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Claims  |
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What is claimed is:
1. An apparatus for entoptically perceiving and mapping the foveal area
vasculature of the retina of a human subject's eye under examination, the
apparatus comprising:
(a) a means to establish and maintain translational and rotational
alignment of said eye with said apparatus;
(b) a main light source which is imaged in or near the eye's entrance pupil
plane and a means of moving said main light source or said image along a
path in space, said main light source being of variable intensity and
shape;
(c) a means of directing said main light image into said eye's entrance
pupil, resulting in an angle of illumination of said eye's retina changing
with time;
(d) a means to image an aperture at optical infinity or other plane of
interest to correct for any refractive error as an optical field stop for
said apparatus, said aperture being of variable size and shape;
(e) a fixation light source and a means to form a fixation light image on
the retina of said eye, said fixation light source being of variable
intensity;
(f) a tracking light source and a means to form an image of tracking light
from said source on the retina of the eye, said tracking light source
being of variable intensity;
(g) a means of moving said tracking light retinal image with respect to
said fixation light retinal image;
(h) a means of transducing movement of said tracking light retinal image to
yield coordinates of its present location on said eye's retina with
respect to said fixation light retinal image;
(i) a means of compiling or displaying said coordinates of said tracking
light retinal image movement, said compilation or display comprising a map
of said tracking light retinal image positions with respect to said
fixation light retinal image; and
(j) a means to detect and indicate magnitude and direction of translation
of said eye with respect to said apparatus.
2. The apparatus of claim 1 wherein the means to establish and maintain
rotational alignment is a bite bar for the subject to orally embrace or a
chin and forehead rest.
3. The apparatus of claim 1 wherein said main light image path is circular,
about 2 to 6 mm in diameter and about centered in the eye's pupil.
4. The apparatus of claim 1 wherein said main light image path is circular,
4 mm in diameter and about centered in the eye's pupil.
5. The apparatus of claim 1 wherein said main light path is circular, being
retraced at the rate of about 0.5 to 10 Hz.
6. The apparatus of claim 1 wherein said main light path is circular, being
retraced at the rate of about 3.5 Hz.
7. The apparatus of claim 1 wherein said aperture is that of an adjustable
iris diaphragm.
8. The apparatus of claim 1 wherein said eye's area of retinal illumination
comprises a circle and said fixation light retinal image is within said
circle.
9. The apparatus of claim 1 wherein said eye's area of retinal illumination
comprises a circle and said fixation light retinal image is centered
within said circle.
10. The apparatus of claim 1 wherein said main light source has a peak
wavelength of about 430 to 555 nm and a half band pass of about +/- 60 nm.
11. The apparatus of claim 1 wherein said main light source has a peak
wavelength of about 470 nm and a half band pass of +/- about 60 nm.
12. The apparatus of claim 1 wherein said main light source image is
circular with a diameter of about 0.5 to 3 mm.
13. The apparatus of claim 1 wherein said main light source image is
circular with a diameter of about 1.0 mm or less.
14. The apparatus of claim 1 wherein said main light source is imaged in or
near said eye's entrance pupil plane.
15. The apparatus of claim 1 wherein said main light source is imaged in or
near said eye's anterior focal plane.
16. The apparatus of claim 1 wherein said main light source image is
diffuse and of uniform intensity.
17. The apparatus of claim 1, where in step (j) said indication of
magnitude and direction of translation of said eye with respect to said
apparatus is visible.
18. The apparatus of claim 1, where in step (j) said indication of
magnitude and direction of translation of said eye with respect to said
apparatus is sensed by an external operator.
19. The apparatus of claim 1 where in step (j) said indication of magnitude
and direction of translation of said eye with respect to said apparatus is
sensed by a sensor and monitored by a computer.
20. The apparatus of claim 1 wherein said tracking light retinal image
coordinates of steps (h) and (i) are corrected manually for retinal image
translation caused by any translation of said eye with respect to said
apparatus.
21. The apparatus of claim 1 wherein said tracking light retinal image
coordinates of steps (h) and (i) are corrected by automatic computation
for retinal image translation caused by translation of said eye with
respect to said apparatus.
22. The apparatus of claim 1 wherein said tracking light retinal image
coordinates in step (i) are calibrated in units of length measured on said
retinal surface.
23. The apparatus of claim 1 wherein said tracking light retinal image
coordinates in step (i) are calibrated in units of angular subtense.
24. The apparatus of claim 1 wherein said means of moving said tracking
light retinal image is a joy stick or similar x-y manual controller.
25. A method for entoptically perceiving and mapping the foveal area
vasculature in the retina of the eye of a human subject under examination
with respect to the retinal point of fixation of said eye, the method
comprising:
(a) optically directing a main light source image within said eye's
entrance pupil to illuminate a portion of said eye's retina defined by a
field stop, said image being of variable intensity;
(b) causing said main light source image to move along a main light path in
space, said main light path in space resulting in said eye's angle of
retinal illumination changing with time;
(c) optically directing a fixation light source on said eye's retina and
forming thereon a fixation light retinal image, said fixation light source
image being of variable intensity;
(d) having said subject visually fixate on said fixation light source
image;
(e) having said subject report descriptions of entoptically perceived
vascular features of interest near the foveal area of said eye;
(f) optically directing a tracking light source on said eye's retina and
forming thereon a tracking light retinal image, said tracking light source
image being variable in intensity and movable over said eye's retinal
surface with respect to said fixation retinal image through a tracking
light controller operated by said subject;
(g) causing said fixation light, and tracking light source images to be
viewed by said eye through an optical field stop aperture illuminated by
the main light, said aperture being imaged at infinity or other plane of
interest;
(h) scaling outputs of said tracking light controller to correspond to
distances and angles measured on said eye's retinal surface;
(i) having said subject move said tracking light retinal image along or
around said entoptically perceived vascular features of interest while
said subject maintains constant translational and rotational alignment of
said eye with said fixation light retinal image;
(j) using said scaled outputs of said tracking light controller to create
position reports of said entoptically perceived vascular features of
interest on said eye's retina;
(k) combining said position reports with said descriptions of said
entoptically perceived vascular features of interest to create a retinal
vascular map;
(1) having said subject adjust said intensities of said main, fixation and
tracking light sources for best entoptic visualization;
(m) having said subject adjust complete rotation frequency and thus
velocity of the main light source image;
(n) observing said eye to detect misalignment with said fixation light
retinal image due to any combination of translation or rotation of said
eye;
(o) determining corrections required to said position reports resulting
from said misalignment of said eye with respect to said fixation light
retinal image; and
(p) applying said corrections to said position reports.
26. The method of claim 25 wherein said main light path in space is
circular.
27. The method of claim 25 wherein said main light path in image space is
circular, 2 to 6 mm in diameter, and about centered in the eye's pupil.
28. The method of claim 25 wherein said main light path in image space is
about 4 mm in diameter and about centered in the eye's pupil.
29. The method of claim 25 wherein said main light path in space is
circular, being retraced at the rate of about 0.5 to 10 Hz.
30. The method of claim 25 wherein said main light path in space is
circular, being retraced at the rate of about 3.5 Hz.
31. The method of claim 25 wherein said field stop aperture is an
adjustable iris diaphragm.
32. The method of claim 25 wherein said eye's area of retinal illumination
comprises a circle and said fixation light retinal image is within said
circle.
33. The method of claim 25 wherein said eye's area of retinal illumination
comprises a circle and said fixation light retinal image is centered
within said circle.
34. The method of claim 25 wherein said main light source image has a peak
wavelength of about 430 to 555 nm and a half band pass of about +/- 60 nm.
35. The method of claim 25 wherein said main light source image has a peak
wavelength of about 470 nm and a half band pass of about 60 nm.
36. The method of claim 25 wherein said main light source image is circular
with a diameter of about 0.5 to 3 mm.
