U.S. patent application number 12/213451 was filed with the patent office on 2008-12-18 for method for preventing myopia progression through identification and correction of optical aberrations.
This patent application is currently assigned to New England College of Optometry. Invention is credited to Jane E. Gwiazda, Ji C. He, Richard Held, Frank Thorn.
Application Number | 20080309882 12/213451 |
Document ID | / |
Family ID | 22632675 |
Filed Date | 2008-12-18 |
United States Patent
Application |
20080309882 |
Kind Code |
A1 |
Thorn; Frank ; et
al. |
December 18, 2008 |
Method for preventing myopia progression through identification and
correction of optical aberrations
Abstract
A method for at least one of preventing myopia and retarding the
progression of myopia is provided. The method includes measuring
optical aberrations in a human eye and correcting the optical
aberrations. Measuring optical aberrations may include measuring
wavefront aberrations of parallel light rays entering the eye.
Inventors: |
Thorn; Frank; (Newton,
MA) ; Held; Richard; (Cambridge, MA) ;
Gwiazda; Jane E.; (Brookline, MA) ; He; Ji C.;
(Somerville, MA) |
Correspondence
Address: |
COVINGTON & BURLING, LLP;ATTN: PATENT DOCKETING
1201 PENNSYLVANIA AVENUE, N.W.
WASHINGTON
DC
20004-2401
US
|
Assignee: |
New England College of
Optometry
Boston
MA
|
Family ID: |
22632675 |
Appl. No.: |
12/213451 |
Filed: |
June 19, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10169418 |
Oct 2, 2002 |
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PCT/US00/35582 |
Dec 28, 2000 |
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12213451 |
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60173582 |
Dec 29, 1999 |
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Current U.S.
Class: |
351/246 ;
128/898; 351/159.06 |
Current CPC
Class: |
G02C 2202/22 20130101;
G02C 7/04 20130101; G02C 2202/24 20130101; A61F 9/013 20130101 |
Class at
Publication: |
351/246 ;
128/898; 351/160.R |
International
Class: |
A61B 3/10 20060101
A61B003/10; A61B 19/00 20060101 A61B019/00; G02C 7/04 20060101
G02C007/04 |
Claims
1. A method for retarding or eliminating the progression of myopia
in a patient, comprising: measuring at least one optical aberration
present in an eye of the patient; correcting the at least one
optical aberration to thereby impede the progression of myopia in
the patient, wherein correcting the at least one optical aberration
is carried out without treatment of refractive error in the
patient.
2. The method of claim 1, wherein correcting the at least one
optical aberration is carried out using a contact lens.
3. The method of claim 1, wherein correcting the at least one
optical aberration is carried out by surgery.
4. The method of claim 2, wherein measuring at least one optical
aberration comprises measuring deviations at the retina of parallel
light rays entering the eye.
5. The method of claim 2, wherein measuring at least one optical
aberration comprises measuring deviations at the pupil of parallel
light rays entering the eye.
6. A method for retarding or eliminating the progression of myopia
in a patient, comprising: screening the eyes of the patient to
detect optical aberrations; measuring the detected optical
aberrations; correcting at least one of the detected optical
aberrations to thereby impede the progression of myopia in the
patient, wherein correcting the at least one optical aberration is
carried out without treatment of refractive error in the
patient.
7. The method of claim 6, wherein correcting the at least one
optical aberration is carried out using a contact lens.
8. The method of claim 6, wherein correcting the at least one
optical aberration is carried out by surgery.
Description
TECHNICAL FIELD
[0001] The present invention pertains to measuring optical
aberrations in the human eye and, more particularly, to preventing
myopia and retarding the progression of myopia by correcting the
aberrations.
BACKGROUND ART
[0002] Myopia (or near-sightedness) has become the most pervasive
visual disorder in the world. About twenty-five percent of people
in industrialized countries in the Western world, and more than
fifty percent of people in industrialized Asian countries require
optical correction for myopia. With increasing educational demands,
the prevalence of myopia is increasing steadily. Extensive reading
by children and adolescents appears to cause progressive myopia.
Since increasing educational demands have increased the prevalence
of myopia, optical correction, such as eyeglasses, contact lenses,
and refractive surgery for myopia is a major health care
expense.
