in the right eye on clinical examination. The tracing on the left is
performed during stimulation of the left eye for 2 seconds (horizontal bar)
with a mini-Ganzfeld stimulus while recording from the right eye. The tracing
on the right is performed while stimulating the right eye and recording from
the left eye.
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Following enucleation, histopathologic evaluation of treated eyes disclosed
complete loss of the normal retinal architecture within the macula (Figure 3A). The retina was replaced by avascular
tissue composed of glial cells and pigmented macrophages. A thin preretinal
membrane was present overlying the scar.
For an area extending approximately 
mm on either side of the scar, there was loss of the photoreceptor layer with
some remaining cells in the inner nuclear and ganglion cell layers. The retinal
pigment epithelium was absent in the area of the scar. The underlying choroid
was thickened with pigmented cells. The temporal aspect of the optic nerve
was atrophic with thinning of the temporal nerve fiber layer. Examination
of the optic nerve cross sections disclosed a C-shaped area of atrophy with
vacuolization of the nerve fiber bundles and gliosis (Figure 3C). Optic nerve cross sections stained with toluidine blue
demonstrated some residual degenerating myelin bundles (Figure 4).
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Figure 3. Histopathologic analysis of the
retina and optic nerve from monkey 89-126/R99-27 (hematoxylin-eosin). A, Within
the macula, there was complete loss of the normal retinal architecture (original
magnification x25). The retina was replaced by avascular tissue composed
of glial cells and pigmented macrophages (arrows). A focal, thin preretinal
membrane was present overlying the scar (arrowheads). The retinal pigment
epithelium was absent in the area of the scar. The underlying choroid was
thickened with pigmented cells (asterisks). B, The temporal aspect of the
optic nerve was atrophic (arrowheads) with thinning of the temporal nerve
fiber layer (original magnification x10). For an area extending approximately
mm on either side of the scar, there was loss of the photoreceptor layer with
some remaining cells in the inner nuclear and ganglion cell layers. C, Microscopic
examination of right optic nerve cross sections (original magnification x10)
disclosed a C-shaped area of atrophy (between arrowheads) temporally with
vacuolization of the nerve fiber bundles and gliosis.
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Figure 4. Right optic nerve (monkey 89-140).
A, Injured area demonstrates mostly degenerating myelin profiles (arrow) and
glial cells (asterisk). Some residual normal myelinated axons (arrowhead)
are present, particularly at the margin of injury, as in this area. B, Normal
area demonstrates well-formed myelinated axons (arrowhead) (toluidine blue,
original magnification x400).
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Quantification of axon loss between eyes in experimental monkeys and
control monkeys demonstrated a significant axon loss in treated eyes (Table 4). The mean ± SD fiber loss
for the 4 treated eyes with 0.6 log unit RAPDs in comparison with fellow eyes
was 53.3% ± 8.0% (95% CI, 45.0%-61.6%). Compared with the difference
between pairs of untreated eyes of 4 control monkeys, this loss significantly
exceeded the normal intereye difference in axon counts of 12.8% ± 2.8%
(95% CI, 10.0%-15.6%) (P<.001, 2-tailed t test). Axon loss was greatest in the temporal sector
of the optic nerve (Table 5; Figure 3). Estimation of the retinal ganglion
cell death prior to euthanasia based on projection of monkey retinal ganglion
cell isodensity maps of Perry and Cowey11 onto
the retinal photomontage resulted ina mean estimated percent ganglion cell
loss of 30.0% (range, 24.8%-35.3%) (Table
2), lower than what was observed for axon counts.
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Table 4. Total Retinal Ganglion Cell Axonal Counts and Mean Axonal
Diameters in Treated and Untreated Monkeys*
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Table 5. Retinal Ganglion Cell Axonal Counts in Various Sectors of
the Optic Nerves in Treated and Untreated Monkeys
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Comparison of mean axonal diameters between pairs of eyes in treated
monkeys and pairs of eyes in untreated monkeys demonstrated no significant
difference (Table 4). The distribution
of the diameters for treated and untreated eyes was similar (Figure 5).
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Figure 5. Graph of axonal diameters of treated
right and untreated left eyes.
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COMMENT
The principal finding of this study is that an RAPD, measuring 0.6 log
units, developed after an approximate loss of between 25% and 50% of retinal
ganglion cells. To determine the amount of retinal ganglion cell loss using
the present model, full-thickness destruction of the retina within a circumscribed
area had to be achieved followed by an adequate interval of time for ascending
atrophy to take place. Complete destruction of all retinal layers was achieved
by repeated laser treatments and confirmed by histopathologic examination.
The area of injury was essentially confined to the treatment area as only
a thin rim of outer retinal injury, with some remaining inner nuclear and
ganglion cell layers was present. The monkeys underwent euthanasia 2 months
after the detection of an RAPD. To ensure complete destruction within the
scar, the previously treated area was retreated 4 weeks prior to euthanasia.
