Combining Fresnel and Block Prisms Journal
Tammy Miller
Date Aug 2024
This journal could not have come at a better time! I just saw a gent with a very large RXT and R Hypot deviation, ptosis and Marcus Gunn jaw wink (from memory he measured about 75 XT with 35-40 R HypoT at near, and 50XT with 25 R HypoT at distance). He was diagnosed as a child with a right Double Elevator Palsy, but I’m kind of questioning whether he is more of a R III given he also has a bit of a right adduction deficit aswell. Having said that, I must go back to check his vertical saccades and Bells. Anyway, I was stacking prisms and thinking to myself this is really inaccurate and wasn’t even sure how I’d measure him in other positions of gaze with angles this big.
Has anyone tried either of these methods described in the journal?
1) Fresnel prism fixed – where a Fresnel prism of 30D is attached to the posterior surface of the spectacle lens in advance. While the patient wears the spectacles, the examiner holds a block prism before the spectacle lens in the frontal plane position and performs the prism and alternating cover test.
Or
2) Block prism fixed – where Fresnel prisms with different powers are attached to the posterior face of a 50D block prism in advance (Figure 3). The examiner holds
these prisms (“piggyback prism”) in the same way as for a single block prism and can perform the prism and alternating cover test.
If so how did you find it? I’d be interested to hear more, or any other advice how you’d measure such large angles in free space as I do not have a Synoptophore down here – ideally that would be the way.
Thanks
Tammy
7 responses to “Combining Fresnel and block prisms to measure large angles of strabismic deviation”
No, I haven’t tried either of these methods, but surely the optics of refraction don’t change.
also have you tried looking through a 30D Fresnel the blur and distortion make it really hard to see!
I don’t have a synoptophore either and would tend to ‘chicken out’ and state approximate measurement only! and would use 2 loose prisms I hold one and patient holds the other over the second eye to achieve these high strengths. If a vertical NEEDED too I’d be the one to hold the H and V prism.
regards Sally
My technique for peripheral measurements ask patient to move head to endpoint where both eyes ‘see’ target to do side gaze much more repeatable.
similar with up and down -nothing flash. Sally (interested in others methods)
I haven’t tried these methods either but like Sally I think trying to use a strong fresnel like a 30 would make the vision very blurry and therefore fixation difficult
For large angles I tend to get the patient to hold about a large prism in front of one eye and then I either use block prisms or a prism bar in front of the other eye to measure, that way I can also measure the vertical component if needed.
I know not ideal in incomitant deviations, but I can’t help think might be as accurate as trying to measure though huge thick prisms and all the aberrations involved
I think any measurement of these huge angles are approximate anyway and does it really make much difference as there is only so much surgery you can do on a muscle before you end up with limitations?? And no one wants that.
To measure in different positions, I do like Sally and get the patient to fix on a distance target and then turn their head as far as they can go and still maintain fixation with either eye in the relevant positions. As she says more repeatable in lieu of not having a synoptophore.
Jo
That’s a nifty way of measuring large angles, but like Jo and Sally I would be concerned about the quality of fixation because of the induced blur a 30 dioptre prism would produce. I use the same methods that have already been described i.e having the pt hold the prism and using either loose of prism bar for other eye myself. There is a gadget that you can buy to hold the loose prisms in this circumstance. Has anyone ever used it?
This idea is interesting ! We definitely had several large angle patients in the last few months! I don’t know how easy it will be to see the pt’s eye with a strong stick on prism though-30 diopter prisms and over are terrible…
Got me thinking I used to have rigid fresnels to use on a trial frame and halbert clips to latch on top of the glasses to hold rigid prisms for the same purpose- however the prisms in this case still stack and face in the same direction as the block prisms so don’t think these calculations would apply and I would still be underestimating the angles by quite a lot. I have sold those unfortunately after I moved to NZ! otherwise would be neat to compare their blurriness with the stick-ons option. They need a frame to sit on, which makes them less practical than the piggyback option, which you can use without the glasses frame being in the way. I have not used or seen the frame support that holds a block prism by itself.
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Thanks Tammy. I’ve never used that technique and concur with above. The article does discuss the fixation target being a light source – so maybe for those poor acuity/fixation strabismus patients??? Lora