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The anterior chamber angle is formed where the iris meets the wall of the eye. The insertion of the iris is usually just behind the start of the cornea into, or just in front of, the inner surface of the ciliary body (Figure 1). In the recesses of the angle is located the trabecular meshwork, which largely governs the ease at which aqueous can drain from the eye. The method of visualising the anterior chamber angle is known as gonioscopy, which derives from Greek and literally translates as angle-observation. Gonioscopy facilitates appraisal of the angle configuration and the inspection of its structures.
In clinical practice, gonioscopy is supplemented by the assessment of central anterior chamber depth. The latter may be done either using an optical section and making a comparison with the thickness of the cornea and with observations in the fellow eye, or, in a quantitative manner, with the Smith technique.1
In recent times cross-sectional imaging techniques have become available that are able to image the angle, namely ultrasound biomicroscopy, Scheimpflug photography and optical coherence tomography. These often provide better information of gross angle anatomy and have enhanced measurement capabilities. However, the new technology does not supplant gonioscopy, which provides an unrivalled perspective of the iris root, angle and peripheral cornea. With gonioscopy it is as though you are standing on the crystalline lens and sticking your head up through the pupil. Even in regards to examining access to the angle, with cross-sectional imaging techniques it is not always possible to accurately determine the insertion of the iris in relation to the angle structures and, therefore, the grade of the angle. It follows that advancements in imaging of the angle should be considered a useful adjunct to gonioscopy in some situations, but never a replacement for gonioscopy.
Indications for gonioscopy
Gonioscopy is essential in the assessment of narrow angles and management of glaucoma. Its other main indications are when deciding on the appropriateness of pupil dilation and in the follow-up of patients at risk of angle neovascularisation. Clinical signs and symptoms that indicate gonioscopy are listed in Table 1.
Narrow angle
The Van Herick method of estimating peripheral angle configuration is a commonly undertaken screening test (Figure 2).2 It can be argued that it is preferable to gonioscopy in the first instance in primary care owing to its speed and simplicity. Indeed, relative to gonioscopy the test has reasonable sensitivity and specificity at detecting occludable angles.3-5 The test is, however, only an estimation method that can sometimes be misleading and so is not a substitute for visualisation of the angle. To carry out the Van Herick technique a narrow slit beam is traversed from the sclera onto the cornea, stopping just past the limbus when an optical section of the cornea is first seen. At this point, if the width of aqueous is a quarter or less than the width of the cornea, designated grade 2 or less, closure of the angle is considered possible. These eyes require gonioscopy to determine whether they have angle-closure or are at risk of angle-closure in the future. If this cannot be done by the primary care clinician then the patient should be referred to a practitioner who is competent in gonioscopy. This referral can be routine in asymptomatic patients, but should be urgent in the presence of measured high IOP or symptoms that are suggestive of periods of high IOP and, thus, intermittent angle-closure.
Pupil dilation
A common indication for gonioscopy is before instilling dilating eyedrops to assess the likelihood of causing angle-closure. In many cases gonioscopy is prompted by concern raised by the Van Herick method of estimating peripheral angle configuration. In this regard, it is important to appreciate that angle-closure following dilation is very rare even in eyes that do have an angle that is considered closable. Indeed, the risk of precipitating an attack of acute angle-closure is invariably less than the risk of eye disease being missed by not dilating. As a caveat, if dilation is not demanded by clinical circumstances then patients with anterior chamber angles narrow enough to cause concern on the safety of dilation are best served by referral to a specialist who can decide on the appropriateness of a prophylactic peripheral iridotomy or cataract extraction.
Glaucoma workup
Gonioscopy should be undertaken in all patients at risk of glaucoma and with established disease. The technique should be undertaken when establishing a diagnosis and, thereafter, as indicated by the clinical situation. Its principle aim is to determine if the angle is wide open, closable, or closed over all or some of its circumference. The status of the angle directs appropriate management in both primary and secondary care. It is important in determining the urgency of referral recommended and subsequent triage, and critically influences whether any treatment offered is medical or surgical. The definitive treatment of a closed or closable angle caused by pupil block is the creation of a peripheral iridotomy and/or the removal of any cataract.
