WO1997039675A1

WO1997039675A1 - Ophthalmic apparatus and test method

Info

Publication number: WO1997039675A1
Application number: PCT/GB1997/000418
Authority: WO
Inventors: Andrew Mcnaught; Mark Westcott
Original assignee: Moorfields Eye Hospital
Priority date: 1996-04-22
Filing date: 1997-02-14
Publication date: 1997-10-30
Also published as: AU1802397A; GB9608295D0

Abstract

Apparatus and methods for the detection of visual field loss in humans characterized by projection of an annular stimulus (20) onto the retina (18).

Description

Ophthalmic Apparatus and Test Method The present invention concerns a test for visual field loss in humans and apparatus for performing the test.

The presence of disease of the eye which produces loss of all or part of the visual field may not be immediately apparent to the sufferer and treatment may be delayed, particularly if the visual loss is gradual and the disease has no other warning symptoms. This is true of chronic glaucoma which is an important cause of blindness worldwide. Other, less common, diseases involving the visual pathways through the brain may also produce gradual visual field loss. Such loss may be the only early symptom and ideally should be detected at the earliest opportunity.

Surveillance for the early features of asymptomatic ocular disease is performed by optometrists and primary care physicians. In the case of glaucoma, measurement of the intraocular pressure and ophthalmoscopic examination of the optic nerve head are the most commonly performed screening procedures. Unfortunately, the sensitivity and specificity of these tests either alone, or in combination, are low. It has been recommended that an estimation of the visual field should also be performed. However, even relatively simple automated perimetry test programmes are time consuming, require expensive equipment, a mains power supply and are difficult for some, particularly elderly patients, to perform.

There is a need for a test of visual function which is rapid, simple to perform, which shows sensitivity and specificity which exceed that of the current non- perimetric methods and may be performed using portable equipment which is familiar to the examiner.

The present invention provides a method of fundus perimetry comprising the steps of:

(a) projecting an annular stimulus on the retina of a patient's eye; and

(b) requesting the patient to report what is seen.

The request for the patient to report what is seen may, obviously, be made before, during, or after projection of the annular stimulus so long as a report is obtained.

Advantageously, the annular stimulus has an outer diameter which subtends a visual angle of 10 to 50 degrees of arc.

The report by the patient may be verbal, but preferably takes the form of a diagram. Pre-prepared diagrams of an annular form may be used on which the subject may mark any breaks, changes in colour or dimness.

Patients with a normal field of vision see an unbroken annular shape of uniform brightness. A patient with visual field defects sees a broken shape and/or areas of different colour or brightness. 7/39675 T/G 9 /00418

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The invention further provides apparatus suitable for fundus perimetry, the apparatus comprising an annular light source and focusing means for focusing light from the light source, the apparatus being so constructed that, in use, the apparatus projects an annular stimulus on to the human retina.

Preferably the annular light source comprises a light source and an opaque mask with a transparent or translucent annular portion.

Preferably the focusing means comprises at least one converging lens.

The invention also provides apparatus suitable for fundus perimetry comprising a slit lamp incorporating an opaque mask with a transparent or translucent annular portion, being so constructed that, in use, the apparatus projects an annular stimulus on to the human retina. Advantageously, the annular stimulus has an outer diameter which subtends a visual angle of 10 to 50 degrees of arc. The slit lamp will generally be used with a hand held converging lens.

Preferably the annular portion of an opaque mask used in the apparatus of the invention is transparent.

A present inventor has observed that if a horizontally orientated slit beam is projected onto the retina by means of standard unmodified biomicroscopic apparatus (slit-lamp in combination with hand-held converging lens) above and/or below the optic nerve head in glaucoma patients, a subject may report the presence of a gap or focal dimming of the perceived line of light. The position of the defect in the perceived line is found to correspond to the site of focal visual field defects which had been previously documented using conventional automated perimetry.