[0003] Myopia is due primarily to an elongation of the posterior
pole of the eye during the school age years. Structures in this
region of the eye tend to be stretched during development, and
their integrity is compromised. This causes greater risk to the
effects of ocular trauma, diabetes, macular degeneration, and other
diseases. This means that myopia is also a major contributor to
irreversible blindness.
[0004] Referring to FIG. 1, in a normal eye, the cornea 10 and lens
12 at the front of the eye focus an image of the visual world on
the retinal receptors 14 at the back of the eye. At the retinal
receptors the image begins to be processed and sent on to the brain
as a complex neural signal. A myopic eye is too long, so that the
image of most of the visual world is focused in front of the
retina. Consequently, myopia is treated by weakening the optical
power in the front of the eye so that the image is focused on the
retina. This means that eyeglasses, contact lenses and refractive
surgery are not treating the basic disorder, but are merely
counteracting the effects of ocular elongation. Each of these
treatments has its own problems, is expensive, and in no way
reduces the likelihood that the myopic person will contract one of
the blinding diseases which are secondary to myopia later in
life.
[0005] Eyeglasses, contact lenses, and to a lesser extent,
refractive surgery can accurately correct myopic defocus (often
referred to as the spherical error of the eye) by placing as much
of the focused image as possible on the retina. Some eyes have an
aberration that creates a difference in optical power between one
meridional orientation and another. This aberration is known as
astigmatism and is correctable with eyeglasses (although eyeglasses
cause visual distortion) and with specialty contact lenses (which
may be uncomfortable).
[0006] At least thirty other "higher order" aberrations can be
measured and quantified in the human eye. Each of these aberrations
contributes a different type of degradation to the retinal image.
These aberrations are usually measured in the laboratory with a
complex optical instrument in which a laser beam is aimed at the
retina of people who have their pupils dilated with drugs. However,
such aberrations can now be measured in children and adolescents
without the use of bright light or the need for pupillary dilation
with drugs.
[0007] Traditional clinical correction of optical defocus places
the average position of an image of the visual world on the retina.
However, parts of that image may be in front of or behind the
retina due to the refractive properties of the eye's aberrations.
Thus, most of the image in a "perfectly" corrected eye may be
significantly out of focus due to these aberrations.
[0008] A small number of people have myopia due to rare inherited
diseases or, in old age, in conjunction with diabetic crystalline
lens changes. More than ninety percent of the people with myopia,
however, develop it during their school age years. It has been
shown that this progressive myopia is clearly related to a genetic
predisposition (Pacella et al., "Role of Genetic Factors in the
Etiology of Juvenile-Onset Myopia Based on a Longitudinal Study of
Refractive Error," Optom. Vis. Sci. 76, 381-386, (1999)) and to an
intensity of school work, especially reading.
[0009] Animal studies have shown conclusively that blurring the
visual world by scattering an image through the use of eyelid
closure or smoked or sandblasted eyeglass lenses leads to myopia.
Similarly, defocusing the visual world with minus lenses induces
myopic response. Both blurring (i.e., general image degradation)
and myopigenic defocus affects myopigenesis more in some species
than in others and more in some breeds than in others within the
same species. This suggests that the myopic response to
environmental influences is genetically dependent.
[0010] Epidemiological studies and other studies demonstrate the
same type of environmental-genetic interaction
SUMMARY OF THE INVENTION
[0011] A method is provided for preventing myopia and/or retarding
the progression of myopia by measuring optical aberrations in a
human eye and correcting the optical aberrations.
[0012] In accordance with one embodiment of the invention, the step
of measuring includes measuring wavefront aberrations of parallel
light rays entering the eye. The step of measuring may also include
measuring deviations at the retina of parallel light rays entering
the eye, as well as measuring deviations at the pupil of parallel
light rays entering the eye.
[0013] In accordance with another embodiment of the invention, the
step of measuring may include providing a multi-channel optical
system wherein an aperture is moved to multiple positions with
respect to the pupil of the eye and alignment parameters for each
aperture position is recorded at least one optical distance. A
system of equations is then solved to derive a set of aberration
constants based on the alignment parameters. The step of measuring
may also include detecting first and higher order astigmatism
and/or detecting at least one of first and higher order coma,
spherical aberrations and other aberrations.
