The present invention relates to a device for use with the indirect ophthalmoscope which is an instrument used to allow visualisation of the retina . In particular, the device allows accurate localisation of the image of the retina in three-dimensions so that measurements relative to anatomical features of the retina can be carried out.

It is important to be able to inspect the retina of the eye for the diagnosis of a number of ophthalmological conditions, such as retina detachment, tumours, and other lesions. ^' This is usually carried out by illuminating the eye with a strong light source and inspecting the retina with the aid of a convex lens, known as a condensing lens. This technique, however, requires great manual dexterity, and considerable mental co-ordination for integration and interpretation of the images of the various parts of the retina as they are inspected. Moreover, it is difficult to make quantitative measurements of the eye, for example, to assess the rate of increase in size of a tumour.

It is an object of the present invention to mitigate these disadvantages and to provide a device with which quantitative measurements of the retina can be made. One aspect of the present invention provides an optical spacing device for use with an indirect opthalmoscope which allows accurate localisation of the

image of the retina. The spacing device comprises a body adapted to be coupled to a convex lens of focal length suitable to allow inspection of the retina and a transparent screen spaced from the lens by the body at the focal length of the lens such that the image produced by the lens is focused in plane of the screen.

Preferably the optical spacing means includes means to vary the distance of the screen from the lens to allow for condensing lenses of different focal length to be used.

The primary purpose of the optical spacer body is to allow accurate positioning of the lens and the screen. An additional spacer may also be provided in front of the lens to locate the optical spacer and lens at the correct distance from the eye. The spacer can be positioned using the patients orbital rim as a reference point to compensate for axial refractive error.

It is necessary to illuminate the eye and a coaxial illumination source may be combined with the optical spacing device. In another embodiment, the light source may be arranged to shine directly through the convex lens by using a beam splitter prism interposed in the optical pathway.

The convex lens has a focal length suitable to allow convenient inspection of the retina by the ophthalmologist, so that the lens power has a rating of usually 15 - 30 Dioptres but higher power lenses may be used with a slit lamp microscope.

The screen is formed of a transparent material, such as a plastics material or glass. The screen is preferably curved to take account of the curvature of the retina. The screen may be tinted or treated with an anti-reflective material to reduce spurious reflections. If the screen is tinted green, blood vessels appear dark and are thus easier to distinguish. Generally,the screen will.be disc-shaped and may be provided with calibration markings such as a grid of orthogonal lines or concentric circles to facilitate direct measurement of retinal features. It is not possible to visualise the entire retina at once, so it is necessary to inspect a number of areas of the retina individually, and from these to build up a composite view of the entire retina. In other circumstances, it may only be necessary to study and measure one retinal feature, for example to monitor the progress of a tumour.

The screen may also include a fixation target which is brought to a focus on the retina of the patient's unaccommodated eye. In conventional ophthalmoscopy, the patient is asked to look at a point in a darkened room (which he cannot see) in order to keep the eye still. This generally proves very difficult for the patient to do for any length of time. However, because any target on the screen will automatically be brought into focus by the convex lens on the retina of the eye, it is most effective to provide a fixation target on the screen itself. This may be in the form of a simple cross. If

the lens with spacer and screen is rotated such that the patient's line of sight is varied, this allows the ophthalmologist to inspect different parts of the retina.

The ophthalmologist may trace the outline of a feature directly on the screen. The screen can be removeable if it is desired to have a permanent record of the retinal image at a particular time. This allows follow-ups to be easily made and to assess the progress of lesions and for the effects of therapy. A further aspect of the invention provides a method of obtaining a record of information from the retina of an eye, which comprises the steps of, placing a convex lens and an imaging screen in front of the eye; focusing an image of the retina in the plane of the screen; and

(1) recording information indicative of the retinal condition directly on the screen, or (2) taking measurements indicative of the retinal condition from the image on the screen and recording the measurements.

In order to compensate for patients who are long sighted or short sighted and to satisfy the condition that the first principal focus coincides with the anterior focus of the eye, an additional spacer may be coupled in front of the lens and spacer using the orbital rim as a reference point. This is only necessary for refractive

errors greater than 3 Dioptres.

Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, wherein Figure 1 is a schematic view of an ophthalmologist using the device according to an embodiment of the invention to examine the eye of a patient; Figure 2 is an enlarged exploded view of the device coupled to the lens, as shown in Fig. 1; Figure 3 is an optical ray diagram illustrating the principles of the indirect ophthalmoscope;

Figures 4a, b and c are an optical ray diagram depicting ^' the effect of focusing using a lens with normal, long and short-sighted eyes; Figure 5 shows an optional fixation attachment;

Figure 6 shows two different screens for use with the optical spacing device; and

Figure 7 shows a composite retinal map made up of thirteen individual retinal images. Reference is first made to Figures 1 and 2 which shows a Nikon convex condenser lens 1 in a lens holder 2. The holder is spaced by hollow opaque cylinder 3 from a circular screen holder 4 to minimise stray light. The lens holder 2, cylinder 3 and screen holder 4 constitute the optical spacing device. Transparent screen 5 is curved to take account of the retina curvature, and is formed of a plastics material upon which a fixation target

6 in the

form of a cross (x) is marked.

