Vascular entoptoscope - Patent Review 5360010

Claims

We claim:

1. An apparatus for entoptically perceiving the macular area retinal vasculature of a human subject's eye under examination, the apparatus comprising:

(a) a light beam source; and

(b) a means of continuously directing a light beam from said source into a subject's eye, such that with time the light beam illuminates a same retinal area from constantly varying angles.

2. The apparatus of claim 1 wherein the light beam from said source is variable in intensity and shape.

3. The apparatus of claim 1 wherein the light beam source is a light generator or a light field narrowed through a constraining aperture.

4. An apparatus for entoptically perceiving the macular area retinal vasculature of a human subject's eye under examination, the apparatus comprising:

(a) a means of directing a light beam into a subject's eye, at angles such that the angle of illumination of said eye's retina by the light beam changes with time; and

(b) a means to image an aperture of fixed or variable size and shape at optical infinity or other plane of interest as an optical field stop for said apparatus.

5. The apparatus of claim 4 wherein the light beam is of variable intensity and shape.

6. An apparatus for entoptically perceiving the macular area retinal vasculature of a human subject's eye under examination, the apparatus comprising:

(a) a means of directing a light beam into said eye, at angles such that the angle of illumination of said eye's retina changes with time;

(b) a means to image an aperture of fixed or variable size and shape at optical infinity or other plane of interest as an optical field stop for said apparatus; and

(c) a luminous or non-luminous fixation point with a means to image said fixation point on the retina of said eye.

7. The apparatus of claim 6 wherein the field stop is sectionalized to facilitate location of vessels or vessel defects with respect to the fixation point.

8. The apparatus of claim 6 wherein a movable luminous or non-luminous point is contained in the optical field stop.

9. The apparatus of claim 1, 4 or 6 wherein a means is provided to correct for refractive error of said eye.

10. An apparatus for entoptically perceiving the macular area retinal vasculature of a human subject's eye under examination, the apparatus comprising:

(a) a means of directing a light beam into said eye at angles such that the angle of illumination of said eye's retina by the beam changes with time;

(b) a means to image an aperture at optical infinity or other plane of interest as an optical field stop for said apparatus, said aperture being of fixed or variable size and shape; and

(c) a luminous or non-luminous fixation point with a means to image said fixation point on the retina of said eye.

11. The apparatus of claim 10 wherein said light beam is directed in a circular path 2 to 6 mm in diameter.

12. The apparatus of claim 11 wherein said circular path is 3.5 mm in diameter and about centered in said eye's pupil.

13. The apparatus of claim 11 wherein said light beam is directed in an irregular path of greatest dimension about 2 to 6 mm such that all angles of retinal illumination obtainable with a circular path are present.

14. An apparatus for entoptically perceiving the macular area retinal vasculature of a human subject's eye under examination, the apparatus comprising:

(a) a means of directing said light beam into said eye in a circular path 2 to 6 mm in diameter and about centered in the eye's pupil, said beam being retraced at the rate of about 0.5 to 10 Hz, resulting in an area of retinal illumination in which the angle of illumination of said eye's retina changes with time;

(b) a means to image an aperture at optical infinity or other plane of interest as an optical field stop for said apparatus, said aperture being of fixed or variable size and shape; and

(c) a luminous or non-luminous fixation point with a means to image said fixation point on the retina of said eye.

15. The apparatus of claim 14 wherein said light beam is directed in an irregular path of greatest dimension about 2 to 6 mm such that all angles of retinal illumination obtainable with a circular path are present.

16. The apparatus of claim 14 wherein said circular path is retraced at the rate of about 3.5 Hz.

17. The apparatus of claim 14 wherein said aperture is that of an adjustable iris diaphragm.

18. The apparatus of claim 14 wherein said eye's area of retinal illumination comprises a circle and said fixation point is centered within said circle.

19. An apparatus for entoptically perceiving the macular area retinal vasculature of a human subject's eye under examination, the apparatus comprising:

(a) a means of directing a light beam into said eye in a circular path 2 to 6 mm in diameter and about centered in the eye's pupil, said beam being retraced at the rate of about 0.5 to 10 Hz, and said light having a peak wavelength of about 430 to 555 nm and a half band pass of 0 to 100 nm, resulting in the angle of illumination of said eye's retina changing with time;

(b) a means to image an aperture at optical infinity or other plane of interest as an optical field stop for said apparatus, said aperture being of fixed or variable size and shape; and

(c) a luminous or non-luminous fixation point with a means to image said fixation point on the retina of said eye.

20. The apparatus of claim 19 wherein said light beam is directed in an irregular path of greatest dimension about 2 to 6 mm such that all angles of retinal illumination obtainable with a circular path are present.

21. The apparatus of claim 19 wherein said light beam has a peak wavelength of about 470 nm and a half band pass of .+-. about 60 nm.

22. An apparatus for entoptically perceiving the macular area retinal vasculature of a human subject's eye under examination, the apparatus comprising:

(a) a means of directing a light beam into said eye through a constraining aperture about 0.1 to 3 mm diameter;

(b) a means of directing a light beam into said eye in a circular path 2 to 6 mm in diameter and about centered in the eye's pupil, said beam being retraced at the rate of about 0.5 to 10 Hz, and said light having a peak wavelength of about 430 to 555 nm and a half band pass of 0 to 100 nm, resulting in an angle of illumination of said eye's retina changing with time;

(c) a means to image an aperture at optical infinity or other plane of interest as an optical field stop for said apparatus, said aperture being of fixed or variable size and shape; and

(d) a luminous or non-luminous fixation point with a means to image said fixation point on the retina of said eye.

23. The apparatus of claim 22 wherein the constraining aperture is an exit pupil.

24. The apparatus of claim 22 wherein said constraining aperture is of irregular shape with greatest dimension about 0.1 to 3 mm.

25. The apparatus of claim 22 wherein said constraining aperture is of irregular shape with greatest dimension about 1 mm.

26. The apparatus of claim 22 wherein said constraining aperture is imaged between said eye's retina and said eye's anterior focal plane.

27. The apparatus of claim 22 wherein said light beam enters said eye through a rotating constraining aperture placed in front of said eye.

28. The apparatus of claim 22 wherein said light beam is diffuse and of uniform intensity.

29. An apparatus for entoptically perceiving and mapping the macular area retinal vasculature 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 light source of variable intensity illuminating an aperture which is imaged in or near the eye's entrance pupil plane to form an exit pupil of the apparatus and a means of moving said exit pupil along a path in space;

(c) a means of imaging said exit pupil into said eye's entrance pupil at angles such that the angle of illumination of an illuminated area of said eye's retina changes with time;

(d) a means to image an aperture stop 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 luminous or nonluminous fixation point and a means to image said fixation point on the retina of said eye;

(f) a luminous or nonluminous tracking point and a means to form an image of the tracking point on the retina of the eye;

(g) a means of moving said tracking point retinal image with respect to said fixation point retinal image;

(h) a means of transducing movement of said tracking point retinal image to yield coordinates of its location on said eye's retina with respect to said fixation point retinal image;

(i) a means of compiling or displaying coordinates of said tracking point retinal image movement, said compilation or display comprising a map of tracking point retinal image positions with respect to said fixation point retinal image; and

(j) a means to detect and indicate magnitude and direction of translation of said eye with respect to said apparatus.

30. The apparatus of claim 29 wherein the aperture in step (b) is an entrance pupil of the apparatus.

31. The apparatus of claim 29 wherein the tracking point is a light source.

32. The apparatus of claim 29 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.

33. The apparatus of claim 29 wherein path of movement of the apparatus exit pupil is circular, about 2 to 6 mm in diameter and about centered in the eye's pupil.

34. The apparatus of claim 29 wherein the path of movement of the apparatus exit pupil is circular, 4 mm in diameter and about centered in the eye's pupil.

35. The apparatus of claim 29 wherein the path of movement of the apparatus exit pupil is circular, being retraced at the rate of about 0.5 to 10 Hz.

36. The apparatus of claim 29 wherein the path of movement of the apparatus exit pupil is circular, being retraced at the rate of about 3.5 Hz.

37. The apparatus of claim 29 wherein said aperture stop is that of an adjustable iris diaphragm.

38. The apparatus of claim 29 wherein the area of retinal illumination is a circle and said fixation point retinal image is within said circle.

39. The apparatus of claim 29 wherein the area of retinal illumination is a circle and said fixation point retinal image is centered within said circle.

40. The apparatus of claim 29 wherein said light source has a peak wavelength of about 430 to 555 nm and a half band pass of 0 to 100 nm.

41. The apparatus of claim 29 wherein said light source has a peak wavelength of about 470 nm and a half band pass of about .+-.60 nm.

42. The apparatus of claim 29 wherein the exit pupil of the apparatus is circular with a diameter of about 0.1 to 3 mm.

43. The apparatus of claim 29 wherein the exit pupil of the apparatus is circular with a diameter of about 1.0 or less.

44. The apparatus of claim 29 wherein the exit pupil of the apparatus is imaged in or near said eye's entrance pupil plane.

45. The apparatus of claim 29 wherein the exit pupil of the apparatus is imaged between said eye's anterior focal plane and the said eye's retina.

46. The apparatus of claim 29 wherein said light source is diffuse and of uniform intensity.

47. The apparatus of claim 29 where said indication of magnitude and direction of translation of said eye with respect to said apparatus in step (j) is visible.

48. The apparatus of claim 29, where said indication of magnitude and direction of translation of said eye with respect to said apparatus in step (j) is sensed by an external operator.

49. The apparatus of claim 29 where said indication of magnitude and direction of translation of said eye with respect to said apparatus in step (j) is detected by a sensor and monitored by a computer.

50. The apparatus of claim 29 wherein tracking point retinal image coordinates of step (h) are corrected manually for retinal image translation caused by any translation of said eye with respect to said apparatus.

51. The apparatus of claim 29 wherein tracking point 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.

52. The apparatus of claim 29 wherein tracking point retinal image coordinates in step (i) are calibrated in units of length measured on said retinal surface.

53. The apparatus of claim 29 wherein tracking point retinal image coordinates in step (i) are calibrated in units of angular subtense.

54. The apparatus of claim 29 wherein said means of moving said tracking point retinal image of step (h) is a joystick or similar x-y controller.

55. An apparatus for entoptically perceiving and mapping white blood cell circulation and macular retinal vasculature of a human subject's eye under examination, the apparatus comprising:

(a) a means to establish and maintain translational alignment of a subject's eye with the apparatus;

(b) a light source of variable intensity illuminating an aperture which is imaged in or near the eye's entrance pupil plane to form an exit pupil of the apparatus, and a means of moving said exit pupil along a path in space;

(c) a means of imaging said device apparatus exit pupil into said eye's entrance pupil at angles such that the angle of retinal illumination of said eye's retina changes with time;

(d) a means to image an aperture stop at optical infinity or other plane of interest to correct for refractive error as an optical field stop for said apparatus, said aperture being of variable size and shape;

(e) a luminous or nonluminous fixation point and a means to image said fixation point on the retina of said eye;

(f) a luminous or nonluminous tracking point and a means to form an image of the tracking point on the retina of the eye;

(g) a means of moving said tracking point retinal image with respect to said fixation point retinal image;

(h) a means of transducing movement of said tracking point retinal image to yield coordinates of its location on said eye's retina with respect to said fixation point retinal image;

(i) a means of compiling or displaying said coordinates of said tracking point retinal image movement, said compilation or display comprising a map of said tracking point retinal image positions with respect to said fixation point 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 of variable intensity and a means to illuminate said eye's retina with said blue-field light source;

(l) a luminous or nonluminous speed-comparator and a means to form a retinal image of said speed-comparator on said eye's retina, said speed-comparator 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 fixed or variable path on said eye's retina at a fixed or variable velocity; and

(n) a means of rotating and translating said speed comparator retinal image on said eye's retina.

