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A black and white case? Snellen acuity and cataract surgery

snellen chart cataract surgery

Australia’s eyecare profession is clear about how much weight should be given to 6/12 visual acuity when determining a patient’s suitability for cataract surgery, but this is potentially at odds with a new clinical care standard in development. Insight dissects the issue.

The contentious role of 6/12 visual acuity or worse as an indication for cataract surgery has been in the spotlight after featuring in an initial version of what is to become Australia’s first Cataract Clinical Care Standard, currently under development.

Led by the Australian Commission on Safety and Quality in Health Care, it is being developed in light of a recommendation in the Second Australian Atlas of Healthcare Variation (2017). The Atlas reported seven-fold variation in the rate of MBS-funded services for cataract surgery across 320 local areas, and recommended the development of a clinical care standard to address this variation.

The standard’s ultimate goal is to support clinicians and health service organisations to improve their pathways of care and access for people with clinically significant cataract. It also aims to ensure patients with cataract are offered surgery according to their clinical needs, and that they can make an informed choice suitable to their circumstances, in consultation with their healthcare professional.

The Cataract Clinical Care Standard was developed in collaboration with a working group of clinicians, researchers and consumers. A public consultation was carried out between February and April in 2019. But it prompted submissions from leading ophthalmic bodies who were particularly concerned about the implications of using 6/12 visual acuity or worse (Snellen chart) as an indication for surgery.

The standard has since been finalised taking into account feedback from the consultation process and recommendations of international evidence-based clinical guidelines, while also weighing up the goals of the standard, and the role of the commission.

At the time of writing, the commission wasn’t prepared to say whether 6/12 visual acuity has remained in the final document, which it said would be released shortly.

RANZCO confirmed it had been consulted and is aware the standard has not been released in its final form. President Professor Nitin Verma says the college is therefore not able to make specific comments about what might be in the final standard and whether RANZCO will endorse it.

With respect to Snellen acuity, which drew opposition from the eyecare sector during the public consultation phase, Verma says RANZCO does not believe that it is the most important factor in determining the threshold for cataract surgery.

“Many factors need to be taken into consideration, visual acuity being just one,” he says, adding that RANZCO will make a more detailed statement when the final version of the standard has been released, depending on what it contains.

Is 6/12 a reliable measure? 

At its core, the Cataract Clinical Care Standard aims to address current variation in rates of cataract surgery between the public and private sectors, urban and remote areas, and for Aboriginal and Torres Strait Islander people compared with non-Indigenous Australians.

Vision Eye Institute (VEI) consultant and one of Australia’s leading ophthalmic surgeons Professor Gerard Sutton, who is based in Sydney, has a keen interest in equal access to cataract surgery.

He says addressing healthcare variation is important and much of the clinical care standards are worthwhile, but the eyecare profession is wary of the standards being too proscriptive.

He shared with Insight some of his concerns – held also by the Australian Society of Ophthalmologists and Optometry Australia – about assessing visual acuity when determining a patient’s suitability for cataract surgery.

The initial draft of the Cataract Clinical Care Standard proposed a patient is offered cataract surgery when they have a lens opacity that limits their vision-related activities and causes clinically significant visual impairment involving reduced visual acuity of 6/12 or worse, or disabling glare or contrast sensitivity. (Although not explicitly stated in the initial draft standard, the Snellen chart – invented in 1862 by Dutch ophthalmologist Herman Snellen – remains the most widespread technique in clinical practice for measuring monocular and binocular visual acuity.)

The problem, Sutton says, is that visual acuity measured on a Snellen chart is “only a measure of high contrast visual acuity” and a patient with a clinically significant cataract can have better than 6/12 visual acuity. This means a patient can fail the proposed standard’s visual acuity threshold, jeopardising their contention for cataract surgery, despite their vision deteriorating to a point where cataract surgery would not only improve their vision but their quality of life too.

“There is good evidence to support the theory that if you treat a clinically significant cataract, the flow-on impact, in terms of reduced hip fractures from falls and so on, means patients can maintain independence,” he says.

A statement from the commission on 21 May clarified the standard emphasises that a comprehensive assessment of both objective and subjective factors is undertaken when considering a patient for cataract surgery, to ensure a complete understanding of the impact of cataract on the patient’s quality of life.

It went on to state the standard recognises “even with good visual acuity, some people can have disabling visual impairment due to cataract-related glare sensitivity or contrast sensitivity”, and that the impact of visual deficits can vary according to individual needs and a person’s reliance on their vision for work, or for maintaining their independence or quality of life.

If this is the case, Sutton questions why 6/12 visual acuity needed to be included in the standard in the first place.

He is concerned that if the standard is finalised in its original form, the majority of ophthalmologists in Australia would be considered to be performing outside of clinical care standards because in-practice, they take a more nuanced approach to assessing a patient’s visual acuity (an assertion supported by a recent survey detailed below).

“There is an implication that if you don’t have visual acuity less than 6/12, the surgeon will then need to justify their decision to offer cataract surgery. Apart from constricting the surgeon’s medical sovereignty in the decision making it will also potentially allow health funds to start to restrict access. My position is that 6/12 should be out,” he says.

Sutton describes the standard in its original form as potentially trying to restrict people having cataract surgery with an arbitrary number.

Strong in his convictions about the inadequacies of the Snellen chart in cataract surgery, Sutton and his VEI colleague Mr Chris Hodge surveyed RANZCO Fellows to canvass their views on the use of Snellen acuity – and how much weight it should have in the prioritisation of cataract surgery.

The survey results, published in RANZCO’s Eye2Eye magazine and reported by Insight, demonstrated that while 45.9% of respondents agreed Snellen acuity may be considered one part of an assessment to prioritise surgery lists, most (80.4%) indicated 6/12 is not the most important determinant for surgical intervention.

