Interpreting visual field loss

Attention to the right metrics overcomes confounding effects of cataract. Cheryl Guttman Krader reports

Assessing patients with glaucoma and cataract can be particularly challenging, since cataract formation can mimic visual field loss and progression, emphasised Hans G Lemij MD, PhD, during a session on glaucoma pearls for the cataract surgeon at the 2020 ESCRS Virtual meeting.

Dr Lemij, a glaucoma specialist at The Rotterdam Eye Hospital, Rotterdam, the Netherlands, began his presentation by showing street scene images to depict how patients with vision impairment from glaucoma with or without cataract perceive their vision in the real world. The examples demonstrated that patients with cataract and a centrally located scotoma from advanced glaucoma may be disappointed by their functional outcome after cataract surgery because of persisting glaucomarelated blurred central vision.

PEARLS FOR READING THE VISUAL FIELD TEST REPORT Because a generalised depression of the entire visual field can be associated with glaucoma as well as with cataract, Dr Lemij recommended paying attention to the pattern deviation plot when interpreting the visual field printout in order to assess the potential functional outcome after cataract surgery in a patient with glaucoma.

The pattern deviation plot is based on the total deviation plot; it shows values that are corrected following a ranking of the field’s test locations. The seventh best point (i.e., the 85th percentile of the retinal sensitivities) is compared to the age-corrected normative database. The difference, intended to be caused by media opacities (such as cataract), a narrow pupil or refractive error, is then added to the values of retinal sensitivities for correction.

“By looking at the pattern deviation plot on the visual field printout, clinicians can get some insight into the contribution of cataract because the pattern deviation compensates for confounding from cataract,” Dr Lemij said.

In a case example, he demonstrated how this analysis can help clinicians parse out focal losses due to glaucoma in a patient with cataract. However, Dr Lemij also noted the need to consider the information in the context of the patient’s other findings. Although the visual field of the patient in the case appeared greatly improved based on the pattern deviation analysis, closer inspection showed that some of the individual retinal sensitivity data points had a value less than 0, indicating that the patient failed to perceive even the brightest stimulus at those points during the testing.

“Retinal sensitivity data point values less than 0 would hardly ever occur because of cataract. Therefore, this case demonstrates why you cannot rely only on the pattern deviation value and instead need to factor in all other aspects collected during the examination,” Dr Lemij said.

“In this case, the patient had a visual acuity of 6/6, which is not consistent with cataract.” DETECTING VISUAL FIELD PROGRESSION Detection of progression is critical in managing patients with glaucoma, but when interpreting the visual field, clinicians must recognise that the ability to identify real progression can be confounded by measurement variability, which can mimic or mask true glaucomatous progression, and cataract formation, which can also mimic progression.

When trying to determine the rate of glaucomatous functional loss, repeating the visual fields will circumvent the confounding associated with measurement variability. Dr Lemij cited a paper by Chauhan and colleagues (Chauhan BC, et al. Br J Ophthalmol. 2008;92(4):569-573) that provided recommendations on the frequency of visual field examinations required to identify clinically meaningful rates of change in glaucoma.

“In this very important paper, the authors pointed out that you need to repeat the visual field tests very often to be certain or somewhat certain that you are dealing with true progression. Depending on your target, however, the required testing can be highly impractical and probably not realistic,” Dr Lemij said.

Illustrating the challenge, Dr. Lemij explained that to accurately detect an overall change in the mean deviation of -1.0 dB over two years (-0.50 dB/year) a patient would have to complete seven visual field tests per year. Detecting progression occurring at a rate of -1.0 dB/year over a three-year period would require that the patient undergo three visual field tests per year.

The information provided by progression software, and particularly the Glaucoma Progression Analysis, offers a helpful alternative. When assessing patients with In case of younger cataract for glaucoma progression, Dr Lemij suggested focusing on the patients, with a longer life Visual Field Index (VFI) because unlike the mean expectancy, one deviation (MD), the VFI should obviously is relatively insensitive to confounding by cataract. be stricter in “Using the rate of change in the VFI in a patient with glaucoma accepting any progression than in and cataract, you can still monitor for very old patients glaucomatous visual field Hans G Lemij MD, PhD loss over time,” he said.

For interpreting the VFI rate of progression value, Dr Lemij noted that a decline of more than 1% per year would be considered clinically meaningful.

“In case of younger patients, with a longer life expectancy, one should obviously be stricter in accepting any progression than in very old patients,” he added. Hans Lemij: H.Lemij@oogziekenhuis.nl