Following the above observation it has been found that an advantage results from the use of an illuminated annular stimulus projected concentric with the optic disc, instead of the linear slit-beam. Use of the annular stimulus is sensitive to the anatomy of the nerve fibre layer which radiates from the optic disc. Defects in this nerve fibre layer are associated with similarly arranged visual field defects and it has been found by the inventor that the sensitivity in detecting these field defects is dependent on placement of the stimulus such that the defect(s) and the stimulus intersect at an angle close to 90 degrees: this requirement is satisfied by use of an annular stimulus, especially a circular annular stimulus as is shown by the Example described below.

Further modification of the annular stimulus parameters was performed to optimise the efficiency and ease of use and to ensure acceptable sensitivity and W 7/39 75

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specificity in diagnosis. For example, the annular stimulus should be visible to the examiner to allow positioning and to check for vignetting by the pupil.) It has been confirmed that the method correctly identifies patients previously diagnosed, using conventional, hospital-based automated perimetry, as either patients having a normal visual field or as patients with established cases of glaucoma.

Although the technique according to the invention is of primary value in diagnosis of glaucoma, any disease of the eye manifesting itself partly or wholly as visual field loss may be detected, to a greater or lesser extent, using this technique. It should be noted that slightly different stimulus parameters may be required for optimal detection of a disease other than glaucoma. Optimal parameters may be found by simple experimentation using a test group of subjects of known diagnosis.

In this specification, the term "annular" is used to describe the space between the outer periphery of a first smaller body and the inner periphery of a second coplanar larger body, the peripheries being substantially similar. Preferably the annulus is circular. Other shapes may be used, for example, an ellipse, a triangle, a square, or another polygon having more than 4 sides, but preferably the annulus will be at least substantially circular.

The annular stimulus projected on the retina may have an outer diameter which has a visual angle in the range 10 to 50 degrees of arc. Preferably the visual angle is at least 20 degrees of arc. An annular stimulus having an outer diameter having a visual angle in the range 30 to 40 degrees of arc, especially about 30 degrees of arc, is especially preferred. An annular stimulus of diameter 30 degrees centred on the optic nerve just overlaps the fovea.

The annular stimulus should preferably be as narrow as possible, i.e., the difference between the inner and outer diameters should be as small as possible. Preferably the width of the annular stimulus, i.e., the difference between the visual angle of the inner diameter and the visual angle of the outer diameter, is 5 degrees of arc or less. More preferably it is 1 degree of arc or less and especially preferred is a width of 0.5 degrees of arc or less. The width may be as small as 0.1 degree of arc. It has been found that reducing the width of the annular stimulus increases the sensitivity of the test.

The luminance of the annular stimulus as projected into the eye may be in the range 1 x IO³ to 1 x IO⁶ candelas/m² and is preferably in the range 0.5 x IO⁵ to 2 x IO⁵ candelas/m². A luminance of 1.2 x 10^s candelas/m² is particularly preferred. Lowering the luminance has been found to increase the sensitivity of the test but can lead to a reduction in specificity. Luminance may be measured by Maxwellian projection; the annular stimulus is projected onto substrate, for example a piece of paper, positioned where the patient's retina would be and the luminance is measured at that position with a hand-held photometer.

The light source may be of any suitable type of visible light. For example, an incandescent bulb may be used. A source of monochromatic light may be used if desired. The light source may comprise a light emitter that produces a particular wavelength or a range of wavelengths or filters may be placed in the apparatus to give a projected stimulus of a particular wavelength or range of wavelength.

There is evidence that tests which selectively test the blue pathway may be more sensitive in detecting early glaucomatous damage, hence it may be advantageous to project a blue annular stimulus. Use of a projected stimulus of 475 nanometres or less may be preferred, for example, light having a wavelength in the range 380 to 475 nanometres. This may, for example, be provided by the use of a filter having a band width of 380 to 475 nanometres.