56. The apparatus of claim 55 wherein the exit pupil of the apparatus has a circular path of movement about 2 to 6 mm in diameter and about centered in the eye's pupil.

57. The apparatus of claim 55 wherein the exit pupil of the apparatus has a circular path of movement 4 mm in diameter and about centered in the eye's pupil.

58. The apparatus of claim 55 wherein the exit pupil of the apparatus has a circular path of movement which is retraced at the rate of about 0.5 to 10 Hz.

59. The apparatus of claim 55 wherein the exit pupil of the apparatus has a circular path of movement which is retraced at the rate of about 3.5 Hz.

60. The apparatus of claim 55 wherein said aperture stop is that of an adjustable iris diaphragm.

61. The apparatus of claim 55 wherein said eye's area of retinal illumination is a circle and said fixation point retinal image is within said circle.

62. The apparatus of claim 55 wherein said eye's area of retinal illumination comprises a circle and said fixation point retinal image is centered within said circle.

63. The apparatus of claim 55 wherein said light source has a peak wavelength of about 430 nm and 555 nm and a half band pass of 0 to 100 nm.

64. The apparatus of claim 55 wherein said light source has a peak wavelength of about 470 nm and a half band pass of about .+-.60 nm.

65. The apparatus of claim 55 wherein the exit pupil of the apparatus is circular with a diameter of about 0.1 to 3 mm.

66. The apparatus of claim 55 wherein the exit pupil of the apparatus is circular with a diameter of about 1.0 mm or less.

67. The apparatus of claim 55 wherein the exit pupil of the apparatus is imaged in or near said eye's entrance pupil plane.

68. The apparatus of claim 55 wherein the exit pupil of the apparatus is imaged between said eye's anterior focal plane and said eye's retina.

69. The apparatus of claim 55 wherein said light source is diffuse and of uniform intensity.

70. The apparatus of claim 55 where said indication of magnitude and direction of translation of said eye with respect to said apparatus in step (j) is visible.

71. The apparatus of claim 55 where said indication of magnitude and direction of translation of said eye with respect to said apparatus in step (j) is sensed by an external operator.

72. The apparatus of claim 55 where said indication of magnitude and direction of translation of said eye with respect to said apparatus in step (j) is detected by a sensor and monitored by a computer.

73. The apparatus of claim 55 wherein said tracking point retinal image coordinates of step (h) are corrected manually for retinal image translation caused by translation of said eye with respect to said apparatus.

74. The apparatus of claim 55 wherein said tracking point 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.

75. The apparatus of claim 55 wherein said tracking point retinal image coordinates in step (i) are calibrated in units of length measured on said retinal surface.

76. The apparatus of claim 55 wherein said tracking point retinal image coordinates in step (i) are calibrated in units of angular subtense.

77. The apparatus of claim 55 wherein said means of moving said tracking point retinal image of step (h) is a joystick or similar x-y controller.

78. The apparatus of claim 55 wherein said blue-field light source has a dominant wavelength of about 430 to 500 nm.

79. The apparatus of claim 55 wherein light from said blue-field light source is directed coaxially with the instrument's optical axis.

80. The apparatus of claim 55 wherein said blue-field light source is about 50% of total light.

81. The apparatus of claim 55 wherein said blue-field light is applied to said retina constantly or intermittently in alternation with the light source of step (a) at a rate which minimizes perceptual flicker and having a duty cycle variable to optimize perception of both retinal vessels and white blood cells.

82. The apparatus of claim 55 wherein said retinal path of said speed-comparator light image is curved to mimic the course of a retinal vessel.