[0014] In accordance with a further embodiment of the invention a
method for preventing myopia and/or retarding the progression of
myopia includes screening for aberrations in a human eye, measuring
the aberrations and correcting the aberrations. The step of
screening may include detecting a depth of focus by measuring
visual acuity, contrast sensitivity and/or blur sensitivity and the
visual acuity, contrast sensitivity and/or blur sensitivity may be
measured with a psycho-physical test.
[0015] In accordance with other embodiments of the invention, the
step correcting may include providing an optical device, providing
at least one optical lens and/or providing at least one contact
lens. The step of correcting may also include altering an optical
surface in the eye and/or performing corneal surgery. The step of
correcting may further include providing intra-ocular implants.
[0016] In accordance with additional embodiments of the invention,
the step of correcting includes providing adaptive optics, and the
adaptive optics may include deformable mirrors, systems of multiple
lenslettes, micro-mirror electro-machined components, optically
addressed liquid crystal spatial light modulators, membrane
mirrors, and/or piezoelectric bi-morph mirrors. The adaptive optics
may produce periods of clear vision and may be miniaturized so as
to be wearable on the face of a person
[0017] The step of correcting may also include providing high
illumination levels to reduce the pupil of the eye.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 shows a structure of an eye relevant to the present
invention;
[0019] FIG. 2 is a flow chart representing the steps involved in a
method of the present invention;
[0020] FIG. 3 is a flow chart representing another embodiment of
the present invention;
[0021] FIG. 4 is a flow chart representing an embodiment of the
invention that employs an apparatus such as a multi-channel optical
device;
[0022] FIG. 5 is flow chart representing an embodiment of the
invention wherein aberrations of a human eye are screened by
determining depth of focus;
[0023] FIG. 6 is a schematic of a multi-channel optical device that
may be used to perform the measurements of the present
invention;
[0024] FIG. 7A shows a front view of a prior art pupil sampling
aperture;
[0025] FIG. 7B shows a pattern of entry positions at the pupil of
the subject of a sampling beam using the apparatus of FIG. 7A;
and
[0026] FIG. 8 shows frequency histograms of the root-mean-square
(RMS) of wavefront aberration in the human eye for 280 subjects in
four groups. The number of subjects (N) and the mean RMS with
standard deviation are indicated for each group.
DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS
[0027] The inventors have shown that myopic children and
adolescents tend to under accommodate when looking at near targets
during and prior to the period in which they are developing myopia.
This under accommodation results in a myopigenic defocus similar to
that which induces myopia in animal experiments. The inventors have
also demonstrated that children and adolescents who are becoming
myopic have certain binocular anomalies (near esophoria and high
AC/A ratio) which tend to cause under accommodation (Gwiazda, J.,
Thorn, F., Bauer, J. and Held, R., "Myopia Children Show
Insufficient Accommodation to Blur," Invest. Opthalmol. Vis. Sci.,
34, 690-694 (1993); Gwiazda, J., Bauer, J., Thorn, F., and Held,
R., "A Dynamic Relationship Between Myopia and Blur-Driven
Accommodation in School Aged Children," Vision Res., 35, 1299-1304
(1995); Gwiazda, J., Grice, K., and Thorn, F., "Response AC/A
Ratios are Elevated in Myopic Children," Physiol. Optics., 19,
173-179 (1999) all of which are incorporated herein be reference.).
This suggests that myopigenic defocus due to under-accommodation
induces myopia in children and adolescents.
[0028] When children must look intensely at near patterns, for
example, during reading, their eyes accommodate (focus) and
converge (turn in together) on the text being read. This
accommodation effort tends to increase optical aberrations, causing
increased blur, and the eyes tend to under accommodate, causing a
myopigenic defocus. These innate factors, plus extensive reading in
children and adolescents whose eyes are still young enough to grow,
appear to cause progressive myopia.
[0029] The inventors have shown that all adults with high amounts
of optical aberrations are myopic. In fact, about 25% of myopic
children and adults have optical aberrations that are greater that
those in non-myopic adults. The aberrations in myopic eyes were
often two or three times as large as the upper limit in non-myopic
adults.
[0030] In accordance with the present invention, it is taught that
if large aberrations in the eyes of children are detected and
treated, the progression of myopia may be retarded or
eliminated.
