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The use of the slit lamp biomicroscope in optometric practice is now more widespread than ever before. Increasing contact lens work and practitioner confidence in advanced examination techniques, such as indirect ophthalmoscopy and contact tonometry, has meant more extensive use of the slit lamp during a routine eye examination.

However, without regular use, many of the skills required to use the instrument for successful ocular assessment can be lost, along with practitioner confidence.

The aim of this article is to help the practitioner develop good slit-lamp technique, concentrating on the practical application in routine eye examinations.

Short History

The earliest slit lamps have been around from the beginning of the 20th century and today's modern high-tech examples still use the same basic principle taken from the original Gullstrand model designed in 1911.1 Improvements in materials and design mean that the original illumination system employing a 'Nernst Line' filament lamp was replaced with a coil filament, giving rise to the Koeller (Vogt) short-focus projector system2 still in use in today's slit lamp biomicrosopes. Linking the illumination system to a corneal microscope produces the confocal arrangement used on modern equipment and allows for high precision - high magnification images which can be readily manipulated by the skilled practitioner during patient examination.

Slit Lamp Design

Two different designs are available, named after the type of illumination system developed by their owners: 'Zeiss' and 'Haag-Streit'. The main difference between them is that for the Haag-Streit design the illumination system sits above the observation system, allowing the illumination column to be 'decoupled' in the vertical meridian as well as the horizontal. The advantage of this is that it allows for greater flexibility when minimising unwanted reflections during advanced ophthalmic investigations such as gonioscopy and increases the peripheral field of view during indirect ophthalmoscopy.3 This is not possible to do with the Zeiss type because its illumination system sits level with the observation system. Despite this difference both types of instrument lend themselves well to producing high quality optical images of an evenly illuminated sharply demarcated 'slit' using a halogen lamp. A more recent development is the introduction of a convex mirror to enhance light transmission through the system. Choosing between the two systems may come down to personal preference and/or cost.

The illumination system itself contains a number of features which are usually incorporated into a slit-lamp routine and the effectiveness of this is increased with the flexibility of the observation system. In generic terms most modern slit lamps incorporate the features (and sometime more) detailed in Table 1.

Slit Lamp Set-up

Familiarity with the controls and features of the slit lamp (Table 1 and Figures 1 and 2) is required. Ensure the instrument is optimally focused and coupled around its centre of rotation.

1 Focusing the instrument

The most accurate method of doing this is using the focusing rod supplied with the instrument.

Place the rod in its pre-determined slot in the central pivot of the instrument with the flat surface facing towards the eyepieces. Remove any spectacles as moving closer to the instrument allows a greater field of view (unless the practitioner has a high degree of astigmatism which could lead to image distortion). 5

To increase accuracy and reduce accommodative fluctuations 'rack out' both eyepieces so they are at maximum plus and therefore giving a 'blurred' image of the focusing rod.

Starting on approximately X10 magnification, look through ONE eyepiece at a time and adjust it gradually until a sharp image of the slit just comes into focus. Do not over-adjust the eyepiece as this will increase the likelihood of accommodation (instrument myopia) and may result in fluctuations of image clarity during patient examination.

Some eyepieces have a built-in refractive error correction marked on them but for maximum accuracy do not simply adjust the eyepiece to the known refractive error as this does not take into account proximal accommodative changes and therefore is deemed to give a less accurate end point.6

2 Repeat the process for the second eye

Once two clear monocular images are obtained, increase the slit lamp magnification to approximately X24-X32 and fine-tune the focus of each eyepiece in turn. The purpose of this step is to further increase the accuracy of the end point.

3 Obtaining a binocular single-vision image

Only when two clear monocular images of the slit have been achieved should the eyepieces then be adjusted to give a binocular single view. The eyepiece construction allows for the distance between both eyepieces to be modified to reflect the interpupillary distance of the user. A sharply defined single image of the slit should be seen.

