Disc photography

Baseline optic disc photographs, obsolete or still essential?

Roibeard O’hEineachain

Posted: Friday, September 1, 2017

George Spaeth MD

Modern imaging technology can provide an increasingly well defined representation of the optic disc, and the software used in interpreting the images is also gaining in sophistication and the accuracy with which it can distinguish between glaucomatous and normal eyes. That raises the question of whether glaucoma specialists should still rely on disc photography. That was the topic of a debate held at the 7th World Congress of Glaucoma in Helsinki, Finland.

George Spaeth MD maintained that optic disc photographs do indeed remain an essential tool in the diagnosis and continuing care of patients with glaucoma.

“The ideal diagnostic test should be diagnostically valid and relevant, allow staging the severity of the condition, and provide a permanent baseline against which future tests can be compared. It should be readily available everywhere, it should not require instrumentation that is expensive and difficult to maintain and should be easily learned and easily interpreted,” said Dr Spaeth, Wills Eye Hospital, Philadelphia, Pennsylvania, USA.

Identifying patterns
He noted that identifying glaucoma through ophthalmoscopy is a skill that one can acquire through viewing of optic discs of patients who definitely have glaucoma. He suggested that rather than deconstructing the images and looking for specific features like disc haemorrhages, one should learn to recognise the patterns glaucomatous eyes have in common. He likened it to the way art critics learn to identify the artist who painted a painting without having seen it before.

“There is no reason for you as an ophthalmologist to learn to identify paintings, but there is every need to learn the method of identifying patterns and turn it into something meaningful,” Dr Spaeth said.

He also maintained that data obtained from newer imaging technologies like optical coherence technology (OCT) and Heidelberg Retinal Tomography (HRT) instruments is not truly diagnostic regarding glaucoma. Opinions vary on the threshold thinness of the retinal nerve fibre layer (RNFL) for glaucomatous damage. Moreover, there are other pathologies, such as myopia, that also cause RNFL thinning.

Another problem with the newer technologies is that companies keep changing their methodology, making it impossible to compare the data over time. Furthermore, as disc cupping develops, the retinal nerve fibres are pulled towards the disc margin, making registration uncertain. Thus, following patients so as to identify the rate of change is better done with photographs than with OCT.

Optic disc photographs do not require expensive equipment that becomes obsolete. Photography is cost-effective. Ophthalmoscopy for the purpose of examining the optic disc and photography to provide a permanent record are essential aspects of the examination of a patient with, or suspected of having glaucoma, even though the skill is difficult to learn and even though as with every technique the disc appearance will not always provide us with the answer we need.

Newer technology making older techniques less relevant
Felipe Medeiros MD said that while he still obtains baseline disc photographs for most of his patients, he questioned whether they are still essential for everyone.

He noted that while his own research has shown that changes detected in disc photographs are highly predictive of visual field progression, those studies involved film photography obtained by simultaneous stereophotography cameras, and highly trained graders for assessing those photographs. Nowadays, most ophthalmologists use digital photography that does not seem to provide the same level of quality for assessing glaucomatous change to the optic disc as conventional stereoscopic film photographs, he said. His statement was based on research from his database of thousands of patients with glaucoma followed for several years, which showed that digital photography could detect only very few subjects that showed progression over time.

He also pointed out that the European optic disc assessment trial showed a large variability in diagnostic accuracy among clinicians when grading optic disc stereophotographs, and that imaging devices actually outperformed those clinicians in distinguishing eyes with glaucoma from those without the disease.

He added that unlike imaging technology, disc photographs do not provide an objective quantification of change. As a result, clinicians limited to stereophotography will have difficulty in determining the rate of progression as well as in detecting small changes in the optic nerve. He emphasised that measuring the rate of progression is a fundamental variable in the longitudinal care of patients with glaucoma.

Dr Medeiros showed recent evidence on the use of imaging devices for detecting progression and measuring rates of change, such as spectral-domain (SD) OCT. “We need to be able to identify those patients who are progressing fast, as they will be those at the highest risk for developing vision impairment from glaucoma. Imaging devices are ideally suited for quantifying rates of progression and, in general, will do a better job than disc photographs in identifying patients who are progressing fast. This is what really matters”, he said.

Dr Medeiros also maintained that the technological advances in imaging devices is something that ophthalmologists should welcome. “People have criticised imaging tests based on the fact that devices change over time. However, imaging platforms such as SDOCT have been stable for quite some time, and efforts have been made to maintain backwards compatibility even when there are hardware and software improvements.

“This has allowed continuous follow-up of patients for many years, within the time frame that clinical decisions are usually made for most cases,” he concluded.

George Spaeth:
Felipe Medeiros: