Download Naturally occurring vitreous chamber-based myopia in the

Naturally Occurring Vitreous Chamber—Based Myopia in
the Labrador Retriever
Donald 0. Mutti,1'2 Karla Zadnik,2 and Christopher J. Murphy5
investigate whether myopia is present in a breed of domestic dog, the Labrador
retriever, and how the ocular components are related to refractive error in this breed.
PURPOSE. TO
Cycloplegic refractive error was measured in 75 Labrador retrievers by retinoscopy.
Corneal and crystalline lens radii of curvature were measured in the right eyes of 57 of these dogs
using a video-based keratophakometer, with axial ocular dimensions measured using A-scan ultrasonography.
METHODS.
RESULTS. Of the 75 dogs tested, 11 (14.7%) were myopic by at least —0.50 D in one eye, and 6 (8.0%)
were myopic in both eyes (full range of refractive errors, +3-50 D to —5.00 D). Of the 57 dogs with
ocular component measurements, seven (12.3%) were myopic by at least —0.50 D in the right eye.
There was a significant negative correlation between refractive error and vitreous chamber depth
(Spearman r = —0.42; P < 0.001). Myopic eyes had an elongated vitreous chamber depth (10.87 ±
0.34 mm for myopic dogs, 10.02 ± 0.40 mm for nonmyopic dogs; P < 0.0001, Kruskal-Wallis test).
There was also a significant quadratic association between lens thickness and vitreous chamber
depth (P < 0.005; R2 — 0.11), indicating that thinner lenses occurred at both shorter and longer
vitreous chamber depths.
CONCLUSIONS. Myopia
in the Labrador retriever is analogous to human myopia in that it is caused by
an elongated vitreous chamber. Thinner crystalline lenses found at longer vitreous chamber depths
may be analogous to lens thinning documented in human ocular development. The Labrador
retriever warrants investigation as a potential model of myopia that is naturally occurring rather
than experimentally induced. {Invest Ophthalmol Vis Set. 1999;40:1577-1584)
M
yopia is one of the most prevalent human visual
disorders, affecting 25% of people in the United
States between the ages of 12 and 54 years.1 The
study of myopia has been facilitated by the development of
experimental models. Myopia may be induced in various animal species, either by deprivation of form vision through lid
suture or occlusion, or by optical defocus through the application of negative-powered lenses. Deprivation and defocus
result in an elongated vitreous chamber in the macaque,2'3 in
the common marmoset,4'5 in other mammals such as the tree
shrew, 67 and in the chick.8'9 The applicability of experimental
myopia to human myopia is an important issue.10 These animal
models are currently used in experimental evaluations of pharmaceuticals whose ultimate destination may be clinical trials
testing their efficacy in retarding or preventing human myopia."-' 3
Experimentally induced myopia is analogous to human
myopia, in that both are characterized by an increase in the
overall length of the eye, particularly in the size of the vitreous
14-17
chamber.
Human juvenile-onset myopia may be contrasted with experimental myopia in several areas. Experimen-
From the ' School of Optometry, University of California, Berkeley;
The Ohio State University College of Optometry, Columbus; and the
^Department of Surgical Sciences, School of Veterinary Medicine, University of Wisconsin, Madison.
Submitted for publication September 1, 1998; revised February 5,
1999; accepted February 24, 1999.
Proprietary interest category: N.
Reprint requests: Donald O. Mutti, School of Optometry, University of California, Berkeley, CA 94720-2020.
