NANOSNET

First year resident question:

Which is better for a total of, for example, 3 base out prism total split between two eyes, to write?

"1.5 base out OD, 1.5 base out OS"

"2 base out OD, 1 base out OS"

Factors being what is customary or if there's a difference in accuracy, or other factors I'm not aware of;.

Mitch

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Original Message:
Sent: 10/16/2025 9:25:00 PM
From: Tatiana Bakaeva
Subject: RE: Splitting prisms

Optically, there's no difference in how the prism is split between the two eyes.

In some cases, though, unequal splitting can be helpful: for instance, placing the larger prism to the paretic side in paralytic strabismus to better address the primary deviation, or adjusting for anisometropia to balance lens thickness and weight.

For a small total amount like 3PD, though, these distinctions are unlikely to be meaningful, so, as it was nicely put, one could indeed "leave it to the optical dispenser and write 'divided as you like.'"

Tatiana Bakaeva, MD, PhD

Director of Neuro-Ophthalmology

Assistant Professor of Surgery

Assistant Professor of Neurology

The Warren Alpert Medical School at Brown University

Tatiana, it’s interesting that you mention that because it was the reason I made the post - I generally put the larger of the unevenly split prism, sometimes all of it, on the side with the thinner spectacle lens edge where it’s more hidden.

Russ, I was hoping this seemingly innocent “first year resident” question as I put it, would expose some subtleties, and using the greater prism on the subnormal vision side is brilliant. And I second the Reubens book, I read it cover to cover eagerly when I was a resident, it’s very interesting and well-written, I’ve got to find my copy and see what it says on this topic, though I’m afraid I may have loaned it to someone and can’t remember who :(

Another question just popped up - in the case if vertical prism, which is less objectionable, the base down or base up, in terms of function and cosmetics? II’ve always felt base up is better so preferentially split more to the base up side.

Another question I have, and I suppose I could call a good optician someplace, is whether there’s a difference in the accuracy of OD BD 1.5/OS BU 1.5 vs. OD BD 2/OS BU 1, i.e. are the odds of an accurate 3 BD OD total better with one or the other.

For those of you who are a little wet behind the ears, be sure to tell patients, and document in the chart, that there’s a significant probability of needing to tweak the prism in followup and it’s the patient’s responsibility to be mindful of the lad’s expiration of warranty for free doctor prescribed changes. It’ll save some grief.

Mitch


Original Message:
Sent: 10/16/2025 9:36:00 PM
From: Russell Edwards
Subject: RE: Splitting prisms

This can be complicated. If there's no anisometropia, splitting them equally usually makes sense. If there's subnormal vision in one eye, putting more prism over the eye with worse vision may reduce symptoms associated with distortion. There are additional considerations with vertical prisms, paretic muscles, restrictive strabismus, significant anisometropia. An experienced and interested optician can help - it will pay to talk to the optician in complex cases. Consider reading Reuben's " How to prescribe glasses without making a spectacle of yourself". An old text, but thorough. 

Russ Edwards

I prefer an integer number rather than a fraction, I'll do 1 BO on the lens with the higher  prescription and 2BO on the lens with the lowest prescription 

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Carlos W Vazquez west Texas Eye Associates
Pediatric- Neuropthalmology
El Paso Texas
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