Download The effects of choroidal or ciliary nerve transection on myopic

The Effects of Choroidal or Ciliary Nerve Transection on
Myopic Eye Growth Induced by Goggles
Yung-Feng Shih, * Malinda E. C. Fitzgerald,-\ and Anton Reiner\
Purpose. To determine the role of the choroidal and ciliary nerves and the functions they
control, choroidal blood flow (CBF) and accommodation - pupil diameter, respectively, in
myopia induced by form-vision deprivation.
Methods. Three groups of chicks were studied: chicks with choroidal nerves cut in the right
eye, chicks with ciliary nerves cut in the right eye, and sham control chicks that received the
same surgical preparation but no nerve cuts. A plastic, dome-shaped goggle was glued over
the right eye of birds in all three groups after orbital surgery, and, 2 weeks later, CBF was
measured using laser Doppler flowmetry. Refractive status was then measured using streak
retinoscopy, and axial, nasotemporal, and dorsoventral lengths were measured using vernier
calipers after enucleation. The eyes were also weighed.
Results. In the sham control birds, considerable ocular enlargement in all dimensions and a
high degree of myopia ( — 14.68 diopters) was observed in the goggled eye, and CBF in the
goggled eye was 66% of that in the nongoggled eye. In birds with choroidal nerve cuts, the
degree of enlargement of the goggled eye was less in all dimensions, and the myopia in the
goggled eye (—4.74 D) was attenuated compared to that observed in the sham coiitrols. CBF
in the goggled eye was 21% of that in nongoggled eye. Finally, in the birds with ciliary nerve
cuts, nasotemporal and dorsoventral enlargement of the goggled eye were similar to that in
the shams, but the axial elongation and the degree of myopia (—9.57 D) were less than
observed in sham control eyes. As in the shams, CBF in the goggled eye was reduced to 59%
of that in the nongoggled eye.
Conclusions. These results show that although elimination of accommodation and severe reductions in CBF do affect eye growth (the latter more so), they do not prevent form-vision
deprivation-induced myopia. Thus, either the mechanism of visual deprivation-induced myopia is different from that in idiopathic human myopia, or CBF levels and accommodation do
not play a major role in either. Invest Ophthalmol Vis Sci. 1994;35:3691 -3701.
I n previous studies, we have examined the relationships between choroidal blood flow (CBF) and accommodation on one hand, and eye growth and the genesis of myopia on the other.1"3 We found that myopic
eye growth in chicks results in decreases in CBF as a
direct consequence of the ocular enlargement. We
found that dramatically reducing CBF in chicks by
From the *Department of Ophthalmology, National Taiwan University Hospital,
Taipei, Taiwan, and the f Department of Anatomy and Neurobiology, University of
Tennessee - Memphis, Memphis, Tennessee.
Supported by National Institutes of Health grants EY05298 (AR) and AG10538
(MECF), by the Baptist Memorial Health Care Foundation Clinical Innovations
Fund (MECF/STC), and by a fellowship from the National Taiwan University
Hospital (YFS).
Submitted for publication October 6, 1993; revised January 28, 1994; accepted
April 7, 1994.
Proprietary interest categmy: N.
Repiint requests: Dr. Malinda E. C. Fitzgerald, Department of Anatomy and
Neurobiology, 855 Monroe Avenue, University of Tennessee - Memphis, Memphis,
TN 38163.
Investigative Ophthalmology & Visual Science, September 1994, Vol. 35, No. 10
Copyright © Association for Research in Vision and Ophthalmology
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transecting the choroidal nerves, which arise from the
ciliary ganglion and provide a major vasodilatory drive
to the choroid,4"7 results in diminished ocular growth.
This further supports the notion that the decreased
CBF is a consequence of myopic eye growth. We have
also examined the role of accommodation in the control of eye growth in chicks by disabling accommodation (by transecting the ciliary nerves, which arise
from the ciliary ganglion and innervate both the muscles of accommodation and the pupilloconstrictor
muscles) .3 Surprisingly, in these studies, we found that
severing the ciliary nerves yielded increased eye
growth in the treated eye, accompanied by increased
CBF in both eyes. We interpreted the effects of ciliary
nerve transection as suggesting that accommodation
was not necessary for growth to a normal size or beyond, because these eyes were enlarged in all dimen3691
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Investigative Ophthalmology & Visual Science, September 1994, Vol. 35, No. 10
animals, dome-shaped, acrylic plastic goggles were
glued to the circumorbital feathers and skin of the
operated right eye immediately after the orbital surgery in all chicks, according to the method of Hodos
and Kuenzel.10 Thus, three groups of chicks were studied: birds with sham nerve transection and goggling
of the right eye (n = 14), birds with choroidal nerve
transection and goggling of the right eye (n = 17),
and birds with ciliary nerve transections and goggling
of the right eye (n = 18). The efficacy of our nerve
transections has been confirmed and described in a
previous study.3
The birds were maintained on a 12-hour light/
12-hour dark cycle. After 2 weeks, the birds were
weighed and anesthetized with ketamine and xylazine,
and the heads were positioned in a stereotaxic device.
