Optical coherence tomography alone is insufficient for glaucoma management

Optical coherence tomography (OCT) entered glaucoma practice promising an objective measure of damage to the primarily affected structures – RNFL and also ganglion cells. This was in sharp contrast to the prevalent gold standard, perimetry, that is highly subjective, with a variable learning effect, and which required significant patient cooperation and reproducibility over time. The widespread availability of retinal OCT machines and ease of use, together with color-coded quantitative printouts offering an analysis, have led to the overwhelming acceptance of OCT for glaucoma management. It is unfortunately often being used as a standalone test for diagnosis of glaucoma and its progression. What have we learnt over time? Where does OCT stand in glaucoma management today? It is apparent that there is significant interindividual variability of OCT measurements of affected tissues, making comparisons across individuals difficult. It is also known that OCT from different machines is not comparable.1 Staging of glaucomatous loss with RNFL was found to be possible to a degree after the ‘floor effect’ measuring nonvisual components of the retina was recorded and the degree of loss of the remaining visual tissue was noted.2,3 A deep learning model using OCT thickness maps outperformed clinicians in diagnosing the early stage and also overall glaucoma diagnosis with 10.4–11.2% higher accuracy.4 No definitive numbers or areas could be defined in different severities of glaucoma on GCC or RNFL. Wong et al.5 have provided a real world look at evaluating ocular hypertensive and glaucoma suspect eyes in 2025, stating that Spectralis OCT parameters show some potential for early glaucoma detection and monitoring, but their current diagnostic effectiveness remains limited. Comparing POAG and CPACG, PACG eyes were found to have smaller discs, a smaller cup volume, a smaller cup-to-disc ratio, a larger rim area, and a thicker global retinal nerve fiber layer when compared with eyes with POAG.6 Single OCT parameters identifying early diagnosis of glaucoma in POAG, PACG, and NTG were identified in one study as IT BMO-MRW, inner temporal mGCL, ST cpRNFL, and mean global BMO-MRW. A combination of these features may enhance their diagnostic capacities.7 In myopia, diagnosing glaucoma is difficult, more so in the presence of secondarily enlarged optic discs or a large delta zone. Longitudinal evaluations have indicated that myopic eyes with glaucoma exhibit a steeper decline in RNFL thickness than nonmyopic glaucomatous eyes. There are frequent artifacts seen in OCTs for glaucoma, commonly due to ERM, vitreomacular traction, fixation problems, decreased media clarity, unstable tear film, disc drusen, and so on, which influence both diagnosis and definitive identification of progression in glaucoma. About half of OCT scans in myopes showed artifacts, seen in 43.68% of circumpapillary RNFL, and 30% of macula OCTs. On evaluating the scans with artifacts, 23.9% masked true progression, 36.5% showed false progression, and 39.6% did not change progression analysis.8 Careful examination of the raw B-scan images is therefore critical to identify artifacts and true glaucoma progression. Retinal OCT has also been shown to be altered in systemic diseases9 such as diabetes mellitus, hypertension, chronic kidney disease, neurodegenerative diseases such as Alzheimer’s and Parkinson’s, multiple sclerosis, rheumatologic diseases, SLE, Behcet’s, and also drug toxicity. The greatest utility of OCT is in longitudinal monitoring for both glaucoma progression and conversion from disc suspect or OHT to glaucoma. This negates the effect of individual variability in RNFL or GCC/GCIPL and allows evaluation for minute structural changes and loss. Studies note that OCT changes largely precede visual field loss but have been found to be variable and asynchronous in pattern and timing.10,11 GCC is thought to be better than RNFL in following patients with preperimetric and early glaucoma; both work equally well in moderate glaucoma, while GCC is again more useful in eyes with severe damage, as RNFL reaches ‘floor effect.’ Changes in common artifacts such as epiretinal membranes or posterior vitreous detachment together with possible loss of the RNFL due to systemic diseases occur in the same age group and therefore limit its usefulness in the diagnosis of glaucoma progression, if the ophthalmologist is not carefully evaluating scans. There is also variable age-related loss across machines. The RNFL thickness change per year was -0.47 ± 0.27 μm/year for Spectralis, -0.44 ± 0.17 µm/year for Cirrus, and -0.25 ± 0.21 µm/year for Optovue. The ganglion cell-inner plexiform layer change was -0.24 ± 0.08 µm/year on Cirrus, while the ganglion cell complex change was -0.40 ± 0.26 µm/year on Optovue.12 Many advances in OCT technology are providing better data analysis, deep learning for predictive analytics, enhanced depth imaging for lamina cribrosa thickness, BMO, and the prelaminar neural tissue but need further evaluation. European Glaucoma Society guidelines 202513 state – Agreement between classification with quantitative OCT imaging and VF testing is only moderate. Diagnosis of glaucoma based only on OCT exam should be avoided. Agreement between structural progression and functional deterioration, over the relatively short duration of reported studies, is only partial or poor because of the measurement variability of both structural and functional tests. Most commercially available software does not compensate for aging. Results acquired with different instruments are not interchangeable. Perimetry provides an assessment of functional impairment that affects quality of life and has a long track record of being used to determine the degree of IOP reduction required and long-term prognosis of different stages and types of glaucoma. It therefore continues to be the gold standard in glaucoma care, with OCT today, as a complementary tool, providing structural information of minute changes, especially over time. All clinical features are essential in the glaucoma management – history, ocular and systemic examination, intraocular pressure, and pachymetry. To conclude, OCT in glaucoma is useful as a complementary tool. Ophthalmologists need to be aware of its limitations – interindividual variations, variability of data between machines, artifacts, and reporting errors. As of now, OCT alone should not be used for diagnosis or follow-up of glaucoma patients.14About the authorProf Ramanjit Sihota Prof Ramanjit Sihota, MD, FRCS, FRCOphth, was heading the Glaucoma research facility and clinical services at Dr Rajendra Prasad Centre for Ophthalmic Sciences, All India Institute of Medical Sciences, New Delhi, for 14 years. She is currently a consultant at Shroff Eye Centre, N Delhi. She completed her MD Ophthalmology in 1982 and her FRCS, Edinburgh in 1984. She has won the World Glaucoma Association ‘Research recognition award’ 2011, and numerous national and international awards for research in glaucoma, especially pertaining to primary angle closure disease, glaucoma surgery, and imaging in glaucoma. Dr Sihota spearheaded the National Task force in Glaucoma, which trained Distric Ophthalmologists across India in glaucoma management. She has also taken a lead in rationalizing the treatment of primary angle closure disease in South East Asia. She extensively revised ‘Parson’s Diseases of the Eye’ and has written chapters in books and published over 225 papers in peer-reviewed, indexed international journals. She has written ‘Practical approach to Glaucoma” for use by PGs and general practitioners. She is a member of the International Glaucoma Research Society and committees of the World Glaucoma Association. She has been on Indian Council of Medical Research, Medical Council of India, and Asia Pacific Glaucoma Society committees. She is a past president of the Glaucoma Society of India. She was the recipient of the ARVO India 2024 Dr Badrinath oration. Prof Sihota has been in the lifetime and annual Stanford lists of the top 2% of researchers worldwide for Ophthalmology.