2
Investigative Ophthalmology & Visual Science, June 1999, Vol. 40, No. 7
Copyright © Association for Research in Vision and Ophthalmology
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tal myopia displays an age-related loss in sensitivity to both
deprivation and defocus.9 Human analogues to deprivationinduced eye elongation from sources such as cataract,18 hemangioma,19 corneal opacity,20 and vitreous hemorrhage21 are
effective between birth and 6 years of age, well before the age
at which the prevalence of myopia begins to increase in children.22'23 Therefore, the period of sensitivity to deprivation or
defocus does not correspond with the onset of human juvenile
myopia. Hyperopic defocus from negative lenses in animal
models has been compared with the 0.22- to 0.39-D average
hyperopic defocus from accommodative lag experienced by
myopic children under normal viewing conditions.24 Yet, it is
unclear whether the amount of defocus used to produce elongation in the chick and the monkey, 2 D to 3 D, is analogous to
prolonged exposure to smaller amounts of defocus in children.3'25 Finally, human refractive error is influenced by heredity. Identical twins resemble each other in refractive error
more often than do fraternal twins,26'27 and the prevalence of
myopia increases in children with the number of myopic parents.28'29 Animal experimentation to date has been limited to
evaluating environmental influences.
A naturally occurring, heritable form of myopia due primarily to enlargement of the vitreous chamber depth would be
a useful tool for research into the mechanism of ocular enlargement and the efficacy of growth-modulating pharmaceutical
treatments for myopia in humans. Spontaneous myopia in
animals is rare, but domestic dogs may be a promising source.
We have accumulated a database of retinoscopic refractive
error in 771 domestic dogs of numerous breeds (Fig. 1). Several
common breeds are myopic on average, including the minia1577
1578
Mutti et al.
IOVS, June 1999, Vol. 40, No. 7
Breed of Dog (number of animals refracted)
Alaskan Malamute (21)
-
American Cocker Spaniel (16)
-
Australian Shepherd (37)
-
Basenji(16)
-
Belgian Tervuren (8)
-
Bernese Mountain Dog (11)
-
Border Collie (7) Chesapeake Bay Retriever (13) Clumber Spaniel (5.) Doberman (6) English Springer Spaniel (34) Field Spaniel (11) German Shepherd (36) German Shepherd Guide Dogs (53) Golden Retriever (67) Great Dane (5) Labrador Retriever (115) Miniature Dachshund (Smooth-Haired) (5) Miniature Poodle (18) Miniature Schnauzer (18) Mixed Breed (9) Norwegian Elkhound (6) Nova Scotia Duck Tolling Retriever (10) Papillon (5)
-
Pug (7) Rottweiler (23) Samoyed (20) Schipperke (7) Shar-Pei(7) Shetland Sheepdog (22) Siberian Husky (15) Smooth-Coated Collie (49) Soft-Coated Wheaton Terrier (13) Other Terriers (34) Toy Poodle (42)
i
i
i
i
i
I
i
i
i
-4 .0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5
i
I
i
i
I
r
1.0 1.5 2.0 2.5 3.0 3.5 4.0
Refractive Error (Diopters)
FIGURE 1. The average (±SD) refractive error of numerous breeds of domestic dogs screened by cycloplegic and noncycloplegic retinoscopy. The
number of dogs examined for each breed is in parentheses. These data include all dogs measured and presented in our previous article30 reporting
that German shepherd guide dogs have a lower prevalence of myopia than unselected German shepherds. Labrador retriever data include the 75
dogs reported within the present study.
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Myopia in the Labrador Retriever
IOVS, June 1999, Vol. 40, No. 7
1579
TABLE 1. Ocular Component Values as a Function of Refractive Status
Ocular Component
Vitreous chamber depth (mm)
Anterior chamber depth (mm)
Lens thickness (mm)
Keratometer power (D)
Equivalent refractive index
Anterior lens radius (mm)
Posterior lens radius (mm)
Equivalent lens power (D)
Nonmyopes (n = 50)
10.02
4.29
7.85
36.94
1.535
7.61
8.12
48.26
±
±
±
±
±
±
±
±
0.40
0.36
0.51
1.98
0.027
0.55
1.29
3.69
Myopes (it = 7)
10.87
4.16
7.70
37.12
1.543
7.94
8.87
47.00
± 0.34
±0.33
± 0.56
± 2.45
± 0.027
± 0.57
± 0.93
± 4.43
<0.0001
0.47
0.26
0.67
0.55
0.23
0.14
0.45
P by Kruskal-Wallis.