83. The apparatus of claim 55 wherein said retinal path of said speed-comparator light image is straight.

84. The apparatus of claim 55 wherein said retinal path of said speed-comparator retinal image is about 10.sup.-4 to 10.sup.-3 m in length.

85. The apparatus of claim 55 wherein said velocity of the speed comparator can be adjusted to mimic velocity of a white corpuscle passing through vasculature.

GENERAL

The present application relates to a psychophysical method and apparatus for entoptically evaluating and mapping the human macula area vasculature with respect to the retinal point of fixation (RPF), as well as low-cost, simpler devices (i.e., no automatic hard copy provided) for self-visualization of the retinal vasculature or abnormalities thereof. Both mapping and simpler instruments incorporate similar principles and techniques as described herein to optimize the visual percept of the retinal vasculature and vascular abnormalities. In all embodiments of the Vascular Entoptoscope, light from a short wavelength source enters the eye through a small aperture, moving along a regular or irregular path at an optimized velocity. The essential difference between mapping and simpler instruments lies in the requirement to establish and monitor precise eye-instrument alignment and data display for accurate location of the RPF macular area retinal vascular defects.

MEDICAL APPLICATIONS

Knowing the precise location of the RPF is essential in modern ophthalmic surgery because the RPF 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.

A therapeutic trend in Ophthalmology is to photocoagulate (treat by burning) retinal abnormalities closer and closer to the retinal point of fixation (RPF). However, since treatment to the RPF can blind the eye, protocols advocate avoiding direct treatment to the RPF if at all possible. Since an examiner can not visualize the exact location of the RPF, protocols advocate using the center of the foveal avascular zone (FAZ) as a guiding landmark for RPF localization. That is, treatment should avoid the center of the FAZ on the assumption that the RPF is located at the center of the FAZ. Recent evidence using a mapping version of the Vascular Entoptoscope revealed that the RPF is not always centered within the FAZ (in fact not all eyes even have a FAZ). As a result, photocoagulation treatment in the foveal area may actually be causing blindness by burning the RPF unintentionally in about 20% of the cases where foveal area photocoagulation is the treatment of choice. This specific application of the Vascular Entoptoscope is discussed in detail in the section labelled "Alternative Methods and Prior Art" on page 10.

An important use for both precise mapping and simpler versions of the present invention will be in monitoring and treatment of diabetic retinopathy. Diabetes is the leading cause of blindness among working-age Americans, causing 8,000 new cases of blindness and 65,000 new cases of proliferative eye disease each year. In 50% to 84% of the cases, laser photocoagulation can prevent (or markedly reduce) adverse consequences of the proliferative stage of diabetic retinopathy. Receiving proper and timely treatment not only saves vision but reduces the socio-economic cost of diabetes-related visual impairment, currently estimated at $75 billion per year.

How is the Vascular Entoptoscope useful in the context of the diabetic? Current recommendations for medical care of diabetics include routine eye exams every 6 months to monitor the state of retinal vascular disease; however, many diabetics do not comply. If patients do not actually experience adverse effects (generally a visual acuity loss), many will assume nothing is wrong and conclude that a routine eye exam is a waste of time and money; this attitude can and often does lead to blindness. By the time a patient notices vision loss, significant damage has usually already occurred. Further, even for patients receiving regular care, the disease may progress undetected between appointments. Regular retinal examinations using the Vascular Entoptoscope, however, could alleviate these problems by allowing both convenient self-examination and more accurate and timely laser photocoagulation.