Gonioscopy can also assist in the detection of secondary glaucomas. Examples of secondary open-angle glaucoma where it is valuable include pigment dispersion syndrome, pseudoexfoliation syndrome, trauma, inflammation and raised episcleral pressure. Examples of secondary angle-closure that it may reveal include neovascular glaucoma and posterior pushing mechanisms. Gonioscopy can be particularly insightful in puzzling cases of unilateral glaucoma.
Angle-closure glaucoma
Primary acute angle-closure glaucoma is easily recognised by its characteristic constellation of clinical signs. Examination reveals very high IOP, oedema of the corneal epithelium, a distorted and non-reactive pupil, and mild anterior chamber inflammation. In these cases, gonioscopy usually only confirms the suspicion of the clinician. Indeed, it may not be possible to perform gonioscopy initially due to haziness of the cornea, although in monocular presentations it is reasonable to make inferences from the state of the fellow eye because treatment needs to be started without delay. Angle assessment must, however, be undertaken as soon as possible in these eyes because other entities can masquerade as primary acute angle-closure. Less common conditions that present with similar clinical signs to primary angle-closure but require different treatment include neovascular glaucoma, phacolytic glaucoma and glaucomatocyclitic crisis.
The value of gonioscopy is exemplified in the detection of chronic and intermittent angle-closure. If not performed these diagnoses will be missed and appropriate treatment delayed. The patient would thus be exposed to an avoidable risk of significant pain and irreversible sight loss from an attack of acute angle-closure. Chronic closed-angle glaucoma often presents without symptoms in a near-identical manner to chronic open-angle glaucoma with moderately raised IOP. Its defining characteristic is a narrow angle that is closed over some of its circumference. Intermittent angle-closure may present with either no symptoms, or complaints of headache or episodic ocular pain associated with blurred vision and haloes, possibly only on specific questioning, normal or high IOPs, and healthy or glaucomatous optic nerves. In this condition the angle is narrow, possibly with some areas that are closed, and commonly has an irregular line of pigment above the trabecular meshwork as a telltale sign of previous iris contact. There may also be evidence of ischaemic damage to the iris and lens from past episodes of angle-closure. The iris may exhibit whorling of its normally radial iris fibres, iris transillumination defects and iridoschisis. The lens may have irregular, grey-white, anterior subcapsular opacities, termed glaukomflecken.
Secondary open-angle glaucoma
Pigment dispersion syndrome (PDS) and pseudoexfoliation (PXF) syndrome have characteristic angle appearances. In PDS the trabecular meshwork is darkly pigmented, as though smeared with dark chocolate. In PXF syndrome the trabecular meshwork has clumps of fibrillar material mixed with pigment that have the appearance of brown sugar. Gonioscopy is thus indicated if pigment is seen on the cornea, in the presence of iris transillumination defects, or if PXF material is identified/suspected on the anterior lens surface or at the pupillary border.
As discussed later, gonioscopy is also useful in identifying glaucoma caused by trauma, inflammation, raised episcleral pressure, developmental angle anomalies and neoplasia.
Vascular occlusive disease
Gonioscopy is crucial in patients with vascular occlusive disease at risk of neovascularisation. In these cases, pan-retinal photocoagulation or cryotherapy may prevent rubeotic glaucoma and its attendant sequelae of pain, blindness and, ultimately, phthisis. It is important to appreciate that neovascularisation of the angle is not always accompanied by neovascularisation of the iris. Eyes at risk of neovascularisation include those with a history of retinal vascular occlusion, diabetic retinopathy and carotid artery disease. Ischaemic central retinal vein occlusion and proliferative diabetic retinopathy account for almost all cases of neovascular glaucoma.