ture schnauzer, rottweiler, smooth-coated collie, miniature
dachshund, German shepherd, and toy poodle. A previous
report of myopia in German shepherds suggested that myopia
in dogs could be lenticular in origin.30 The present study
examined the prevalence of myopia in Labrador retrievers and
evaluated the ocular components, including the crystalline
lens, to determine the source of myopia in this breed.
METHODS
Although the Labrador retriever is emmetropic on average (Fig.
1), it was known from previous examinations that a cohort of
dogs in the surrounding community (Madison, WI) were myopic. All dogs were brought by their owners for a Canine Eye
Registry Foundation examination held at the School of Veterinary Medicine at the University of Wisconsin and were returned to their owners at the completion of the examination.
Refractive error was measured in 75 Labrador retrievers by
cycloplegic retinoscopy for both eyes, then ocular component
dimensions of right eyes were measured in 57 of those 75 dogs
using A-scan ultrasonography and a video-based keratophakometer (measuring corneal and lenticular radii of curvature in
the horizontal meridian). Of the 75 dogs refracted by retinoscopy, 45 were female. The average age was 35 years (range,
0.13-10.5 years). For the 57 dogs with ocular component
measurements, the average age was 31 years (range, 0.3-7.0
years), and 35 were female. Dogs were included in the ocularcomponent measurement phase of the examination after study
procedures were explained, and written consent was obtained
from the owner. All procedures adhered to the ARVO Statement for the Use of Animals in Ophthalmic and Vision Research and were reviewed and approved by the Animal Use
Committee for the University of Wisconsin.
One drop of 1% cyclopentolate (Cyclogyl; Alcon, Fort
Worth, TX) was instilled in each of the right and left eyes.
Thirty minutes after drop instillation, streak retinoscopy was
performed on each eye with handheld trial lenses in the horizontal and vertical meridians. Corneal and crystalline lens radii
of curvature were measured on the right eye only using a
portable, custom handheld video-based keratophakometer.
The details of the design of this device are reported elsewhere.31
Purkinje images were photographed in the optimal focal
plane for each pair with refocusing between viewing Purkinje
III and IV, recorded on videotape, then digitized by a frame
grabber (Data Translation, Marlboro, MA) with the separation
between the center of each image in the pair measured by
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image-processing software (Image Analyst, ver. 7.22; Billerica,
MA). This distance yields an equivalent mirror radius of curvature in air that may then be refracted through the optical
elements preceding the reflecting surface, giving the radius of
curvature in the eye and an equivalent refractive index that
creates agreement between measured components and refractive error.23
Ocular axial dimensions (anterior chamber depth, vitreous
chamber depth, and crystalline lens thickness) were measured
on the right eye only by A-scan ultrasonography (model 820,
Humphrey, San Leandro, CA), consisting of five readings using
a handheld probe in semiautomatic mode. The gates were
altered to allow for the deeper anterior chamber, increased
thickness of the crystalline lens, and shorter vitreous chamber
in dogs. Distances were corrected for the different acoustic
velocities reported for ocular tissues of the dog.32 Dogs were
not sedated for ultrasound. Topical anesthesia was provided by
1 drop of 0.5% proparacaine. Statistical analyses were performed using statistical software (SAS JMP, ver. 3.1.5, SAS
Institute, Cary, NC).
