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There are no short-cuts to gaining proficiency and so when starting it is necessary to practice gonioscopy with as many eyes as possible, rather than reserving the technique for just those eyes in which it is especially important. Contact lens patients are often good for improving familiarity with the technique because these individuals are used to having something in the eye, and no-fluid goniolenses can even be applied to the surface of a soft lens without the instillation of an anaesthetic. Images are better than any textual description and so I recommend viewing the excellent video clips at www.gonioscopy.org. If possible it is also extremely helpful to arrange to observe at a local glaucoma clinic.
Preparation
Instruction
Patients need to be told why it is necessary to put a lens on their eye. As a suggestion of what to say: 'I want to look in the corners of the eye where its internal fluid drains. This is because problems with the drainage of fluid from the eye can cause glaucoma. The drain of the eye is hidden from view and to see it I need to put a lens on to the tear film, on the eye's surface'. Patients should be reassured that the eye will be numbed and so the procedure will not be painful, although they may have an awareness of gentle pressure or coldness. Some clinicians suggest that warming the goniolens improves tolerance. If a Goldmann-type lens is to be used then patients should also be warned that during the procedure coupling solution may dribble onto the face, and that this is not harmful.
Set up
A topical anaesthetic is instilled before gonioscopy. Alternatively, with no-fluid goniolenses it is possible to not anaesthetise the eye by the prior insertion of a soft contact goniolens. Anaesthetic instillation does not need to be repeated if it has already been done for applanation tonometry, although in patients with twitchy eyelids or where a longer than normal examination is anticipated I prefer to instil a second drop. One of the keys to successful gonioscopy is that the clinician and patient be in a correct and comfortable position. Some clinicians find it helpful to support the elbow to enable steady holding of the goniolens on the eye.
Technique
Goldmann-type lens specifics
The goniolens should be positioned with its concave surface upwards to receive a viscous coupling solution, such as carboxymethylcellulose (eg Celluvisc) or carbomer gel (eg Viscotears). The lower the viscosity of the coupling solution the greater the chance of the goniolens lifting from the eye during the examination and so introducing air bubbles that disturb viewing, particularly when turning the lens on the eye. However, advantages of lower viscosity coupling solutions are that they are less likely to interfere with subsequent viewing of the fundus or leave the patient with sticky eyes and blurred vision. It is useful to store the bottle of the coupling solution upside-down to avoid air bubbles. With very viscous solutions it may also be of value to start a stream of fluid on a tissue before transferring the flow to the lens. If air bubbles do gather beneath the goniolens when on the eye that obstruct viewing the only remedy is to remove the lens, wipe away the coupling solution and start again.
To apply the goniolens the patient should be positioned at the slit lamp and asked to look upwards. While looking around the side of the biomicroscope and holding the lens (left hand for right eye and vice versa), corneal side up and tilted at about 45° towards the globe, the little finger of the hand holding the goniolens is used to retract the lower eyelid of the patient. Then the lower edge of the lens engages and fully retracts the lid downward. Finally, the goniolens is touched onto the eye and rapidly tilted into an upright position as the patient is asked to slowly look straight ahead. This should expel any trapped air upwards.
When viewing the angle, most clinicians apply the lens so that the mirror is at the top of the eye. This is to allow the inferior angle to be examined first. The inferior portion of the angle is typically the widest and is where the trabecular meshwork has the most pigment, and so is the easiest to identify structures and become familiar with the appearance of a patient's anatomy. After the inferior angle has been assessed, the goniolens is rotated to view another portion of the angle. When the angle has been examined in its entirety the goniolens is sometimes suckered on the eye and resists removal. The air-tight seal between the goniolens and the eye can usually be broken by asking the patient to blink hard. If this is not successful, the lens can be slid towards the sclera across the change in surface curvatures. Rarely it is necessary to apply pressure through the edge of the lens to indent the globe to break suction.