An alternative method of focusing the instrument has been suggested and in the absence of a focusing rod may be used. However, this method is considered to be less accurate than using the focusing rod which consists of a flat plane which slots into the instrument at the point where the two main systems of the slit lamp met - the central pivot. The most commonly used alternative method is using the patient's eyelid6 instead of the focusing rod but this relies on the patient remaining very still as any inadvertent movements will result in the slit lamp no longer being precisely focused.

The eyelid is not a flat plane but a curved surface (not situated exactly at the point of common focus) therefore only that particular point will be in focus at any one time.

To carry out this alternative method, follow the steps detailed above for monocular focusing only to ensure image clarity move the slit lamp backwards and forwards while oscillating the illumination column from side to side until the point at which the slit beam does not move off the eyelid but remains at the focused point. If a 'with' movement is seen, the instrument is within the focus (that is, too close). To correct this the instrument needs to be pulled back towards the practitioner. If an 'against' movement is seen the instrument is outside the focus (ie too far away) so this can be corrected by moving closer to the patient.7

4 Check the slit lamp is coupled

For the Haag-Streit design, ensure the large knob on the front of the illumination column located approximately halfway down is tight (Figure 3). On attempted rotation of the illumination column around its base, the slit remains focused on the focusing rod as it is locked in position, allowing the observation system and illumination system to stay focused at the same point.

On rotation of the illumination system relative to the observation system (altering the angle between the two) the image of the slit should remain in the centre of the focusing rod. A slight increase and decrease of the slit width may be observed, but essentially the slit position should remain unaltered if the instrument is coupled.

For the Haag-Streit design the vertical adjustment of the illumination column should also be locked to avoid unwanted tilting of the light source away from the focusing rod during setup and use. To check this, gently attempt to rock the illumination column back and forth in the vertical plane - if it remains at 90 degrees then the lock (Figure 3) is in operation and the instrument is fully coupled.

Patient Instructions

In order to maximise information gathering during a slit lamp routine it is important to explain the procedure to the patient before beginning to ensure their understanding and cooperation and to allay any fears they may have about the examination.

Patient Setup

A comfortable patient means a cooperative patient!

◆ Adjust patient's chair height

◆ Adjust chin rest level either up or down until the outer canthus of the patient's eye is aligned with the outer canthus marker

◆ Ensure the patient is in the centre of the vertical travel of the instrument

◆ Fixation target is often available if required.

Illumination

The different types of illumination commonly used are outlined in Tables 2 and 3. A practitioner's slit-lamp routine often incorporates many of these. Image quality of ocular structures is influenced by the type of lamp the instrument has (modern instruments usually have a halogen lamp which provide a brighter more even light source than a tungsten one).8

Controlling the light levels falling on the eye is also an important part of any slit-lamp routine. The most sophisticated method of achieving this is using a rheostat which allows precision delivery of different amounts of light.

An alternative method is to use a neutral density filter but this is not as flexible or as fast as the rheostat. Altering the rheostat is often underutilised and undervalued in the course of a slit-lamp examination when subtle changes to ocular structures can be missed.

By controlling the amount of light falling on the eye the practitioner can minimise any glare from stray light falling on surrounding structures and improve visibility of those structures under observation eg limbal blood vessels. Illumination may also be varied through altering the slit width and height.

Slit-Lamp Manipulation

Familiarity with the slit lamp will enable the practitioner to develop the most efficient and informative routine. For both instrument designs the simplest method of manipulating the controls is to place one hand on the joystick to control the movement of the instrument in three dimensional space, namely horizontally, vertically and forwards/backwards. The other hand should be placed on the illumination column, initially on the slit width knob to allow for adjustments to this and additionally the slit height, the use of filters, adjusting the angle between the eyepiece and illumination systems, manipulating the patient's eyelids and so on during the routine. It is usually easier to reach around the instrument from the temporal side of the patient, meaning that when examining the patient's right eye the practitioner's right hand should be on the joystick and their left hand on the illumination column. This arrangement should then be reversed when examining the patient's left eye.

Every patient is different and therefore the main emphasis during a slit-lamp routine may not necessarily be the same each time. With this in mind it is important to tailor the slit-lamp examination to the patient's needs. It is beyond the scope of this article to cover all applications of the slit lamp in detail but the following generic routine (Table 4) and application of slit-lamp techniques should provide a suitable level of guidance for any practitioner hoping to improve their skills.