RESULTS
The average spherical equivalent refractive error was +0.55 ±
1.05 D for the right eye and +0.37 ± 1.20 D for the left eye,
ranging from —5.00 D to +3-50 D of hyperopia. Dogs were
generally isometropic, although there was a small, but statistically significant, difference between the spherical equivalent
refractive error of the two eyes of +0.17 ± 0.47 D (right minus
left; P < 0.0022, paired f-test). There was a significant negative
association between the spherical equivalent refractive error
and age in each eye, indicating a reduction in hyperopia with
increasing age (right eye: Spearman correlation = —0.25, P =
0.030; left eye: Spearman correlation = -0.31, P = 0.007). The
average refractive astigmatism was small in magnitude in'each
eye. The amount of astigmatism was statistically significant in
the right eye at 0.11 ± 0.32 D (with-the-rule orientation, i.e.,
vertical meridian more myopic; P < 0.005, Wilcoxon signedrank test), but not in the left eye at 0.08 ± 0.38 D (P < 0.10,
Wilcoxon signed-rank test).
The overall prevalence of myopia among Labrador retrievers examined to date is 24.7% (54/219 eyes, Fig. 1). In the
present study of 75 dogs refracted by cycloplegic retinoscopy,
a myopic refractive error consisting of at least —0.50 D in each
principal meridian occurred unilaterally in 17 eyes (11.3%; 8
right, 9 left), in at least 1 eye of 11 dogs (14.7%), and in both
eyes of 6 (8.0%) of 75 dogs. The following analyses examine
1580
Mutti et al.
9.0
JOVS, June 1999, Vol. 40, No. 7
9.5
10.0
10.5
11.0
11.5
12.0
VITREOUS CHAMBER DEPTH (mm)
FIGURE 2. The right eye spherical equivalent refractive error (Sph.
Eq.) as a function of vitreous chamber depth for 57 Labrador retrievers.
The four puppies in the study are indicated by open circles, and the
mature clogs by points. The correlation is significant (Spearman r =
-0.42; P < 0.001).
the associations between refractive error and ocular biometric
data from the right eyes of 57 of these 75 dogs, including 7
animals (12.3%) with myopia in the right eye.
The only significant difference in ocular component values as a function of whether the eye was myopic or not was a
longer vitreous chamber depth among myopic dogs (P <
0.0001; Kruskal-Wallis test). The mean values for ocular components as a function of refractive status are listed in Table 1.
The association between an enlarged vitreous chamber and a
more negative refractive error in these dogs can be seen in
Figure 2. The scatterplot indicates that the relation is neither
linear, nor has equal variance around a regression line over the
range of vitreous chamber depth values. Therefore, a nonparametric correlation was calculated at —0.42 (Table 2; Spearman
r; P < 0.001). As in human refractive error,17 the most important ocular component contributing to the range of refractive
error in the Labrador retriever is vitreous chamber depth.
Significant correlations occur among a number of ocular
components in the eye of the dog and are presented in Table
2. Derived variables such as the equivalent refractive index and
the equivalent lens power are omitted, because any correlations are most likely an artifact of calculation from other component values. The human eye is characterized by negative
correlations between the length of the eye and the following
three variables: refractive error, lens power, and corneal power. 1533 Vitreous chamber depth also has a negative correlation
with refractive error and lens power in the dog. The sign on
the correlation between vitreous chamber depth and the crystalline lens is positive, because lenses are described in terms of
radii rather than power, but the positive correlation suggests
that flatter lenses are associated with longer vitreous chambers,
as in the human eye. The positive correlation between anterior
and posterior lens radii suggests that they vary together, with
flatter lens radii contributing to a shallower anterior chamber,
perhaps through the positive association between a flatter
anterior lens radius of curvature and greater lens thickness.
The negative correlation between vitreous chamber depth and
corneal power seen in humans did not reach statistical significance in this sample of Labrador retrievers. The correlation
between corneal and lens power is significant in human eyes,
but small (—0.12).33 This correlation is much higher in dogs,
with flatter, less powerful corneas associated with flatter and
thicker lenses.
Although there is no significant linear correlation between
vitreous chamber depth and lens thickness, examination of
Figure 3 indicates that this relation is nonlinear—that is, that
increasing vitreous chamber depths are associated with increasing lens thicknesses up to the point at which the longest
vitreous chamber depths are associated with thinner lenses.
