Continuing Education

Author: Tina Romanay and Bill Harvey

Tina Romanay and Bill Harvey give some straightforward instruction to using slit-lamp biomicroscopy in practice (C4713, one standard CET point)

This module is now closed - you cannot earn any CET points.

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DIRECT VERSUS INDIRECT

Direct ophthalmoscopy, as the name suggests, allows the practitioner to view the fundus of a patient directly.

The direct ophthalmoscope consists of a lamp, an array of condensing lenses mirror filters stops rheostat and sight hole contained within a tight plastic casing.

The direct image is achieved by the light from the lamp focusing via the condensing lenses onto a mirror which reflects light to a focus on the patient retina. This is then viewed via the sight hole by the observer, usually through a hole in the mirror. A battery of lenses ensures adequate focus of the patient retina irrespective of patient and observer ametropia, and filters and stops allow a variety of fundus views.

The positioning of the hole usually above the light source at the mirror leads to an uneven retinal illumination with inferior views often darker. This is intensified by the increasingly elliptical pupil as one looks further down into the inferior fundus leading to the familiar 'shadow' seen in many patients.

Indirect ophthalmoscopy requires the observer to view an image of the fundus that has been projected to a point in space.

Most commonly this is done by means of a high powered positive lens which forms an inverted and laterally reversed aerial image immediately in front of the lens held before the patient's eye. The image is then observed by a viewing instrument either in the form of a slit-lamp or a head set. In this case the light source is nearer to the patient's eye but laterally displaced such that the incident and reflected paths are different. This optical arrangement has the advantage of both increasing the fundus view through media changes (such as cataract or corneal transparency loss) and improving the contrast view of various lesions (such as naevi) which may not be visible under direct view.

As will further be discussed, the technique also offers the advantages inbuilt in the viewing system, such as binocularity, filter and rheostat adjustment, easy image capture, and a more comfortable viewing distance (for both parties!).

SLIT-LAMP BINOCULAR INDIRECT OPHTHALMOSCOPY (BIO)

This technique has gained such huge popularity in recent times, that many practitioners have incorporated the use of hand-held indirect lenses with the slit-lamp to view the fundus into their general routine. It is also now a compulsory General Optical Council core competency skill for qualifying optometrists.

The condensing lenses used in indirect ophthalmoscopy are high-plus lenses, when used with a slit lamp produce a virtual image of the fundus that is laterally reversed and inverted. Therefore the clinician that is new to this technique has to adjust their observation to account for an upside down, mirror image.

There are a number of lenses available on the market with a number of modifications. These include:

◆ Yellow filters - that are either fixed or detachable. This reduces the amount of blue light impacting on the fundus, which is important in prolonged examinations

◆ Lid adapters - that help separate the lids and set the lens at the correct working distance

◆ Graticules - that are useful for measuring the size of structures viewed

◆ Mounts - which steady the lens on the slit lamp.

The higher the dioptric power of the lens the greater the field of view, however the working distance is then reduced. Each lens produces a stereoscopic fundus view that is only marginally magnified, with the greatest magnification being achieved with the lower powers. The slit lamp's optical system also allows additional magnification of the image.

Table 1 lists some of the properties of lenses that are commercially available. The information has been collated from the manufacturer's data. Note how, generally speaking, the higher the power of the lens, the greater the field of view but the lower the magnification. This is important. It is tempting with a slit lamp to keep using one single viewing lens for all occasions and to adjust the magnification using the slit lamp. This, however, reduces the crispness of the image. It is therefore preferable to keep a lower power lens (say a 60D) for accurate viewing of specific areas or lesions, such as disc or macular, and a higher powered lens (say a SuperField) for wider views, as would be needed for a multifocal lesion disorder such as diabetic retinopathy. Most regular users would confess to owning at least two lenses.

Condensing lenses described with the 'super' prefix, such as the SuperPupil and the SuperField, are not equi-convex so need to be held with the tapered edge (Figure 1) towards the patient. The other lenses may be held either way.

Examination technique

◆ For the best results, the pupil is dilated with an appropriate pharmaceutical agent(s).

◆ It is important to carry out "pre-dilation" checks prior to dilation. Ensure you question the patient on previous history i.e. adverse reactions, allergies and medications that may contraindicate the use of dilating drops. It is also vital to check the intra-ocular pressures and anterior chamber angles, as raised pressures and narrow angles may prohibit dilation.