[0031] Definitions and proposed standards relating to measurement
of optical aberrations are available in (Atchison et al.,
"Mathematical Treatment of Optical Aberrations: A User's Guide,"
Trends in Optics and Photonic, Optical Society of America, 35,
110-130 (2000) and Thibos et al., "Standards for Reporting the
Optical Aberrations of the Eyes," Trends in Optics and Photonics,
Optical Society of America, 35, 232-244 (2000) which are hereby
incorporated herein by reference.
[0032] FIG. 2 illustrates a method for preventing myopia and/or
retarding the progression of myopia. Optical aberrations are
measured in step 21. Aberrations may be expressed in terms of
Zernike polynomials; however, the use of other representations is
within the scope of the invention. The optical aberrations are then
corrected in step 22 using optical correcting devices well known in
the art including, but not limited to, spectacles, contact lenses,
adaptive optics, corneal surgery, laser surgery, and intra-ocular
implants.
[0033] FIG. 3 illustrates a method for preventing myopia and/or
retarding the progression of myopia wherein deviations, at a
pupillary or retinal location, of parallel light rays (e.g., 16 in
FIG. 1) entering a human eye are measured in step 31. The optical
aberrations are then calculated, step 32, from the measurements
taken in the previous step and the aberrations are corrected in
step 33 with optical correction devices as described in connection
with FIG. 2.
[0034] FIG. 4 illustrates a procedure for measuring wavefront
aberrations using a three channel system as shown in FIG. 6. The
procedure is discussed in He et al., "High Optical Quality is a
Necessary Condition for the Human Eye to Maintain Emmetropia,"
(1999), which is attached hereto and incorporated herein by
reference. Referring to FIG. 6, the system has separate channels
61, 62, 63 for test, fixation-stimulus, and pupil-monitoring
respectively. A pupil of a subject is located at P.sub.0 and a
retina of the subject is located at R.sub.0. A Badal optometer
(focusing block) 64 allows an operator to change the refractive
state of the test channel 61 and the pupil-monitoring channel 62
together, without changing the location of the pupil conjugate
planes (P.sub.1, P.sub.1', P.sub.2, and P.sub.2') and retinal
conjugate planes (R.sub.1, R.sub.2, R.sub.2', and R.sub.3)
[0035] A 543-nm He--Ne laser 60 produces light for the test channel
61. The coherence of the laser 60 is broken by a rotating diffuser
65. The light from the laser 60 is collimated by a lens 66 and 12
mm steel ball 68. The reflection from the ball 68 produces a
divergent, high-numerical aperture beam 67 that the subsequent
optics image as a point source. A gimbaled mirror 69 is controlled
by an analog joystick (not shown) that allows the subject to change
the angle of the mirror 69 rapidly in two dimensions. Tilting the
mirror 69 changes the angle at which the test beam enters the eye
and therefore changes the retinal location of the test spot.
[0036] At the pupil-monitoring channel 62, a pupil entry position
of the test beam is selected from a set of 1 mm holes 72 (shown in
FIG. 7A) that tile the pupil of the eye by rotating a aperture
metal wheel 70 (see FIG. 7A) that is optically conjugate to the
pupil. The aperture wheel 70 is constructed such that it can be
rotated to one of 37 preset locations.
[0037] A fixation target, typically a cross, is provided at a
fixation-stimulus channel 63. The fixation-stimulus channel 63 is
illuminated by a light source, such as a fiber-optic illuminator
75. Light from the illuminator 75 is collimated and then passes
through a filter holder-slide holder 74 located in a retinal
conjugate plane R.sub.2'. The light from the illuminator 75 is then
imaged on an adjustable iris diaphragm 76 located in a pupil
conjugate plane P.sub.1. The iris diaphragm 76 is set to 1 mm
diameter to match the size of the pupil sampling. However, for
conditions in which the wavefront properties of the eye are
measured when the eye is accommodated by high illumination levels,
the diameter is increased to 6 mm to provide a better stimulus. The
fixation-stimulus channel 63 is combined with the pupil-monitoring
channel 62 at beam splitter 77.