Body temperature was maintained at 38°C with a Harvard (South Natick, MA) homeothermic heating blanket with the thermoprobe placed under the wing. The
scalp and skin were incised and reflected to each side
to expose the upper part of both eyes, and the fascia
MATERIALS AND METHODS
of the eyeballs was cut to expose the sclera. Laser
White Leghorn chicks (Gallus domesticus) were Doppler flowmetry using a Laserflo blood perfusion
monitor (model BPM 403 A; now produced by Vahatched in our laboratory. Four days after hatching,
samedics,
Minneapolis, MN) was employed to measure
chicks were anesthetized with ketamine (Ketaset; 67
the
CBF
in
both eyes. Laser Doppler flowmetry and
mg/kg, intraperitoneally; Aveco, Fort Dodge, IA) and
the
assumptions
that are inherent to the use of this
xylazine (Gemini; 6.6 mg/kg, intraperitoneally; Butinstrument
have
been
described in detail by other auler, Columbus, OH), and nerve transection surgery
31314
thors.
Note
that
we
use the term laser Doppler
was performed. Under aseptic conditions, a small inciflowmetry
(as
do
other
authors)
even though we measion was made in the skin between the eye and audisure
flow,
volume,
and
velocity.
The laser Doppler
tory canal, the right eye was retracted, and the lateral
probe
was
held
in
a
stereotaxic
carrier,
and the tip of
rectus muscle was cut from its ocular attachment to
the
laser
probe
was
positioned
close
(approximately
gain access to the ciliary ganglion and its postgangli1 to 3 mm) to the scleral surface. A small amount of
onic nerves.3 The ciliary ganglion, which lies on the
ultrasound
gel was used in the interface between sclera
temporal side of the optic nerve at the posterior pole
and
the
probe
tip to increase signal transmission. The
of the eye, is located between the optic nerve and the
Laserflo
indicates
blood flow measurements in milliinferior rectus and lateral rectus muscles. The avian
meters
per
minute
per 100 grams of tissue. Measureciliary ganglion gives rise to two distinct sets of postments
are
displayed
both digitally and on a chart reganglionic nerve fibers: choroidal nerves arising from
corder.*
Volume
and
velocity data from the Laserflo
the choroidal neurons and ciliary nerves arising from
4
are
expressed
as
Doppler
shifts per photon and mean
the ciliary neurons. The choroidal nerves leave the
frequency
of
Doppler
shifted
light, respectively. Thus,
ciliary ganglion as a set of five to seven thin nerve
although
the
Laserflo
provides
quantitative blood volbundles that enter the posterior pole of the eye near
ume
and
velocity
information,
this
information is not
the optic nerve head and disperse within the choroid.
expressed
in
standard
blood
volume
or velocity units.
The ciliary nerves enter the eye more laterally as three
The
data
were
recorded
for
each
eye
from the same
thick bundles that course along the lateral side of the
area
of
the
superior
anterior
part
of
the
eye, anterior
eye within the outer part of the choroid. After expoto
the
superior
rectus
muscle.
Five
pairs
of measuresure of the ciliary ganglion, all evident choroidal
ments
each
were
made
of
blood
flow,
blood
velocity,
nerves along the optic nerve head were transected in
and
blood
volume
for
each
eye.
Each
pair
of reone group of chicks, and in a second group of chicks
cordings
consisted
of
the
initial
and
final
blood
flow
the ciliary nerves distal to the ciliary ganglion were
values
during
a
30-second
period.
After
a
pair
of
flow
cut as they entered the temporal part of the eye. In a
measurements,
initial
and
final
blood
volume
values
third group of chicks (termed "sham group"), no
during the next 30-second period were recorded for
nerve transection was carried out after exposure of
the ciliary ganglion. The orbital skin incision in all
chicks was then sutured with 4 - 0 silk. To induce myo*Data in this paper are presented in values as displayed on
pic eye growth by means of form deprivation in these
Laserflo; no other computations were conducted.
sions. The above normal eye growth may stem from
the increased CBF in these birds, which is likely a
centrally mediated bilateral response to the increased
light on the retina due to the pupil dilation caused
by ciliary nerve transection.4-7
Although these studies showed that accommodation might not have been necessary for growth to a
normal size and that normal CBF was necessary for
such growth, they did not clarify whether normal CBF
and accommodation were necessary for the excessive
ocular enlargement commonly associated with myopia. In chicks (which have been used extensively in
myopia research8"12), high myopia and eye enlargement can be induced by lid suturing or by placing
goggles or occluders that impair form vision over an
eye. To explore the mechanisms underlying myopic
eye growth with form-vision deprivation, we have examined the effects of choroidal and ciliary nerve transection on ocular enlargement induced by goggles.
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Choroidal Nerve Cut and Myopic Eye Growth
the same eye, and then initial and final blood velocity
values during the next 30-second period were recorded for the same eye. At this point, the probe was
positioned over the other eye, and the same three
pairs of flow, volume, and velocity measurements were
taken. The probe was then repositioned over the initial eye, and the process was repeated until all measurements had been completed for both eyes. To ensure repeatability of probe placement for each eye of
an animal, stereotaxic coordinates were used for the
probe positioning. The mean value of the five pairs
of recordings of CBF, volume, and velocity were calculated for each eye in each bird.
After the CBF measurements, the refractive status
was measured using streak retinoscopy. The animals
were then killed with an overdose of anesthetic, and
the eyes were immediately removed, cleaned of extraocular tissue, and weighed. Measurements of the anterior-posterior dimension (termed "axial length"),
the nasotemporal dimension, and the dorsoventral dimension (the latter two termed "equatorial dimensions"), as well as of horizontal corneal diameter, were
made using vernier calipers. Data were analyzed using
a two-way analysis of variance for repeated measures
with a priori planned comparisons to test the differences between goggled and nongoggled eyes within
groups, among goggled eyes of different groups, and
among nongoggled eyes of different groups.15
Some additional animals with choroidal nerve cuts
and some with ciliary nerve cuts were processed for
light microscopic examination of the pathologic effects of the nerve transections on the retina. Some of
these animals were perfused transcardially 2 weeks
after nerve transection and goggling, using 2% glutaraldehyde/2% paraformaldehyde/0.5% acrolein in
0.1-M sodium cacodylate buffer (pH 7.4) as the fixative. After removal, the eyecups were washed in 0.1-M
cacodylate buffer (pH 7.4), divided into quadrants,
postfixed in 1% osmium tetroxide, dehydrated in an
ascending series of alcohols, infiltrated, and embedded in Epon - Araldite (Electron Microscopy Sciences,
Fort Washington, PA).316"18 For several quadrants for
each eye, 0.5-micron sections were obtained from two
to three blocks on a Reichert Ultracut E (ReichertJung, Vienna, Austria) and were stained with toluidine
blue-azure II for light microscopic examination. In
addition, some animals that had received choroidal
nerve transections and goggling were perfused with
0.75% saline followed by PLP fixative (4% paraformaldehyde in 0.1-M D,L-lysine, 0.01-M sodium periodate,
0.1-M sodium phosphate buffer; pH 7.4). After fixation, the eyecups were removed and stored at 4°C in
a 0.1-M phosphate buffer, 0.02% sodium azide, 25%
sucrose solution until sectioned. The eyes were sectioned in the horizontal plane at 20 //m on a HackerBright (Hacker Instruments, Fairfield, NJ) cryostat at
-25°C and stored at -20°C until stained. A series of
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these sections was stained with hemotoxylin and eosin
to reconstruct the observed retinal damage. All animals were handled in accordance with the National
Institutes of Health's Guide for the Care and Use of
Laboratory Animals and the ARVO Resolution for the
Use of Animals in Ophthalmic and Vision Research.
RESULTS
Ocular Histology
In a previous study,3 we used histologic methods combined with immunohistochemical staining of choroidal and ciliary nerve fibers to confirm that ciliary nerve
section as carried out by us destroys the fibers distal
to the ciliary nerve transections, and choroidal nerve
transection as carried out by us does destroy the choroidal nerve fibers distal to the transection. Consequently, we did not seek to confirm the efficacy of
the nerve cuts in the present study. We did, however,
examine the retinal histology of the eyes with nerve
cuts. With gross examination, it was evident that the
goggled eyes of the birds with choroidal nerve cuts
showed depigmentation in the temporal retina. The
depigmentation was largely confined to the central
portion of the temporal retina. Examination of the
hemotoxylin - eosin-stained sections confirmed this
and revealed that the damage also typically extended
slightly into the lateral and inferior part of the superior retina and into the lateral and superior part of
the inferior retina (Fig. 1). Examination of plasticembedded sections from the affected part of the temporal retina revealed that the outer retina had degenerated (Figs. 2, 3). All photoreceptor outer and inner
segments and all photoreceptor cell bodies (and the
entire outer nuclear layer) were absent in this part of
the temporal retina. The outer plexiform layer and
most of the inner nuclear layer were also absent. Ganglion cells remained, and a disorganized remnant of
the innermost part of the inner nuclear layer was also
typically present (Figs. 2A, 2B, 3A, 3B). The histologic
correlate of the gross depigmentation was observed to
be the loss of some cells from the retinal pigment
epithelial (RPE) layer, resulting in nonpigmented
gaps in the RPE layer (Figs. 2A, 3B). In addition, ectopic RPE cells were observed within all layers of the
neural retina, presumably indicating that they had migrated from their normal position. As a consequence
of these various changes, the retina displayed an abnormal appearance. This was particularly marked in
the outer retina, where the region normally occupied
by photoreceptors was instead occupied by migrated
RPE cells, macrophages, fibroblasts, and Milller cells
and their processes (Figs. 2A, 2B, 3A, 3B). The choroid
also showed abnormalities, with the choriocapillaris
reduced or absent and the amount of extracellular
matrix around choroidal vessels gready increased
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Investigative Ophthalmology & Visual Science, September 1994, Vol. 35, No- 10
T
N
FIGURE l. Schematic diagram illustrating the location of retinal pathology (region of parallel diagonal lines) in an eye
that, 2 weeks before histologic processing, had received choroidal nerve transection and goggling. The region of retinal
damage showed gross depigmentation of the RPE layer and
loss of the outer retinal layers.