37. The method of claim 25 wherein said main light source image is circular
with a diameter of about 1.0 mm or less.
38. The method of claim 25 wherein said main light source image is focused
in or near said eye's entrance pupil plane.
39. The method of claim 25 wherein said main light source image is focused
in or near said eye's anterior focal plane.
40. The method of claim 25 wherein said main light source image is diffuse
and of uniform intensity.
41. The method of claim 25 wherein said corrections in step (o) of said
position reports are determined by a human operator.
42. The method of claim 25 wherein said corrections in step (o) of said
position reports are determined by an automatic computer.
43. An apparatus for entoptically perceiving and mapping the white blood
cell circulation and foveal area vasculature in the retina of a the human
subject's eye under examination, the apparatus comprising:
(a) a means to establish and maintain translational alignment of said eye
with said apparatus;
(b) a main light source which is imaged in or near the eye's entrance
pupil, and a means of moving said main light source or its image along a
main light path in space, said main light source being of variable
intensity;
(c) a means of optically directing said main light image into said eye's
entrance pupil, resulting in said eye's angle of retinal illumination
changing with time;
(d) a means to image an aperture at optical infinity or other plane of
interest to correct for any refractive error as an optical field stop for
said apparatus, said aperture being of variable size;
(e) a fixation light source and a means to form a fixation light image on
the retina of said eye, said fixation light source being of variable
intensity;
(f) a tracking light source and a means to form a tracking light image on
the retina of said eye, said tracking light source being of variable
intensity;
(g) a means of moving said tracking light retinal image with respect to
said fixation light retinal image;
(h) a means of transducing movement of said tracking light retinal image
with respect to said fixation light retinal image to yield coordinates of
its location on said retina with respect to location of said fixation
light retinal image;
(i) a means of compiling or displaying said coordinates of tracking light
image movement, said compilation or display comprising a map of said
tracking light retinal image locations with respect to location of said
fixation light retinal image;
(j) a means to detect and indicate magnitude and direction of translation
of said eye with respect to said apparatus;
(k) a blue-field light source and a means to illuminate said eye's retina
with said blue-field light source, said blue-field light source being of
variable intensity;
(l) a speed-comparator light source casting an image and a means to form a
retinal image of said speed-comparator light on said eye's retina, said
speed-comparator light source being of variable intensity and said
speed-comparator light source retinal image being of a size about equal to
said entoptically perceived white blood cells;
(m) a means of causing said speed-comparator retinal image to move along a
path on said eye's retina at a fixed velocity, said path and velocity
being variable; and
(n) a means of rotating and translating said speed comparator light retinal
image on said eye's retina.
44. The apparatus of claim 43 wherein said main light path image space is
circular, about 2 to 6 mm in diameter and about centered in the eye's
pupil.
45. The apparatus of claim 43 wherein said main light path image space is
circular, 4 mm in diameter and about centered in the eye's pupil.
46. The apparatus of claim 43 wherein said main light path is circular,
being retraced at the rate of about 0.5 to 10 Hz.
47. The apparatus of claim 43 wherein said main light path is circular,
being retraced at the rate of 3.5 Hz.
48. The apparatus of claim 43 wherein said aperture is that of an
adjustable iris diaphragm.
49. The apparatus of claim 43 wherein said area of retinal illumination
comprise a circle and said fixation light retinal image is within said
circle.
50. The apparatus of claim 43 wherein said area of retinal illumination
comprises a circle and said fixation light retinal image is centered
within said circle.
51. The apparatus of claim 43 wherein said main light source has a peak
wavelength of between about 430 nm and 555 nm and a half band pass of
about +/- 60 nm.
52. The apparatus of claim 43 wherein said main light source has a peak
wavelength of about 470 nm and a half band pass of about +/- 60 nm.
53. The apparatus of claim 43 wherein said main light source image is
circular with a diameter of about 0.5 to 3 mm.
