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Do I need gonioscopy when I have an OCT?

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Figure 1. Typical gonioscopic view of a wide open angle with light pigmentation
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Alex Petty
BOptom(Hons)
Innovative Eye Care, Adelaide SA

 

Traditionally, the gold standard technique for examining the anterior chamber angle is gonioscopy. In the hands of a skilled practitioner the use of a gonioprism can quickly and accurately ascertain if an angle is open, narrow or closed and what other pathology may be present.

My personal observation is that some optometrists, young and old, are using gonioscopy less frequently and are less confident in their ability to interpret what they see when carrying out an examination.

Two possible explanations that may be drawn from this are that the time-restrictions placed on eye examinations makes gonioscopy impractical or that the information gained from gonioscopy is not deemed useful by practitioners as part of a comprehensive eye examination. A survey published in Clinical and Experimental Ophthalmology in 2008 has shown that only 56 per cent of ophthalmologists performed gonioscopy.1

Both of these trends are concerning, given that the knowledge of the anterior chamber angle characteristics is essential when evaluating a patient with suspected glaucoma.  The increased adoption of spectral-domain optical coherence tomography (SD-OCT) in general ophthalmic practices raises the question of whether anterior-segment SD-OCT can replace gonioscopy for anterior chamber evaluation in clinical practice or whether gonioscopy is still an important technique that an optometrist or ophthalmologist should regularly use.

Gonioscopy has many advantages including its lack of expense, its reasonably short procedural time, the ability to dynamically see features across an entire angle quadrant and, when using a non-flange gonioprism, the ability to indent the cornea to differentiate between appositional and synechial angle closure.

Gonioscopy does have its shortfalls. It is a contact procedure that many patients find uncomfortable, its results are affected by factors such as examiner skill, patient co-operation, environmental light exposure and inadvertent corneal pressure, and it is a qualitative assessment, making comparison over time or between practitioners potentially unreliable.

Other techniques to evaluate the anterior chamber angle include estimation of the Van Herrick anterior chamber to cornea ratio at the limbus with the slitlamp, ultrasound biomicroscopy and anterior OCT. Van Herrick is a technique often used by optometrists and ophthalmologists but is no substitute for angle visualisation due to its lesser ability to detect primary angle closure.2,3,4

107 Figure 2_F

Figure 2. Comparison of a narrow angle (< 0.2:1) in an older patient with a wide open angle (1:1) in a young patient (left and right, respectively) as measured with Van Herrick estimation of anterior chamber width to corneal width

UB has been used since the early 1990s to look into anterior eye structures but these machines are rarely seen outside ophthalmological centres. SD­OCT is swiftly becoming common­place in optometric practices and many of these machines have versatile anterior segment modules and analysis software, making imaging of the anterior chamber realistic. Some machines such as the Tomey Casia can even simulate a 3-D gonioscopic view from its scans.

The advantages of OCT for anterior chamber angle imaging include the speed and ease of image capture (which could be carried out by a technician during pre­testing), the ability to quantify the information with useful parameters such as the TIA (trabecular­iris angle, also used in the Schaffer and Spaeth gonioscopy grading scales) or AOD500 (angle opening distance), and the control of environmental light.

Disadvantages include the fact that angle structures are still altered by pupil size, accommodation and position of gaze, and when compared to ultrasound, structures such as the ciliary body and supra­choroidal space cannot be visualised. Anterior OCT is often used as a line scan to give a cross-­section in a particular meridian and therefore, cannot show information across the entire quadrant without numerous scans, potentially missing areas of trabecular contact or peripheral anterior synechiae.

OCT will also not give information about coloured pathological features within an angle such as pigmentation or new blood vessels. (Figure 3)

107 Figure 3_F

Figure 3. Useful landmarks and parameters when examining an AS­OCT of an angle (Nidek RS­3000)

There are several scenarios in which anterior OCT is particularly useful.

  • Evaluating lens vault

Lens vault is defined as the distance measured between the anterior surface of the crystalline lens and the perpendicular line connecting opposite scleral spurs. Foo et al 2012 showed that lens vault was one of the main predictors of angle closure.5 This value essentially relates to the bulk of the lens taking up space in the anterior chamber and is more predictive than just lens thickness, as thick lenses do not necessarily move the peripheral iris closer to the trabecular meshwork. Patients with lens vault as a dominant factor in their angle closure glaucoma may benefit more from cataract surgery, as it has been shown that the amount the angle opens corresponds to the drop in IOP following lens extraction.6 (Figure 4)

