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Generally, it is appropriate to examine areas either side of the corneal midline with the illumination system on the same side as that which is being examined. So for temporal cornea the illumination system should be positioned on the temporal side of the observation system, and for nasal cornea it should be on the nasal side.¹

However, this does not have to be strictly adhered to as one of the major advantages of the slit lamp is its versatility. The movement of the coupled system around a central pivot offers the flexibility to manoeuvre the illumination system relative to the observation system. This allows the practitioner to refine and simultaneously employ more than one technique.

Parallelepiped (Figure 1)

Setup:

Corneal examination should be carried out with direct illumination and a parallelepiped setup. This allows a good quality three-dimensional image of the corneal layers – epithelium, stroma and endothelium (and the limiting borders between each) – to be viewed. The wider the angle, the greater the separation of the structures and, therefore, the easier it is to differentiate them.

The initial magnification should be medium, and then if necessary increased to closely examine any areas requiring further investigation (Figure 2). The cornea should be scanned systematically using this technique.

Optic section

Any corneal abnormalities detected while using the parallelepiped technique need to be accurately examined. One of the most important decisions with regard to this is to determine the location of any lesion, that is, the depth.

To do this, a modified version of the parallelepiped is adopted – the optic section (Figure 3). The slit width is narrowed to a ‘knife edge’ (1-2mm) and the rheostat setting turned to maximum intensity. This gives a very bright, thin, sharp slit within which a high quality ‘slice’ of the corneal tissue can be seen.

The magnification should be high and the angle between the illumination system and the eye-pieces increased to around 60 degrees.

The difference in the refractive index of each corneal layer changes the property of the light as it passes through them resulting in a high resolution image of each.

Apart from showing the depth of any foreign body or lesion this also gives a good indication of the regularity of the corneal curvature showing any areas of thinning (ectasis) or protuberance.

FURTHER INVESTIGATION OF THE CORNEA

In contact lens work or certain ocular conditions, such as pigment dispersion syndrome or the presence of keratic precipitates, it is important to take a close look at the corneal endothelium. More than one technique may be used to do this.

Specular reflection

Specular reflection has already been mentioned in the section on tear film assessment in Part 1. To examine the endothelium in this way the slit lamp should be set up in a similar fashion to that used for examining the tear film, except the instrument should be focused on Purkinje image II, located on the back surface of the cornea – the second most anteriorly placed image of the filament bulb. The corneal section will be slightly out of focus initially.

Under medium magnification with a slit width of 3-4mm (parallelepiped), the angle is then narrowed between the illumination column and the eyepieces until the section lies almost on top of Purkinje I and II. At this point, a dull greyish area can be seen to the nasal side of the section (Figure 4). This area represents a tiny snapshot of the endothelial layer which is small and limited by the size of the filament image. Most slit lamps do not offer high enough magnification to see the individual cells which make up the endothelial structure (around 80X is required to see this). Some slit lamps have a maximum magnification of around 40X which gives some idea of the integrity of the endothelium, showing up any anomalies in the structure such as polymegathism or polymorphism.² This manifests as areas which appear optically empty to the practitioner; these areas do not equate to ‘missing’ cells but rather are due to the irregularity of the endothelial layer at this point.

When cells become damaged the surrounding cells adapt in terms of their size and shape to repair the endothelium. Any consequent irregularity created by this process means that the endothelial surface can no longer act as a mirror. The light striking these areas is not reflected back down the observation system but instead bounces off these irregular cells in different directions resulting in the observer having the impression they are seeing a ‘gap’ where this has occurred.

Retroillumination

This illumination technique relies on using light bouncing off a structure located more posteriorly to that which is under observation. One such ocular structure used to create this setup is the iris, another example of such an object is the fundus.

Some of the light which falls onto the iris is diffusely reflected back towards the observer and in doing so illuminates any structures sitting more anterior to it, such as the cornea.

This technique is known as retroillumination. This type of illumination can be carried out with the instrument either ‘coupled’ or ‘decoupled’ depending on the location of the object under observation. For those areas of interest lying close to the limbus and, therefore, closer to the iris it is possible to examine this when the instrument is coupled, under low magnification. This is known as ‘direct retroillumination’. This technique improves contrast and allows any subtle changes to be seen in this diffuse light without unwanted reflections obscuring the view.

Alternatively, if the area of interest lies more centrally³ or higher magnification is required, then the instrument needs to be decoupled to ensure that the area under observation is sharply in focus.

By focusing the coupled instrument on the area under examination, the observer should then decouple the illumination system and manipulate it so that the light hitting the area of interest now bounces off a posterior region to the side (not directly behind it) of the structure being assessed. This allows the structure to be observed against a dark background and this method is known as ‘indirect retroillumination’. The observation system should remain focused and locked in the original position (Figure 5). A combination, therefore, of direct, indirect and retroillumination is required to fully assess all aspects of the cornea.

EXAMINATION OF THE LIMBAL VESSELS

Indirect illumination

Setup:

Limbal vasculature can be hard to assess using direct illumination – vessel location relative to the higher contrast cornea and white sclera means that differentiating the limbal detail can be difficult due to the bright reflection and glare off the surrounding tissue.

To counteract these problems, the easiest method is to use indirect illumination. The slit height can be lowered to help reduce unwanted reflections and glare from the adjacent tissues.

Using this set up to examine, for example, nasal limbal vessels, the illumination column can be moved to the nasal side, the same side as that being assessed.

This further reduces unwanted reflections further and with the beam forming a clear image of nasal cornea light on either side of the bright image shows up as a greyish glow in which the limbal vessel arcade can be seen along with any vascular changes which may be found, for example, with contact lens induced hypoxia (Figure 6).

The vessels are difficult to see and this technique requires practice to master. To enhance the image the red-free filter can be used to improve the contrast and increase the magnification to give a more detailed examination.

The vessels should be assessed around the entire limbus as localised changes may be apparent.

ANTERIOR CHAMBER ASSESSMENT

Assessment of the dimensions of the anterior chamber is important but as part of a slit-lamp routine any anterior chamber activity should also be noted.

Certain eye conditions may present with anterior chamber activity or evidence of previous episodes of anterior chamber inflammation such as those found with anterior uveitis. The anterior chamber is normally an optically empty space which allows light to pass straight through it as it does not contain anything for light to reflect off.

Anterior chamber activity may present with characteristic features such as cells and flare⁴ which the practitioner can assess in a number of ways using the slit lamp.

Conic section

The conic beam should be initially focused on the front surface of the cornea, and the instrument then moved forward until the focus is directed halfway between the cornea and the lens behind, so that both structures are equally out of focus.

A quiet anterior chamber will show a dark space in between the two blurred images of the cornea and the lens. Any activity such as flare and cells will show up in the beam of light projecting across the anterior chamber as grey ‘mist’ with white cells moving within it illuminated in the beam (Figure 7).

The cells and flare show up due to the Tyndall effect – when the light strikes these objects in the anterior chamber the light is scattered and reflected back down the observation system.

Oscillating the slit-lamp system back and forth allows the visibility of the cells and flare to be increased using a combination of direct and indirect illumination.⁵

To further increase visibility of any objects in the anterior chamber ask the patient to look up and then straight ahead as this causes the contents to move around and settle back down under the influence of gravity, stirring up any objects of interest which may then show up within the light beam.

Grading cells and flare

It is useful to measure the degree of anterior chamber activity using a known grading system.⁶ The number of cells visible within the light beam should be counted and graded accordingly (Tables 1 and 2).

The final part of this series will address assessment of the retrolental area and will provide a review some of the filters and attachments in common use.

References