(Figs. 2A, 2B, 3A, 3B). In addition, the sclera was thinner in the affected portion of the eye, typically because
of a loss of chondrocytes and matrix from the outer
sclera. The nondepigmented portions of the temporal
retina adjacent to these degenerative zones appeared
completely normal, except for a narrow transitional
zone in which the retina graded back into a normal
appearance (Fig. 2B). The majority of the superior,
inferior, and nasal retinal quadrants appeared normal
at both gross and histologic levels (except for the
above-noted portions of the superior and inferior
quadrants and their surrounding transitional zones)
(Fig. 1). No morphometric studies were performed at
this time, however, to determine whether some photoreceptor cells had been lost from these seemingly unaffected parts of the retina. We have also previously
reported depigmentation and histologically verified
retinal degeneration in the temporal retina of nongoggled chick eyes with choroidal nerve transections.3
In contrast to the goggled choroidal nerve cut eyes,
the goggled eyes with ciliary nerve cut and those with
sham treatment did not exhibit any gross structural
changes such as those observed in the choroidal nerve
cut eyes, and the retinas appeared histologically normal.
Choroidal Blood Flow
Choroidal blood flow was decreased in all goggled
eyes (Table 1; Fig. 4) (P < 0.0016). In the sham con-
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trol group, the CBF of the goggled eye was 66% of
that of the nongoggled eye. This result is consistent
with those in our previous studies,12 in which we have
shown that myopic ocular enlargement leads to decreased CBF in the myopic eye. In contrast, CBF in
the goggled eye in the choroidal nerve cut group was
only 21% of that of the nongoggled eye. In these choroidal nerve cut eyes, the decrease in CBF in the right
eye compared to that in the left eye appeared to be the
consequence of reductions in both choroidal blood
volume (P = 0.0001) and choroidal blood velocity (P
= 0.0001). The CBF in the goggled eye of the choroidal nerve cut group was also significantly less than the
CBF in the goggled eye of shams (P = 0.0001). Thus,
the choroidal nerve cut reduced choroidal blood flow.
In the ciliary nerve cut group, the CBF in the goggled
eye was 59% of that in the nongoggled eye (P =
0.0016). The CBF in the goggled eye in the ciliary
nerve cut group, however, was not significantly different from the CBF in the goggled eye of the sham birds
(P = 0.98), indicating that the ciliary nerve cut itself
did not alter choroidal blood flow. Finally, CBF in the
nongoggled eyes was not significantly different among
the ciliary nerve cut, choroidal nerve cut, and sham
groups (P s= 0.2421), indicating that choroidal nerve
cut and ciliary nerve cut do not affect CBF in the
contralateral eye.
Myopia and Ocular Enlargement
Because the chicks varied in body size and body
weight, and therefore mean eye size varied between
groups, we compared the differences between the goggled eye and the nongoggled eye for each measured
parameter from each nerve cut group with the goggled eye - nongoggled eye difference for these same
parameters for the sham control group (Table 2;
Fig. 5).
In the sham control group, there was a significant
difference between the goggled eyes in the sham control birds, which were highly myopic ( — 14.68 D), and
the nongoggled left eyes, which were moderately hyperopic (+4.0 D) (P= 0.0001). Thus, the mean difference in refractive state between the goggled and nongoggled eyes was —18.68 ± 1.46 D. Direct caliper measurements showed that the axial, nasotemporal, and
dorsoventral dimensions of the goggled eyes in the
sham control birds were considerably and significantly
greater than those of the contralateral eye (P =£
0.0001). Eye weight (P = 0.0001) and corneal diameter (P = 0.044) were also significantly greater in the
goggled eye, reflecting the overall enlarged state of
the goggled eye. The degree of enlargement in the
goggled eyes of the sham group is consistent with that
observed in previous studies using this manipulation.10
In contrast to the sham group, the choroidal nerve
cut eyes were only moderately myopic (—4.74 D), and
the nongoggled contralateral eyes in this group exhib-
Choroidal Nerve Cut and Myopic Eye Growth
FIGURE 2. Low power photomicrographs showing degenerated retina (A, B) and normal
retina (C) from a chick that had sustained choroidal nerve transection and goggling of the
eye shown in (A, B). The normal retina is from the contralateral eye, temporal retina.