The points in Figure 3 are best fit with the following quadratic
equation:
Lens thickness = -O.51(VCD)2 + 10.3(VCD) - 44.4
where VCD is vitreous chamber depth.
Each coefficient in this model is significant (JP < 0.005)
with an adjusted R2 of 0.11. The inclusion of higher order
terms does not significantly improve the fit, nor are higher
order terms indicated for any other relations between components.
The eye of the Labrador retriever grows rapidly. Of the 57
eyes, 4 belonged to puppies younger than 6 months (open
circles in Fig. 2). Both the vitreous chamber depth (mean, 9-2
TABLE 2. Matrix of Spearman Correlation Coefficients between Ocular Components in 57 Labrador Retrievers
Variable
Vitreous chamber (mm)
Anterior chamber (mm)
Lens thickness (mm)
Spherical equivalent refraction (D)
Keratometer power (D)
Anterior lens radius (mm)
Posterior lens radius (mm)
Vitreous
Chamber
(mm)
-0.076
-0.041
-0.42*
-0.24
0.28t
O.34f
• p < o.ooi.
t P < 0.01.
* P < 0.05.
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Anterior
Chamber
(mm)
-0.14
0.17
0.043
-0.31t
-0.41*
Lens
Thickness
(mm)
Sph. Eq.
Refraction (D)
Keratometer
Power (D)
Anterior
Lens Radius
(mm)
-0.19
-0.10
-0.16
-0.53*
-0.47*
0.39*
0.13
-0.29t
0.404:
-0.024
IOVS, June 1999, Vol. 40, No. 7
Myopia in the Labrador Retriever
9.5
1581
TABLE 3. Parameters of a Schematic Eye for the
Labrador Retriever Based on In Vivo Measurements
9.0
E
E, 8.5
CO
CO
HI
FHICh
z
CO
z
HI
8.0
7.5
7.0
6.5
6.0
I ' '
8.5
I.... I•... I.... I.... I •
9.0
9.5
10.0 10.5 11.0 11.5 12.0
VITREOUS CHAMBER DEPTH (mm)
FIGURE 3. Lens thickness as a function of vitreous chamber depth.
The association is quadratic rather than linear (JP < 0.005).
mm) and the axial length (mean, 20.8 mm) of puppies are less
than those of mature dogs. We looked for age-related trends in
the mature eyes of the Labrador retrievers in the present study,
excluding the four eyes of the dogs less than 6 months of age.
As expected, anterior chamber depth showed a significant
decrease with age (Fig. 4; Pearson r = —0.32; P < 0.02).34
Keratometer power, anterior and posterior lens radii of curvature, lens thickness, and vitreous chamber depth did not display age-related changes between 6 months and 7 years of age
in these dogs (JP < 0.06). The absence of age-related trends in
lens thickness and vitreous chamber depth in mature dogs in
this study may be because of the younger maximum age of
dogs used in this study compared with those used previously (7
years compared with 13 years), breed differences between
Labradors and Samoyeds, or a smaller sample size resulting
from using only the right eye instead of both eyes of each
dog.34
Refractive indices
Cornea (single surface)
Aqueous
Crystalline lens
Vitreous
Distances from anterior cornea (mm)
Anterior lens
Posterior lens
Retina
Radii of curvature (mm)
Cornea
Anterior lens
Posterior lens
Distances from anterior cornea (mm)
Primary nodal point
Secondary nodal point
First principal plane
Second principal plane
Distances from principal planes (mm)
Primary focal distance
Secondary focal distance
Equivalent power of the eye (D)
Ametropia (D)
1.3375
1.3333
1.5361
1.3333
4.27
12.10
22.12
9.13
7.65
-8.20
8.22
8.95
3-75
4.48
-13-42
17.89
74.52
+ 1.06
Ocular components are rarely measured in vivo in the dog,
and all reported crystalline lens radii have been from in vitro
data, to our knowledge. We use in vivo phakometric and
ultrasonographic data to construct a schematic eye for the
Labrador retriever (Table 3, Fig. 5). With the sole difference
between myopic and nonmyopic eyes being an elongated vitreous chamber depth, average values for each ocular component are used with the average value for a nonmyopic vitreous
chamber depth in this model. The eye having an average
myopic vitreous chamber depth is presented for comparison in
Figure 5. Modeled ametropias compare well with measured
average nonmyopic and myopic refractive errors of +0.86 D
and —2.07 D, respectively.