General Overview

Setup: ◆ Diffuse or wide beam

◆ High intensity if using a diffusing filter, otherwise a moderate to low intensity

◆ Wide and variable angle

◆ Low magnification

◆ Observation unit central and normal (at 90 degrees) to the cornea.

The purpose of this initial step is to ascertain the condition of the patient's ocular features and to highlight any particular areas which require further investigation. It is also useful for ocular photography and CL fit assessment.

There are two commonly used methods of doing this: the first is achieved with the use of a ground glass filter found on the reverse side of the mirror on the illumination system (not all slit lamps have this feature). The use of the diffusing mirror allows a wide beam and high level of illumination to be used (to counteract the diffusing properties of the mirror).

The second method, where there is no diffusing filter available, is to open the slit as wide as possible and REDUCE the illumination by adjusting the rheostat (the neutral density filter may be used instead if there is no rheostat). Too much light in this instance will result in an uncomfortable patient and may create lots of glare and reflections which obscure the practitioner's view of the patient's eye. Apart from this it is important to remember that with the more modern slit lamps the intensity of the lamp is high and prolonged exposure to it may cause irreversible damage to the ocular tissue.9

Using either of the suggested setups, begin examining the patient's eye by scanning across either the upper lid or lower lid tempro-nasally or naso-temporally, varying the angle between the illumination column and microscope during the scan. Either raise or lower the slit lamp and repeat the procedure across the middle of the eye, taking in the appearance of the conjunctiva and cornea while the patient is looking straight ahead. One further scan is recommended at this stage to examine the remaining eyelid (Figure 4). During this time it is recommended that the practitioner has a good look at the eye in situ. This technique highlights any areas which may require further investigation and gives an overall impression of the health of the ocular structures.

This procedure is then repeated, only this time the practitioner asks the patient to change their direction of gaze in order to assess more of the ocular surface. Any abnormalities or features should be noted.

Detailed Examination of the Lids and Lashes

Setup: ◆ Wide parallelepiped beam

◆ Moderate intensity

◆ Wide and variable angle

◆ Low-medium magnification

◆ Observation unit central and normal (at 90 degrees) to the cornea.

Once the general overview has been done, a more detailed look at the lids and lashes should be carried out. The slit beam should be narrowed slightly and the intensity adjusted so it is of medium brightness. Again the angle between the illumination system and the eyepieces is variable, around the 40-70 degrees mark. The magnification should be low to medium. As the practitioner scans across the surfaces of the eyelid using direct illumination, the lid position, any lumps or bumps, lid margin regularity and colour should be assessed along with the meibomian glands and lash follicles.

Manipulation of the lids should allow the patency of the glands to be assessed through gentle squeezing. The integrity and position of the puncta can also be checked at this time. Meibomian gland dysfunction may be graded using Table 6.10

Detailed Examination of the Conjunctiva and Sclera

Setup: ◆ Wide parallelepiped beam

◆ Moderate intensity

◆ Wide and variable angle

◆ Low-medium magnification

◆ Observation unit central and normal (at 90 degrees) to the cornea.

Using a similar setup to that for examining the lids and lashes, the conjunctiva can be assessed (Table 3). Any hyperaemia, degenerative changes such as pingeculae or pigmentation should be noted and where relevant the extent of the change and location eg diffuse, sectorial or focal, using the CCLRU grading scale11 or Efron's grading scale12 to help differentially diagnose any ocular condition. The view of any hyperaemic changes can be enhanced with the red-free filter (green filter) which cuts out any background light scatter, increasing the contrast between the conjunctival tissue/white sclera and the blood vessels which show up 'black'. Gentle movement of the transparent conjunctival membrane can be achieved through lid margin manipulation. The more anterior conjunctival blood vessels will move with the conjunctival tissue across the surface of the sclera, while the deeper scleral vessels stay in situ. A colour variation between the vessels will also be seen under white light, where the deeper scleral vessels tend to have a purplish-blue hue compared to the reddish-pink superficial conjunctival vessels.