The apparatus may have fixed values for luminance, wavelength, outer diameter, inner diameter and thickness of the annular stimulus but preferably at least one is variable.

The apparatus is advantageously such that the luminance is variable. One method of providing variable luminance is to provide two rotable cross polarisers in - 8 -

the path of the light source, it then being possible to vary the luminance by changing the angle of the polarisers relative to each other. Such an arrangement is described below with reference to the drawings. This particular feature, which allows optimisation of the luminance of the stimulus in any given clinical situation as well as providing a mechanism for estimation of the luminance threshold of a patient's response may also be emulated by a variable intensity illumination source or a series of (rotating) interchangeable neutral density filters. Variations of these strategies form the basis of conventional perimetry.

The opaque mask may comprise an at least partially transparent or translucent disc having a central locating means and an opaque disc of smaller diameter located on the locating means. This mask is then held in place in a housing, the annular portion being formed by the difference in diameter of the housing and the opaque disc. If such a form of mask is used with a slit lamp then the mask may be located in a filter ring provided in the slit lamp.

The invention provides a modifier system comprising an at least partially transparent or translucent disc of a diameter suitable to fit in a filter housing on a slit lamp, the disc having a central locating means and an opaque disc of smaller diameter locatable on the locating means. The at least partially transparent or translucent disc of the mask or modifier system may have a diameter in the range 11 to 15 mm. The difference in radius between the opaque disk and the opening in the filter housing in the slit lamp may be in the range 0.05 to 2.5 mm.

To alter the diameter of the annular portion a large opaque washer may be used. Such washers have an internal diameter which is greater than the external diameter of the opaque disk and are placed on the at least partially transparent disc thereby forming an annulus between the outer edge of the opaque disk and the inner edge of the washer. In order to locate the washers accurately they should fit snugly into the housing. Another way in which the diameter of the annular portion may be altered is to provide the partially transparent or translucent discs with an opaque ring around the edge. By altering the width of the opaque ring, for example, by having a number of different partially transparent or translucent discs, the diameter may be changed.

Advantageously, a further small light source centred on the centre of the annular light source is also provided. This light is projected on to the retina as a "spot". This central light beam assists the user in locating the annular stimulus in the correct position on the retina. For example, when testing for visual field defects caused by glaucoma, it is desirable that the annular stimulus be centred on the optic nerve head. The central light "spot" may be observed by the person carrying out the test and used as a guide, location of the central "spot" on the optic nerve head meaning that the annular stimulus is in the correct place for testing for most glaucoma-related defects.

The opaque mask may comprise a transparent or translucent portion positioned at the centre of the annular portion such that, in use, a light beam is projected on to the retina in the centre of the annular stimulus. Preferably the transparent or translucent portion is a disk shape of relatively small diameter. It is especially preferred that the central light beam is projected as a "spot", especially a circle, having a diameter which subtends a visual angle of less than 6 degrees of an arc. For example, a circle having a diameter of 5 degrees of an arc is suitable.

In the modifier system and mask described above which comprises an at least partially transparent or translucent disc having a central locating means and an opaque disc of smaller diameter located on the locating means, the locating means is preferably transparent or translucent thereby providing the central light beam.

Preferably the test is conducted in the following way: A focused image of the annulus is positioned so that the central beam illuminates only the optic disc. It is usually unnecessary to dilate the pupil. To maintain 97/39675 P T/GB97/00418

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fixation the patient is asked to gaze at a suitable object with the eye that is not being tested. The patient is then asked what is seen: he/she is asked to describe whether the ring (annulus) is seen and whether it is complete. Normal patients report seeing a complete ring (100% specificity in study described in the Example below) . If a gap/dimming/colour change of part of the annulus is described, the patient may then be asked to draw the position of the defect on a printed representation of the ring. This may be directly compared with any available previous visual field tests. If desired the test may be repeated with the annular stimulus centred on a different position, for example, the fovea, as discussed further below. The presence of any perceived focal disturbance in the ring constitutes an abnormal test result and warrants further investigation using more detailed conventional perimetry.