History or signs of trauma
In cases of recent trauma it is important to confirm the integrity of the globe before undertaking gonioscopy. After this cautionary step, gonioscopy may aid the detection of small hyphaema and the search for a foreign body. In new and older presentations, the technique can identify iridodialysis and angle-recession, which are risk factors for the development of chronic open-angle glaucoma. Open-angle glaucoma resulting from trauma may not manifest itself for months or even years after the traumatic event. Gonioscopy should be undertaken to specifically exclude angle-recession when there is a history of ocular trauma, signs suggestive of previous trauma, and unilateral glaucoma. Signs that should arouse suspicion of previous trauma, not always volunteered or recalled by patients, include scarring of the skin around the eye, corneal scarring, an asymmetrically deep anterior chamber in the absence of cataract surgery, iris or pupil damage, and unilateral cataract.
History or signs of uveitis
When inflammation is only seen at the slit lamp as cells and flare in the anterior chamber, examination by gonioscopy may reveal inflammatory precipitates or peripheral anterior synechiae (PAS) in the angle, usually inferiorly. In Fuch's heterochromic iridocyclitis, fine blood vessels may be seen to traverse the angle.
Anterior chamber dysgenesis
Ocular developmental anomalies are a risk factor for an abnormal trabecular meshwork and, thus, glaucoma at a relatively young age. Some anomalies have been grouped into clinical syndromes (Table 2). Iris anomalies include hypoplasia, discerned by enhanced visibility of the sphincter pupillae and iris vasculature, and diffuse transillumination defects pupil displacement, termed corectopia and secondary pupils, termed polycoria, referred to as stretch and melt holes when caused by traction and ischaemia, respectively. Corneal anomalies include congenital opacities and posterior embryotoxon. The latter is a prominent and anteriorly displaced termination of Descemet's membrane, seen on slit lamp as a thin white line at the level of the endothelium, encircling the cornea, adjacent to the limbus. A posterior embryotoxon as an isolated finding is both common and benign, but is linked with glaucoma when accompanied by attached strands of peripheral iris.
Iris anomalies
Subtle elevations are better seen when looking along the plane of the iris than with a straight ahead view. Sometimes, when the iris is being pushed from a mass on its underside, gonioscopy enables visualisation of the causative lesion. Apparent dragging of the pupil, possibly associated with ectropion uveae, indicates gonioscopy to investigate the source of traction. Possible causes of a dragged pupil include contractile membranes and neoplasia in the angle.
Contraindications for gonioscopy
Gonioscopy should not be performed when there is a possibility that the globe has a laceration or penetrating injury. Similarly, gonioscopy is typically not recommended immediately post-cataract surgery or following other penetrating intraocular surgeries. This is because in these circumstances pressure from the goniolens may force aqueous from the eye, dangerously lowering IOP and, possibly, collapsing the anterior chamber and so lead to damage of the corneal endothelium. Gonioscopy should also be avoided in the presence of a known hyphaema, or soon after, because it could initiate a re-bleed. Additionally, in patients with a compromised corneal epithelium, particularly those with a history of recurrent corneal erosions or epithelial basement membrane dystrophy, the benefits of gonioscopy must be balanced with worsening corneal disease.
Gonioscopy optics
The angle recess cannot be visualised in most eyes because light emerging from the angle is internally reflected where the pre-ocular tear film contacts air. This occurs because the angle of incidence of these light rays, the angle formed with a line dropped perpendicular to the corneal surface, exceeds the critical angle of the tear-air interface. This problem is overcome by placing a lens in optical continuity with the cornea and tear film that, by means of a steep anterior surface or tilted mirror, reduces the incidence angle formed at the surface contacting air, permitting a direct or indirect view of the angle structures, respectively (Figure 3).
Direct gonioscopy is used when examining and operating on infants under general anaesthesia. In other circumstances indirect gonioscopy is preferred, owing to its speed, advantageous utilisation of the optics and magnification of the slit lamp, and non-requirement for the patient to be supine. Only techniques for indirect gonioscopy are detailed herein.