“This finding appears to reflect global guidelines which do not place a hard limit on corrected visual acuity prior to undertaking surgery,” Sutton and Hodge reported.

“The number of available, validated quality of vision questionnaires has increased in recent years, potentially providing more information on subjective complaints. Although only 8.3% suggested these are used routinely, 41.7% confirmed the use of their own non-standardised questions focussing on patient visual-based limitations in daily life.”

Visual function in the real world 

In March 2019, the Australian Society of Ophthalmologists (ASO) raised its concerns about the Cataract Clinical Care Standard consultation draft in a letter signed by former president (now vice president) Dr Peter Sumich.

ASO’s submission made clear its position on visual acuity: “We believe Snellen acuity does not have a role in the indications for cataract surgery and should be removed from the criteria.”

The ASO denounced Snellen acuity as an outdated measure of visual function and suggested it remains in use because GPs and allied health workers understand it as a notion of vision.

“Snellen acuity is only a test of black on white contrast in a dim room at standardised distance which does not equate to visual function in the real world,” it said.

“In the real world, patients must function with glare, reflected light, oncoming headlights, dusk and darkness and rainy nights. Low contrast contour detection such as a shallow dip in a grey pavement which denotes a lower ground level can be missed by a cataract patient with the attendant risk of a stumble and fall.”

ASO’s submission goes on to state that some patients are asymptomatic and record good Snellen acuity until they drive into the sun and the internal light scatter of the crystalline lens creates a white out. In the above situations, ASO’s submission states Snellen acuity may be better than 6/12 and may indeed be 6/6.

In short, Snellen acuity is said to be unreliable and therefore has the potential to be misleading.

“Snellen acuity does not allow for any nuance or for any individual variation in the visual requirements of a patient. It is the most basic, non-discriminating way to measure visual acuity. These assessments, these judgements, should be left to the eye professionals who know their patients, not bureaucrats,” Sumich says.

Without the support of organisations within the sector at present, Sumich says it’s difficult to comprehend how the standard can be deemed relevant with 6/12 visual acuity.

He says the eyecare profession has a unified position on combining modern clinical – and subjective – techniques to assess a patient’s eligibility for cataract surgery.

“We know from studies that patients with untreated cataract are eight times more likely to fall, three times more likely to fall and fracture a bone, and have increased rates of depression and social isolation,” Sumich says.

He says the standard in its current iteration is not based on medical advice despite the commission appointing experts for the exact purpose of providing said advice.

An optometry perspective 

Optometry Australia (OA) also made a submission to the Cataract Clinical Care Standard consultation draft in 2019.

Mr Simon Hanna, professional development and clinical policy manager at OA, represented the organisation on the standard’s working group established in 2017, and remains a member.

“Our [OA] submission noted support for many elements of the draft standard and primarily raised concerns with the requirement that patients have a visual acuity of 6/12 or worse before they are considered eligible for surgery,” Hanna says. He says waiting for patient’s visual acuity to be 6/12 or worse is effectively putting them “behind the eight-ball” to receive cataract surgery in a timely fashion.

“A patient going on to a cataract surgery waiting list when their visual acuity is 6/12 or worse means the chances of them losing their driver’s license have sky-rocketed,” he says.

“They could be waiting six, 12, 18 months, or two years, depending where they live,” he says. “For some, losing their license during that period can be isolating if they are unable to rely on others – friends or family – to drive them places.”

Hanna says OA’s submission cited a study demonstrating that fall rates are higher for people on cataract surgery waiting lists.

“The profession needs a cataract clinical care standard but assessing visual acuity to determine eligibility for surgery needs to reflect an individual, case-by-case assessment. It can’t be too proscriptive, it needs to be more open to an eyecare practitioner’s discretion,” he says.

Cataract referral form 

In a separate clinical undertaking, the NSW Government’s Agency for Clinical Innovation (ACI) prioritised developing and piloting a cataract surgery referral form for outpatient clinics based on the recommendations from the Increasing Access to Cataract Surgery forum held in August 2018.

Ms Audrey Molloy represented Optometry NSW/ACT on the ACI committee in 2019, specifically involved in revising the Cataract Referral Form.

Molloy, an optometrist at The Eye Practice in Sydney’s CBD and strategic communications and member liaison at Optometry NSW/ACT, says the cataract referral form does not include any mention of 6/12 (or any other) visual acuity level.

“As the form implies, referral for cataract needs to consider aspects of a patient’s vision and quality of life other than visual acuity,” she says. “The form also records the impact on quality of life as well as driving license status and the number of falls the patient may have had over the previous year.”

Molloy says there were a large number of stakeholders involved in the discussions, but the outcome was to ensure the form included several criteria for referral in addition to best corrected visual acuity.

Importantly, as she points out, ‘best corrected visual acuity’ is not the same as ‘pinhole acuity’ – the latter being measured with the pinhole occluder.

Quoting from Review of Optometry, Molloy notes: “The pinhole occluder works along the same basis as pupil constriction in bright conditions causing an improvement in visual acuity. Through a smaller pupil,
the effects of minor ocular irregularities—such as refractive error or paracentral cornea or lens opacities—are diminished.”

She adds: “Pinhole acuity is commonly better than best corrected visual acuity as measured by a referring optometrist, yet pinhole acuity is commonly the criterion used during triage for surgery waiting lists
at the hospital. This can result in a patient with best corrected visual acuity of 6/15, for example, measuring 6/9 on pinhole acuity and being declined surgery.”

The ACI work on the cataract referral form was completed in mid-2019 and Molloy has not been involved since.

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