107 Figure 4_Lens Vault In A Closed Anterior Chamber Angle Tomey Casia _F

Figure 4. Lens vault diagram in a closed angle eye (Tomey Casia)

  • Documenting narrow angles

Gonioscopic grading can be confusing as the three main scales are conflicting and can be laborious to master (Table 1). Being able to quantify the extent at which an angle is open and compare this value consistently over time and between observers potentially offers a more accurate method of angle assessment as reports suggest intra­observer variation in AS­OCT analysis of the anterior chamber angle is low.7

107 - Gonioscopy Table 1
Table 1. Comparison of the three main gonioscopy grading scales

Several studies have looked into how anterior segment OCT compares with the gold standard of gonioscopy. Sensitivity for detecting angle closure was shown to be as high as 98 per cent although specificity was lower at 55 per cent.8 Indeed, AS­OCT has been shown to classify a higher proportion of eyes in the sample as having one or more closed quadrants (59 per cent) versus gonioscopy (33 per cent).9  Interestingly in this paper, the superior, inferior and nasal quadrants appeared more frequently closed with AS­OCT yet the temporal quadrant was more often closed with gonioscopy. (Figure 5)

107 Figure 5_F

Figure 5. AS­OCT showing the effect on anterior chamber angle opening when light is shone in the pupil. The image on the left has a pen­light being shone in the fellow eye to simulate poor gonioscopy technique, whereas the right image is in a dark room (Nidek RS­3000).

Conclusion

OCT adds a very useful tool to the arsenal of the ophthalmic practitioner for anterior chamber angle screening and monitoring changes to these structures over time. Any test is more useful than no test, so for an individual not confident in their gonioscopic technique, OCT could provide a potentially more accurate result. However, do not forget that especially in glaucoma suspects and patients diagnosed with glaucoma, OCT in its current form cannot replace gonioscopy. If you individualise your procedures to use OCT as a screening tool and confirm your findings with dark­room gonioscopy on suspicious angles, then you can still provide excellent patient care while not disrupting your busy practice schedule.

The final point to make is that images in medicine are very powerful tools. Using the OCT scans to educate patients about their condition will reinforce their understanding in a way simply describing the gonioscopy result cannot. If our patients are well educated during their consultations, they will become more compliant and recognise the importance of their routine eye examinations in the future.

  1. L Liu. Australian and New Zealand survey of glaucoma practice patterns. Clin Exper Ophthal 2008; 36: 19-25: 1804-2264
  2. Alsbirk PH. Limbal and axial chamber depth variations. A population study in Eskimos. Acta Ophthalmol (Copenh) 1986; 64: 6: 593-­600.
  3. Thomas R., George T., Braganza A, Muliyil J. The flashlight test and van Herick’s test are poor predictors for occludable angles. Aust N Z J Ophthalmol 1996; 24: 3: 251­-256.
  4. Congdon NG, Youlin Q, Quigley H, Hung PT, Wang TH, Ho TC, Tielsch JM. Biometry and primary angle­closure glaucoma among Chinese, white, and black populations. Ophthalmology 1997; 104: 9: 1489-­1495.
  5. Foo LL, Nongpiur ME, Allen JC, Perera SA, Friedman DS, He M, Cheng CY, Wong TY, Aung T. Determinants of angle width in Chinese Singaporeans. Ophthalmology 2012; 119: 2: 278­-282.
  6. Huang G, Gonzalez E, Peng PH, Lee R, Leeungurasatien T, He M, Porco T, Lin SC. Anterior chamber depth, iridocorneal angle width, and intraocular pressure changes after phacoemulsification: narrow vs open iridocorneal angles. Arch Ophthalmol 2011; 129: 10: 1283­-1290.
  7. Tan AN, Sauren LD, de Brabander J, Berendschot TT, Passos VL, Webers CA, Nuijts RM, Beckers HJ. Reproducibility of anterior chamber angle measurements with anterior segment optical coherence tomography. Invest Ophthalmol Vis Sci 2011 ;52: 5: 2095­-2099.
  8. Nolan WP, See JL, Chew PT, Friedman DS, Smith SD, Radhakrishnan S, Zheng C, Foster PJ, Aung T. Detection of primary angle closure using anterior segment optical coherence tomography in Asian eyes. Ophthalmology 2007: 114: 1: 33-­39.
  9. Sakata LM, Lavanya R, Friedman DS, Aung HT, Gao H, Kumar RS, Foster PJ, Aung T. Comparison of gonioscopy and anterior segment ocular coherence tomography in detecting angle closure in different quadrants of the anterior chamber angle. Ophthalmology 2008a; 115: 5: 769­-774.


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