◆ The patient is instructed on the need for maximum compliance and asked to fixate the slit lamp fixation target if available, alternatively when viewing the right eye the patient may fixate on the practitioner's right ear and vice versa.

◆ The illumination and observation system is placed in the straight-ahead position (allowing binocular viewing). The magnification is set to it lowest position and the slit height and width adjusted to around 5mm and 3-4mm respectively (to fill the pupil diameter).

◆ The slit is focussed on the patient's cornea or iris and centred. The lens is then introduced close to the patient's eye (figure 2). The lens needs to be orientated accordingly if it is not equi-convex as mentioned earlier.

◆ As shown in Table 1, condensing lenses have a working distance varying typically from 4-11mm, at which it should be ideally held from the patient's eye. It is important to maintain this distance to offer a binocular view. As this is much less that most people might imagine, many practitioners happily use their lenses and achieve monocular views. Therefore it is always important to check that the fundus view is visible to both eyes and if not, the lens needs to be positioned closer to the patient.

◆ The slit lamp is moved away from the patient until the inverted image of the retina (usually a thin vertical strip at this stage) is seen (Figure 3). At this stage, fine movements of the joystick allow finer focusing, and adjustment of the slit width and rheostat may be made optimal for the needs of viewing but also to minimise discomfort for the patient (the brightest widest image may not always be appreciated)

◆ Focusing just before the retinal surface allows investigation of the vitreous, which can be particularly useful in posterior vitreous detachment (PVD) where a Weiss ring might be visible (figure 4).◆ By bringing the lens closer to the eye the field of view can be increased. Any reflections from the slit lamp can be minimised or eliminated by slightly tilting the lens either vertically or horizontally.

◆ The image can be optimised by adjusting the magnification and slit width/height on the slit lamp.

◆ The eye is examined in various positions of gaze. The lens is repositioned each time to optimise the view. If the observer requires the image to move then the lens is moved in the same direction as the desired movement.

A good systematic approach for full fundus examination might be:

◆ Start at the disc and note all salient features - check that the view is stereoscopic as this will help to interpret disc topography.

◆ Move the slit lamp nasally to the disc and cross the foveal region, noting all vascular, pigmentary and other features, and noting the foveal quality, avoid prolonged assessment to avoid patient discomfort. This same method should allow scanning of most of the macular and paramacular and peripapillary region without patient eye or lens movement, bar any subtle tilting to minimise reflections

◆ Asking the patient to look up allows viewing of the superior retina. Once they look up their pupil will obviously move up too so the lens will accordingly have to be moved up until in front of the pupil at which point a retinal view will again be seen. By tilting the lens top away from the patient slightly, excessive reflections may be minimised by maintaining coaxial illumination. The same should be repeated for all 8 positions of gaze.

◆ Remember the view is laterally and vertically reversed. Some practitioners prefer to invert the record card when noting appearance. In the superior position of gaze, as described above, the view is still that of superior retina, but, the far periphery is in the inferior field of the viewer and the mid-periphery in the superior field. When a patient looks nasally, the nasal retina is viewed, the lens should be tilted so that the most nasal aspect of the lens is tilted away from the patient, and the far nasal periphery is in the temporal field of the viewer.

Although ideally used following dilation, the slit-lamp BIO can also be used to examine a patient with undilated pupils. The lenses, such as the SuperPupil which are specifically designed for use on a small pupil, work best in these instances. Nonetheless, a good view of the disc can be obtained in most patients. BIO has many advantages but also has some disadvantages. These are listed in Table 2.

Whatever lens is used, it is essential to keep it completely clean at all times. A good quality lens cloth and cleaning fluid is advised. Alcohol based cleaners (for example, a Medi-Swab) are never a good idea as they may dissolve the cement holding the lens in its case and result in the all too familiar 'rattle' that many older lenses emit in use.

Following examination it is important to inform the patient of ocular side affects that may occur, given that dilating drops were used. It is good practice to give patients an advice sheet that explains what drops were used, how long the drops take to abate, what precautions to take while the pupils are dilated and what medical advice to seek in case of an adverse ocular reaction (though the risks are minimal).

◆ Tina Romanay is Director of City University Fight for Sight Optometry Clinic. Bill Harvey is clinical editor of Optician

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