Ординатура / Офтальмология / Английские материалы / Ophthalmology Investigation and Examination Techniques_James, Benjamin_2006

Box 2.3

Corneal and conjunctival dyes

■First make sure that patients are not wearing contact lenses. These dyes will stain soft lenses!

■Warn patients that the drop will cause the vision to change, appearing yellow (fluorescein) or crimson (rose Bengal)

■Warn the patient that the drops will sting, particularly rose Bengal

■A complete assessment requires that the upper lid is gently raised

Fluorescein

This is readily available as a strip which requires wetting with normal saline or a 2% sterile unit dose solution. It absorbs light in the blue wavelength and emits a green fluorescence. At 0.1% it is highly fluorescent but at 2% it is non-fluorescent

Precorneal tear film

Following application of a dilute solution of fluorescein (e.g. by applying a drop from a wetted strip to the lower tarsal conjunctival surface), patients are asked to blink and then keep the eye open. The fluoresceinstained tear film, illuminated with a cobalt blue filter, initially forms a uniform coating over the eye which gradually breaks down. The time it takes for the film to start to degrade is recorded. It should normally be more than 10 s.

Topical fluorescein is also used in the assessment of epiphora (see Ch. 8)

Epithelial defects

A dilute solution is again applied to the eye and the cobalt blue filter used for illumination. Conjunctival and corneal epithelial lesions are highlighted (Fig. 2.21). Fluorescein stains cellular defects and intercellular spaces, not cells themselves. It will thus show corneal abrasions. It is also used to assess patients with dry eyes. It is important to observe the staining pattern as quickly as possible following application because the

dye rapidly diffuses and punctate staining will become poorly delineated. This observation is usually performed immediately after the tear break-up time has been measured. To maximise the visibility of any staining, a yellow-orange barrier filter should be used over the slit-lamp objective to remove the scattered blue light from the sclera, which degrades the image

Aqueous leaks

A more concentrated 2% solution is used to detect aqueous fluid leaking from the eye. The leaking aqueous dilutes the fluorescein, which starts to fluoresce as the concentration falls, becoming bright green (Fig. 2.22)

Rose Bengal

This iodine derivative of fluorescein stains dead and dying epithelial cells of the cornea and conjunctiva which have lost their mucin coating. It will not stain cells that are covered by mucin. It will also stain the mucus of the precorneal tear film. A 1% solution is applied to the upper bulbar conjunctiva of the eye following the application of a topical local anaesthetic (the drop is more painful than fluorescein). Staining of the skin can be avoided by pulling the lower lid down at the time of instillation; a tissue is used to mop up excess dye before the lid is released. Any further spillage of dye should also be rapidly mopped up. The staining pattern can be viewed using red-free light; if a white light is used, contrast is improved by placing a green filter over the microscope objective lens. To avoid prolonged discomfort for the patient it is better to wash the dye out of the eye at the end of the examination with normal saline solution. The dye is particularly useful in examining patients with keratoconjunctivitis sicca (Fig. 2.23). An alternative dye is lissamine green, which is less irritant. It can be obtained in strips or solution. The staining pattern of both these dyes persists longer than fluorescein

The lens is best viewed through a dilated pupil. This is particularly important if looking for pseudoexfolliative material on the anterior lens surface (Fig. 2.26). Look for abnormalities in the shape

of the lens (e.g. posterior lenticonus). The type and position of any lens opacity are next observed using both direct and retinal retroillumination (Fig. 2.13). Determine the type of cataract present. This can be

Fig. 2.26 Pseudoexfoliation.

graded according to the lens opacities classification system (LOCS III). This uses a system of coloured direct and retroillumination pictures to grade degrees of nuclear, cortical and subcapsular cataract.

If the patient has an implant lens following cataract surgery, determine the clarity of the posterior capsule. This may again be made easier if it is viewed by retinal retroillumination.

Abnormal movements of the lens (phakodonesis) and iris (iridodenesis) should also be recorded. The observer focuses on the pupil and the patient is asked to look to the side and then straight ahead again. The observer notes any iris or lens shake as the eye comes to a sudden stop in the primary position.