Figure 3 is an optical ray diagram showing the retinal plane R _s and lens P _s of the eye having anterior focus F _a . Convex lens 1 of the device is arranged so that its first principal focal plane Fi coincides with focus F _a of the eye. Thus, when the eye is focused on infinity as in the resting condition, an image of the retina is brought into 'focus at the second principal focal plane F2 of lens: this is where the screen 5 is located. The image formed in the plane of the screen is viewed by the ophthalmologist.

The optics of the convex lens are such that when placed before an emmetropic (i.e. a normal sighted eye when resting and focused at infinity) the emerging parallel rays originating from the retina are brought to a focus at the second principal focal plane F2 as shown in Fig. 4a. Because the rays emerging from the eye are parallel, the size of the image formed is independent of the distance of the lens from the eye. However, rays emerging from an ametropic (i.e. long (hypertropic) or short sighted (myopic)), because they are not parallel, form an image the size of which and whose distance from the lens depends on the distance the lens is held from the eye as-shown by planes marked M (myopic) and H (hypertropic) in Figs. 4a and 4b.. However, if the lens is arranged such that its first principal focus coincides with the anterior focus F _a of the eye the image, although

still formed at a variable distance from the lens, always subtends the same angle at the principal focal plane and is therefore always the same size independent of the axial refractive error of the eye. By use of a fixation target 6, as best seen in Figure 6 (a), 120° field of the retina may be examined.

Figure 5 shows a fixation light 8 mounted on the end of a ^' telescopic arm 9 depending from a slideable mounting 10 arranged to slide around a ring 11. The ring is intended to fit around screen holder 4. Use of the fixation light which extends beyond the ambit of the screen allows the more peripheral parts of the retina to be inspected. As the arm 9 is extended, with the eye fixed on the light 8, the eye becomes more and more deviated from its normal line of sight so that the outer regions of the retina can be seen. Figure 7 shows a typical result of a mapping of the inner and outer regions of the retina out to 200° by building up a series of overlapping disc maps of the entire retinal region acquired by use of the different fixation targets shown in Figure 6 (a)(b) and light source of Figure 4.

Figure 6b shows a screen 5 having a series of concentric circular graduations 12 inscribed thereon at constant radial spacing. These graduations are helpful in taking absolute measurements of the retina.

It will be appreciated that various modifications may be made to the optical spacing device without departing

from the scope of the invention. For example, the tube may be replaced by spaced legs of a fixed length and the screen graduations do not have to be constant. Also the device may include micrometer adjustments to move cross-wires on the screen to facilitate measurement. Certain applications of the ophthalmoscope of the present invention will now be described by way of example only.

Example 1 (use of fixation device)

The patient is positioned and the condensing lens, with spacer and screen attached, introduced in front of his eye. ^' The patient immediately becomes aware of the central fixation target and as his attention is directed to it, it becomes aligned with his fovea quickly centring his posterior pole in the doctor's field of view. Having examined the posterior pole his attention is directed to the second fixation target located at the upper extreme of the screen. As his eye rotates upwards to take up fixation a new 60° field is brought into view still including the fovea but with, in addition, an extra 30° of superior fundus not previously visualised. The condensing lens is now rotated slowly clockwise about its principal axis while the patient maintains fixation. As the eye rotates gradually in the orbit the doctor's field of view also progresses in a clockwise manner until he has systematically examined a 120° field. By employing an

optional illuminated fixation target attached to the rim of the lens (Figure 4) the entire fundus may be examined in a similarly controlled way.

Example 2 (use as a visuscope)

The central fixation target may be used to assess the point and steadiness of fixation e.g. in a child with amblyopia (a squint or lazy eye) or indeed in any condition where acular function (e.g. detached retina) may be felt to be impaired.

Example 3 (measurement of optic disc size)

The deficiences of current methods of assessment of optic disc size are described in a publication by Alvarez et al,

"The Disc-Macula Distance to Disc Diameter Ratio: A new test for confirming optic nerve hypoplasia in young children." Journal of Paediatric Ophthalmology & Strabismus. 1988; 25, 151-154.

Diagnosis or exclusion of optic nerve hypoplasia (i.e. under-development) may have important clinical implications for a patient, particularly in children. The present device can provide accurate measurements as follows:

Adequate pupil dilatation and cycloplegia are achieved with an approriate drop. The child is positioned for indirect ophthalmoscopy and the condensing lens with appropriate spacer and screen (b) (Figure 5) attached are introduced. (The +14 D lens may be used to obtain higher magnification of the posterior pole.) Where the child's

coperation may be obtained his attention is directed to the central fixation target. This automatically aligns the optic disc graticule 15° from his fixation point. By then rotating the condensing lens about its principal axis the graticule is readily superimposed upon the disc which may be directly measured with an accuracy of +0.05mm. If a significant refractive error has been uncovered by retinoscopy this must be taken into account by ensuring that the first principal focus of the condensing lens coincides with the anterior focus of the eye thereby ensuring that the graticule calibration accurately corresponds to the angle subtended by the disc at the nodal point of the eye. In the pha ic eye because the nodal point ^' is located at a fairly constant position behind the corneal surface the anterior focus can be adequately localised by means of a second spacer between the rim of the condensing lens and the lateral orbital rim. In the aphakic eye, however, the nodal point is correspondingly more anterior, therefore an appropriate spacer has to be employed.

