It Takes Two: Clinical Diagnosis of Glaucoma

Lisa Thompson,* a 74-year-old female presented, reporting no problems with her eyesight. She wears progressive spectacles and has a history of bilateral cataract surgeries at age 67. Prior, she had been a moderate myope. She reported good health but had a family history of myopia (mother and maternal uncle).

Vision unaided was R 6/9, L 6/9=.

Refraction was R plano/-0.75×147 (6/7.5), L plano/-1.00×60 (6/7.5), near add +2.50DS.

Examination revealed clear, well-positioned intraocular lenses (IOLs) in both eyes. Anterior chambers were deep, and the angles were open. There was no sign of any pseudoexfoliation or pigment dispersion.

Fundus examination revealed cup-to-disc ratio estimates of R 0.75 (Figure 1) and L 0.6 (Figure 2).

Optical coherence tomography (OCT) of the macula showed suspicious R > L retinal nerve fibre layer (RNFL) thinning with matching R > L ganglion cell complex (GCC) thinning (Figure 3). OCT of the optic nerve head revealed thin optic nerve rims.

Central corneal thickness measures were R 541 µm, L 532 µm.

Intraocular pressures (IOP) were R 19mmHg, L 16 mmHg.

Visual field testing with the Medmont M700 revealed full fields in both eyes with perfect reliability indices.

Given the cupped optic nerves, RNFL, and GCC thinning but normal visual fields, Lisa was classified as a ‘glaucoma suspect’, and monitored on a six-monthly basis. Her visual field, RNFL, and macular GCC remained normal. However, two-and-a-half years later, she returned with IOP elevated to R 29 mmHg and L 16 mmHg. OCT revealed progression of the superior and inferior RNFL thinning, with right superior and inferior pericentral scotomata.

Xalatan eye drops were initiated with a target IOP of R and L 16 mmHg.

Mrs Thompson was reviewed eight weeks later. Her IOP had reduced to R 20, L 16 mmHg.

Visual fields both showed progression on the right with inferior and superior retinal nerve fibre layer thinning and early right superior and inferior pericentral scotomata. Mrs Thompson was diagnosed with primary open angle glaucoma (POAG), and after discussion with her glaucoma specialist regarding the benefits and downfalls of changing medical treatment or considering selective laser trabeculoplasty (SLT), she decided to proceed with SLT.

SLT proved successful. Mrs Thompson continued on Xalatan eye drops nocte and her IOPs were consistently around 15–16 mmHg in both eyes, with no change in OCT RNFL, macular GCC or visual field (Figure 4). Fortunately, the left eye exhibited no change in IOP, visual field, or on the OCT and was able to be closely monitored along with the right eye.

While no one test can provide a reliable diagnosis on its own, each has its advantages and its challenges

Discussion

Left untreated, glaucoma can cause irreversible vision loss and eventually lead to blindness. Early detection and intervention can prevent significant loss of vision. Progression is unpredictable, and there is no gold standard test for diagnosis or progression. However, glaucoma is normally diagnosed by assessing the optic nerve, visual field, IOP, and RNFL and GCC on OCT. While no one test can provide a reliable diagnosis on its own, each has its advantages and its challenges.

Optic Nerve Evaluation

Optic nerve evaluation is important in monitoring glaucoma, however there can be poor agreement between clinicians; as well as being a relatively crude measure, optic nerve evaluation is unable to detect minor changes in disease.

Visual Field Testing

Visual field testing is currently the only clinical test for evaluating functional loss in glaucoma, however it is highly subjective and can have poor reproducibility, especially in the elderly due to reduced attention span, psychological disorders such as dementia, and poor hand-eye coordination resulting from physiological ageing or systemic comorbidities. Additionally, visual field testing is more difficult to perform in patients with advanced disease. Moreover, visual field testing requires active and accurate patient participation and has been shown to induce anxiety in patients, which can further hinder accuracy of results. Visual field testing may also require several tests to establish baseline, resulting in an increased time burden to patient and practitioner.¹

Optical Coherence Tomography

OCT allows for precise, high-resolution visualisation of the optic structures. Objective, and quick to perform, modern OCT instruments enable earlier disease detection and monitoring of progression with far less chair time.

In glaucoma management, OCT is used to measure and monitor changes in RNFL and GCC, which are highly correlated with disease progression. As measurements are objective, i.e. independent of patient responses, results are generally more accurate and highly reproducible. OCT has demonstrated it is more sensitive to detecting progression in earlier stages of the disease with good accuracy. It can improve predictions of converting from pre-perimetric glaucoma to perimetric glaucoma,² as was the case with Mrs Thompson, who exhibited OCT changes well before any visual field defects.

However, despite the level of accuracy, repeatability, and sophistication of modern OCT instruments, OCT alone cannot be used for glaucoma diagnosis and management. Unlike visual field testing, OCT tells nothing of functional vision loss, i.e. the impact of the disease. ‘Red disease’ – a false positive diagnosis – may occur due to issues with image acquisition giving rise to artifacts, or from anatomical variations and comorbidities. Red disease can also occur when patients fall outside the normative database of the instrument, however results may be perfectly ‘normal’ for them and not indicative of actual disease.

Glaucoma is a chronic disease so patients may suffer the burden of lifelong treatment and follow-up. To prevent unjustified psychological and financial burden from unnecessary treatments, OCT results need to be carefully interpreted in conjunction with results from other testing. For practices, OCT instruments are expensive and often can take up significant space, potentially limiting access in resource-constrained settings.

Complementary Roles

Before the advent of OCT, threshold visual field testing with standard automated perimetry, optic disc evaluation, and IOP were used for diagnosis and detection.

However, visual field testing and OCT are not mutually exclusive; rather they play complementary roles in glaucoma management. Combining both methods provides a comprehensive understanding of the disease. OCT allows for accurate and earlier detection of disease as well as assessing for pre-perimetric glaucoma. It may guide the interpretation of visual field changes, especially in early or ambiguous cases, while visual field testing can validate the functional significance of OCT-detected structural abnormalities. Together, these tools allow for more precise monitoring and, subsequently, timely intervention to preserve vision for glaucoma patients.

*Patient name changed for anonymity.

Jessica Chi is the Director of Eyetech Optometrists, an independent speciality contact lens practice in Melbourne. She is the current Victorian, and a past National President of the Cornea and Contact Lens Society, and an invited speaker at meetings throughout Australia and beyond. She is a clinical supervisor at the University of Melbourne, a member of Optometry Victoria Optometric Sector Advisory Group and a Fellow of the Australian College of Optometry, the British Contact Lens Association, and the International Academy of Orthokeratology and Myopia Control.

Reference

1. Kaliaperumal S, Janani VS, Kattamani S et al. Study of anxiety in patients with glaucoma undergoing standard automated perimetry and optical coherence tomography – A prospective comparative study. Indian J Ophthalmol. 2022 Aug;70(8):2883-2887. doi: 10.4103/ijo.IJO_508_22.
2. Zhang X, Dastiridou A, Huang D et al. Advanced Imaging for Glaucoma Study Group. Comparison of glaucoma progression detection by optical coherence tomography and visual field. Am J Ophthalmol. 2017 Dec;184:63-74. doi: 10.1016/j.ajo.2017.09.020.