At the completion of this article, the reader should be able to improve their detection of glaucoma using gonioscopy, including:
- Review the types of gonio lenses
- Understand the Preferred Practice Pattern (PPP) Classification for Primary Angle Closure Spectrum
- Understand the limitations of anterior segment imaging and how gonioscopy can complement its use
- Develop evidence-based management plans based on gonioscopy and other clinical findings.
Dr Jeremiah Lim
BOptom, MPhil, PhD (Melb)
Senior Lecturer, School of Allied Health, Optometry
The University of Western Australia, Perth WA
Lisa Jansen
BOptom(Hons), CertOcTher
Lecturer, School of Allied Health, Optometry
The University of Western Australia, Perth WA
Pauline Xu
M.Optom, CertOcTher, BOptom
Senior Lecturer, School of Allied Health, Optometry
The University of Western Australia, Perth WA
As primary eyecare providers, optometrists require every tool at their disposal to meet the rising incidence of glaucoma. Yet gonioscopy, one of the oldest – and arguably one of the most illuminating – techniques isn’t performed as often as it should be. In this two-part series, the authors share learnings from interactions with optometrists attending their gonioscopy workshops at the University of Western Australia optometry facility in 2023.
In 2023, we had the opportunity to conduct a series of gonioscopy workshops at the University of Western Australia. The workshops included a gonioscopy and primary angle closure theory refresher followed by a 30-minute guided gonioscopy practice session.
The optometrists who attended our gonioscopy workshops were WA-based and ranged in experience from new graduates to some with more than 30 years of experience. Most attendees acquired their degree in Australia with good representation across all states, others did their training overseas in countries such as New Zealand and the UK.
The attendees brought with them a shared curiosity about gonioscopy and a determination to master its use in their clinics. Because of that, they were also eager for us to discuss – and debunk – a number of gonioscopy myths.
Myth: Anterior segment imaging can replace gonioscopy
Anterior segment imaging has improved in recent years with faster anterior-segment OCT (AS-OCT) devices. However, one of the challenges with traditional AS-OCT is the limited number of radial B-scans used to acquire the image. This may give the clinician a false impression of the angle configuration given that virtually only a thin slice of each angle is examined.
This means isolated regions of peripheral anterior synechiae (PAS) or patchy pigment dispersion appearing between the image planes will be missed. The resolution may be improved by increasing the radial sampling such as through the use of radial rotating Schiempflug cameras (e.g. Pentacam) or swept-source OCTs, which can acquire in excess of 128 radial B-scans. Even so, gaps between scans are still not visible, resulting in only moderate agreement with gonioscopy.1 Another barrier to high-density AS scanning is artefacts caused by eye movements or blinking. Furthermore, critical features such as trabecular meshwork pigments, blood and neovascularisation might not be appreciated well via an AS-OCT scan due to its absence of colour.
Types of gonioscopy lenses
The iridocorneal angle plays an important role in the drainage of aqueous humour, accounting for up to 90% of aqueous outflow via the trabecular meshwork. The remaining outflow occurs through the ‘non-traditional’ or uveoscleral route.2 It is the balance between aqueous production and drainage that gives rise to the intraocular pressure. Consequently, an obstruction or narrowing of the iridocorneal angle may give rise to increased intraocular pressure – though this is not always the case clinically.
Due to the unique properties of the cornea, direct visualisation of the iridocorneal angle is not straightforward. Light entering the limbus travels to the other limbus through the transparent cornea in a phenomenon known as ‘total internal reflection’. This is partly due to the difference in refractive indices between the cornea/tear-air interface. To break free of total internal reflection, light striking the corneal limbus has to return to the viewer at an angle > 46°. This may be achieved using direct or indirect forms of gonioscopy.
Direct gonioscopy
In direct gonioscopy, a high powered ‘contact’ lens such as the Koeppe gonio lens is introduced to neutralise the air-cornea interface and to bend light away from the cornea and into the clinician’s view (Figure 1A). This often requires the patient to lie on their back while the practitioner views through the lens using a free space light source such as a Barkan illuminator. Direct gonioscopy is used mainly in operating theatres.