The anterior vitreous is the last part of the eye that is readily visible without the aid of special lenses. Look for the presence of cells or pigment granules and condensations of the vitreous gel. The angle between the microscope and the illumination will have to be small, particularly if the deeper vitreous is to be visualised. The illumination column should also be tilted. Lenses used to examine the retina (for example, the 90 D; see Ch. 5) can be used to examine the posterior vitreous.

The examination continues with tonometry if appropriate. The retina and iridocorneal angle are

then viewed through special lenses (gonioscopy, see Ch. 4).

The experienced slit-lamp user will be able to perform an accurate assessment of an eye quickly: it requires much practice, however.

Summary

It is important to spend time learning the basic controls of the slit-lamp methods of illumination and use of attachments and special lenses. Correct examination technique and an orderly progression through the structures of the eye will enable what might otherwise be subtle abnormalities to be readily seen.

■The slit lamp is essential for a detailed examination, in three dimensions, of the anterior segment

■It utilises reflections from various optical interfaces to visualise different structures

■Illumination may be varied in height, intensity, angle, colour and concentricity

■Magnification to a cellular level (endothelium) is available

■Various pieces of adjunctive equipment allow examination of the posterior segment, the drainage angle, corneal thickness, anteriorchamber depth and intraocular pressure

Further reading

Bron AJ, Evans VE, Smith JA. Grading of corneal and conjunctival staining in the context of other dry eye tests. Cornea 2003; 22: 640–650.

Smith RJH. A new method of estimating the depth of the anterior chamber. Br J Ophthalmol 1979; 63: 215–220.

van Herick W, Shaffer RN, Schwartz A. Estimation of width of anterior chamber. Incidence and significance of the narrow angle. Am J Ophthalmol 1969; 68: 626–629.

CHAPTER 3

Tonometry

BRUCE JAMES

Introduction

Measurement of intraocular pressure is key to the diagnosis and treatment of glaucoma. Manometric measurement, whilst being the most accurate method, requires the insertion of a cannula into the eye and is not clinically possible. Indirect methods (tonometers) for assessing intraocular pressure have thus been developed. There are two major types of tonometer: indentation and applanation.

Indentation tonometry

The Shiotz indentation tonometer measures the extent to which a plunger of known weight indents the globe in a supine patient (Fig. 3.1). The higher the intraocular pressure, the less the plunger is able to indent the cornea. Intraocular pressure is measured by converting the scale reading with a nomogram. Errors in measurement occur because not all eyes will respond to indentation in the same way. Variation in ocular rigidity and corneal shape will necessarily lead to errors in measurement. The weight of the tonometer on the eye is also responsible for a gradual reduction in pressure. This

Fig. 3.1 The Shiotz tonometer.

3

method of intraocular pressure measurement has largely been replaced by applanation tonometers.

Applanation tonometry

The Goldmann applanation tonometer remains the gold standard against which other tonometric methods are assessed (Fig. 3.2). It is used in conjunction with the slit lamp. Its principle is based on the Imbert–Fick law, which states that:

An external force (W) against a sphere equals the pressure in the sphere (Pt) times the area of flattening (A) (assuming the sphere to be perfectly spherical, dry, so no other forces such as surface tension effects are involved, infinitely thin and flexible).

W = Pt × A

The eye does not fulfil any of these criteria, however! The technique relies on an assumption that for a particular area of flattening of the inner aspect of the cornea, surface tension pulling the head of the tonometer towards the eye is balanced by the lack of flexibility of the cornea. An external diameter of corneal flattening of approximately 3 mm (no greater than 3.5 mm) allows this assumption to be made. Additionally, allowance must be made for the effect of the thickness of the cornea, which is obviously not infinitely thin, thus the area flattened on the outside of the cornea is not the same as the area flattened on the inside. It is further assumed that if the displaced volume resulting from corneal flattening is small, then ocular rigidity will not significantly affect the result.

