IMAGING GLAUCOMA

 David F Garway-Heath MD

Modern imaging techniques are becoming increasingly accurate in identifying structural changes in the optic nerve head that correspond to, or are predictive of, visual field changes in glaucoma patients, said David F Garway-Heath MD, FRCOphth, International Glaucoma Association Professor of Ophthalmology at the UCL Institute of Ophthalmology, London, UK, in a Glaucoma Day session at the XXXIII Congress of the ESCRS in Barcelona, Spain.

“Over the last 20 years, new technologies have become available that have enabled us to quantify imaging data with increasing precision, and the question we need to ask is: ‘With this sophisticated technology, are we actually any better off?’” Dr Garway-Heath said.

He noted that optic nerve head stereo photography is likely to remain a mainstay of glaucoma diagnosis and monitoring. It is the only technique that can provide true-to-life colour imaging and therefore it is the only means for detecting features like optic nerve haemorrhages, which are an important predictor of visual field progression.

Meanwhile, newer technologies like the GDx (Carl Zeiss Meditec) and the Heidelberg Retina Tomograph (HRT, Heidelberg Engineering), and optical coherence tomography (OCT) are providing increasingly detailed imaging and have at their command a large normative database with which to compare individual images. As a result, the machines can outperform most clinicians using stereoscopic photographs in distinguishing eyes with glaucoma from healthy eyes (Reus et al, Ophthalmology 2010;117:717-723).

Stereo photograph interpretation can be unreliable

Studies show that changes in the optic nerve head can be highly predictive of future visual field loss, whether detected by stereo photography (F Madeiros et al, Arch Opthalmol 2009, 127:1250 1256) or newer technologies like HRT (BC Chauhan et al, Ophthalmology 2009; 116:2110-2118).

However, because of the subjective nature of optic nerve head photograph interpretation, its accuracy in the detection of glaucomatous change is highly dependent on the expertise of the clinician viewing the images. Furthermore, even glaucoma specialists viewing serial optic disc photographs will not always agree as to whether glaucomatous change has occurred, Dr Garway-Heath pointed out.

For example, among the trained technicians at the optic disc reading centre for the Ocular Hypertension Treatment Study (OHTS), the kappa value for the agreement between repeat gradings of stereoscopic photographs was relatively high, ranging from 0.65 to 083 (RK Parrish et al . Am J Ophthalmol 2005;140:110-344).

By comparison, in another study, the inter-observer kappa value was only 0.2 for non-expert ophthalmologists, and 0.51 for glaucoma experts. Moreover, the level of agreement between non-experts and the experts was only 68 per cent (Breusegem et al, Ophthalmology 2011;118:742-746). An agreement level of 50 per cent is the same as would be expected from flipping a coin.

In another study, involving 10 glaucoma experts simultaneously presented with sets of two time-distinct disc photographs from each of a series of eyes, the level of inter-observer agreement as to whether progression had occurred was only modest to fair, with kappa values ranging from to 0.34 to 0.68 (Azuaro-Blanco et al, Am J Ophthtalmol 2003:136:949-950).

He suggested that modern imaging technologies might therefore be a useful adjunct to optic disc photographs in determining if glaucoma progression has occurred. He cited a study which showed that HRT is at least as accurate in detecting evidence of progression as are experts using monoscopic photographs (BC Chauhann et al, Invest Ophthalmol Sci 2009; 50:1682).

Modern imaging devices also include software that maps the structural glaucomatous changes over time to corresponding areas of the visual field. In this way, imaging can be used to help tip the balance one way or the other in cases where visual field testing yields equivocal results, he said.

When structure and function disagree

Dr Garway-Heath noted that, in some eyes with glaucoma, the visual field loss will appear to occur prior to structural changes, while in others the opposite will occur. There are several possible reasons why these discrepancies may occur, but the most likely explanation is measurement error. A sure sign that this is the case in a particular patient is a seeming fluctuation of structural and/or functional change over time.

Factors leading to disagreement include poor image quality and normative databases which are inappropriate for a particular patient because of the patient’s age, their level of refractive error or other factors. Some errors of interpretation can therefore be avoided by taking those factors into consideration.

For example, in an eye with an abnormal HRT scan but normal visual fields, if the patient is 95 years old and has an intraocular pressure (IOP) of 10mmHg, it could just be the effect of age on the optic nerve. However, if the patient was 35 years old and had a high IOP, then they would be a glaucoma suspect.

“So, to conclude, imaging can support a diagnosis, but it cannot make the diagnosis for you. Similarly, imaging can provide additional evidence on patients’ progression status, but clinicians need to be aware of the source of error in interpreting the reports and consider clinical context,” he said.

David F Garway-Heath:

david.garway-heath@moorfields.nhs.uk