Tear Film Assessment (White Light)

Setup: ◆ Horizontal narrow beam/Reduced height vertical beam

◆ High intensity

◆ 40 degree angle (variable)

◆ Low-medium magnification

◆ Observation unit central and normal (at 90 degrees) to the cornea.

The slit lamp can be used to evaluate volume and quality of tears. Tear volume can be assessed by measuring the 'tear prism height'. To do this the slit beam can either be rotated through 180 degrees so that it is now horizontal and the slit width reduced to match the tear film volume along the lower lid. The 'height' can then be read off the slit width scale. Alternatively, with the slit beam at 90 degrees, the slit height can be reduced until it just matches the height of the tear prism and again the volume in millimetres can be read off the slit height scale. The normal height of the tear prism is usually considered to be between 0.2 and 0.4 mm.13 The regularity of the tear prism should also be examined. Tear quality can be ascertained through direct illumination and, on asking the patient to blink, any debris or the degree of oiliness of the tears can be seen as the film moves across the ocular surface. An idea of how well the tears perform in terms of the wettability of the ocular surface can also be gained from this.

Tear film assessment may also be carried out using specular reflection. The front surface of the eye acts like a mirror when a certain optical condition is met: the angle of incidence (i) = the angle of reflection (r). In practical terms it is often easier to create the specular reflection effect using a natural phenomenon known as the 'Purkinje images'. There are four such images of the slit lamp filament which coincide with four of the ocular surfaces of the eye (Table 5). To assess the tear film a bright image of the lamp filament is seen (Purkinje 1) at the junction of the tears/front surface of the cornea when the angle between the two systems meets the optical criterion.

The practitioner looks for the first Purkinje image by subtly adjusting the separate of the two systems until a clear image of Purkinje 1 is seen down ONE eyepiece. This should be down initially under low-medium magnification with a narrow beam of reduced height and once clear the magnification should be increased to obtain greater detail of the image. Any debris will show up very brightly, moving in the tears and the quality of the tear film may be considered poor if coloured fringes are seen around the Purkinje image.14

Further tear film assessment with fluorescein and invasive tear break-up time (non-invasive tear break-up time can be measured using the mires on a keratometer but is not discussed here) should be left until the end of the routine once the white light examination is complete - this will be covered in part II.

Corneal Overview

Sclerotic scatter - coupled technique

Setup: ◆ Vertical narrow beam

◆ High intensity

◆ 40-60 degree angle

◆ Naked eye

◆ Instrument coupled.

Sclerotic scatter utilises the principle of total internal reflection15 to create a characteristic ring or glow around the limbus of a healthy cornea. Any breach in the corneal structure will result in the loss of this total internal reflection due to a change in the refractive index of the corneal layers and present itself as a bright area or patch within an otherwise 'dark' cornea because of the light scattering effect.

This is observed using indirect illumination because the area where the light source is directed differs to that under observation.

A thin beam (optic section) of light (reduce slit height to avoid glare from the bright white scleral surface) is shone onto the limbus, usually from the temporal side.

A wide angle, high intensity and low magnification should be used and the instrument is COUPLED.

The practitioner may choose to observe the result down the microscope (although this is usually considered less accurate16) or look around the side of the instrument with the naked eye at the patient's cornea to see the resultant limbal glow.

Alternatively, when wanting to examine tissue integrity under higher magnification with emphasis on the central corneal area, a different slit lamp setup is required:

Sclerotic scatter - decoupled technique

Setup: ◆ Vertical narrow beam

◆ High intensity

◆ 40-60 degree angle

◆ Med-high magnification

◆ Instrument decoupled

◆ Observation unit central and normal (at 90 degrees) to the cornea.

Any lights in the consulting room should be fully extinguished. Initially, the instrument should be coupled. Focus the instrument under low magnification with a high intensity, narrow beam on the area to be observed eg corneal apex.

While this area is sharply in focus lock the instrument in place using the locking screws on the instrument base.

Decouple the instrument (refer to the section on slit lamp design). Rotate the illumination column manually until the light just hits the limbus. Continue observing the corneal apex through the eyepieces.