The Goldmann tonometer

The Goldmann tonometer allows the application of a variable force to a transparent head or prism which

Tonometry

a

b

Fig. 3.2 (a) The Goldmann tonometer;

(b) disposable prisms for the tonometer.

contacts or applanates the cornea. The diameter of the head is set at 3.06 mm. This meets the theoretical requirements laid out above and simplifies the conversion of the applied force into an intraocular pressure measurement. An external force of 1 g flattening an area of cornea with a diameter of 3.06 mm corresponds exactly to an intraocular pressure of 10 mmHg. If the pressure is higher, more

force must be applied to the head to flatten this uniform area of cornea; if the pressure is lower, less force must be applied.

When the head touches the eye a meniscus forms. The applied force is increased to enlarge the area of contact and size of meniscus until the endpoint is reached, when both have a diameter of 3.06 mm. Observing the meniscus through a split prism contained within the head reveals it displayed as two separate semicircles (Fig. 3.3a). These gradually increase in size until they just overlap. This is the point at which the correct area of the cornea has been applanated (Fig. 3.3b). The force required to achieve this uniform area of corneal flattening, and thus the corresponding intraocular pressure, can then be read from the tonometer scale. The semicircles must be equal; if not, the cornea has not been properly applanated (Fig. 3.3c).

Measurement of intraocular pressure with the Goldmann tonometer

•The patient is seated at the slit lamp; any tight neckwear should be loosened. The technique is explained and the patient asked to breathe normally. The patient is warned that the anaesthetic drops will sting but that the procedure itself does not hurt; the fluorescein will make things appear yellow for a short time. The slit lamp is set as usual on low magnification. The cobalt blue filter is inserted in the illumination path and the slit diaphragm opened completely, with the lamp switch turned to maximum illumination. Reduced illumination may cause intraocular pressure to be underestimated.

•A drop of local anaesthetic is instilled in both eyes with the addition of a weak fluorescein solution (proxymetacaine 0.5% + fluorescein is probably the least painful and is convenient, ensuring a repeatable concentration of fluorescein every time). Applanation without fluorescein significantly underestimates intraocular pressure.

•The tonometer head is cleaned. Solutions of 70% isopropylalcohol and 3% hydrogen peroxide can be used to sterilise the tip, but it is also vital to ensure that there is no organic matter on the surface of the tonometer and that any disinfectant is removed prior to applanation. Concern about the spread of slow viruses and

possible transmission by tears has led to the production of disposable tonometer heads and disposable caps (Fig. 3.2b). It is likely that these will become increasingly used.

•The tonometer is located on the slit lamp base plate. It is viewed uniocularly, set either for the observer’s right or left eye by placing the pin on its base into either the right or left hole of the horizontal guide plate on the slit lamp. The microscope is perpendicular to the eye. This may require the microscope to be turned nasally to allow for convergence by the patient. The correct positioning of the tonometer arm is indicated when it clicks into position. The illuminating column is placed at 60º to illuminate the end of

the head. On some heads a coloured band indicates where the illumination should be focused. In some cases the tonometer is attached to the accessory peg and is swung into position. The measuring drum is set to about 10 mmHg (Fig. 3.4).

•The tonometer head is slowly advanced until it just contacts the eye; observing from the side, the limbus will suddenly glow with a blue light. The examiner may have to hold the lids apart gently. If this is done it is important not to use force or apply pressure to the globe, as this may cause an artificial elevation of intraocular pressure. It is usually possible to rest the fingers holding the lids on the orbital rim. The observer

Fig. 3.4 Setting up the tonometer for use.

now looks through the eyepieces, gently manipulating the microscope until the two semicircles are viewed. They should be equal in size and shape, about 0.3 mm thick and in the middle of the field of view.

•By rotating the measuring drum the semicircles are made to overlap on their inner edge

(Fig. 3.3). It is usual to see a pulsatile movement of the semicircles. This corresponds to the inflow and outflow of blood from the eye during each heart beat (Fig. 3.5). If there is significant pulsatile movement the semicircles are adjusted so that the inner-edge overlap represents the midpoint of their excursion.