Photomicrograph (A) shows a field of view through the center of the most damaged part
of the temporal retina. Note that the RPE layer is discontinuous, and many RPE cells have
migrated into the retina (arrows), some as far as the optic nerve fiber layer. Although
ganglion cells and part of the inner plexiform layer are evident, the cells of the inner nuclear
layer (bipolar and amacrine cells) and those of the outer nuclear layer (photoreceptors) are
completely absent. Note also the absence of choriocapillaris vessels along Bruch's membrane.
Photomicrograph (B) shows severely damaged retina at the left-hand edge that grades into
less damaged retina along the right-hand edge. Note the presence of choriocapillaris vessels
throughout, and an intact inner plexiform layer and a largely intact inner nuclear layer on
the right. Photomicrograph (C) shows normal retina from the temporal quadrant of the
contralateral eye. Bar =100 (xxn. OLM = Outer limiting membrane; GCL = ganglion cell
layer; RPE = retinal pigment epithelium; NFL = nerve fiber layer; IPL = inner plexiform
layer; INL = inner nuclear layer; CC = choriocapillaris; BM = Bruch's membrane.
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Investigative Ophthalmology 8e Visual Science, September 1994, Vol. 35, No. 10
,CC
I-RPE
NFL
FIGURE 3. This set of light-microscopic micrographs shows the same retinal levels as in figure
2, but at slightly higher magnification, showing greater detail of the damaged retina. Photomicrograph (B) is from the center of damaged temporal retina, (A) is from the beginning
of the transition to normal retina, and (C) is from normal retina, from the temporal quadrant
of the contralateral eye. Bar = 50 (xm.
D, significantly less (48.2% of sham; P= 0.0001) than
the sham goggled - nongoggled refractive difference.
Thus, the goggled eyes were much less myopic in these
chicks than in the sham chicks. Further, the extent of
axial (46.5% of sham) and nasotemporal (72.4% of
ited essentially the same degree of hyperopia as observed in the nongoggled control eye in the sham
group (+4.28 D). Thus, the mean difference in refractive status between goggled and nongoggled eyes in
the choroidal nerve cut group was only —9.01 ± 1.47
TABLE l.
Means for Choroidal Blood Flow, Volume and Velocity for Right and Left Eye of
the Three Groups of Chicks (sham nerve cut control, choroidal nerve cut,
and ciliary nerve cut)
Nerve Cut Groups
Sham Control
(n- = 14)
Choroidal Cut
(n = 17)
Ciliary Cut
(n = 18)
Blood Flow
Parameter
Rig)
Left
Right
Left
Right
Left
FLOW
VOLUME
VELOCITY
15.17 ± 1.53
.4190 ± .0350
.6422 ± .0495
22.91 ± 2.67
.5808 ± .0384
.8421 ± .1178
5.13 ± 0.94
.2523 ± .0206
.3984 ± .0489
24.92 ± 2.53
.4815 ± .0411
1.041 ± .0702
15.13 ± 1.94
.3865 ± .0257
.7271 ± .0641
25.49 ± 2.95
.5007 ± .035
.9773 ± .0883
Each value represents the mean (±SE). The Laserflo gives blood flow measurements in ml/min per 100 g tissue. Volume data and
velocity data from the Laserflow are expressed as Doppler shifts per photon and the mean frequency of the Doppler shifted light,
respectively.
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Choroidal Nerve Cut and Myopic Eye Growth
SHAM CONTROL
CHOROIDAL CUT
3697
CILIARY CUT
FIGURE 4.
CBF was decreased in all operated eyes compared
to the nonoperated eyes of the same group. The decrease
in CBF in the treated eye of the choroidal, ciliary, and sham
groups compared to the untreated eye within each group
was statistically significant (*P < 0.0016). Additionally, the
CBF in the treated eyes of the choroidal nerve cut group
was significantly less than that in the sham treated eyes (#P
= 0.0001).
sham) enlargement of the goggled eye compared to
the nongoggled eye was significantly less than that
observed in sham control (P == 0.0308). The difference in weight (52.1% of sham; P = 0.0001) and in
corneal diameters (200% less than sham; P= 0.0001)
between goggled eyes and nongoggled eyes were significantly less than in the shams. The dorsoventral
enlargement of the goggled eye (compared to that of
the nongoggled eye) also appeared less in the choroidal nerve cut birds (68.4% of sham), although this
effect was not statistically significant (P= 0.1057) (Table 2; Fig. 5).