In testing for visual defects caused by neurological disease of the visual pathways, for example, a brain tumour, it is necessary to test for loss of peripheral vision and hence the annular stimulus should not be centred on the optic nerve but instead should be around the fovea.

The steps (a) projection of the annular stimulus and (b) requesting a report from the patient may be carried out more than once if desired. It may, for example, be advantageous to project an annular stimulus centred on the optic disc and to obtain a report from the patient and to project an annular stimulus centred elsewhere on the retina, preferably on the fovea, and to obtain a report from the patient. This then gives a broader area of coverage of the retina. Although the test time will inevitably be increased, testing with the annular stimulus centred in two different positions should give improved sensitivity, for example, centring on the fovea should allow detection of nasal step defects that may not be detectable by use of an annular stimulus centred on the optic disc. (The nine false negative patients found in the study described in the Example below were all demonstrated to have focal field defects beyond the perimeter of the annular stimulus used; in the Example the stimulus was centred on the optic disc.)

It is usually unnecessary to dilate the subject's pupil, however, it may be necessary to do so in a small number of patients, particularly those who are undergoing treatment which, as a side effect, reduces the size of the pupil, in order to be able to project an annular stimulus of sufficient diameter on to the retina.

The apparatus may take any suitable form. It will usually, however, take the form of a biomicroscopic apparatus or of a direct or indirect ophthalmoscope.

The fundus perimetry technique of the present invention is particularly applicable to a hospital clinic setting where slit-lamp biomicroscopy is routinely performed and the required equipment is readily available. Suitable modifications for a standard slit- lamp are described below.

A slit lamp is generally used with a hand-held converging lens. The strength of the lens may be varied and is chosen to fulfil the desired requirements. A lens having a strength of 20 to 90 dioptres may be suitable. A lens having a strength of 60 to 90 dioptres, especially a strength in the range 70 to 90 dioptres, may be preferred.

Also described are suitable modifications for a standard direct or portable indirect ophthalmoscope. Such an ophthalmoscope allows the same test to be performed using hand-held apparatus, thereby extending the application of the test to primary care and "field" conditions.

The ability to perform fundus ring perimetry using a single, portable, hand-held instrument greatly enhances the utility of the technique. This enables testing to be performed in the primary care or "field" setting with possible implications for the detection of ocular disease in a larger proportion of the "at risk" population.

The direct ophthalmoscope is used by both primary care physicians and optometrists to examine the ocular fundus. Optical characteristics include: 1) Separation of illumination and viewing optical trains to minimise the effect of the corneal light reflex in degrading the fundal image.

2) Relatively small field of view (up to 10-15 degrees of arc) which is limited by the diameter of the viewing aperture and objective lens, distance between viewing aperture and the subject's pupil plane and acceptable displacement of viewing and illumination beams. The area of retinal illumination is limited by similar factors which include the characteristics of the condensing lens and mirror.

Examination of the data from the slit-lamp pilot study carried out by the inventor suggests that the sensitivity of the ring perimetry test may be reduced if the angular diameter of the ring stimulus is greatly reduced. To ensure similar performance as measured by sensitivity and specificity a direct ophthalmoscope mounted system requires a larger field of view than is found in a currently available unmodified model: a minimum of 20 degrees is preferred. One way in which this may be achieved is by the application of modern aspheric lens design to the basic plan of the direct ophthalmoscope. An instrument according to the invention preferably conforms to the following specifications:

1) Field of illumination in excess of 10 degrees, preferably in excess of 20 degrees, of arc.