[0038] In step 41 of FIG. 4, the subject's eye is first aligned to
the optical axis of the system by using an infrared sensitive CCD
video camera 78. The camera 78 provides a magnified view of the
pupil. By looking at a monitor screen of a computer (not shown) and
adjusting the Badal system 64 to clarify the screen, the eye is at
its resting state. Measurements are referenced to the entry
location within the pupil. The measurements consist of a few
practice trials and six tests, three for each eye. In step 42 an
aperture 72 is moved to multiple positions with respect to the
pupil to produce a pattern as shown in FIG. 7B. A test may consist
of thirty-nine trials with the first and the last trials for the
center of the pupil. The other thirty-seven trials randomly sample
the entire pupil with a 7.times.7 matrix in 1 mm steps except the
twelve points in the four corners. The subject's task is to align a
cursor with the center of the fixation target and click a mouse of
the computer on each trial. Each test usually lasts about three
minutes, and the entire session requires approximately thirty
minutes.
[0039] In step 43, the shifts in the fixation target are recorded
by the computer and translated into the slope of the wavefront at
the thirty-seven pupil locations. In step 44 a system of equations
is solved using a least square procedure to fit the slope
measurements to the derivative of thirty-five terms of the Zernike
polynomial functions. The derived coefficients provide estimates of
the weights of the individual aberrations, and are used, in step
45, to reconstruct the overall wavefront at the pupil plane. The
aberrations are corrected in step 46 in the same manner as
described in step 22 of FIG. 2.
[0040] Procedures and devices for measuring wavefront aberrations
are further discussed in Liang et al., "Objective Measurement of
Wave Aberrations of the Human Eye With the Use of a Hartmann-Shack
Wave-Front Sensor," Journal of the Optical Society of America A,
11, 1-9 (1994); Thibos, "Principles of Hartmann-Shack Aberrometry,"
Trends in Optics and Photonics, Optical Society of America, 35,
163-169 (2000); and He et al, "Measurement of the Wave-Front
Aberration of the Eye by a Fast Psychophysical Procedure," Journal
of the Optical Society of America A, 15, 2449-2456 (1998) each of
which is incorporated herein by reference.
[0041] FIG. 5 illustrates a method for retarding the progression of
myopia wherein a human eye is screened for aberrations. Visual
acuity, contrast sensitivity, and/or blur sensitivity are measured
in step 51. Depth of focus is determined in step 52 from the
measurements performed in step 51 using procedures described in
Thorn et al., "Myopia Adults See Through Defocus Better than
Emmetropes," Myopia Updates, Springer, Tokyo, 368-374, T. Tokoro
(ed.) (1998) and Rosenfield and Abraham-Cohen, "Blur Sensitivity in
Myopes," Optometry and Vision Science, 76, 303-307 (1999) which are
also incorporated herein by reference. Deviations in parallel light
rays (e.g., 16 in FIG. 1) entering a human eye at the retina or
pupil of the eye are measured in step 53. Aberrations are then
calculated, step 54, from the measurements taken in the previous
step. The aberrations are precisely measured as described above and
then corrected in step 55.
[0042] Optical correction for aberrations within the human eye may
be provided through several different optical procedures. In one
embodiment of the invention, spectacle lenses are used to reduce
astigmatic aberrations. In another embodiment of the invention,
contact lenses are used to reduce second and third, and perhaps
higher orders of aberrations because contact lenses move with the
eye, thereby preserving the alignment of their optical surfaces
with the optical surfaces of the eye during eye movements. See,
(Bartsch et al., "Resolution Improvement in Confocal Scanning Laser
Tomography of the Human Fundus," Technical Digest Series 2, Optical
Society of America, 2, 134-137 (1994) and Guirao et al, "Effect of
Rotation and Translation on the Expected Benefit of Ideal Contact
Lenses," Trends in Optics and Photonics, Optical Society of
America, 35, 324-329 (2000) which are incorporated herein by
reference.
[0043] Lenses may also be used to change (usually reduce) optical
accommodation levels in order to reduce optical aberrations.
Several studies have used this procedure for various purposes. See,
for example, the study by Jackson and Brown, "Progression of Myopia
in Hong Kong Chinese School Children is Slowed by Wearing
Progressive Lenses," Optometry and Vision Science, 76, 346-354
(1999) which is incorporated herein by reference.