In the animals with ciliary nerve transections, the
goggled ciliary nerve cut eyes were highly myopic
(—9.57 D), and the nongoggled normal eyes exhibited
essentially the same degree of hyperopia as observed
in both the sham group and choroidal nerve cut group
nongoggled normal eyes (+4.40 D). The mean goggled - nongoggled difference in refractive status was
thus —13.97 ± 0.95 D, greater than that observed in
the choroidal nerve cut birds but only 74.8% of that
observed in sham control birds. Similarly, the extent
of axial elongation was greater than in the choroidal
nerve cut birds, but only 71.3% of that observed in
the shams. Both the degree of myopia and the axial
elongation were significantly less in the ciliary nerve
cut birds than in the shams (P s 0.0156). The degree
of myopia and axial elongation, however, were significantly greater than in the choroidal nerve cut birds
(P < 0.0278). The extent of nasotemporal and dorsoventral enlargement was similar, though, to that in the
sham control group (nasotemporal, 98.9% of sham;
dorsoventral, 102.6% of sham). These differences
were not significant ( P > 0.3874). The goggled-nongoggled corneal diameter difference was, however, significantly less than that in the shams (162.5% less than
sham; P = 0.001), and the goggled - nongoggled eye
weight difference was significantly less in the ciliary
nerve cut birds than in the shams (78.3% of sham; P
= 0.0329), but was significantly greater than in the
choroidal nerve cut birds (P = 0.004).
In summary, choroidal nerve cuts retarded the
overall myopic eye growth produced by the goggling,
whereas ciliary nerve cuts attenuated only the axial
elongation produced by goggling (Table 2; Fig. 5).
DISCUSSION
In the present study, we found that goggles that degrade the visual image result in myopic eye growth,
even in an eye without accommodative ability and with
TABLE 2.
Mean Values Obtained for Measured Ocular Parameters of the Right and Left Eye
from the Three Groups of Chicks (sham nerve cut control, choroidal nerve cut,
and ciliary nerve cut).
Nerve Cut Groups
Sham Control
(n = 14)
Ocular Dimensions
Right
Left
Choroidal Cut
(n = 17)
Right
Left
Ciliary Cut
(71 = 18)
Right
Left
Refractions (D)
-14.68 ± 1.55 +4.00 ± 0.26 -4.67 ± 1.07 +4.34 ± 0.50 -9.57 ± 1.59 +4.40 ± 0.30
9.81 ± 0.07 10.17 ± 0.09
9.53 ± 0.09
9.58 ± 0.08 10.45 ± 0.16
Axial length (mm)
11.10 ± 0.18
Nasotemporal length (mm)
14.12 ± 0.15 13.25 ± 0.11 13.44 ± 0.14 12.81 ± 0.13 13.72 ± 0.16 12.86 ± 0.13
Dorsoventral length (mm)
13.62 ± 0.12 12.86 ± 0.10 12.90 ± 0.10 12.38 ± 0.14 13.28 ± 0.15 12.50 ± 0.13
5.65 ± 0.05
5.57 ± 0.04
5.27 ± 0.05
5.35 ± 0.05
5.28 ± 0.05
5.33 ± 0.04
Corneal diameter (mm)
1.01 ± 0.03
0.79 ± 0.02
0.84 ± 0.02
0.72 ± 0.02
0.89 ± 0.03
0.71 ± 0.02
Eye weight (g)
Each value represents the mean (±SE).
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Investigative Ophthalmology 8c Visual Science, September 1994, Vol. 35, No. 10
SHAM CONTROL
120-,
FIGURE 5.
Effect of manipulations on myopic eye growth.
Because the chicks varied in body size and body weight,
thereby resulting in between-group variability in mean eye
size, we compared the differences between the goggled eye
and the nongoggled eye for each nerve cut group to the
differences between the goggled eye and the nongoggled
eye for the sham control group. Note that in the sham
group, the goggled eye showed significant growdi in all dimensions, weight gain, and increased pupil diameter compared to nongoggled eye. Significant reductions were observed in axial elongation and eye weight gain for the goggled eye in the choroidal and ciliary nerve cut groups,
compared to that observed in the sham goggled eye (*P <
0.0156). Additionally, in the choroidal nerve cut group, a
significant reduction in nasotemporal elongation was observed (*P= 0.0306) compared to the shams. Dorsal-ventral elongation of the goggled eye compared to the nongoggled was not significantly different from the sham group in
either nerve cut group (P > 0.1057). NasoTemp = Nasotemporal; DorsoVent = dorsal-ventral.
extremely low levels of CBF. Nonetheless, the loss of
accommodation attenuated the myopic eye growth by
reducing axial elongation by 25%, with no evident
effect on growth in the other dimensions. In contrast,
reducing CBF to 21% of normal by choroidal nerve
transection reduced the myopic eye growth by 50% in
the axial dimension and by approximately 25% in the
other dimensions. Thus, ciliary nerve transection and
choroidal nerve transection have different effects on
myopic eye growth.