54. The apparatus of claim 43 wherein said main light source image is
circular with a diameter of about 1.0 mm or less.
55. The apparatus of claim 43 wherein said main light source is imaged in
or near said eye's entrance pupil plane.
56. The apparatus of claim 43 wherein said main light source is imaged in
or near said eye's anterior focal plane.
57. The apparatus of claim 43 wherein said main light source image is
diffuse and of uniform intensity.
58. The apparatus of claim 43 where in step (j) said indication of
magnitude and direction of translation of said eye with respect to said
apparatus is visible.
59. The apparatus of claim 43 where in step (j) said indication of
magnitude and direction of translation of said eye with respect to said
apparatus is sensed by an external operator.
60. The apparatus of claim 43 where in step (j) said indication of
magnitude and direction of translation of said eye with respect to said
apparatus is sensed by an external computer.
61. The apparatus of claim 43 wherein said tracking light retinal image
coordinates of step (h) are corrected manually for retina image
translation caused by translation of said eye with respect to said
apparatus.
62. The apparatus of claim 43 wherein said tracking light retinal image
coordinates of step (h) are corrected by automatic computation for retinal
image translation caused by translation of said eye with respect to said
apparatus.
63. The apparatus of claim 43 wherein said tracking light retinal image
coordinates of step (h) are calibrated in units of length measured on said
retinal surface.
64. The apparatus of claim 43 wherein said tracking light retinal image
coordinates of step (h) are calibrated in units of angular subtense.
65. The apparatus of claim 43 wherein said means of moving said tracking
light retinal image of step (h) is a joy stick or similar x-y manual
controller.
66. The apparatus of claim 43 wherein said blue-field light source has a
dominant wavelength of about 430 to 500 nm.
67. The apparatus of claim 43 wherein light from said blue-field light
source is directed coaxially with the instrument's optical axis.
68. The apparatus of claim 43 wherein said blue-field light source
comprises about 50% of total light.
69. The apparatus of claim 43 wherein said blue-field light is applied to
said retina constantly or intermittently in alternation with the eccentric
moving light source at a rate of 40 to 70 Hz, the duty angle being
variable to optimize perception of both the retinal vessels and the white
blood cells.
70. The apparatus of claim 43 wherein said retinal path of said
speed-comparator light image is curved to mimic the course of a retinal
vessel.
71. The apparatus of claim 43 wherein said retinal path of said
speed-comparator light image is straight.
72. The apparatus of claim 43 wherein said retinal path of said
speed-comparator light image is about 10.sup.-3 to 10.sup.-2 m in length.
73. The apparatus of claim 43 wherein said velocity of the speed comparator
can be adjusted to mimic velocity of a white corpuscle passing through
vasculature.
74. A method for entoptically perceiving and mapping the white blood cell
circulation and foveal area vasculature in the retina of the eye of a
human subject under examination with respect to the retinal point of
fixation of said eye, the method comprising:
(a) optically directing an image of a main light source within or near said
eye's entrance pupil so as to illuminate a portion of said eye's retina
defined by a field stop, said main light source image being of variable
intensity;
(b) causing said main light image to move along a main light path in space,
said path resulting in said eye's retinal illumination angle changing with
time;
(c) optically directing a fixation light source on said retina, resulting
in the formation of a fixation light retinal image, said fixation light
source being of variable intensity;
(d) having said subject visually fixate on said fixation light retinal
image;
(e) having said subject provide descriptions of entoptically perceived
vascular features of interest near said eye's foveal area;
(f) optically directing a tracking light source on said eye's retina,
resulting in the formation of a tracking light retinal image, said
tracking light retinal image being movable over said retinal surface with
respect to said fixation light retinal image through a tracking light
image controller operated by said subject;
(g) computing scaled outputs of said tracking light image controller to
correspond to distances and angles measured on said retinal surface;
(h) having said subject move said tracking light retinal image along or
around said entoptically perceived vascular features of interest while
said subject maintains constant translational and rotational alignment of
said eye with said fixation light retinal image;
(i) using said scaled outputs of said tracking light retinal image
controller to create position reports of said entoptically perceived
vascular features of interest on said retina;
(j) combining said position reports with said descriptions of said
subject's entoptically perceived vascular features of interest to create a
retinal map;
(k) observing said eye to detect misalignment with the instrumentation or
said fixation light retinal image due to any combination of translation or
rotation of said eye with respect to said fixation light retinal image;
(1) determining corrections required to said position reports resulting
from said misalignment of said eye;
(m) applying said corrections to said position reports;
(n) illuminating said eye's retina in alternation with said main light
source or constantly with a blue-field light source, said blue-field light
source being of variable intensity and, if alternating, having a duty
cycle of less than 100%;
(o) having said subject report entoptic perception of said white blood cell
circulation;
(p) causing said fixation light and tracking light images to be viewed by
said eye through an optical field stop aperture, said aperture being
imaged at infinity or other plane of interest; and
(q) having said subject adjust one or both of intensity and duty cycle of
said light sources for best entoptic perception;
75. The method of claim 74 wherein said main light path in space is
circular.
76. The method of claim 74 wherein said main light path in image space is
circular, about 2 to 6 mm in diameter, and about centered in the eye's
pupil.
77. The method of claim 74 wherein said main light path in image space is
circular and 4 mm in diameter and about centered in the eye's pupil.
78. The method of claim 74 wherein said main light path in space is
circular, being retraced at the rate of about 0.5 to 10 Hz.
79. The method of claim 74 wherein said main light path in space is
circular, being retraced at the rate of about 3.5 Hz.
80. The method of claim 74 wherein said aperture stop of step (p) is that
of an adjustable iris diaphragm.
81. The method of claim 74 wherein said illuminated portions of said eye's
retina by said main light source in step (a) comprise a circle and said
fixation light retinal image is within said circle.
82. The method of claim 74 wherein illuminated portions of said eye's
retina by said main light source comprise a circle and said fixation light
retinal image is centered within said circle.
83. The method of claim 74 wherein said main light source has a peak
wavelength of between 430 nm and 555 nm and a half band pass of about +/-
60 nm.
84. The method of claim 74 wherein said main light source has a peak
wavelength of about 470 nm and a half band pass of about +/- 60 nm.
85. The method of claim 74 wherein said main light source image is circular
with a diameter of 0.5 to 3 mm.
86. The method of claim 74 wherein said main light source image is circular
with a diameter of about 1.0 mm or less.
87. The method of claim 74 wherein said main light source image is focused
in or near said eye's entrance pupil plane.
88. The method of claim 74 wherein said main light source image is focused
in or near said eye's anterior focal plane.
89. The method of claim 74 wherein said main light source image is diffuse
and of uniform intensity.
90. The method of claim 74 wherein said determining of corrections of said
position reports is made by a human operator.
91. The method of claim 74 wherein said determining of corrections of said
position reports is made by an automatic computer.
92. The method of claim 74 wherein application of said corrections to
position reports is made by a human operator.
93. The method of claim 74 wherein application of said corrections to
position reports is made by an automatic computer. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
The present invention relates to use of a psychophysical method and
apparatus for entoptically evaluating and mapping the human foveal area
vasculature with respect to the retinal point of fixation (RPF). Knowing
the location of the RPF is essential in modern ophthalmic surgery because
it can be inadvertently damaged during photocoagulation, resulting in
marked vision loss. Unfortunately, the RPF is not visible on direct
examination of the eye; its location must be determined subjectively and
then related to observable landmarks. The present invention uses
well-known techniques of entoptic visualization in a novel way to
accomplish this goal.