[0044] It is also within the scope of the invention to perform
corneal surgery to reduce optical aberrations. This procedure is
described in Hamam, "A Quick Method for Analyzing Hartmann-Shack
Patterns: Application to Corneal Surgery," Trends in Optics and
Photonics, Optical Society of America, 35, 187-198 (2000); Hong and
Thibos, "Optical Aberrations Following Laser in Situ Keratomileusis
(LASIK) Surgery," Trends in Optics and Photonics, Optical Society
of America, 35, 220-226 (2000); and Munger, "New Paradigm for the
Treatment of Myopia Refractive Surgery," Trends in Optics and
Photonics, Optical Society of America, 35, 227-230 (2000) all of
which are incorporated herein by reference.
[0045] Further, intra-ocular implants a may be used to reduce
optical aberrations. The rationale, measurement, and analysis used
for the intra-ocular implant embodiment is the same as that in the
refractive surgery reduction of aberrations discussed above.
[0046] In another embodiment adaptive optics are used to reduce
optical aberrations. Adaptive optics may employ deformable mirrors,
micro-mirror electro-machined components, lenslette arrays,
optically addressed liquid crystal spatial light modulators,
membrane mirrors, or piezoelectric bi-morph mirrors to correct the
eye's aberrations. Adaptive optics may be used in devices that can
be worn when they are miniaturized to the point of wearability.
Adaptive optics devices may also be used in instruments that allow
patients to experience periods of clear vision through reduced
optical aberrations. See, for example, Roorda and Williams,
"Adaptive Optics and Retinal Imaging," Trends in Optics and
Photonics, Optical Society of America, 35, 151-162 (2000) and
Munger, "New Paradigm for the Treatment of Myopia Refractive
Surgery," Trends in Optics and Photonics, Optical Society of
America, 35, 227-230 (2000) each of which is incorporated herein by
reference.
[0047] In another embodiment high illumination levels are used to
reduce pupil size thereby reducing the amount of optical
aberrations. This method is discussed in Campbell, "Contributions
to the Optical Quality of the Eye: Implications for `Perfect`
Optical Correction," Trends in Optics and Photonics, Optical
Society of America, 35, 227-230 (2000) which is also incorporated
herein by reference.
[0048] It should also be noted that the embodiments described
herein are not mutually exclusive and can be used in combination.
For example, visual acuity, contrast sensitivity and/or blur
sensitivity may be measured in combination with measurement of
wavefront aberrations. Likewise, the numerous devices mentioned
above in connection with correcting the aberrations may be used in
combination with one another to produce equivalent or superior
results.
[0049] Although the above embodiments are preferred, many
modifications and refinements which do not depart from the true
spirit and scope of the invention may be conceived by those skilled
in the art. It is intended that all such modifications, including
but not limited to those set forth above, be covered by the
following claims.
High Optical Quality is a Necessary Condition for the Human Eye to
Maintain Emmetropia
Ji C. He*, Pei Sun.sctn., Richard Held*, Frank Thorn*, Editha Ong*,
Xiuru Sun.sctn., Jane E. Gwiazda*
*New England College of Optometry, 424 Beacon Street, Boston, Mass.
02115, USA
[0050] .sctn.Institute of psychology, Chinese Academy of Science,
P.O. Box 1603, Beijing, Beijing 100012, P. R. China
[0051] Vision is optimized when the focal plane of the eye's optics
is coincident with the retina so that the image of a distant object
falls on the photoreceptor layer: a condition called emmetropia. A
mismatch between the focal and axial lengths of the eye causes
refractive errors in the forms of either hyperopia
(far-sightedness), when the focal plane lies behind the retina, or
myopia (near-sightedness), when it is in front of the retina. Most
children's eyes approach emmetropia at about 5 years of age from a
mismatch in infancy.sup.1-2. While many children maintain their
emmetropia into adulthood, others become myopic because the eye
rows too long. Animal studies indicate that degrading image quality
can cause myopia.sup.3-8. A similar causation for the human eye is
less clear.
[0052] Human eyes recently have been found to have irregular
aberrations.sup.9-16, which degrade image quality, thereby making
them candidates for myopization. We measured monochromatic
aberrations in myopic and emmetropic children and adults, and found
that adult emmetropes had less aberrations than either myopes or
emmetropic children. These results indicate that high image quality
is necessary for maintaining emmetropia.