Zeiss-type lens specifics
Coupling solution is generally not needed with these flatter goniolenses. However, a small drop of artificial tears to the concavity of the lens improves optical contiguity in patients with dry eye or marked corneal irregularity, and may minimise epithelial trauma. To apply the goniolens the patient is positioned at the slit lamp and asked to look straight ahead. The handle of the goniolens, or the lens itself, is held between thumb and forefinger, with the remaining three fingers braced against the patient's face (left hand for right eye and vice versa). I personally use the index finger to support the upper eyelid against the upper bony rim of the orbit in all but the most cooperative of patients. While looking around the side of the biomicroscope the goniolens is applied directly onto the centre of the cornea. A delicate touch is needed to apply the correct level of pressure with the lens. Too much pressure causes Descemet's membrane to wrinkle, which prevents a quality view of the angle and, of more concern, forces aqueous into the recesses of the angle and artifactually deepens the angle. Too little pressure allows air bubbles to gather beneath the goniolens, which prevents viewing of the angle. The optimal level of pressure has very narrow tolerance limits, and so air should constantly be seen darting in and out under the goniolens when viewing through the slit lamp. The mirrors of the goniolens are initially put on in a square position, rather than a diamond configuration, because it is more comfortable if the patient blinks. The inferior angle is usually examined first using the top mirror. Other portions of the angle are then examined, but without the rotation required for most Goldmann-type goniolenses. When all mirrors have been inspected, if it is considered necessary to view the small portions of the angle not visible when the goniolens is held squarely, the goniolens can be minimally turned on the eye, or, to allow the patient to blink and reduce epithelial damage, removed and applied again.
Mirrored imagery
The portion of the angle viewed is that opposite the mirror. A mirrored image of the angle is seen, which differs to an inverted image as is formed with indirect ophthalmoscopy. For example, when viewing the superior angle in the bottom mirror of the goniolens the image is upside-down but not laterally reversed (Figure 1). Relating what is seen in the mirrors of a goniolens to a location in the angle can be confusing due to mirrored imagery. A helpful strategy when unsure is to turn the goniolens so that the lesion is seen in the centre of the mirror, because at this point the location of the lesion in the angle is exactly opposite.
Viewing methods
Gonioscopy is a dynamic technique. The illumination characteristics are varied and the goniolens is manipulated to help appreciate subtle findings. Regarding lighting, the room lights can be on or dimmed, and the brightness, size and orientation of the slit beam can be adjusted. The attention of the clinician may be focused on the area directly illuminated or just off to the side.
When assessing if an angle is open or closed, and if open the likelihood of closure, the testing conditions are critical. The room lights are dimmed, the illumination and the height of the slit beam are decreased so that it does not impinge on the pupil and cause pupillary constriction with attendant artifactual opening of the angle. In this regard, there is a conflict with ease of viewing. Having assessed the occludability of the angle, the next task is to ascertain other features of interest in the angle: pigment, pseudoexfoliative (PXF) material, anterior synechiae, recession, neovascularisation, etc. For this purpose it is permissible, and often helpful, to increase the room and slit lamp illumination. Indeed, the view of angle structures is improved when the pupil is constricted, particularly in narrow angles. Some clinicians like to rotate the slit beam so that it is always perpendicular to the angle, whereas others feel that this additional manipulation is not necessary.
A specific technique that is valuable in identifying structures in eyes with very little or excessive trabecular pigmentation, or other causes of confusing anatomy, is the corneal wedge. To set up the corneal wedge the slit beam is narrowed and the illumination system is displaced to create an optical section of the cornea (Figure 2). When correctly focused, the corneal light reflex from the inner aspect of the cornea will be sharp, and that from the outer corneal surface will be slightly hazy. The latter curves as it approaches the angle to meet the reflex from the inner surface, meeting at Schwalbe's line, where after they continue as one line down the angle structures and onto the iris. By pointing to Schwalbe's line the corneal wedge locates the top of the trabecular meshwork. Viewing of the corneal wedge is challenging. Its identification is assisted by turning room lights down and ensuring adequate brightness of the slit beam. The beam must be narrow, and the observation and viewing systems of the slit lamp must be offset, but not so much that the two no longer point down the barrel of the goniolens. The corneal wedge is easier to see with Goldmann-type goniolenses because in these the mirror has a relatively low angle of inclination and so gives a better cross-sectional view of the cornea, which in turn can be followed down to locate the wedge. With Zeiss-type goniolenses the tilt of the mirror may be reduced by sliding the goniolens in the direction of the mirror being used. It should be noted that the corneal wedge never appears as obvious as it does in non-photographic representations in textbooks. Owing to the design of slit lamps, it is usually only possible to view the corneal wedge in the superior and inferior angle because decoupling of the illumination and observation system is only possible horizontally. However, this is normally sufficient to familiarise oneself with the appearance of the angle structures in an individual.