While entoptic (referring to visual phenomena having their seat within the
eye) observations of retinal vessel shadows were recorded more than 100
years ago, these subjective sensations have thus far found limited use in
modern medical practice. In part, this is because convenient objective
photographic methods have been developed for studying the retinal
vasculature, thus relegating subjective observations (with their lack of
objective controls) to a secondary role. In practice, for example,
entoptic perceptions of the geometric patterns of blood vessels have been
described only schematically, while fluorescein angiography has produced a
wealth of detailed photographs. But what is lacking in the photographs
(and other direct observations) as well as the entoptic perceptions to
date is quantification of the location of the RPF with respect to the
vasculature architectures.
Instead of the RPF, what is evident on most retinas (and what is usually
assumed to be concentric with the RPF) is the foveal avascular zone (FAZ),
an area surrounding the fovea where retinal vessels are absent or
decreased. Recent research has shown that the FAZ and the RPF are not, as
previously assumed, always concentric. The assumption has persisted
because it is approximately correct in most people and, prior to the
present invention, there was no convenient way to check it in a given
patient. Now that a simple, accurate mapping system has been developed and
tested, there are data to suggest that therapeutic failures following
photocoagulation (defined as loss of 6 or more lines of visual acuity) may
be reduced up to 20% in specific groups of patients having preoperative
mapping of the retinal vasculature with respect to the RPF.
Studies of the Foveal Avascular Zone (FAZ)
Techniques to study the FAZ can be classified into 3 categories: anatomic,
angiographic, and psychophysical. Anatomic studies in human and other
primates include whole mount and flat mount following trypsin digestion,
and injection with india ink, neoprene latex and derivatives of
methacrylic esters. While anatomic studies often provide eloquent detail
of the vasculature surrounding the foveal area, they do not allow in vivo
monitoring of changes in the vasculature and may be misleading. For
example, latex injection under pressure may open anatomic connections
which are not operative under normal physiologic conditions.
Fluorescein angiography, on the other hand, is generally accepted as the
standard procedure for in vivo study of the human retinal vasculature, but
it is invasive and not generally repeated daily or even weekly.
Furthermore, to obtain the capillary detail necessary to study the FAZ and
the vasculature near the fovea requires clear optical media and skilled
photographic personnel. And even if photographic conditions are ideal, the
angiographic detail of the foveal area vasculature may be variable in
quality, depending on the density of the macular pigment and variations in
normal fundus pigmentation.
Nevertheless, fluorescein angiography as well as angiography with other
dyes have been used extensively to study the retinal vasculature and FAZ
in both healthy and diseased eyes in vivo. Laatikainen and Larinkari (1)
reported FAZ diameters around 0.57 mm for 167 eyes of 158 healthy patients
(mean=0.572, range 0.23 mm to 0.83 mm). Bresnick et al. (2), in a study of
the FAZ in diabetics, reported FAZ diameters between 0.58 and 1.00 mm with
a mean of 0.73 mm for the normal control group (non-diabetic). Together
these findings are consistent with the anatomic findings of Bligard et al.
(3) where post-mortem human eye FAZ diameters were reported to range from
0.12 to 1.2 mm (mean 0.65 mm) using trypsin-digest. In diseased eyes, the
FAZ has been reported to be smaller than normal in patients with
cicatricial retinopathy of prematurity and larger than normal in vascular
occlusive diseases such as diabetes, sickle cell retinopathy, talc embolic
retinopathy and retinal branch vein occlusion. Taken together, all of
these data provide a basis for comparison with retinal maps made possible
by the psychophysical techniques of the present invention. Psychophysical
procedures, however, can provide data unobtainable with angiography.
For example, even in the presence of cloudy ocular media, viewing a bright
uniform blue field (430 nm) allows the entoptic visualization of
leucocytes ("flying corpuscles") in the retinal capillaries surrounding
the foveal area. Careful observation of the phenomenon reveals an area
apparently centered on the RPF where no leucocytes are seen; presumably
the FAZ. Yap et al. capitalized on this phenomenon to measure, in one eye
of 22 normal subjects, FAZ diameters ranging between 1.92 and 2.86 degrees
(0.59 to 0.83 mm on the retina assuming a secondary nodal point-to-retina
distance of 16.67 mm). Earlier estimates using the same entoptic phenomena
found the diameter of the FAZ to be approximately 1.5 degrees as measured
in object space or 0.44 mm on the retina (Weale (4) quoted by Dartnall and
Thomson (5)). But, while entoptic visualization of leucocytes provides a
non-invasive method for making inferences about the FAZ and the
vasculature of the foveal area, it does not provide a view of the retinal
vessels themselves.