Why are regular exams so important and diagnostically significant for the diabetic patient and why is the Vascular Entoptoscope a significant improvement over current techniques? Regular eye exams are routinely performed because diabetes is a vascular disease and eye care specialists can monitor the vascular disease by directly observing the eye's vascular supply. In no other part of the body are working vessels so easily viewed without invasive procedures. The Vascular Entoptoscope will allow patients to monitor his or her own vessels down to the smallest 7.mu. capillaries as often as they like. These capillaries are about 1/3 the size of the smallest vessels an eye care specialist can see by looking into the eye with an ophthalmoscope. Yet they are easily observed with the simpler embodiments of the present invention; sophisticated eye alignment systems are not required. Inexpensive (simpler) Vascular Entoptoscopes can thus be used in screening clinics or as take-home units, detecting the presence or absence of retinal vascular abnormalities (including changes in the size of the foveal avascular zone), which can then be evaluated more completely by a professional follow-up exam. If an abnormality is detected, the patient can approximately locate the vascular abnormality and direct the examiner's attention to that area. If treatment is warranted, the mapping Vascular Entoptoscope may be used to ensure maximum protection is given to the RPF.

MUSEUM APPLICATION

While our laboratory work at first focused on medical applications, the first potential buyer of a Vascular Entoptoscope was a museum. It was believed that seeing one's own smallest capillaries in a noninvasive fashion would be an excellent hands-on display that museum patrons would enjoy. In view of this, simpler versions of the device were designed as suitable for a museum environment (e.g., they do not contain sophisticated alignment and mapping systems). Such a device operates on the identical fundamental design principles of the more sophisticated Vascular Entoptoscope originally built.

NOVELTY APPLICATION

In developing the medical and museum applications as well as working with museum staff in developing the display, it became clear that if an inexpensive simple, hand-held, take-home device could be developed, it would not only have a significant medical market but would also have significant appeal as a novelty item. Therefore, we have attempted to reduce the physical embodiment of the fundamental design principles of the original vascular entoptoscope to simpler hand-held, low cost, take-home devices suitable for personal use and novelty markets.

DATA ON NORMAL PATIENTS

The Vascular Entoptoscope subsystem has been used to test normal and diabetic eyes. All subjects easily saw the Purkinje image of their retinal capillaries. Ten of the 14 normal subjects tested to date graphed details of the shadow of their FAZ in both eyes, and 2 (due to personal time constraints), in only one eye. Another subject observed a traditional FAZ in one eye, but saw capillaries running through what should have been the FAZ in the other; and one subject saw capillaries running through the fixation point in both eyes. FIG. 19 (a-d) depicts a sample of the variety of FAZ tracings obtained. FIG. 19a displays the tracing of one of only 3 eyes with a retinal point of fixation located in the geographic center of the FAZ as classically described anatomically. FIG. 19b displays a tracing from an eye with the retinal point of fixation located a typical distance from the geographic center of the FAZ, whereas FIG. 19c displays the tracing of the subject with the largest distance (189.mu.) between the retinal point of fixation and the geographic center of the FAZ. FIG. 19d displays the tracing of one of 3 eyes with vessels in the retinal area more commonly occupied by the FAZ. Even by casual observation, it becomes clear that the FAZ boundaries are not always concentric with the fixation point (FIGS. 19b and 19c).

All 23 eyes with FAZs had retinal fixation points located within the FAZ. However, only three eyes from three different subjects had their retinal points of fixation located at the geographic center of the FAZ. Vectors defining the distance from the geographic center of the FAZ to the subject's fixation point and the direction of deviation (with 0.degree. being horizontal to the right) were determined for each FAZ tracing. These distances were then converted to retinal distances using the Gullstrand reduced model eye with a nodal-point to retina distance of 16.67 mm after compensating for the optical magnification factor of the Maxwellian view optical system and the gain of the X-Y plotter. FIG. 20 shows use of a polar coordinate system to illustrate the location of the retinal point of fixation relative to geographic center of the FAZ for each eye tested. Note that while the data as a whole tends to cluster near the origin (i.e., the retinal point used for fixation tended to be nearer the center of the FAZ as opposed to the edge of the FAZ) the distribution of directions of deviation appear random. The largest deviation of the retinal point of fixation from the geographic center of the FAZ was 189.mu.. The average deviation from the geographic center across all subjects is 66.50.mu.. There was no tendency for the eccentricity of the retinal point of fixation to increase with increasing FAZ diameter.