2) Incorporation of an annular occluder (mask) in the illumination optical train. That may be of the same, or 7/3 75

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similar, design as that described for the modified slit lamp. The dimensions and precise placement of the occluder will depend on the final dimensions of the instrument. This allows fundus annular perimetry to be performed according to the invention. Preferably the diameter of the annular stimulus is in excess of 20 degrees of arc and the stimulus may be centred on the optic disc by use of a central "spot" of light when the clinician is seeking to diagnose glaucomatous visual field loss. Other eye disease may be more accurately identified with different stimulus configurations.

Indirect ophthalmoscopy is normally performed using a head mounted illumination source and a large diameter hand-held aspheric converging (condenser) lens. This apparatus may also be modified by the inclusion of a suitable mask or an alternative light source to enable it to be used to carry out fundus annular perimetry. An important characteristic is the large field of view available. Optical principles from this method may be incorporated in the design of the portable instrument described above.

A modified slit lamp in accordance with the present invention and the general principle of the invention will now be described by way of example only with reference to the accompanying drawings in which:

Figure 1 is a plan view of a modifier system for use in a slit lamp; Figure 2 is an expanded perspective view of parts of the modifier system; and

Figure 3 is a schematic representation of one embodiment of the apparatus of the invention.

The following description refers to modifications made to a Haag-Streit 900 slit-lamp in order to fulfil the requirements of the invention. The modifier system is shown in Figures 1 and 2. Similar modifications may be made to slit-lamps produced by other manufacturers. The dimensions of the components may, however, need slight modification to allow generation of the stimulus to the required specifications with such slit-lamps.

The slit-lamp is modified by use of a 15mm diameter transparent plastic disc, 1, which has a 1.5mm diameter (distance d) central, machined, transparent, raised "peg", 2, which is, in turn, supported within an aperture, 3, (diameter b) in the lowest filter ring below the lamp housing in the Haag-streit slit-lamp. Resting on this transparent disc is an annular, plastic, opaque, occluder disc, 4, ("washer") (diameter is distance c, in this particular embodiment c= 10.5mm) which is centred using the "male" peg, 2. Hence an annular aperture, 6, is formed between the inner diameter of the housing, 3, and the outer diameter of the occluder washer, 4. The width of the aperture is half the difference between the distance b and the distance c. This arrangement allows adjustment of the width of the annular stimulus by the 7/39675 T/GB97/00418

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use of a different diameter "washer", 4, which can be exchanged after removal of the lamp housing. An important feature is the presence of the central transparent peg, 2, which allows the passage of a narrow beam of light which is used to illuminate the optic disc during the test and thus ensure accurate centring of the much larger diameter ring stimulus (Figures 1 and 2).

To allow easy adjustment of the luminance of the ring stimulus a 15mm diameter (distance a) annular disc of polarising material, 5, (Fig. 2) is placed beneath the transparent disc, 1, so that the light passing through the ring aperture, 6, also passes through the polariser, 5. A hole 3mm in diameter (distance e) in the centre of the polarising filter, 5, ensures that the beam to illuminate the optic disc is not dimmed. A second disc of polarising material is positioned over the end of the optical column assembly in a holder which allows ease of rotation and hence adjustment in the luminance of the annular stimulus. The precise specification relating to the ring stimulus luminance used in the pilot study described below may be obtained by use of a single neutral density filter instead of the polariser combination described.

The above modifications allow projection of a variable diameter, width and luminance annular stimulus onto the retina with a central narrow beam of constant diameter and luminance to illuminate the optic disc. The stimulus is projected onto the retina of the subject using a converging lens. The study detailed below describes use of a hand-held, uncoated 90D (Volk) aspheric lens.