Indirect gonioscopy
In indirect gonioscopy, a ‘contact’ lens containing mirrors, also known as a ‘gonio prism’, is placed on to the cornea to neutralise the air-cornea interface and to allow reflected light from the iridocorneal angle to be viewed (Figure 1B).
With indirect gonioscopy, the patient sits upright and a slit lamp is used, allowing for comfortable viewing of the angles and better magnification.
Images: Priscilla Ganesan.
Examples of indirect gonioscopy lenses include the large-diameter Goldmann lens and the smaller-diameter Volk G4 (Figure 1C). Indirect gonioscopy is the most commonly used type of gonioscopy in optometry.
Many clinicians would have encountered a ‘three-mirror’ lens, typically a Goldmann lens. The central lens is a contact fundus lens allowing the clinician to view the posterior pole. There are three mirrors; the rectangular mirror is used for the mid-periphery, the trapezoid mirror for the equator, and the apical ‘thumbnail’ mirror for the far periphery/ora serrata under dilation and for viewing the iridocorneal angle.
Because only one mirror is used for gonioscopy, three-mirror lens-users have to rotate the lens on the eye to view each quadrant separately. Due to the large diameter of the Goldmann lens, even without a flange, dynamic gonioscopy is not possible.
Another common lens is the ‘four-mirror’ gonioprism known as the Sussman lens, which comes with a flange (large contact diameter) or without (small contact diameter). In some cases, they come with a handle (Posner style). Popular examples of Sussman lenses include the Volk G4, Ocular 4 mirror Sussman or the Katena Diamond 4 mirror lens.
Sussman style lenses without a flange are most convenient as they do not require a coupling fluid and do not require rotation as all four quadrants are in view simultaneously. They are generally also more comfortable for the patient.
Gonioprisms with flanges are typically more stable on the eye as they vault the cornea, creating a mild ‘suction’ force. The use of a viscous coupling fluid is essential to neutralise the air-corneal interface to ensure an unobstructed view of the angle as well as for comfort. However, as these lenses have a larger diameter than the horizontal visible iris diameter (HVID), this comes at the cost of not being able to perform corneal indentation.
Primary angle closure glaucoma: classification and terminology
To ensure consistency and unambiguity, it’s important that clinicians use consistent definitions for angle closure. One common misconception is to think that primary angle closure glaucoma (PACG) is synonymous with acute angle-closure crisis (AAAC). AAAC is more dramatic, but thankfully rarely encountered in primary practice.
The American Academy of Ophthalmology Primary Angle Closure Disease Preferred Practice Pattern panel of 2019-2020 provided the following definitions of primary angle closure disease (Table 1).
Iridotrabecular contact (ITC), also known as an ‘occludable angle’, is generally defined as an angle ≤20° (Shaffer system) or the non-visibility of the posterior trabecular meshwork (PTM).3.4 Though it is worth noting that the AAO PPP5 defines ITC as the iris “appearing to touch the PTM”.
Given that the PTM is the site of the opening to Schlemm’s canal in which drainage occurs, it makes sense that a visible PTM would allow for aqueous outflow to occur. However, there may be situations where the more conservative definition of the AAO PPP may be better applied. This may include situations where only a small sliver of the PTM is visible, which may mean that drainage is still partially obstructed.
One of the main changes to the definition of primary angle closure disease is the reclassification of angle closure suspects from the original International Society of Geographic and Epidemiologic Ophthalmology (ISGEO) classification of 270° ITC (three quadrants) to the more widely-accepted 180° (two quadrants or six clock hours). One of the reasons for this shift is that population studies6 have found that >50% of primary angle closure disease is missed if the 270° definition was used. Even at 180°, it is believed that primary angle closure disease is missed. [For more information, see the review paper by Sun et al., 20177].
Promising tech developments
One promising development in anterior segment imaging is the hyperparallel OCT (HP-OCT) developed by Cylite. The HP-OCT acquires the entire anterior image instantaneously, thereby reducing the chances of motion artefacts encountered in traditional OCT. This allows for the acquisition of a dense volumetric scan of the AC providing the examiner a qualitative assessment of the full 360° view of the AC angle.