With indirect gonioscopy the anterior chamber angle is reflected in a mirror inclined at an angle between 59° and 64° with the anterior surface of the lens. The portion of the angle viewed is that opposite the mirror. A mirrored image of the angle is seen, which differs from 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 4). The inclination of the mirror is significant because of its influence on the apparent size of angle structures and the ability to examine narrow angles (Figure 5). Light reflected from the angle has to emerge from the anterior lens surface perpendicular to the eye to enter the viewing system of the slit lamp, and from this it follows that the vantage point from which the angle is seen is higher with steeper mirrors. The advantage of a steep mirror is that it allows visualisation into the recesses of narrow angles when the line of sight of a less inclined mirror would be obstructed by an anteriorly bowed iris. However, owing to an increasingly oblique view, the apparent size of angle structures is smaller as the mirror is more steeply tilted. Minification caused by oblique viewing is given by the cosine of the angle of approach with the plane of the iris. It is important to appreciate the influence of mirror angle on the view during gonioscopy, particularly when swapping between lenses, because it can lead to erroneous angle grading.
Indirect gonioscopy
The main practical difference between indirect goniolenses is in their contact curvature, and whether this is steeper or flatter than the cornea. The prototypic lenses are the Goldmann 3-mirror 'Universal' lens and the Zeiss lens. Both lens styles have their advantages and neither has a clear clinical superiority to the other for all circumstances (Table 3). Goldmann lenses have a contact curvature steeper than the cornea and so a viscous gel needs to be used as a coupling medium for optical contiguity. The Zeiss lens avoids the need for coupling fluid because it is relatively flat, which forces out central bubbles, and the space between the cornea and lens edge is filled by a tear meniscus. Other distinguishing characteristics between the two prototypic lenses have become blurred in recent years as new lenses have become available with features of both.
Goldmann-type goniolenses
An advantage of Goldmann-type lenses is that suction on the eye improves stability and makes them harder to eject with a blink, to the point that they may remain in place if the clinician releases their hold. This is very helpful when learning gonioscopy or if the lens has to be turned on the eye, and is essential during laser treatment of the trabecular meshwork. Stability can be improved further with a flange on some designs, but in my opinion this adds bulk unnecessarily in routine clinical use. A steep curvature is relatively protective of fragile epithelium. Furthermore, by avoiding pressure on the central cornea, these lenses are less prone to artificially open an angle by clumsy handling.
The classic Goldmann three-mirror lens is a multipurpose instrument with a central aperture and two mirrors that allow inspection of the posterior pole and peripheral fundus, respectively, and a third small 'thumbnail' mirror that gives a view of the angle. The inclination of the gonioscopy mirror is 59°, which gives a good, near-straight on, view in most eyes, but can cause difficulty in examining very narrow angles. Other disadvantages are that the lens is large, bulky to handle, and must be rotated on the eye almost full-circle to examine all of the angle. Variants of this lens include smaller models, although these have smaller mirrors for viewing the mid-peripheral and peripheral fundus (NB the mirror used for gonioscopy, the smallest of the three, is invariably the same size in smaller models), and also versions with a flatter contact curve that do not require fluid (although I personally am not keen on the notion of rotating a flat lens on the cornea). Dedicated Goldmann-type goniolenses, without any mirrors for assessing the fundus, are available in one-, two- or four-mirror versions. Compared to the original Goldmann three-mirror lens, these goniolenses are less bulky and so are easier to use in smaller eyes. More mirrors also means that less rotation on the eye is needed to assess the entire circumference of the angle. It should also be noted that the angle of the mirrors of these specialist goniolenses are typically steeper than with the gonioscopy mirror of the original Goldmann lens.
Worthy of special mention are the lenses based on the design of Roussel and Fankhauser that have a single mirror and a convex anterior surface (Figure 6).6 The positively powered surface increases magnification and is tilted such that its centre of curvature coincides with the virtual image of the angle formed by the mirror, which reduces spherical aberration and coma. These lenses were conceived for laser treatment of the trabecular meshwork because they preserve the size and power density of the beam as the goniolens is rotated on the eye. However, their optical properties give an unparalleled view of angle structures and allow appreciation of hitherto undetectable subtleties. Unfortunately, these lenses are large and quite cumbersome for routine clinical use. Also, the convex anterior lens surface necessitates that the viewing system of the slit lamp is pushed closer to the patient, which leads to difficulties in obtaining a clear focus when the patient has deep-set eyes.