Figure 3 shows a schematic representation of one embodiment of the invention. The apparatus comprises a light source, 10, a mask, 11, a polariser, 5, a lens, 12, a second polariser, 13, a mirror, 14, and a converging lens, 15. Light from the light source, 10, hits the mask, 11, and passes through the transparent annular portion, 21, and the transparent central portion, 22, thereby producing an annular shape field of light with a central beam of light. (The mask, 11, may be that of the modifier system as described above with reference to Figures 1 and 2.) Beneath the mask, 11, is a first polariser, 5, with a central hole, 23. The central beam of light passes through the hole, 23, without polarisation of the light. The annular field and central beam pass through a first lens, 12, and a second polariser, 13. Rotation of the polarisers in relation to each other allows variance of the luminance of the annular field of light. The light is then reflected off the mirror, 14, and passes through a converging lens, 15, and passes into the eye, 16. The light passes through the eye's lens, 17, to be focused on the retina, 18. An annular stimulus, 20, is seen by the patient and a central "spot" of light, 19, allows centring by the person carrying out the test. The visual angle subtended at the eye by the annular stimulus is the angle θ.

In alternative embodiments of the invention, one or more filters may be placed in the slit lamp, for example, between the mask, 11, and the polariser, 5, to allow control of the wavelength of the light projected onto the patient's retina. Example

A study was performed on patients attending the glaucoma clinic at Moorfields Eye Hospital. All had conventional automated perimetry on arrival and underwent testing, according to the invention, by an observer who was unaware of the conventional perimetry result/diagnosis. The results were analysed to determine the sensitivity and specificity of the test in correctly identifying those patients who had glaucomatous visual field defects on conventional automated perimetry i.e. the more detailed but prolonged hospital-based perimetry test was used as the "gold standard" .

The apparatus used was a modified Haag-Streit slit lamp and a hand held, uncoated 90D (Volk) aspheric lens, as described above. The parameters were as given in the embodiment described in detail above. Patients

79 eyes of 42 patients (26 male, 16 female) were tested (5 eyes of the 42 patients had insufficient vision to allow conventional perimetry) . Their mean age was 64 (ages ranged from 37 to 83) .

Of those eyes tested, 35 were shown to be affected by glaucoma, i.e., were shown to have visual field loss on conventional perimetry and 40 were shown to be normal or glaucoma suspect i.e., were shown to have a normal visual field by prior perimetry.

9 eyes were excluded: 5 eyes had insufficient vision to allow conventional perimetry (as stated above) and of the 79 eyes tested, 4 subjects were unable to complete the ring test due to either poor fixation (2) , high astigmatism (1) or poor fundal view due to media opacity

(1) .

A modified slit lamp and hand-held converging lens were used as described above with reference to Figures 1 and 2. The following parameters were used: Diameter of occluder "washer" (adjustable) : 10.50 mm

Diameter of clear aperture in slit-lamp filter ring : 11.0 mm

Width of ring aperture (adjustable) : 0.25 mm

Diameter of central aperture (constant) : 1.5 mm Luminance of ring stimulus (adjustable)

: 1.2 x IO⁵ candelas/m²

(Measured using Maxwellian projection and photometer)

Angular diameter of ring subtended on retina (adjustable) : 30 degrees of arc

(Angular width of ring (adjustable) : 0.5 degrees of arc)

Angular diameter of central beam on retina (disc) : 5 degrees of arc

Results

Only 11% of eyes required pupil dilation before testing in order to allow the ring stimulus to be projected on the retina.

The "ring" test demonstrated 74% (26/35) sensitivity and 100% (0/40) specificity in the detection of glaucomatous eyes. In the 26 true positive eyes, there was a close spatial correspondence between the sites of focal field loss as demonstrated with conventional perimetry and the section of the ring perceived to be affected. Additionally, the ring test showed higher sensitivity in the detection of deeper field defects than those more subtle defects.

The test was acceptable to all patients and was usually completed within 3 to 4 minutes including the time required to instruct the patient.

The results using the described parameters of ring diameter, thickness and luminance in this group of patients are encouraging. The quoted values of sensitivity and specificity are higher than other non- perimetric methods of glaucoma detection (intraocular pressure measurement and/or ophthalmoscopy) .