DISCUSSION
Q.
LJJ
LJJ
CD
<
I
O
oc
o
LJJ
0
1
2
3
4
5
AGE (Years)
FIGURE 4. The relationship between anterior chamber depth and age.
The correlation is significant (Pearson r = -0.32; P < 0.02).
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We have shown that myopia in the Labrador retriever is
caused by an elongated vitreous chamber and not by excessive crystalline lens or corneal power. This is a key feature
of human myopia' 4 " 17 and other experimental models.2"9
Myopia has been previously reported in the Labrador retriever, although it was associated with retinal detachment
and skeletal abnormalities.35 Ophthalmoscopy, which is part
of the Canine Eye Registry Foundation examination, and a
physical examination showed no retinal disease or skeletal
abnormality in any of the dogs reported here. Seven dogs,
one of which was myopic (—0.75 D), had fine, punctate lens
opacities. The vitreous chamber was significantly longer in
myopic dogs, even if these seven dogs with opacities were
excluded (P < 0.0003; Kruskal-Wallis test).
One limiting factor is that the association between myopia
and excessive overall axial elongation of the eye present in
human myopia was not seen in the Labrador retriever (Fig. 6;
Spearman r = —0.15; P < 0.26). The absence of this association was noted in a previous report on myopia in German
1582
IOVS, June 1999, Vol. 40, No. 7
Mutti et al.
r = 9.13
1 mm
Ametropia = +1.06 D
Ametropia = -2.43 D
ACD
LT
FIGURE 5. A schematic eye for the Labrador retriever. Parameters are provided in detail in Table 3. ACD, anterior chamber depth; LT, lens
thickness; VCD, vitreous chamber depth; H, H', primary and secondary principle planes; N, N', primary and secondary nodal points; r, radius of
curvature; n, index of refraction.
shepherds.30 Thisfindingmay indicate that myopia is different
in the eye of the dog, perhaps involving a process in which the
lens thins and/or moves forward without excessive overall
growth. Results from this study support lens thinning and
longer vitreous chamber depths rather than anterior movement of the crystalline lens as the source of the myopia in the
Labrador retriever. There is a nonlinear association between
lens thickness and vitreous chamber depth (Fig. 3), yet no
significant association between anterior chamber depth and
lens thickness, between anterior chamber depth and vitreous
chamber depth, between the spherical equivalent refractive
error and either anterior chamber depth or lens thickness (see
Table 2), or between the spherical equivalent refractive error
and the sum of these two variables (Spearman r = 0.18; P <
0.18).
The finding of thinner lenses associated with longer vitreous chamber depths has a parallel in human myopia. Lens
thinning occurs in human refractive development in early
childhood up to the age at which the prevalence of myopia
begins to increase.23'36'37 Thinner lenses are associated with
myopic refractive errors throughout childhood.37 It is hypothesized that the crystalline lens is stretched and thinned as the
eye grows both equatorially and axially.23'37 Determining
whether thinner crystalline lenses occurring at longer vitreous
chambers in the Labrador retriever are caused by a similar
mechanism requires further study.