Zeiss-type goniolenses
Zeiss-type lenses have several practical advantages over Goldmann-type lenses. Firstly, without the need for coupling solution, gonioscopy with these lenses is less hassle and does not leave the patient with a sticky eye or coupling fluid on the cheeks. Also, particularly when thick and preserved with benzalkonium chloride, coupling solution may create difficulties obtaining a clear image during subsequent fundoscopy and photography.
Secondly, these lenses have four mirrors that enable assessment of the full circumference of the angle with minimal rotation of the lens on the eye.
Thirdly, these lenses are smaller, specifically in their contact area, which make them easier to use in small eyes (eg hyperopes and Asians, those most at risk of angle-closure) and deep-set eyes.
In combination these factors mean that Zeiss-type lenses can be used more rapidly during clinical examinations. The more convenient a test the more likely it is to be performed, and it is mainly for this reason that Zeiss-type lenses are preferred for routine use by many clinicians.
Non-trivially, patients often prefer examination with these lenses because they are less gunky and the examination is quicker. Clinical benefits of these lenses are that because of their small contact area they can be moved more easily over the surface of the cornea to quickly alter the tilt of their mirrors, without the patient having to change their direction of gaze, and facilitate the technique of indentation gonioscopy described later.
Disadvantages of Zeiss-type lenses are that they are more easily blinked off the eye by patients and are prone to inadvertently opening the angle by excessive pressure. Additionally, the identification of angle structures with these lenses can be more difficult owing to a slight reduction in corneal clarity by compression, and their relatively high angle vantage point dictated by relatively steep mirrors, Some versions of this lens genre are also more prone to damage by aggressive disinfection methods.
The original Zeiss goniolens is no longer manufactured, but there are now many alternatives. Some are available with a handle, while others are designed to be handheld. Some clinicians prefer holding a lens directly because they feel that it gives them greater control of applied pressure on the eye and makes holding the eyelids during the procedure easier.
A Zeiss-type lens is now available with a convex anterior surface to improve magnification, and another lens with a second mirror in the optical path to correct mirrored imagery and so make locating lesions less confusing. Another novel innovation is the construction of goniolenses from high refractive index glass that redirect light using the principle of total internal reflection, rather than mirrors, to avoid the possibility of mirror delamination with aggressive cleaning. However, a higher refractive index increases the negative lens effect at the interface between the goniolens and the cornea, which leads to a significant reduction in magnification.
Summary
Gonioscopy should be performed on all patients with glaucoma, on all individuals suspected of having glaucoma, and on all individuals suspected of having narrow angles. Without gonioscopy, identification of the underlying mechanism and, therefore, reliably ascertaining the appropriate treatment of any glaucomatous condition is impossible. In addition to the diagnosis and management of glaucoma, gonioscopy is often necessary in the diagnosis and management of ocular trauma, inflammation and neoplasia.
Part 2 of this discussion will review the technique of gonioscopy and the recording of observations. Common variations of normal anterior chamber angles and angle abnormalities that are often seen in a primary eye care setting will also be described in Part 2. ?
References
- Smith RJ. A new method of estimating the depth of the anterior chamber. Br J Ophthalmol, 197963:215-20.
- Van Herick W, et al. Estimation of width of angle of anterior chamber. Incidence and significance of the narrow angle. Am J Ophthalmol, 196968:626-9.
- Thomas R, et al. The flashlight test and van Herick's test are poor predictors for occludable angles. Aust N Z J Ophthalmol, 199624:251-6.
- Foster PJ, et al. Detection of gonioscopically occludable angles and primary angle closure glaucoma by estimation of limbal chamber depth in Asians: modified grading scheme. Br J Ophthalmol, 2000 84: 186-92.
- Baskaran M, et al. Comparison of the scanning peripheral anterior chamber depth analyzer and the modified van Herick grading system in the assessment of angle closure. Ophthalmol, 2007114:501-6.
- Roussel P, Fankhauser F. Contact glass for use with high power lasers-geometrical and optical aspects. Solution for the angle of the anterior chamber. Int Ophthalmol, 19836:183-90.
? 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