In the squirrel monkey, Anderson12 observed
that atrophy occurs in the distal axon segment following retinal photocoagulation
between 2 and 4 weeks after injury. Four weeks following retinal laser treatment,
most of the axon debris had cleared without the appearance of phagocytes.12 In the present study, the prior heavy treatment within
the laser scar over the preceding months, the lack of a change in the RAPD
prior to euthanasia, and the ability to distinguish residual degenerating
neural bundle profiles from normal profiles in axon counting suggest that
theinterval between treatment and euthanasia was adequate and did not result
in an underestimation of the extent of damage. Thus, the optic nerve axon
counts accurately reflect the extent of terminal ganglion cell injury at the
time of euthanasia.
Ganglion cell injury beyond the area of treatment may have occurred
from either damage to the nerve fibers passing through the treated area or
from secondary injury.13-16
Although damage to axon processes passing though the treatment area with subsequent
descending atrophy may have occurred, this was minimized by expanding the
lesion temporally rather than superiorly and inferiorly. In addition, one
would expect injury of axons passing through the lesion to be reflected in
both the pupillary responses and axon counts. Secondary retinal ganglion cell
damage, possibly mediated through glutamate excitotoxicity outside the laser
treatment area, may also have occurred. It is difficult to estimate the role
this may have played. If the effect was progressive following injury, it was
not large enough to alter the size of the RAPD over 8 weeks. If secondary
injury took place simultaneous to primary death of the retinal ganglion cells
that were photocoagulated, its effect would be expected to be observed in
both the pupillary responses and in the axon counts.
Overlay of the retinal ganglion cell templates on the retinal photographs
resulted in an underestimation of axon loss in comparison with postmortem
axon counts. This may be owing to loss of retinal ganglion cells outside the
treatment area as discussed or underestimation of the density of retinal ganglion
cells in these templates.
This model, in which ganglion cell loss was associated with outer retinal
damage, differs from most clinical optic neuropathies in which damage is generally
isolated to the retinal ganglion cell and nerve fiber layers. The particular
advantage of the present model is that it allowed ganglion cell damage to
take place in a controlled, graded manner followed by pupil examination. For
the present model to be compared with clinically encountered optic neuropathies,
it is assumed that photoreceptors within a scotoma do not contribute significant
pupillomotor input through lateral transmission of impulses to regions outside
the scotoma.
In humans, RAPDs of 0.3 log units can easily be detected, and defects
as small as 0.1 log units can be measured using cross-polarizing filters.
Good correlation has been demonstrated between intraobserver clinical measurements
as well as between clinical measurement17 and
automated infrared pupillometry.18 In the present
study, pupillary responses were assessed both clinically and using infrared
pupillometry. Assessment of the pupillary responses in this study was performed
while these monkeys were sedated with ketamine. Although this may have influenced
the pupillary response, we would expect the response of each eye to be equally
affected; hence, this would not impair our ability to identify a difference.
Regarding clinical assessment, a 0.6 log unit deficit was the smallest
RAPD that could be reliably detected in any of the monkeys. The development
of a 0.6 log unit RAPD in the setting of significant injury may reflect a
threshold effect with regard to ganglion cell loss and pupillary responses,
or it may be secondary to technical factors. In humans, the ability to detect
subtle abnormalities with the alternating flashlight test is highly dependent
on timing. We suspect that the responses of the monkey pupil are different
from human responses in that they have a shorter latency and a more rapid
recovery phase. Thus, the most sensitive testing paradigm for detecting an
RAPD in humans may be different from that used in monkeys.
In humans, the RAPD correlates with the anatomic extent of retinal damage
in macular degeneration9 and retinal detachment.7-8 In patients with macular degeneration,
an RAPD is observed most often with disciform scars larger than 6 disc diameters.9 In one study of RAPDs in retinal detachments, each
peripheral quadrant of detachment contributed to 0.35 log units of defect,
whereas detachment of the macula caused an additional 0.68 log units.8 In another study of retinal detachments, abnormal
pupillary responses were uncommon in peripheral detachments and occurred in
approximately half of the detachments involving the macula.7
Although the depth of an RAPD does not correlate with visual acuity,
it does correlate with the extent of visual field loss.4-6
The correlation between the RAPD and static threshold as measured by perimetry5-6 is consistent with the observation
that the light reflex is only 0.2 log units above the threshold for light
perception19 and closer to the threshold for
light perception with larger areas of stimulation.20
Visual fields have been used to estimate the amount of ganglion cell loss
in association with RAPDs by superimposing templates of human ganglion cell
densities21 over visual fields. A linear correlation
was observed, which predicted that a 0.6 log unit RAPD would be associated
with an estimated ganglion cell loss of between 6% and 18%. However, the authors
acknowledged that these results should be interpreted with caution because
the study was unable to account for the presence of relative scotomas and
did not take into account the observation of other investigators that the
relationship between ganglion cell loss and visual field loss has been observed
to be nonlinear.22-24
Bilateral quantification of optic nerve axon loss in humans with RAPDs
has been performed. In these cases, the axon loss was severe and bilateral,
but asymmetric. For example, in a case of bilateral anterior ischemic optic
neuropathy, the axon counts were reduced to 4% of normal in one eye and 28%
of normal in the other eye.25 In 3 cases with
compressive lesions of the anterior visual pathways and RAPDs, the reduction
in optic nerve axons, compared with normal optic nerves, was 30% and 70%,
9% and 32%, and 0% and 3% (right and left eyes).26
Thus, in the setting of bilateral optic neuropathies, an RAPD may be present
with an estimated retinal ganglion cell reduction by as much as 57% in one
eye compared with the other. Although RAPDs were present in these cases, the
severity of the RAPDs was not quantified.