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If thinning of the crystalline lens is caused by or simply
accompanies an excessive length of the vitreous chamber
because of excessive growth, then myopic dogs may be
suitable for testing regimens that attempt to control myopia
by curbing excessive growth. Even though the axial length
may not be excessive, the longer vitreous chamber depth
may represent growth that is greater than optimal. The two
animals with higher amounts of myopia had longer than
average axial lengths. A larger sample of moderately myopic
dogs would help to establish the role of overall axial growth
in the development of myopia. Breeding for myopia may be
one strategy to increase the sample of myopic dogs, as
would studying breeds with a higher prevalence of myopia,
such as the toy poodle. It is of interest to note that in our
ongoing studies of dogs we have identified entire litters of
myopic puppies in which one or both of the parents was
myopic. Attempts to breed for this trait would determine
how the heritability of myopia in dogs compares with humans and establish its utility as a naturally occurring model
of myopia.
Axial distances for the Labrador retriever compare well
with results from unspecified breeds and the Samoyed.32'38'34
The largest difference between published values and our results occurred for crystalline lens radii of curvature. Sivak and
Kreuzer39'40 report that the in vitro lens radii of curvature for
six specimens in each of two investigations was 6.60 mm to
Myopia in the Labrador Retriever
IOVS, June 1999, Vol. 40, No. 7
1583
istry of experimental myopia has been that Pharmaceuticals
that affect induced myopia may be good candidates for
arresting developmental myopia in children.1 '"' 3 Evaluating
the effects of such Pharmaceuticals in a naturally occurring
model of myopia may be just such a situation, providing
important information on the efficacy of agents to alter
ocular growth under normal visual circumstances, conditions more similar to those experienced by children at
risk for myopia. Myopia in the dog warrants further evaluation of its potential to serve as a natural model of juvenile
myopia.
References
19.0
20.0
21.0
22.0
23.0
24.0
AXIAL LENGTH (mm)
FIGURE 6. The right eye spherical equivalent refractive error (Sph.
eq.) as a function of axial length for 57 Labrador retrievers. The four
puppies in the study are indicated by open circles, and the mature dogs
by points. The correlation is not significant (Spearman r = —0.15; P <
0.26).
7.29 mm for the anterior surface and 6.32 mm to 6.72 mm for
the posterior surface, a nearly symmetric biconvex lens with
the anterior surface flatter than the posterior surface. Our
phakometric measurements indicate that the lens is more
asymmetric, with the anterior surface being steeper than the
posterior surface (Tables 1 and 3). This is opposite to the
asymmetry present in the human biconvex lens where the
anterior surface is the flatter. Interestingly, the drawing of the
canine eye in Schiffer et al.32 displays a lens with a flatter
posterior than anterior radius.
Anatomic differences also exist between dog and human
eyes. Adult dogs have deeper anterior chamber depths by 0.5
to 1.3 mm, thicker lenses by approximately 4 mm, shorter
vitreous chamber depths by 6 mm to 7 mm, and shorter overall
axial lengths by 1 mm to 2 mm than adult human eyes. Other
anatomic and functional comparisons may be made according
to a recent review.4' A higher percentage of dog photoreceptors are rods, their binocular visual field is less at 30° to 60°,
they possess a limited accommodative amplitude of 2 D to 3 D,
and they have a horizontally oriented "visual streak" of peak
retinal sensitivity instead of a fovea, a lower number of ganglion cells by a factor of 7, a visual acuity of approximately
20/75,42 and a calculated depth of field of up to 0.33 D.43 The
significance of these differences is unknown, but the human
and dog eye are probably no more dissimilar in anatomy and
accommodative mechanism, for example, than the human and
chick eye.44'45
Dogs are certainly more difficult to use in research than
chicks. Experimental myopia in the chick has been a successful paradigm in part because chicks are small and relatively easy to house in the laboratory, and myopia can be
induced rapidly. A different animal model would have to
supply new and important information to replace the efficiency of using the chick in certain experimental situations.
The underlying assumption of evaluations of the biochem-
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