Example 4 (measurement of fundus lesions)

There are innumerable clinical situations where it is desirable for the ophthalmologist to be able to accurately measure a fundus lesion in order to either monitor its progression or regression or else prognosticate and

thereby plan appropriate management (e.g. conditions such as choroidal malignant melanoma) . Traditionally fundus lesions have been described in terms of "disc diameters." With the advent and development of ocular ultrasonography the problem of measuring solid elevated lesions has, to some extent, been obviated. However, even with the highest technology the accuracy of the measurement depends ultimately upon the skill and experience of the operator in interpreting the images he produces. The boundaries of the lesion may be far less obvious to the radiologist than they were to the ophthalmologist who observed the lesion directly. Ultrasonography is unsuitable for measuring non-elevated lesions or lesions with inadequate acoustic impedance. Serial photography may be employed to assess changes in a lesion but this may be fraught with technical problems for the photographer and inconvenience for both patient and doctor. Even then, accurate absolute measurements may not be possible without sophisticated photographic techniques.

The technique for measuring fundus lesions directly with the device and graticule screen is as follows: The size of the lesion is first estimated by conventional ophthalmoscopy. A condensing lens is then selected which will provide the maximum magnification of the lesion while keeping its boundaries within the field of view. The lens is fitted with the appropriate spacer mounted with screen (b) (Figure 6) . Any significant refractive error is taken

into account if necessary as described above. The eye is correctly positioned by adjusting the fixation of the fellow eye with the adjustable fixation target mounted to the condensing lens so that the graticule scale is adequately superimposed upon the lesion to be studied. Accurate measurements may then be made in any meridian as long as the limited of the lesion can be visualised. By this method estimated accuracy of measurement to within 0.1 mm in any meridian may be obtained. Similar accuracy may be likewise obtained for other important dimensions such as the proximity of the lesion to the optic disc or fovea. The image of a lesion can be measured using a suitable caliper device.

Example 5 (retinal drawings.

The production of drawings of the retina showing its important features accurately located and inter-related is important, particularly for surgeons concerned with operations to cure detached retinas. The device of the present invention may be used as follows. Screen (a) of Figure 6 is first selected and the disc and macula are outlined over the etched markings using a water soluble marking pen. The screen is then mounted on the spacer which is then attached to the +20 or +30 D condensing lens. The patient is asked to fixate the foveal target. The accuracy with which he does this will

provide important information as to his foveal function. The lens is then rotated about its principal axis until the inscribed disc is superimposed over that of the patient. While the patient's fixation is held steady the important features of the posterior pole may be traced quickly and accurately directly onto an acetate screen using appropriately coloured pens. The screen is then removed and is replaced with a screen similar to Figure 6(a), selecting the peripheral fixation target. The procedure for visualising the more peripheral regions is followed as described above except that instead of continuously scanning the fundus a series of "still" views is obtained. For each position the fundus detail is carefully traced onto the screen before moving on to the next position with a fresh screen(b) in place. By this means the entire fundus may be accurately and quickly traced onto 13 acetate discs. These may then'be assembled in a manner similar to that used for composite fundus photographs and the fundus then traced onto the definitive diagram as shown in Figure 7. Once completed the screens may be cleaned for further use.

Example 6 (self assessment of central visual field) Targets introduced at the principal focal plane of the condensing lens are brought sharply into focus on the retina by the unaccommodated eye. The +20 D lens has a diameter of 50mm and produces an image on the screen of the central 60° of the posterior pole. This corresponds

almost exactly, therefore, to the angle subtended by the standard Bjerrum screen. By taking up fixation on the central target and then by introducing appropriate sized targets to the screen, with background illumination suitably adjusted a patient may easily plot his own blind spot and then go on to delineate any scotomata present in his own central field. Alternatively the screen may be arranged as an oculokinetic chart combining a central stimulus and a series of peripheral fixation targets as described by Damato B.E. Oculokinetic Perimetry - A simple test for use in the community. Br. J. Oththalmol. 1985. Vol 69, 927 - 931.

Example 7 (visual field assessment under direct vision)

By combining the techniques already described it is possible to introduce the concept of visual field assessment under direct vision. The technique is as follows:

The patient is positioned and the condensing lens with its attachments introduced as previously described. An appropriate fixation target is selected which brings the area of retina of interest to the doctor into the field of view. While the patient maintains fixation a suitable target is introduced to overlie the area of interest (eg. a chorioretinal scar) . The functional boundaries of the

lesion may then be delineated in exactly the way that one would plot a functional deficiency in the visual field with the conventional tangent screen. The important advantage being, however, that one may immediately correlate the functional defect with the morphological appearance of the retina. Careful application of the device as a research tool may improve the understanding of many disease processes which disturb the function of the retina.

It will be understood that the device can be used in the vetinary field to inspect animal retinas as well as those of humans.