Although our study does not demonstrate the retinal ganglion cell loss
that is sufficient to produce the minimum clinically detectable RAPD of 0.3
log units, it does demonstrate that an RAPD developed when retinal ganglion
cell loss was between approximately 25% and 50%. This would be consistent
with a threshold effect regarding estimated ganglion cell loss and central
visual function. A threshold effect may be observed owing to an exponential
relationship between ganglion cell loss and visual function or a limit beyond
which this relationship is linear. A nonlinear relationship has been observed
with regard to visual acuity27-29
and visual fields.22-24
Taken together, these studies imply that disease may affect retinal ganglion
cells prior to a significant decrease in visual acuity, a mild abnormality
on visual field testing, or a mild RAPD.
A threshold effect with regard to retinal ganglion cell damage and the
RAPD may be explained by redundancy in the anterior visual pathways. The neuroanatomic
substrate for this redundancy may be overlapping receptive fields.30-31 The relationship between visual function
and retinal ganglion cell loss may be influenced by receptive field size and
overlap as well as by the pattern of retinal ganglion cell loss, whether it
is focal and complete, as in the present study, or diffuse and partial. The
receptive field size and overlap varies with the ratio of photoreceptors to
retinal ganglion cells, which varies with location (central vs peripheral)
and ganglion cell type. The pattern of retinal ganglion cell loss also depends
on the pathologic process. Thus, the estimation of retinal ganglion cell loss
in association with an RAPD, as determined in the present study, may differ
in comparison to another model, such as glaucoma, which results in a different
pattern of retinal ganglion cell loss.
These data challenge the hypothesis that a mild asymmetry of 53% of
crossed fibers compared with 47% of uncrossed fibers32
underlies the RAPD seen in optic tract injury.33-36
Although asymmetry of fiber crossing at the chiasm is the most plausible explanation
for an RAPD with optic tract lesions, as many as 4 other possible explanations
are possible: (1) the amount of crossing may be greater than previously estimated,
(2) the asymmetry may be greater for pupillomotor fibers,37
(3) a functional asymmetry may be present that is not represented in anatomical
studies, or (4) additional physiologic processes such as inhibition may be
playing a role. Nevertheless, optic tract lesions can be associated with RAPDs
of 0.3 log units, and a threshold effect might allow for an RAPD to be present
in the setting of small amounts of asymmetric injury to the anterior visual
pathways.
We observed that an RAPD developed when retinal ganglion cell loss was
between approximately 25% and 50% in rhesus monkeys. To the extent that this
may be extrapolated to clinically encountered optic neuropathies, it implies
that unilateral retinal ganglion cell damage may occur prior to the development
of an RAPD. Furthermore, the presence of a 0.6 log unit RAPD implies significant
injury that may have prognostic significance for a patient's ability to recover
from future insults.
AUTHOR INFORMATION
Accepted for publication February 23, 2001.
This work was supported by a departmental grant, Uniformed Services
University of Health Sciences, Bethesda, Md (Dr Kerrison); EY02120, National
Eye Institute, Bethesda (Dr Quigley); and EY01765, National Eye Institute
(Core Facility Grant, Wilmer Institute).
We thank Anthony C. Kouzis, PhD, of the Wilmer Eye Institute for statistical
consultation.
Corresponding author and reprints: John B. Kerrison, MD, Wilford
Hall Medical Center, 2200 Bergquist Dr, Suite 1, Lackland AFB, TX 78236
(e-mail: jkerrison{at}yahoo.com).
From the Wilmer Ophthalmological Institute, Johns Hopkins Hospital,
Baltimore, Md (Drs Kerrison, Miller, and Quigley and Mss Kerrigan-Baumrind
and Kerrigan); Uniformed Services University of Health Sciences, Bethesda,
Md (Drs Kerrison, Rosenberg, Andreason, and Alfaro); Armed Forces Radiobiology
Research Institute, Bethesda (Dr Buchanan); New Jersey Neuroscience Institute,
JFK Medical Center, Edison (Drs Rosenberg and Clark); and the Department of
Ophthalmology, Emory University School of Medicine, Atlanta, Ga (Dr Grossniklaus).
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