•The pressure is read from the rotating drum, multiplying the figure by 10 to find the intraocular pressure in mmHg.

Problems encountered in using the Goldmann tonometer

•The fluorescent band is too wide. This usually occurs if there is a deep tear meniscus or if the lids are in contact with the tonometer head. Dry the tonometer head and start again, otherwise the pressure will be overestimated.

•The fluorescent band is too narrow. The tear film is insufficient. Withdraw the prism and ask

Fig. 3.5 The pulsatile variation in intraocular pressure.

the patient to blink several times. A narrow band underestimates the pressure.

•There is a large overlap of the semicircles unresponsive to rotation of the measuring drum. In this case, the tonometer head has been pressed too firmly against the eye. Withdraw the microscope and start again.

•Repeated measurement of intraocular may result in lesions of the corneal epithelium, which will stain with the fluorescein dye. These are rarely severe and cause the patient no distress, although they may cause a little temporary blurring of vision.

•Multiple measurements of intraocular pressure may lead to a gradual reduction in pressure readings due to massaging of the eye (tonographic effect). The first reading in a patient is often higher than repeated second readings, particularly if the patient is anxious and squeezing the eye. If this is the case, the first reading should be discarded.

•The pressure unexpectedly becomes very low. Beware: the patient is probably about to faint!

Measurement in patients with significant astigmatism

The tonometer head is marked in degrees between 0 and 180 (Fig. 3.6). If there is less than 3 D of

Fig. 3.6 Adjusting the tonometer prism in patients with significant astigmatism. Note the white and red lines on the tonometer housing.

astigmatism, the head is aligned horizontally. That is, the 0º mark is aligned with the white line on the head-holder. If more than 3 D of astigmatism is present the semicircles will be elliptical and the pressure not correctly estimated unless the tonometer head is rotated such that it is positioned at 43º to the meridian of the lowest power. The 43º position is indicated with a red line on the prism housing (Fig. 3.6).

Example

Corneal astigmatism 44 D 50º, 40 D 140º: the tonometer head is rotated until the 140º mark is aligned with the red line on the holder. If focimetry is used to determine astigmatism from the patient’s glasses, the head is set to the axis of the minus cylinder.

An alternative technique proposed by Holladay et al. (1983) for regular astigmatism suggests measuring the intraocular pressure with the head in the horizontal position and then at 90º in the vertical position. The two intraocular pressure readings are then averaged.

Patients with irregular astigmatism or abnormal corneas may have unreliable intraocular pressure measurements made with the Goldmann tonometer.

Calibration

The Goldmann tonometer should be regularly calibrated. The calibration arm fits into a slot on

Fig. 3.7 Calibrating the Goldmann tonometer. Ensure correct alignment of the calibrations on the rod and the holder by viewing them perpendicularly.

the side of the tonometer (Fig. 3.7). The rod is positioned so that the central mark is aligned with the mark on its holder. The measuring drum is placed at 0; the pressure arm should gently rock forwards and backwards with slight pressure. Moving the measuring drum between –0.5 and +0.5 mmHg should likewise cause the pressure arm to rock. The rod is advanced to the next mark and the process repeated at 20 mmHg and then 60 mmHg. The tonometer arm should rock between 19.5 mmHg and 20.5 mmHg and 59 mmHg and 61 mmHg respectively. In practice, a slightly greater tolerance may have to be accepted. If the calibration is incorrect, the tonometer must be returned to the manufacturer.

A simple way to test calibration, which should be used at the start of each clinic, is to check that the arm rocks around zero by moving the dial

0.5 mmHg (the width of the calibration mark on the scale) either side of zero with the prism in place. Once again, a unit may decide on a slightly higher tolerance. If the tonometer is calibrated at 0 mmHg it is unlikely to be significantly out at 20 and 60 mmHg, although these levels should be checked periodically, as described above.

The effect of corneal thickness

Recent research has suggested that in patients with thin corneas the intraocular pressure may be underestimated whilst in those with a thick cornea the pressure will be overestimated.