The examination of narrow angles is often difficult because a convex iris obstructs the view of the angle from a mirror located close to the opposite limbus. Owing to their hidden recesses, these angles are susceptible to being misdiagnosed as closed. Disparity or parallax of a slit beam on the inner wall of the eye and the iris, using the set up described above for a corneal wedge, indicate that the depths of the angle are not being seen. The view in these situations can be improved by asking the patient to look towards the mirror being used (Figure 3a). Only a small change in viewing direction is needed. It is my experience that, regardless of the completeness of instruction, most patients move their eyes too much without a visual target, often causing the goniolens to eject. To improve control I find it helpful to ask the patient to follow my finger of the non-goniolens holding hand and slowly move it from the straight ahead position until the desired view of the angle through the slit lamp is obtained. This procedure alters the tilt of the mirror relative to the microscope, such that the line-of-sight of the slit lamp viewing system now strikes higher up the mirror and has a steeper angle of approach into the angle. An alternative technique that does not require the patient to alter their direction of gaze is to slide the mirror of the goniolens centrally, towards the angle being examined, and thus away from the mirror being used (Figure 3b). Note that this is the opposite direction to that recommended to identify the corneal wedge. This second technique is quicker because it avoids an explanation to obtain patient cooperation, but is only possible with goniolenses that have a relatively small area of contact and so allow for mobility on the eye. An inability to see the trabecular meshwork or the apex of the corneal wedge when the angle is viewed optimally indicates angle-closure.
Increasing the light level, which induces miosis and centripetal movement of iris tissue, or applying pressure to the eye through the goniolens, can be used to deliberately try and open a closed portion of the angle. The latter technique is known as indentation gonioscopy and is more dependable, but can only be performed with relatively flat, small contact diameter goniolenses. If structures previously hidden become visible with these methods the angle-closure was appositional (touching) rather than synechial (adherent). This indicates that an iridotomy or cataract extraction would likely be successful in opening up the angle, whereas, in synechial closure the angle would remain closed. Indentation gonioscopy can also be helpful in differentiating relative pupil block from the more rare plateau iris. In relative pupil block the iris indents smoothly, whereas in plateau iris assumes a 'sine wave' configuration as it indents maximally centrally and drapes peripherally over anteriorly located ciliary processes.
Angle grading systems
There are numerous systems for grading the anterior chamber angle, each with their virtues and proponents. It is, however, important to appreciate the limitations of reducing complex anatomical relationships to a single figure. It should not be assumed that angles with the same grade are equivalent, or that they have an equal probability of closure.
The most commonly employed grading systems in use today are based on those developed by Shaffer and Spaeth. The Shaffer method simply estimates the angle that the iris makes with the trabecular meshwork and posterior corneal surface in four quadrants. Practically, the angle width is usually determined by the ease at which the various angle structures can be seen, or conversely, by the structures obscured by the iris (Table 1).
The Spaeth system expands on this method and formally assesses other relevant angle characteristics. It sequentially records the level at which the iris apparently inserts (A-E), the geometric angle of iris contact in degrees, the peripheral iris contour (r, s, or q), and the degree of trabecular pigmentation (TMP 1 minimal to 4 very heavy) (Table 2). For example, a Spaeth grading of B20.r.TMP-2 refers to an iris that inserts just behind Schwalbe's line at an angle of 20°, with a regular contour and moderate trabecular pigmentation. Such a grading would be typical for a hypermetropic eye with a narrow angle.
Angle anatomy
The structures visible in a wide open anterior chamber angle from posterior to anterior are the iris, ciliary body, scleral spur, trabecular meshwork, Schwalbe's line, and cornea.
Iris
When performing gonioscopy, the contour of the iris plane and its surface should be assessed. A slight convexity of the iris is common. This occurs due to relative pupil block and tends to be greater in hypermetropic eyes. This forward bowing is grossly exaggerated when pupil block is absolute, caused either by primary or secondary mechanisms. Conversely, flat or minimally concave irides may occur in myopia, pseudophakia, and aphakia. Pronounced concavity is often encountered in pigment dispersion syndrome (PDS), where in anatomically predisposed eyes, reverse pupillary block is created when aqueous humour is forced from the posterior chamber into the anterior chamber by the action of blinking.