The magnification may now be increased to aid observation of any opacities, corneal tissue irregularities and so on.

Examination of the Anterior Chamber Angle

When a patient presents for a routine eye examination, one of the measurements often taken is that of the anterior chamber angle this is almost certainly required prior to instilling a mydriatic. To accurately assess this, gonioscopy is the best method but this is not routinely carried out in high street practice and it requires addition skill and training. Instead the practitioner relies on a commonly used grading system named after Van Herick, the devisor of the technique. It is accepted that this technique is less accurate than gonioscopy as it is an indirect assessment of the angle (qualitative)17 but if carried out routinely using the following setup it gives a good indication of the width of the angle and therefore any potential problems.

Van Herick Technique

The aim of this measurement is to assess the width of the angle using the slit width as it shines directly on the corneo-limbal junction.

Setup: ◆ Thin vertical (90 degree) beam (1-2mm)

◆ High rheostat intensity

◆ Medium magnification (X16)

◆ Angle 60 degrees - lock slit lamp to maintain separation angle

◆ Observation system central and normal to the cornea.

Ask the patient to look down the observation system and move the illumination system to temporal limbus so that it creates a sharp image on the limbus-temporal cornea at 60 degrees. Lock the observation system to the illumination column to maintain the 60-degree separation.

When assessing the temporal angle the image created with direct illumination on the corneo-limbal junction lies most temporally. A dark area, or 'gap' will be observed more nasally to the observer and more nasally again there will be a reflection off the iris (retroillumination).

The measurement is based on the apparent width of the sharp focused image (the most temporal image) compared to the gap (which represents the optically empty anterior chamber). Van Herick developed a grading scale which is commonly used in practice to record this (Table 7).

Any angle measured as Grade 2 or less should be viewed with suspicion as it represents a narrow angle which can be a risk factor for the development of closed-angle glaucoma.

The main cause of errors and therefore overestimation of the angle is due to the practitioner allowing the slit lamp to travel too far towards the corneal midline ie too far 'in' from the corneo-limbal junction. By moving too far across the angle will be overestimated as the anterior angle becomes progressively deeper, moving more centrally, resulting in the black gap appearing wider than it otherwise would if the slit lamp beam was located correctly. It takes practice to be able to measure this accurately and it is recommended that the practitioner takes three or more readings of the same angle to compare their consistency and therefore accuracy by making small adjustments to the position of the slit lamp at the corneo-limbal junction. The correct position is where the direct light from the illumination column just falls onto the corneal edge of the limbus.

The insertion of the iris is not the same around the entire anterior chamber border. This anatomical variation means that usually the nasal anterior chamber angle tends to be wider (the widest angle - Table 8). If the temporal angle appears to be narrow then the practitioner should measure the nasal angle too. The setup for this is the same as for measuring the temporal angle only the illumination system should be moved so that light is directed from the nasal side onto the nasal corneo-limbal junction. If the patient has larger facial features (for example, a prominent nose) which obscure the light, this problem can be resolved by ensuring the separation angle between the illumination column and observation system is locked at 60 degrees, then swinging the entire locked system towards the temporal side - ask the patient to look keep looking down the observation system as it is rotated temporally. Adjust the slit lamp to ensure the light just bounces off the nasal corneo-junction and measure the angle as before based on Van Herick's grading scale.

Smith's Technique

Some authors suggest that a more accurate assessment of the risk to a patient of developing closed-angle glaucoma is a measurement of the anterior chamber depth as it is a quantitative method. Certain eye conditions may lead to a change in the depth of the anterior chamber over time, for example, development of cataract can led to a bowing forward of the iris due to the increased mass of the lens, resulting in a shallowing of the chamber. There is evidence to suggest that a patient may have a narrow anterior chamber angle but still have a relatively deep anterior chamber. It is not possible, apart from using gonioscopy, to view the iris insertion18 but anterior chamber depth can be assessed using 'Smith's technique' which is a quantitative measurement.