Direct entoptic visualization of the retinal vasculature can be achieved by
allowing light to enter the eye from unusual or constantly varying angles.
This effect, first noted by Purkinje in 1819 (6), is strikingly distinct
and often spontaneously reported by patients during routine
ophthalmoscopy. Bird and Weale (7), using both fluorescein angiography and
entoptic visualization of the retinal vasculature by scleral
trans-illumination, noted that not all normal individuals with excellent
visual acuity have FAZs which are truly avascular. They point out that
unless extreme care is taken during the entire photographic process,
vascular details within the FAZ may not be imaged (or seen) with
fluorescein angiography but are visible entoptically. These findings
corroborate the earlier fluorescein angiographic work of Yeung et al. (8)
and emphasize the potential sensitivity of entoptic viewing of the central
retinal vasculature.
Clinically, entoptic visualization has long been used to help evaluate the
functional status of the retina behind obstructed media. More recently it
has been used as a guide to train eccentric fixators to improve fixation,
and to study the normal variation in the size and shape of the FAZ. To the
best of the present inventors' knowledge, only one study has used entoptic
visualization to monitor an active disease state. Kluxen and Wilden (9)
taught 136 insulin-dependent diabetics how to observe their retinal
vasculature entoptically. In patients with 1-5 microaneurysms, as revealed
by fluorescein angiography, 55% could entoptically detect their own
pathology. In patients with 6-20 microaneurysms the percentage increased
to 77%. In patients with greater than 20 microaneurysms with severe
background and proliferative retinopathy, 90% could reliably detect their
own pathology and many could document the appearance of new and
disappearance of old microaneurysms over time.
While entoptic visualization of the retinal vasculature is impressive in
its apparent detail, capturing this detail in a quantifiable manner is
difficult. First, entoptic visualization is subjective by nature. Second,
foveation of the variety of intricacies of the vascular detail is
impossible because the entoptic image remains fixed with respect to the
retina (i.e, the location of the retinal vasculature is fixed with respect
to the photoreceptors; therefore, eye movements cannot foveate the vessel
of interest.). Together these effects have limited the usefulness of this
phenomenon. To minimize these problems, the present invention includes an
attempt to enhance stimulus effectiveness by presenting the test stimulus
in Maxwellian view and optimizing stimulus movement. The use of Maxwellian
view for entoptic visualization of the retinal vasculature was first
alluded to by Helmholtz (10) in his Treatise on Physiological Optics
where, in discussing entoptic visualization of the retinal vasculature, he
said:
The. . . vascular figure may be seen also by looking through a compound
microscope with nothing upon the stage, the background being the uniformly
bright circular aperture of the diaphragm. When the eye moves to and fro a
little at the ocular, the slender retinal blood vessels appear sharply
delineated in the field, particularly those running at right angles to the
direction of the motion, whereas the others vanish that are parallel to
this direction.
Helmholtz goes on to point out the importance of the size of the Maxwellian
view exit pupil on shadow formation by stating:
If the pupil is perfectly free, and the eye is turned towards the bright
sky, every point of the pupillary plane may be considered as a source of
light sending rays in all directions to the fundus of the eye, just as if
the pupil itself were a luminous surface. The result is that the blood
vessels of the retina project broad hazy shadows on the parts of the
retina immediately behind them, the length of the umbra being only about
four or five times the diameter of the blood vessel. . . . Hence it may be
assumed that the umbra of the vascular shadow does not reach the posterior
surface of the retina at all. But when the light enters the eye through a
narrow aperture in front of the pupil, the shadow of the blood vessel is
necessarily smaller and more sharply defined, and since the umbra is
longer, parts of the retina that were formerly partially shaded are now
completely shaded, while other adjacent parts are not shaded at all.
