Alison Middleton explains how dispensers can manage anisometropia, helping patients adapt to differing lens powers, while improving comfort, binocular function, and communication at the dispensing desk.
In the last few months, I can think of at least three adult patients who have collected their glasses and reported similar observations: “These are very clear, but they feel weird”.
On each occasion we explored what ‘weird’ meant. The first person reported the table ‘tilting’, one talked about peripheral vision being ‘swimmy’ when driving, and one reported a sense of delayed movement when turning their head. One quick glance at each person’s prescription revealed the most likely starting point to explain their issue.
Anisometropia, a significant difference in prescription between the two eyes, can be one of those conditions that sits quietly in the prescription until it becomes the patient’s reality at the dispensing desk. The optometrist has done their job. The refraction is correct. And yet the patient might report possible headaches, double vision, slight nausea, eyestrain, or a feeling that something just isn’t right. If we can understand what’s happening optically in anisometropia, and know what we can do as dispensers, this can be a very satisfying area of clinical knowledge to brush up on.
The image size problem
When two lenses of different powers sit in front of the eyes, they don’t just correct differently, they magnify differently. Each lens produces a different sized image, and the brain must try to fuse these two images that don’t quite match. This condition is called aniseikonia, and image size differences can be clinically symptomatic, producing eye strain, headaches, depth perception issues and diplopia.
As a clinical rule of thumb, 1 dioptre of difference is enough for diagnosis of anisometropia, and some patients may struggle with this, while patients with 3 dioptres of difference will almost definitely experience compromised binocular function. This isn’t a rare case, it’s a patient type we may encounter regularly, and the dispensing conversation can affect how well they cope.
What dispensers can do
Lens magnification is influenced by base curve, centre thickness, and refractive index. A flatter base curve, thinner centre thickness, and a higher refractive index all reduce magnification.
While dispensers can, in theory, make the calculations required, there are specialist labs that make iseikonic lenses specifically to reduce aniseikonia, and most labs now make these calculations required to manage aniseikonia in freeform lenses.
Three factors to control
Frame selection, centration measurements and communication; these are the three factors dispensers can control the most.
Frame selection matters more than many dispensers realise in this patient group. Smaller frames reduce the amount of peripheral lens the patient looks through, which reduces peripheral aberration and the prismatic differences between the lenses in off-axis gaze. In anisometropia, frame size is a clinical decision, not just a cosmetic one. Additionally, choosing frames that can reduce back vertex distance by sitting closer to the eyes will reduce the impact of different image sizes.
Centration stakes are high
Everything we know about monocular measurement accuracy applies with extra force in anisometropia. A small measurement error that might be inconsequential in a low prescription becomes a genuine binocular problem in an anisometropic patient. Monocular PDs and heights, measured on a properly adjusted frame, are non-negotiable here.
Conversations
Careful communication is probably the most powerful tool we have. We need to ensure our patients know what they might expect before they order their glasses.
Someone who has lived for years with uncorrected or under-corrected anisometropia might find a fully corrected prescription harder to adjust to initially. The visual system adapts over time and change often feels wrong before it feels right.
This was the case with all of our recent patients mentioned at the beginning of this article. It was an easy conversation – I was able to remind them of the discussions they’d had with the optometrist and dispensers at the time of ordering. They appreciated the reminder and were keen to continue adapting.
Contact lenses sit at the corneal plane rather than the spectacle plane, which largely eliminates the image size difference (zero back vertex distance), making them highly effective for anisometropia where glasses may struggle. This isn’t our referral to make, but double checking that the option has been discussed is a good conversation to have.
Anisometropia rewards clinical curiosity. The more you understand about what’s happening between those two lenses, the more you can do to help.
About the author: Alison Middleton is an experienced optical dispenser with more than 20 years’ experience. She currently works as a senior optical dispenser at Holdfastbay Optometry in Glenelg, South Australia, and sits on the advisory board of Optical Dispensers Australia (ODA).