Setup: ◆ Thin horizontal (180 degrees) beam (1-2mm)

◆ High rheostat intensity

◆ Medium magnification (X16)

◆ Angle 60º - lock slit lamp to maintain separation angle

◆ Observation system central and normal to the cornea

◆ Patient's right eye examined with right eyepiece on the observation system.

Direct a narrow, high intensity beam onto the cornea. Two horizontal images of the light are seen - one reflecting off the corneal epithelium and the other off the anterior lens/iris. A gap between the two images will be seen (Figure 5). By increasing the slit height adjustment until the two image are seen to just touch, a measurement can be taken (Figure 6). Read this measurement off the slit height scale and use Table 9 to convert this into an anterior chamber depth measurement.

For those instruments without a variable slit height adjustment a modified Smith's technique may be used where the angle of the illumination system and observation system is gradually reduced until the two images are just touching.19

Clinically an anterior chamber depth of 2mm or less should be treated with suspicion. A new technique for non-invasive measurement of the anterior depth has recently been developed using a scanning peripheral anterior chamber depth analyser.20

◆ The remainder of the slit-lamp routine will be covered in Part II.

References

1 Dave T. Advice on choosing the right slit lamp. OT, 2001:37-40.

2 Morris J, Hirji NK. The Slit Lamp Biomicroscope in Optometric Practice. Fleet: AOP/OT, 1998 Second Edition p1-11.

3 Dave T. Advice on choosing the right slit lamp. OT, 2001:37-40.

4 http://www.keeler.co.uk/refraction/slitlampscomparisontable.htm

5 Doshi S, Harvey W. Investigative Techniques and Ocular Examination. London: Butterworth-Heinmann, 2003: p28.

6 Eytan Z Blumenthal and Christos N Serpetopoulos. On Focusing the Slit-Lamp: Part I. An Inaccurate Ocular Setting - What is there to Lose? Survey of Ophthalmology, 1998 42 (4): 351-354.

7 Morris J, Hirji NK. The Slit Lamp Biomicroscope in Optometric Practice. Fleet: AOP/OT, 1998 Second Edition p1-11.

8 Doshi S, Harvey W. Investigative Techniques and Ocular Examination. London: Butterworth-Heinmann, 2003: p33.

9 Kohnen S. Light-induced damage of the retina through slit lamp photography. Graefe's Arch ClinExp Ophthalmol, 2000 238:956-959

10 Ong BL and Larke JR. Meibomian gland dysfunction: some clinical, biochemical and physical observations. Ophthalmic Physiol Opt, 1990 10(2):144-148.

11 CCLRU Grading Scales. Cornea and Contact Lens Research Unit, School of Optometry, University of New South Wales.

12 Efron N. Efron Grading Scales for Contact Lens Complications - Supplement to Contact Lens Complications. London: Butterworth-Heinmann, 2000.

13 Doshi S, Harvey W. Investigative Techniques and Ocular Examination. London: Butterworth-Heinmann, 2003: p38.

14 Morris J, Hirji NK. The Slit Lamp Biomicroscope in Optometric Practice. Fleet: AOP/OT, 1998 Second Edition p1-11.

15 Freeman MH. Optics, 10th edition. Oxford: Butterworth-Heinmann, 1990.

16 Doshi S, Harvey W. Investigative Techniques and Ocular Examination. London: Butterworth-Heinmann, 2003: p28.

17 Van Herick W, Shaffer RN, Schwartz A. Estimation of width of angle of anterior chamber. Incidence and significance of the narrow angle. Am J Ophthalmol, 1969 Oct68(4):626-9.

18 Christina A, Bruno and Wallace L, M Alward. Gonioscopy in primary angle closure glaucoma. Seminars in Ophthalmology, 2002 June17(2):59-68.

19 Doshi S, Harvey W. Investigative Techniques and Ocular Examination. London: Butterworth-Heinmann, 2003: p59.

20 K Kashiwagi, F Kashiwagi et al. Newly developed peripheral anterior chamber depth analysis system: principle, accuracy, and reproducibility. Br J Ophthalmology, 200488:1030-1035.

Charlotte McAllister is joint MSc course director and visiting clinican at City University

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