The Ocular Hypertension Treatment Study has graphically demonstrated the importance of corneal thickness in tonometry. Patients with a thick cornea were much less likely to convert to glaucoma than those with a thin cornea and thus a thicker sclera. Although structural protective effects of a thick cornea cannot be ruled out, it is likely that the finding is most readily explained by an overestimation of intraocular pressure in patients with thick corneas. Similarly, the cornea has been shown to be thinner in patients with normal-tension glaucoma, suggesting that in these patients the intraocular pressure may be underestimated.

There is also concern that corneal laser refractive surgery may alter the accuracy of applanation pressure measurement as the corneal thickness is reduced by laser refractive surgery.

No algorithm enabling an accurate allowance for corneal thickness on intraocular pressure yet exists. Nonetheless, pachymetry (see Ch. 6) is increasingly important in the glaucoma clinic, particularly in ocular hypertension, to allow an approximate correction of intraocular pressure to be made. The automated conversion programs that some ultrasound pachymeters have for adjusting intraocular pressure to the measured corneal thickness should be used with caution.

Additional tonometers

Perkins tonometer

In some patients positioning at a slit lamp may be impossible because of immobility or because they are under an anaesthetic. In these cases a hand-held version of the Goldmann tonometer can be used (Fig. 3.8). The principle of operation is the same.

Fig. 3.8 The Perkins tonometer.

Anaesthetic and fluorescein are applied to the eye. The instrument is switched on by rotating the knurled measuring wheel; again this should be set at 10 mmHg. The light with a prefitted cobalt blue filter illuminates the end of the prism. The forehead rest is fitted to the top of the tonometer.

With the patient comfortably positioned the rest is applied to the forehead and the tonometer head gently moved towards the eye. When the limbal blue flush is seen the observer views the tonometer head through the fitted magnifying lens. The endpoint is exactly the same as with the Goldmann tonometer. Care must be taken to ensure that the light is as strong as possible; even at maximal brightness it is considerably dimmer than that available on the slit lamp.

The Pascal tonometer

This new tonometer (Fig. 3.9) has an electronic pressure sensor embedded within a contour-matched (concave) tonometer tip. This tip has a shape similar to that of the cornea. The cornea takes up the shape of the tip and then the pressure sensor measures intraocular pressure. It is thought that by measuring pressure in this way the effects of corneal thickness and other biomechanical properties of the cornea are removed. The tip is also covered with a disposable rubber membrane, improving the sterility of the procedure.

The rapid acquisition of pressure measurements (100 per second) allows the pulsatile change in intraocular pressure to be measured. A mean pressure, together with a measurement of the difference between systolic and diastolic pressure (pulse amplitude), is displayed on the screen. The tonometer also assesses the quality of the pressure recording. The exact role that this device will have remains to be ascertained.

Non-contact tonometers

These are primarily for screening use. A number of different types are available (Fig. 3.10). They also work on the applanation principle but use a jet of air to flatten a uniform area of the cornea. The intraocular pressure is calculated from the time taken to flatten the cornea or the force of air needed to flatten the cornea. As the reading is instantaneous, the average of three or four readings should be taken. This machine is most commonly used to screen patients for raised intraocular pressure.

Tonopen XL

This is a small, extremely portable machine (Fig. 3.11). The area of contact with the cornea is only 1.5 mm2. It uses a strain gauge working on the Mackay–Marg principle. The pressure exerted by

the applanating plunger on the cornea is recorded electronically from the strain gauge. A liquid crystal display (LCD) screen displays the average result of four readings. A disposable rubber tip makes it particularly useful in patients with possible ocular infection. The small area of contact may also make it suitable for use in patients with corneal abnormalities, although significant measurement errors may still occur in these patients.

The eye is anaesthetised and, once the calibration procedure has been undertaken, the tonopen is gently placed on the cornea. A beep will be heard when the reading is taken and the individual reading will appear on the screen. An average will appear with 4–6 readings together with a percentage indicating the confidence level of the reading.