Modify RX for intermediate

[libellule]

Okay...I know this something I should be able to do in my sleep, but I leant out my resource books to someone studying for the licensing exam.

I have a patient with RX:

OD -0.25-1.50 X 075
OS -0.75-1.00 X 088

Add +2.75

She wants a lined bifocal for intermediate/Near. I modified the RX the typical way...half the add into the distance to get intermediate but this does not work well for her. I had her measure the distance from eye to screen and it is 22 inches. So I need to compensate for this distance.

My doc didn't write the RX and doesn't want to get involved and her doc's office says she needs to come in for a refraction in order to make the modification (which is a bunch of bull since it is a formula).

I can't find the correct formula since I don't have my books...

Can someone please give me the formula? And if anyone is willing to give me the answer so I can be sure I did it right....I would really appreciate it!

This reminds me that I need to keep all of my skills honed and not let basic stuff like this slip from complacency...and not to lend out my books!

[chip anderson]

Find out where in terms of distance and height that she wants to see near and intermediate. Calculate focal lenght for there. This can be quite different for pianist than it is for watchmakers.

Also put the seg. at appropiate height for whatever is being viewed.

This business isn't rocket science but it helps if one takes the time to think any maybe actually talk to the patient.

"I'll take the history, any senior medical student can do the physical."
Sydney Mayo

[drk]

You must convert the desired focal length to metrics:

22 inches x (2.54 cm/inch) = 57.15 cm or 57cm

57cm x (1 M/100 cm) = 0.57 M for desired focal length.


Next, you need the lens power that focuses at that length.
Remember that really, lenses could be specified in focal length. Nevertheless, a lens power is specified as the inverse of the focal length in meters, called a diopter.

1/0.57M = 1.75 D



So, think of it this way:
Step one: correct patient's distance refractive error
Step two: for near power needed (assuming zero help from patient's own accommodation ability), "add" the 1.75D.

You formulaically gave her +1.37. That would seem to work for most cases! Something's fishy...

I really don't like high adds on the computer due to the reduced depth of field. Can't she push the thing back?

It's OK to do your own "exam". Sit her at a computer with your original specs and see what the deal is...does she want to sit closer or farther than the lens allows?

[sharpstick777]

drk is right as usual,

...but I have found that mid range power is more subjective than reading power.

I would fit her with what you gave her, hold something at her desired range and use +-.25/.50 flippers to see if reducing or increasing mid range power improves her perception. Then adjust the powers accordingly (keeping total reading power as perscribed of course).

The other option is an Office Lens that allows her to adjust the power she needs herself just by tilting her head a little.

[OHPNTZ]

"My doc didn't write the RX and doesn't want to get involved and her doc's office says she needs to come in for a refraction in order to make the modification (which is a bunch of bull since it is a formula)."

I would ask "Why is it a 'bunch of bull'"? If it was my patient, I would definately want to see the patient back...preferably to trial frame the patient.

Your calculation is assuming that there are no other etiologies, the refraction is perfect, etc, etc. I would much rather have the patient measure the EXACT distance from the spectacle frame to the computer monitor in his/her home or workplace setting. Then, to insure everything, trial frame the refraction, and have the patient sit at a computer in the office, and show various intermediate power lenses.

Then if the patient complains, you can assure them that you showed the patient the various powers...it may also save you a remake.

[HarryChiling]

Okay...I know this something I should be able to do in my sleep, but I leant out my resource books to someone studying for the licensing exam.

I have a patient with RX:

OD -0.25-1.50 X 075
OS -0.75-1.00 X 088

Add +2.75

She wants a lined bifocal for intermediate/Near. I modified the RX the typical way...half the add into the distance to get intermediate but this does not work well for her. I had her measure the distance from eye to screen and it is 22 inches. So I need to compensate for this distance.

My doc didn't write the RX and doesn't want to get involved and her doc's office says she needs to come in for a refraction in order to make the modification (which is a bunch of bull since it is a formula).

I can't find the correct formula since I don't have my books...

Can someone please give me the formula? And if anyone is willing to give me the answer so I can be sure I did it right....I would really appreciate it!

This reminds me that I need to keep all of my skills honed and not let basic stuff like this slip from complacency...and not to lend out my books!

The add prescribed is going to be a function of the accomadative reserve and the focal length needed for the working distance. Or in lay-opticians terms:

Add = Focal Power - (1/2 to 2/3) * Accomadative Reserve.

So that means you need to know the working distance that the patients current prescription is prescribed for, let assume 30cm in this case, so the focal power for a 30cm lens is:

1/0.3 = 3.33

The patients add in your example is: +2.75, which set's up our equation as:

+2.75 = +3.33 - (1/2 to 2/3) * Accomadative Reserve
so that means that 1/2 to 2/3 depending on how the doctor prescribes is going to be the function of the accomadative reserve which is equivalent to 0.58D, so if the new working distance is 57cm as drk so kindly pointed out in his conversion, then the focal power is going to be:

1/0.57 = 1.75

Runb through our original formula:

Add = 1.75 - 0.58
Add = 1.17 or rounded to 1.25

Keep in mind the accomadative reserve is going to be in the realm of the prescribing doctor and it shoudl not be your intention to modify that, but the focal length is purely optics and is well within your realm and scope. It might be a good idea to send them back to the doctor as the doctor can factor in the length of use and modify the accomadative reserve to account for that.

[libellule]

Lots of great feedback!

With one small exception:
(This business isn't rocket science but it helps if one takes the time to think any maybe actually talk to the patient.)
All I can say to that is that insulting people when they ask for help is actually quite mean-spirited and not at all helpful and doesn't really deserve even the energy I just gave it.

But, anyway...

I have to acknowledge that I may have jumped the gun with my 'that is bull' comment. I just have a hard time sending someone back to the doc (in her case, because of where we live, its a half-day thing) or charging her to be refracted here. I assumed it was a purely optics issue. So, I take my scolding in that matter...

I will take the advice given here and have her come in, pull out the flippers and try a real-life simulation.

And to DRK...she could push it back..she has told me that if she backs up to 29 inches it clears but is unwilling to change the position of her screen. What a surprise, I am sure.

[Jubilee]

The doc I works for is much like OHPNTZ, he insists on getting the working distance measurement.. refracts for that specific distance and then trials it before making..

Somedays, I admit I get a bit aggravated cause we have to schedule a patient in for a brief exam in an already full book and it can take a few extra days then to get them their glasses.. but overall the patient's satisfaction with the actual glasses once in hand is quite high.

If the exam is recent (3-4 months) we don't charge for the refraction. If it is closer to 6 months or more.. we will charge a modest fee.

[Fezz]

I like to call in the experts for advice in these situations.

I fellow Optiboarder, Borysko, may have the formula for this. Sadly, I think that he may be out of the country and unable to post.


;):cheers::cheers::cheers::D

[drk]

Borysko's the man.



I picture him like this...