Thus, the principles of entoptic visualization have been described, but
prior to the present invention, no device had been built or proposed to
optimize the patient's view of the retinal vessel pattern surrounding the
RPF and locate the RPF precisely on a retinal map. The need for this
information, however, is substantial and growing.
Such data would be particularly useful for eye surgeons who, during
photocoagulation therapy, focus high power laser beams on the retina by
using the retinal vessels as landmarks. It is most important for the
surgeon to avoid burning the retinal point of fixation (to avoid vision
loss), but that point is subjectively determined by the patient in all
cases and cannot reliably be assumed to lie at the center of the FAZ or
the anatomical fovea. Thus, the present invention fills an important need
and provides a significant safety factor for the increasing number of
patients who could benefit from foveal area photocoagulation therapy.
Anatomy and Shadows Within the Eye
Within the portion of the retina resting on the choroid (pars optica),
several layers can be distinguished; FiG. 1 shows them schematically. At
the posterior pole, the distribution of retinal components is altered to
form highly specialized structures which maximize visual acuity, the
anatomical fovea and foveola. In the center of the fovea, the inner
retinal layers down to the outer nuclear layer are displaced, forming a
pit, the foveola, which contains the highest density of cones in the
retina (147,000 cones per square mm). The retinal capillaries of the area
of the fovea typically form concentrically arranged channels ending in a
capillary loop approximately 0.5 mm across which outlines a capillary-free
zone, the FAZ. The lateral displacement of inner retinal structures,
including the retinal vasculature, presumably exists to leave an
unobstructed light path to the site of phototransduction, thereby
enhancing image quality.
In a normal observer, visual performance is maximized at the subject's
point of fixation. That is, when asked to fixate a point in space, normal
observers move their eyes such that the point of interest is imaged on the
retinal region providing the highest resolution. The connection between
fixation and optimal resolution, together with the histological
specialization of the retina, suggest that a normal individual will move
the eye to place the image of the RPF on the foveola. Thus the RPF and the
foveola are commonly assumed to be coincident and reasonably centered in
the FAZ. But, experimental use of the present invention has brought this
assumption into sharp question for a significant minority of patients.
Using the methods and apparatus taught herein, these patients may be
identified and their treatment appropriately modified to maximize
preservation of vision. The guideposts in this process are vascular
shadows subjectively perceived by the patient and their relationship to
the RPF.
Since the retinal vasculature lies anterior to the photoreceptors, shadows
of the vasculature are cast in the plane of the photoreceptors. Under
normal viewing and lighting conditions the vascular shadows of all but the
largest vessels have low contrast and all are effectively stabilized with
respect to the photoreceptors. Since patterns which are stabilized in the
plane of the receptors fade and become invisible, vascular shadows are not
perceived under normal lighting and viewing conditions.
Entoptic visualization of the vascular shadows can be achieved by
increasing shadow contrast and breaking shadow stabilization. Contrast can
be increased by placing a small light source in the eye's entrance pupil
and shadow stabilization can be broken by changing the retinal angle of
incident light by constantly moving the light source. There are at least
four parameters of the vessel shadow pattern in the plane of the entrance
aperture of the photoreceptors which will effect shadow visibility: (1)
the width of the shadows; (2) the contrast of the shadows; (3) the spacing
of the shadows; and (4) the speed, and path of shadow movement. In the
present invention, the first goal was to design an illumination procedure
that optimizes these parameters and renders the vascular bed surrounding
the fovea easily visible.
SUMMARY OF THE INVENTION
The present invention includes an apparatus for entoptically perceiving and
mapping the foveal area vasculature of the retina of a human subject's
eye. The apparatus comprises a means to establish and maintain
translational and rotational alignment of said eye with said apparatus.
Such means may be an affixed bite bar preferably of dental | | |