The degree of reduction in CBF observed in the
goggled eyes with choroidal nerve cuts is similar to
that observed in nongoggled eyes with choroidal nerve
cuts in a previous study of ours.3 In that study, we
found that choroidal nerve cuts (alone or in combination with ciliary nerve cuts) resulted in dramatic decreases in CBF in nongoggled chick eyes and led to
reduced ocular growth in all dimensions. Similarly,
reduced ocular growth has been reported by Lin and
Stone19 after ciliary ganglionectomy in nongoggled
chick eyes. Thus, the present study and previous studies show that levels of CBF somewhat greater than 20%
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to 25% of normal are necessary for normal eye growth.
Because we have observed that full myopic eye growth
can be associated with CBF levels between 40% and
60% of normal, it seems likely that normal eye growth
requires CBF at least 25% to 60% of normal. Further,
levels of CBF somewhat greater than 25% of normal
are needed to avoid severe retinal pathology. In both
goggled and nongoggled eyes with choroidal nerve
cuts, severe depigmentation and photoreceptor loss
in the temporal retina were observed. The pattern
of distribution of ciliary ganglion innervation to the
choroid may contribute to the regional specificity of
these histopathologic changes, because the choroidal
innervation from the ciliary ganglion appears most
abundant in the temporal retina in chicks.3 Because
the extent to which photoreceptors are affected in
myopic eyes with 40% to 60% of normal CBF is uncertain, the minimum level of CBF required for full ocular health is uncertain. Finally, we should add that
these data on the levels of CBF required for eye growth
and health may be specific to birds. Birds lack retinal
circulation, and it is possible that species with both
retinal and choroidal circulations are able to withstand greater reductions in CBF than are species without retinal circulations. It has been suggested, however, that the highly vascularized pecten of the avian
eye plays a nutritive and supportive role for the inner
retina in birds.20
We have also previously shown that severing only
the ciliary nerves in nongoggled eyes leads to increased CBF in both eyes and slighdy increased growth
in the eye with ciliary nerve cuts. We have previously
suggested that the increased CBF in these eyes was
the result of the enhanced light-mediated bilateral
reflexive upregulation of CBF3"721'22 that occurs with
the increased light on the retina due to pupil dilation
of the eye with the ciliary nerve cut. We attributed the
enhanced eye growth to the increased CBF. Based on
this finding, we might have expected that ciliary nerve
cuts in goggled eyes would produce enhanced myopic
eye growth. The failure to observe such enhanced myopic eye growth in goggled eyes with transected ciliary
nerves may stem from the fact that our goggles were
not completely translucent. Thus, the goggles may
have counteracted the effect of a dilated pupil in the
goggled eye by attenuating the amount of light falling
on the retina. This, in addition to the myopic enlargement observed in these eyes, may explain why CBF
levels are similar in the goggled eyes in the sham and
the ciliary nerve cut birds. Alternatively, the present
data could be interpreted to suggest that extreme axial
elongation and myopia occurring with goggles may in
part depend on accommodative mechanisms, and that
disabling accommodation may have made it impossible for the vasodilatory drive, induced by pupil dilation, to have caused eye growth greater than in the
shams. Thus, at the present time, it is uncertain to
Choroidal Nerve Cut and Myopic Eye Growth
what extent the disabling of accommodation and the
putative initial increase in CBF brought about by the
ciliary nerve cuts may have interacted to produce the
final level of myopia seen in birds with this manipulation. It can, however, be concluded that myopic eye
growth induced by goggles that degrade the visual
image may in part depend either directly on accommodation or on other (currently unknown) ocular
changes brought about by the loss of accommodative
control. The mechanism regulating the greater part
of myopic eye growth, however, clearly does not depend on accommodation.
Choroidal Blood Flow and the Genesis
of Myopia
Our previous studies suggested that CBF reductions
to 40% to 60% of normal occur with manipulations
that yield myopia, such as goggle wearing and corneal
scarring.12 These reductions are primarily a consequence of the myopia itself, because reducing CBF
artificially does not yield myopic eye growth.3 In this
study, the CBF of the goggled eye in the sham control
group was 66% of that of the nongoggled eye, and
the CBF of the goggled eye in the ciliary nerve cut
group was 59% of that of the nongoggled eye. The
goggled eyes in both groups of birds were myopic and
enlarged. Thus, in this study, myopic eye growth was
also associated with CBF at approximately 40% of normal, as we reported in a previous study. (Note that we
have previously found that CBF in an eye contralateral
to a goggled eye is only approximately 75% of normal1.) Further reduction in CBF (by choroidal nerve
cuts) in goggled eyes in this study did not further
increase myopic eye growth. Rather, eye growth was
attenuated by such a manipulation, as we also found
to be the case with choroidal nerve cuts in nongoggled
eyes. Thus, there is no evidence to suggest that decreased CBF causes myopia, and all evidence indicates
that the reduced CBF observed with myopia is secondary to the ocular enlargement. The precise mechanisms of this reduction remain uncertain, and the
pathologic implications of this for the long-term
health of the retina require examination.