After surveying the general iris contour, attention should be directed to its angle of approach and the position at which it appears to insert into the ciliary body or other angle structures. A narrow anterior chamber angle is more at risk of angle-closure than one that is widely open. Normally, the angle is narrowest superiorly and widest inferiorly, possibly due to a combination of eyelid forces and gravity. As a general rule, anterior chamber depth, iris convexity, and anterior chamber angle width are correlated. However, exceptions do occur, exemplified by iris plateau configuration/syndrome, which is characterised by a normal anterior chamber depth and an iris that remains relatively flat until its periphery, whereupon it sharply turns, draping over anteriorly displaced ciliary processes to create a narrow angle recess.
Blood vessels on the surface of the iris are rarely seen in brown irides, but are relatively common in fair iris colours. Normal iris blood vessels tend to have a relatively thick calibre, and a radial or circular orientation. In contrast, abnormal new blood vessels are finer, lacy, tortuous, and have a random orientation. It is extremely rare to see normal blood vessels crossing the scleral spur to encroach on the trabecular meshwork. Early angle neovascularisation may only be observable as a reddish hue. As neovascular disease progresses, the fibrovascular membrane contracts and leads to the development of posterior anterior synechiae (PAS). The fine, wandering abnormal blood vessels in Fuch's iridocyclitis only rarely cause PAS.
Ciliary body
The ciliary body is most commonly seen as a dull-brown band, although it may appear slate-grey or pink in lighter eyes. Its visibility and width depends on the position of iris insertion. An unusually wide area of ciliary body exposure, particularly when it is irregular or asymmetric, indicates angle-recession. This occurs due to a tear between the longitudinal and circular muscle fibres of the ciliary body following blunt trauma. Although it does not directly cause glaucoma, angle-recession is a marker for concomitant trabecular damage and, by a degree proportional to its circumferential extent, is a significant risk factor for elevated IOP and glaucoma, although this may not occur until years after the original trauma.
Scleral spur
The scleral spur is an internal projection of scleral tissue, identified as a white opaque line that yellows with age. This structure is difficult to see when the angle is narrow, but when visible is of considerable value as a landmark in gonioscopy. Strands of superficial iris tissue often reflect in the angle to extend over the ciliary body to insert into the scleral spur. In light-coloured eyes these iris processes are pale and subtle, and are often missed with diffuse illumination. In brown eyes they are darker and stand out against the scleral spur. Iris processes are benign and inconsequential, unless they extend over the trabecular meshwork to connect to a posterior embryotoxon where they constitute Axenfeld's anomaly. Iris processes need to be differentiated from PAS, which tend to have a broad base and extend beyond the scleral spur, and functionally in that they impede aqueous outflow. PAS with fine points of adhesion can occur following laser trabeculoplasty if the laser is directed too far posteriorly, its energy is too high, or the angle is very narrow.
Trabecular meshwork
The trabecular meshwork extends anteriorly to Schwalbe's line. Unlike other structures in the angle, scrutiny at high magnification reveals this tissue to have texture and depth. In youth the trabecular meshwork has a translucent blue-grey appearance, but with increasing age it darkens in colour. There is little variation in the degree of pigmentation with race and iris colour, but it is increased in PDS and PXF syndrome, and secondary to inflammation, trauma, ocular surgery, iris cysts and melanomas. Of these, PDS is notable in that its dense pigmentation of the trabecular meshwork is relatively uniform in its distribution around the angle's circumference, whereas in other aetiologies it is concentrated inferiorly.
Keratic precipitates are occasionally seen on the trabecular meshwork in cases of trabeculitis, most commonly related to Herpes infections or glaucomatocyclitic crisis.
Internal to the trabecular meshwork is the canal of Schlemm, which can only be seen when filled with blood. This does not usually happen because IOP prevents reflux from the venous system. However, reversal of the usual pressure gradient may occur in the setting of ocular hypotony or raised episcleral venous pressure, from either disease or excessive pressure with the edge of a goniolens. It is important that this sign is not confused with early angle neovascularisation, which can have a similar appearance when its blood vessels are too fine to discern individually and only visible as a reddish flush.
Schwalbe's line
Schwalbe's line consists of a condensation of connective tissue fibres that represents the termination of Descemet's membrane and marks the transition from transparent cornea to opaque scleral tissue. On gonioscopy it is seen as a fine protuberance, encircling the anterior chamber. Its smooth surface may glisten somewhat in comparison with the adjacent trabeculum, and display a smattering of pigment, most prominent in the inferior angle. When its location is uncertain the corneal wedge can help identify this structure.