With time, it is believed that Cylite will develop more software tools to assist in the automatic analysis of the AC for the measurement and quantification of the AC angle. Future studies are needed to validate its agreement with gonioscopy.
Another approach to AC angle imaging is the use of a purpose-built anterior angle imaging device in the market known as the Nidek GS-1000, which utilises a contact 16-mirror gonioprism attached to a camera, allowing for 360° imaging of the angle. Although the closest substitute to actual gonioscopy, this use of a Goldmann mirror precludes corneal indentation, limiting the detection of PAS. Alternatively, practitioners fortunate enough to own a slit lamp camera, may choose to perform goniophotography manually.
At present, the only AAO PPP-endorsed imaging modality is ultrasound biomicroscopy, which is operator-dependent and time-consuming, but allows for better characterisation of plateau iris configuration4 than AS-OCT due to its ability to image the peripheral crystalline lens through the iris root.
Having said that, it is acknowledged that the advantages of AS-OCT, namely the repeatability and quantification of the AC biometry may lead to a better understanding of the natural history of angle closure and its prognosis over time. At this stage, we can only recommend the use of anterior segment imaging after gonioscopy for the documentation of regions of PAS, neovascularisation or anatomical features of clinical interest. In our experience, goniophotos are good for documenting the extent of PAS, the presence of neovascularisation, angle recession, pigment showers or other anatomical features as they cover a large region (typically a quadrant as opposed to a single slice) and you can appreciate colour. AS-OCT, on the other hand serves as a complementary teaching tool for both the clinician (e.g. the gonioscopy novice) and for patient education especially in cases where a referral for treatment is warranted.
Although both old and new devices can complement learning and management, particularly in challenging cases, a skilfully performed gonioscopy examination still remains the gold standard for AC assessments due to its 360° view and dynamic assessment of the AC angle.
Conclusion
In Part 2 of our ‘Gonioscopy Primer’, we challenge the myths surrounding routine IOP measurement and reliance on Van Herrick (VH) angle measurements. Until then, keep practising your gonioscopy skills. The practice and the community you serve will no doubt benefit from it.
More reading
New options in glaucoma: how MIGS is changing the treatment paradigm
Keeping the ocular surface healthy in the glaucoma patient
Pearls for expanding use of OCT-A in optometric practice
References
1.Porporato N, Baskaran M, Tun TA, Sultana R, Tan M, Quah JH, et al. Understanding diagnostic disagreement in angle closure assessment between anterior segment optical coherence tomography and gonioscopy. British Journal of Ophthalmology. 2019.
2. Fautsch MP, Johnson DH. Aqueous humor outflow: what do we know? Where will it lead us?. Invest Ophthalmol Vis Sci. 2006; 47 (10): 4181-4187. doi:10.1167/iovs.06-0830
3. Baskaran M, Kumar RS, Friedman DS, Lu QS, Wong HT, Chew PTK, et al. The Singapore Asymptomatic Narrow Angles Laser Iridotomy Study: Five-Year Results of a Randomized Controlled Trial. Ophthalmology. 2022; 129 (2): 147-58.
4. He M, Jiang Y, Huang S, Chang DS, Munoz B, Aung T, et al. Laser peripheral iridotomy for the prevention of angle closure: a single-centre, randomised controlled trial. Lancet. 2019; 393 (10181): 1609-18.
5. Gedde SJ, Chen PP, Muir KW, Vinod K, Lind JT, Wright MM, et al. Primary Angle-Closure Disease Preferred Practice Pattern. Ophthalmology. 2021; 128 (1): P30-P70.
6. Foster PJ, Buhrmann R, Quigley HA, Johnson GJ. The definition and classification of glaucoma in prevalence surveys. Br J Ophthalmol. 2002;86(2):238-42.
7. Sun X, Dai Y, Chen Y, Yu D-Y, Cringle SJ, Chen J, et al. Primary angle closure glaucoma: What we know and what we don’t know. Progress in Retinal and Eye Research. 2017; 57: 26-45.