A comparison between the position of the "gaps" in the perceived ring and the location of corresponding focal scotomata as shown by conventional perimetry was carried out. Rings of the appropriate 30 degree diameter were drawn concentric with the blind spot on the conventional perimetry printouts from the glaucomatous eyes. The site of the perceived 'gap' in the ring was within the same quadrant of field as the previously documented defect in 26 out of 26 (100%) of true positive eyes, indicating a close spatial correspondence between the two methods.

Claims

1. Apparatus suitable for fundus perimetry, the apparatus comprising an annular light source and focusing means for focusing light from the light source, the apparatus being so constructed that, in use, the apparatus projects an annular stimulus on to the human retina.

2. Apparatus as claimed in claim 1, wherein the annular stimulus has an outer diameter which subtends a visual angle of 10 to 50 degrees of arc.

3. Apparatus as claimed in claim 1 or claim 2, wherein the annular light source comprises a light source and an opaque mask with a transparent or translucent annular portion.

4. Apparatus as claimed in claim 3, wherein the annular portion of the opaque mask is transparent.

5. Apparatus as claimed any one of claims 1 to 4, wherein the focusing means comprises at least one converging lens.

6. Apparatus as claimed in any one of claims 1 to 5, which further comprises a light source at the centre of the annular light source.

7. Apparatus as claimed in claim 6, wherein the annular light source and the central light source comprise a light source and an opaque mask with a transparent or translucent annular portion and a transparent or translucent central portion.

8. Apparatus as claimed in claim 6 or claim 7, wherein in use, the central light source is projected to form an image which subtends a visual angle of less than 6 degrees of an arc on the retina.

9. Apparatus as claimed in any one of claims 1 to 8, wherein the apparatus is a biomicroscopic apparatus.

10. Apparatus as claimed in any one of claims 1 to 8, wherein the apparatus is a direct or indirect ophthalmoscope.

11. Apparatus as claimed in any one of claims 1 to 10, wherein the annular stimulus is circular.

12. Apparatus as claimed in any one of claims 1 to 11, wherein the visual angle subtended by the annular stimulus is at least 20 degrees of arc.

13. Apparatus as claimed in claim 12, wherein the visual angle subtended by the annular stimulus is in the range 30 to 40 degrees of arc.

14. Apparatus as claimed in any one of claims 1 to 13, wherein the width of the annular stimulus is 5 degrees of arc or less.

15. Apparatus as claimed in claim 14, wherein the width of the annular stimulus is 1 degree of arc or less.

16. Apparatus as claimed in claim 15, wherein the width of the annular stimulus is 0.5 degrees of arc or less. W 7/39 75 P T/GB97/00418

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17. Apparatus as claimed in any one of claims 1 to 16, wherein the luminance of the annular stimulus is in the range 1 x IO³ to 1 x IO⁶ candelas/m².

18. Apparatus as claimed in claim 17, wherein the luminance of the annular stimulus is in the range 0.5 x IO⁵ to 2 x 10⁵ candelas/m².

19. Apparatus as claimed in claim 18, wherein the luminance of the annular stimulus is approximately 1.2 x IO⁵ candelas/m².

20. Apparatus suitable for fundus perimetry comprising a slit lamp comprising an opaque mask with a transparent or translucent annular portion, being so constructed that, in use, the apparatus projects an annular stimulus on to the human retina.

21. Apparatus as claimed in claim 20, wherein the annular stimulus has an outer diameter which subtends a visual angle of 10 to 50 degrees of arc.

22. Apparatus as claimed in claim 20 or claim 21, wherein the opaque mask comprises a transparent or translucent portion positioned at the centre of the annular portion such that, in use, a centring spot is projected on to the retina in the centre of the annular stimulus.

23. Apparatus as claimed in claim 22 wherein the transparent portion is a disk shape of relatively small diameter.

24. Apparatus as claimed in claim in any one of claims 20 to 23, wherein the opaque mask comprises an at least partially transparent or translucent disc having a central locating means and an opaque disc of smaller diameter located on the locating means, the mask being located in a housing on the slit lamp.