The present studies, in fact, suggest that CBF less
than 25% of normal is detrimental to eye growth. We
cannot, however, be certain whether this effect
stemmed from the decreased CBF itself or from the
retinal damage with which the decreased CBF was associated. A number of studies have shown that destruction of the outer retina by various neurotoxic agents
hinders normal and myopic eye growth.23"28 Additional studies have shown that blocking the action of
one specific retinal cell population, the dopaminergic
amacrine cells, with haloperidol (a dopamine antagonist)29 or destroying them with 6-hydroxydopamine30
eliminates form-deprivation myopia. In these studies,
however, the retinal damage was much more widely
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3699
distributed than in our choroidal nerve cut chicks, in
which damage was limited largely to temporal retina.
Thus, it seems likely that the impoverished CBF was
the major contributor to the diminished eye growth
observed in the present study. On a final note, although we think the low levels of CBF were causal to
the temporal retinal damage observed in the choroidal nerve cut chicks, we cannot completely rule out
other possibilities. For example, the choroidal nerve
transections may have disturbed some small unseen
vessels feeding the temporal choroid, or some subtle
features of the mechanical forces applied to the eye
during the surgery to cut the choroidal nerves may
have been unique to this surgery. At present, however,
we have no reason to think that either of these possibilities is true.
Accommodation and the Genesis of Myopia
Professions or pastimes requiring near work (causing
excessive time to be spent in accommodation) is
known to be a major risk factor in naturally occurring
idiopathic myopia in humans,31'32 and an accommodative mechanism has been proposed to underlie myopia and axial elongation produced by at least some
experimental manipulations in tree shrews33'34 and
monkeys.35 In some animal models, however, the ocular enlargement produced by the experimental manipulation does not appear to depend on an accommodative mechanism. For example, neither blockade
of accommodation with topical atropine nor blockade
of accommodation by ciliary ganglionectomy eliminated ocular enlargement consequent to unilateral lid
fusion in rhesus monkeys.35 Thus, lid suturing in this
model produced myopic eye growth without the
involvement of accommodation. Similarly, eliminating
accommodation in chicks by lesions of the Edinger Westphal nucleus (the source of preganglionic innervation of the ciliary ganglion) does not reduce visual-deprivation myopia,36 nor do Edinger - Westphal
lesions in chicks have a major effect on hyperopic or
myopic eye growth in compensation for wearing plus
or minus lenses, respectively.37 Similarly, ciliary ganglionectomy does not attenuate goggle-induced19 myopia, and ciliary nerve transections do not attenuate lidsuture38 myopia in chicks. Finally, ocular enlargement
and myopia with ocular occluders in chicks are still
obtained in eyes with transected optic nerves.39 Because accommodation requires centrally mediated
mechanisms, these latter results in chicks again suggest that accommodative control is not necessary for
myopic eye growth with visual deprivation-induced myopia. The results of the present study are similar to
those in these previous studies in some respects. In
the present study, we found that eliminating accommodation in goggled eyes did not prevent the development of severe myopia in those eyes. Nonetheless, the
axial elongation was decreased to approximately 25%
3700
Investigative Ophthalmology 8c Visual Science, September 1994, Vol. 35, No. 10
less than that obtained in goggled eyes possessing a
normal accommodative function. Similarly, in a study
using goggles that blocked lateral but not frontal vision to produce myopic eye growth in chicks, Wallman
et al40 found that myopic eye growth in deprived eyes
whose ciliary nerves were transected was less than that
in nontransected deprived eyes. These results suggest
that accommodation plays some role in the axial elongation in animal models of myopia employing visual
deprivation. Clearly, however, most of the axial and
equatorial eye growth seen in such animal models
does not depend on accommodation. Such results
lead to two interpretations. It is possible that neither
idiopathic myopia in humans (clinical and anecdotal
data notwithstanding) nor myopic eye growth in animals involves a major role for accommodation as its
mechanism. However, it is possible that accommodation plays a major role in idiopathic myopia in humans, but not in the types of myopia induced in animal models by such manipulations as lid suture and
goggles. It is also possible that the mechanisms controlling ocular growth toward emmetropia are different in different animals. For example, the mechanisms
in humans and chicks might be different. Ocular
growth in chicks might be controlled by local ocular
mechanisms not requiring communication between
eye and brain, whereas ocular growth in humans
might require accommodation and eye-brain communication. It would therefore be important to determine whether forced near work or induced accommodation could produce myopia in an animal such as
chick, and to determine whether an observed effect
was actually dependent on time spent in accommodation.
Key Words
choroidal blood flow, choroidal nerve, ciliary nerve, myopia,
myopic eye growth
Acknowledgments
The authors thank Sherry Cuthbertson, Sharon Frase, Hadley Hamilton, Bryan Jackson, and Charity Stewart Brown for
their technical assistance; Dr. Kris Arheart for his expertise
in statistics; Dr. William Hodos for seminal discussions early
in the course of this research and for providing the goggles
used in this study; and Dr. Steve Charles for his availability
and eagerness to discuss the clinical applications of this research.
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