An unusually prominent Schwalbe's line in known as a posterior embryotoxon. Owing to an anterior displacement it is often visible with a normal slit lamp examination as a thin white line at the limbus, typically most prominent temporally. A posterior embryotoxon as an isolated finding is benign, but is linked with glaucoma when accompanied by attached strands of peripheral iris, where it represents Axenfeld-Rieger syndrome. In this anterior segment dysgenesis the prominence of Schwalbe's line is often marked and it may even be detached from the wall of the eye and hang, suspended, in the anterior chamber.
Occasionally pigment extends anterior to Schwalbe's line, patchy and scalloped in appearance, termed a Sampaolesi's line. This may occur from previous episodes of angle-closure, or any condition that results in pigment release, but most commonly in association with PXF syndrome.
Goniolens disinfection
The Medical Devices Agency advise that wherever practicable a device that comes into contact with the ocular surface should not be used on more than one patient, as to do so may expose patients to unnecessary risk through the transmission of disease.1 This is not practicable with goniolenses and so they should be cleaned and disinfected. In this regard, an interesting development is that Haag-Streit have developed a single-use disposable cap (Stery Cup) for some of its lenses.
Guidance on the disinfection of ophthalmic devices has been published by the Department Health's Advisory Committee on Dangerous Pathogens (ACDP). This advice has been strongly influenced by the theoretical risk of transmitting prions from one patient to another, although there are no known cases of transmission of variant Creutzfeldt-Jakob Disease (vCJD) by ophthalmic devices. Until recently the recommended method for decontaminating ophthalmic devices was soaking in sodium chlorite solution providing 20,000ppm of available chlorine (2 per cent solution) for 1 hour. This is was more aggressive than disinfection procedures recommended by the leading goniolens manufacturers (Haag-Streit, Ocular-Instruments and Volk), which for sodium hypochlorite suggests either a 0.5 per cent solution for 25 minutes or a 1 per cent solution for 10 minutes. My experience is that Goldmann-type goniolenses with mirrors enclosed in a plastic casing can withstand the more aggressive cleaning regime, but some Zeiss-type goniolenses that have mirrors protected externally by only a thin coating cannot and quickly develop mirror peeling. As an aside, alcohol wipes are even more corrosive, and may fix prions to the surface of a goniolens, and so should not be used. Advice from the College of Optometrists on the matter is that clinicians should to use their discretion on whether the clinical benefits of an instrument outweigh the theoretical risks of prion transmission.
Recently the advice for the decontamination of ophthalmic devices by the ACDP ophthalmology sub-group has been updated. It now allows for a lower concentration of sodium hypochlorite of 10,000ppm of available chlorine, and immersion for the shorter time of 10 minutes.2 This is combined with prior cleaning with soap/detergent and a series of rinses with water for irrigation (not tap water), and then finally storing the device dry. Soap/detergent is recommended because sodium hypochlorite is not effective against spores and cysts of certain microorganisms. Multiple rinsing is recommended to avoid the accidental damage to subsequent patients with caustic sodium hypochlorite. It is recommended that tap water be avoided to avoid the risk of Acanthamoeba spp. Puzzlingly, this new guidance appears to be inconsistent with another even more recent publication by the ACDP, which continues to state that sodium hypochlorite is considered to be effective at reducing infectivity but only at concentrations of 20,000ppm of available chlorine and when used for at least one hour.3
Summary
Gonioscopy is a critical part of the examination of patients with suspected glaucoma and with established disease, and on all individuals identified as having possibly closable angles with screening techniques. In addition to the diagnosis and treatment of glaucoma, gonioscopy is often necessary in the diagnosis and management of ocular trauma, inflammation and neoplasia. Besides its functional importance at the business end of aqueous dynamics, no doubt in part due to difficulty in its visualisation, the angle of the anterior chamber is an immensely rewarding and sometimes beautiful part of ocular anatomy to study. ?
References
- Medical Devices Agency. MDA AN1994 (04). Single patient use of ophthalmic medical devices: Implications for clinical practice, October 1999.
- Guidance from the ACDP TSE Working Group: Annex L Managing CJDvCJD risk in ophthalmology. www.dh.gov.uk/ab/ACDP/TSEguidance/index.htm, September 2009.
- Guidance from the ACDP TSE Working Group: Annex C - General principles of decontamination and waste disposal. www.dh.gov.uk/ab/ACDP/TSEguidance/index.ht, November 2009.
? Dr Michael Johnson is an optometrist at Bristol Eye Hospital and a research fellow at Bristol University. He has no commercial interest in any product mentioned.