25. Apparatus as claimed in claim 24, wherein the locating means is transparent or translucent.

26. A modifier system comprising an at least partially transparent or translucent disc of a diameter suitable to fit in a filter housing on a slit lamp, the disc having a central locating means and an opaque disc of smaller diameter locatable on the locating means.

27. A modifier system as claimed in claim 26, wherein the at least partially transparent or translucent disc has a diameter in the range 11 to 15 mm.

28. A modifier system as claimed in claim 26, wherein difference in radius between the opaque disk and the opening in the filter housing in the slit lamp is 0.05 to 2.5 mm.

29. A modifier system as claimed in any one of claims 26 to 28, wherein the locating means is transparent or translucent.

30. Apparatus substantially as herein described with reference to or as illustrated in the accompanying figures.

31. A modifier system substantially as herein described with reference to or as illustrated in the accompanying figures.

32. A method of fundus perimetry comprising the steps of:

(a) projecting an annular stimulus on the retina of a patient's eye; and

(b) requesting the patient to report what is seen.

33. A method as claimed in claim 32, wherein the patient is requested to mark on a pre-prepared diagram of annular form any breaks, changes in colour or dimness in the image of the annular stimulus seen.

34. A method as claimed in of claim 32 or claim 33, wherein the annular stimulus has an outer diameter which subtends a visual angle of 10 to 50 degrees of arc.

35. A method as claimed in any one of claims 32 to 34, which further comprises projecting a light beam at the centre of the annular stimulus.

36. A method as claimed in claim 35, wherein the central light beam is projected to form an image which subtends a visual angle of less than 6 degrees of an arc on the retina.

37. A method as claimed in any one of claims 32 to 36, wherein the annular stimulus is circular.

38. A method as claimed in any one of claims 32 to 37, wherein the visual angle subtended by the annular stimulus is at least 20 degrees of arc.

39. A method as claimed in claim 38, wherein the visual angle subtended by the annular stimulus is in the range 30 to 40 degrees of arc.

40. A method as claimed in any one of claims 32 to 39, wherein the width of the annular stimulus is 5 degrees of arc or less.

41. A method as claimed in claim 40, wherein the width of the annular stimulus is 1 degree of arc or less.

42. A method as claimed in claim 41, wherein the width of the annular stimulus is 0.5 degrees of arc or less.

43. A method as claimed in any one of claims 32 to 42, wherein the luminance of the annular stimulus is in the range 1 x IO³ to 1 x IO⁶ candelas/m².

44. A method as claimed in claim 43, wherein the luminance of the annular stimulus is in the range 0.5 x IO⁵ to 2 x IO⁵ candelas/m².

45. A method as claimed in claim 44, wherein the luminance of the annular stimulus is approximately 1.2 x IO⁵ candelas/m².

46. A method as claimed in any one of claims 32 to 45, wherein the annular stimulus is projected so as to be centred on the optic disc.

47. A method as claimed in any one of claims 32 to 45, wherein the annular stimulus is projected so as to be centred on the fovea.

48. A method as claimed in any one of claims 32 to 47, wherein the steps (a) and (b) are carried out at least twice, once with the annular stimulus projected so as to be centred on the optic disc and once with the annular stimulus projected so as to be centred elsewhere on the retina.

49. A method as claimed in claim in claim 48, wherein the steps (a) and (b) are carried out at least twice, once with the annular stimulus projected so as to be centred on the optic disc and once with the annular stimulus projected so as to be centred on the fovea.

50. A method as claimed in any one of claims 32 to 49, wherein the apparatus or modifier system of any one of claims 1 to 31 is used.

51. Use of a method as claimed in any one of claims 32 to 50, in the diagnosis of glaucoma.

52. Any new feature herein described or any new combination of herein described features.