This data indicate that the retinal point of fixation deviates from the geographic center of the FAZ by about 65.mu. (SD.+-.50.mu.) with a range of 0 to 190.mu.. These findings suggest that laser burns centered on retinal points less than 300.mu. from the geographic center of the FAZ run a significantly higher risk than intended of falling directly on or nearer to the retinal point of fixation. Further, the risk of burning the point of fixation can markedly increase as burns are placed closer to the geographic center of the FAZ. The implications of this finding are profound and find support in current literature.

To illustrate, assume that, as in recent clinical trials, retinal lesions up to 200.mu. from the FAZ center are treated with burns which overlap the lesion by up to 100+.mu.. Further, the data presented herein form a representative sample of the population for the location of the retinal point of fixation with respect to the geographic center of the FAZ (data points, FIG. 20). With these assumptions, 6 of 24 eyes (25%) have retinal points of fixation which are potentially vulnerable to being burned (FIG. 21, lightly shaded area). However, since photocoagulation treatment is generally limited to or slightly overlaps the area of frank pathology (e.g., neovascular membrane, histo-spot, etc.), it is likely that a series of burns will be placed only within the sector of the macular area containing the site of the lesion (FIG. 21, darkly shaded circles schematically show both a 200 and 100.mu. burn). Under these criteria and if the treatment sector of the macular area is limited to 90.degree., it is likely that for any one particular series of burns, one or two eyes out of our sample of 24 (4 to 8%) would have their retinal point of fixation adversely affected by photocoagulation therapy.

The obvious question arises as to what percentage of treatment failures (loss of 6 lines of visual acuity or more at first follow-up) can be accounted for by variations in the location of the point of fixation with respect to the geographic center of the FAZ. While this question cannot be definitively answered with the data collected to date, it is interesting to note that with argon laser treatment it has been reported that 9% of eyes treated for neovascular maculopathy (13), 9% of the eyes treated for macular area ocular histoplasmosis (14), and 10% of those eyes treated for macular area idiopathic neovascularization (15), lost 6 or more lines of visual acuity at first follow-up despite "successful" treatment of the pathology. Further, this order of magnitude of initial follow-up failure is not unique to argon treatment.

Studies using krypton laser therapy for histoplasmosis have reported a similar percentage of patients (8%) with a 6 line loss in visual acuity at first follow-up (16). The argument is further fueled by the fact that the best predictor of visual acuity loss despite adequate therapy is treatment proximity to the center of the FAZ (17). That is, when analysis is limited to eyes with lesions within 375.mu. of the FAZ center, between 8 and 33% of the eyes successfully treated lost 6 lines or more at first follow-up depending on the particular study. While this could be accounted for by assuming the lesions within 375.mu. are more likely to affect the retinal point of fixation, the data presented herein suggest another possibility. Given the uncertainties of thermal spread, dose specification, actual spot size in the plane of the retina, variation in pigment absorption and accuracy of burn placement with respect to desired location, the speculative estimate of 4 to 8% may indeed be an underestimate of the number of retinal fixation points at risk. Simply allowing for 50.mu. of uncertainty would raise the number of retinal fixation points at risk from 4 to 8% up to 16 to 20%. To the extent this analysis is correct, it suggests that therapeutic failures at first visit for eyes with retinal lesions between 200 and 375.mu. could be reduced by as much as 20% by using the actual retinal point of fixation as a reference as opposed to the geographic center of the FAZ.

In summary, the normal patient data collected using the Vascular Entoptoscope of the present invention indicate the retinal point of fixation is not always centered within the FAZ. Further, the deviations of the retinal point of fixation from the center of the FAZ can be large enough to jeopardize the retinal point of fixation during foveal area laser photocoagulation therapy which avoids the center of the FAZ as opposed to locating