[0053] A single lens forms an image of a distant object at its
focal plane. The distance between the lens and the focal plane is
the focal length, a characteristic parameter of the lens. The focal
length of the human eye is determined by both the geometric
curvature of corneal and lens surfaces and the refractive indices
of their ocular media. In most infants, the focal length is greater
than the axial length so that the focal plane lies behind the
retinal plane Hyperopia). Eye growth in childhood tends to match
the focal plane with the retina so as to achieve emmetropia. The
match, however, is not maintained in animal experiments if image
clarity is disrupted by either lid fusion.sup.3-5 or otherwise
depriving the eye of spatial information.sup.6-8. These
manipulations cause the eye to grow too long so that the focal
plane lies in front of the retina (myopia). This dependence of
myopia development on image quality has been observed in various
species ranging from chicken to monkey, but the underlying
mechanisms are not fully understood. Although experimental
manipulation on the human eye is not possible, Nature poses its own
tests of this issue. The human eye is not an ideal optical system.
Its defects are called aberrations and are caused by local
variations in both surface curvatures and refractive indices in the
cornea and the lens and/or misalignment of the optical axes of the
cornea and lens relative to the visual axis of the eye. The
aberrations cause the light rays passing through the pupil to
divert from their ideal paths, and proportionately degrade image
quality so that clearest vision can not be reached even though the
focal plane matches the retina perfectly. Recent measurements in
human eyes have shown that the aberrations vary substantially from
one individual to another in their form and amount.sup.9-16. In
this study we measured aberrations for 280 subjects with different
ages and different refractive errors in order to demonstrate the
effect of aberrations, hence image quality, on the match between
the focal plane and the retina in the human eye.
[0054] Wavefront aberrations at the pupil plane have recently been
used to characterize the overall effect of aberrations.sup.9-11,
13-16. The wavefront represents an equal-phase surface for the
light rays passing the pupil at any given time, and forms a flat
surface on the pupil plane if the eye is ideal. Deficiencies in the
optics of the eye cause the wavefront to deviate from the ideal
surface, and the degree of the wavefront deviations, or wavefront
aberrations, directly depends on how the optics are flawed. We used
a psychophysical ray-tracing technique with natural pupils to
measure wavefront aberrations.sup.9-16, and used a root-mean-square
(RMS) of the deviated wavefront, relative to the ideal flat
wavefront as an estimate for the effect of wavefront aberrations.
Subjects were divided into four groups according to their age and
refractive error as shown in Table 1. Among the 280 subjects,
eighteen percent are Caucasian and eighty-two percent are
Chinese.
[0055] Frequency histograms of the RMS of wavefront aberrations in
the worst eye for each subject are shown in FIG. 8 for the four
groups of subjects. As shown in FIG. 8, every subject has RMS of
wave-aberration greater than 0.5. This result means that the human
eye is not perfect but suffers image degradation resulting from the
deficiency in optics. Adult emmetropes, however, have the lowest
mean RMS of wave-aberration which is significantly different from
the means in the other groups (vs children's emmetropic group,
t=5.55 p<0.0001; vs myopic adult group, t=4.85, p<0.0001; and
vs myopic children's group, t=6.45, p<0.0001.). They have the
smallest standard deviation and it is also significantly different
from the other groups (vs emmetropic children's group, F=2.39,
p<0.005; vs myopic adult group, F=8.89, p<0.001; and vs
myopic children's group, F=10.72, p<0.001). The highest RMS
value in the adult emmetropic group is 1.62 which is exceeded by
forty percent of myopic adults, 37.5% of myopic children, and 23%
of emmetropic children in our sample. The results indicate that
adult emmetropes suffer the least image degradation and that
stronger image degradation occurs for about forty percent of myopes
who have aberrations greater than all adult emmetropes.
[0056] Compared with the wave-aberrations for emmetropic children,
some of whom may develop myopia later, the limited wave-aberration
for adult emmetropes indicates that for the human eye to maintain
emmetropia image degradation must be small. People with strong
wave-aberration, who suffer strong image degradation, may fail to
maintain the match between the focal plane and the retinal plane,
and thereby develop myopia. These findings are in agreement with
the evidence from animal studies.
[0057] It has been suggested that myopia causes aberrations, but if
the elongation of the eye in myopia generally caused optical
deficiencies in the cornea and lens, we would expect all or most
myopes to have more aberrations than emmetropes. But this is not
true for the sixty percent of myopes who have aberrations no
greater than the adult emmetropes.
[0058] Genetic contributions to myopia have long been
recognized.sup.17-19, but the underlying mechanisms are unclear.
Aberrations of the eye caused by defects in the cornea and lens may
be inherited. Thus aberrations causing image degradation may be one
of the genetic mechanisms leading to myopia. Meanwhile, the role of
near-work can not be excluded. Stronger aberrations have been
reported for an accommodated eye.sup.20-21. Near-work would expose
the eye to stronger image degradation and thus impose a higher risk
of developing myopia. Besides the contribution of aberrations
either inherited or near-work associated, the existence of sixty
percent of myopes with less aberrations necessarily indicates a
contribution from other factors on myopia development.
[0059] Small aberrations with reduced variability in adult
emmetropes, as found in this study, suggest that high optical
quality of the image is a necessary condition for the human eye to
maintain emmetropia. Our results also suggest that severe
aberrations are associated with the development of myopia. The
existence of strong aberrations in myopia, which can not be
corrected with available techniques, necessitates the development
of new techniques for vision care in clinical practice. The results
also provide important information about the optics of the human
eye for designing experiments in vision research and visual
instruments in the optical industry.
METHODS
[0060] Apparatus. The apparatus used in this study is a three
channel optical system, including a test, a reference and a pupil
monitoring channel, which share the design of the subjective
wavefront sensor described in a previous study.sup.16 in principle
but was changed to a computer monitor version. The test channel
provides a green cross target on the retina via a movable aperture
with 1 mm diameter, As the aperture is moved from trial to trial
among 37 locations within the subject's natural pupil, the cross
shifts its retinal location accordingly due to the aberration of
the eye. The cross shifts were traced by the subject via a cursor
on the monitor of a computer provided in the reference channel. The
subject's pupil was monitored by a CCD camera and a monitor in the
pupil-monitoring channel during the experiment and any eye movement
relative to the optical axis of the system was compensated by
moving a 3D translator on which the subject's head rested. In the
systems there is a movable stage with two mirrors on the common
pathway set as a Badel system for compensating the subject's
refractive error.
[0061] Procedure. The subject's eye was first aligned to the
optical system. By looking at the monitor screen via a 1 mm
aperture and adjusting the Badal system to clarify the screen, the
eye was at its resting state. The measurements consisted of a few
practice trials and six tests, three for each eye. Each test
consisted of 39 trials with the first and the last trials for the
center of the pupil. The other 37 trials randomly sampled the
entire pupil with a 7.times.7 matrix in 1 mm steps except the 12
points in the four corners. The subject's task was to align the
cursor with the center of the cross and click the mouse on each
trial. Each test usually lasted about 3 minutes, and the entire
session took about a half hour.
[0062] Data Analysis. The shifts in the cross target recorded by
the computer were translated into the slope of the wavefront at the
37 pupil locations. A least square procedure was used to fit the
slope measurements to the derivatives of 35 terms of the Zernike
polynomial functions. The derived coefficients provide estimates of
the weight of individual aberrations, and were used to reconstruct
the overall wavefront at the pupil plane.
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[0084] Acknowledgements. We thank Jinhua Feng for help in data
acquisition. This work is supported by a grant from the National
Eye Institute of National Institutes of Health, U.S.A., the
National Natural Science Foundation of China and K. C. Wang
Education Foundation of Hong Kong.
TABLE-US-00001 TABLE 1 Subjects' information Spherical Number of
Number of Age Age equivalent Subjects- Subjects- (years) (years)
error Male Female Mean Range (Diopter) Emmetropic 20 25 21.5 19-27
0.75 to -0.5 Adults Emmetropic 45 35 15.1 12-17 0.75 to -0.5
Children Myopic 39 41 21.4 19-29 -0.6 to -9.0 Adults Myopic 35 40
15.0 11-18 -0.6 to -7.0 Children
* * * * *