Slab Off- When Is It Really Necesaary?

[willsaake]

At work I run into a lot off prescriptions that look like this:

OD +5.50 -1.75 170 2.25 Add

OS +0.50 -1.25 005 2.25 Add

I know from old rules I learned that any 2-3 D difference slab off would be prescribed. I have seen 4-5 jobs this month (different people) and have yet to see slab off prescribed. Is there a reason why the doctors or the optician would not be prescribing. They always make the lenses ST-28's.

Thanks,

Will Saake

"I don't have a cool quote yet."

[David Wilson]

Will,
The prescription you have given would certainly warrant the use of slab-off, or some other way of addressing the differential prismatic effect at the near visual point. I can only assume that the reason they are not being ordered or dispensed is because the optometrist or optician had not, unlike you, considered the option. Anisometropes will cope with their anisometropia prior to presbyopia by using the paraxial region of the lens. Bifocals and progressives do not allow them to do this and so they experience diplopia or asthenopia caused by struggling to overcome the differential prismatic effect. Lenses like the Younger reverse slab-off deserve more use than they get and would certainly make reading more comfortable for the likes of the person whose prescription you have given.
Regards
David Wilson

[Corey Nicholls]

GO SLAB OFF!!!!! Sounds pretty bad doesn't it?

But seriously, I think we don't see many "Slab Offs" these days, as not many people even know what they are, never mind working out how to use one! I have also noticed the amount of tolerance that people have in their reading in regard to verticle prism, sometimes it is truly amazing!

As David Wilson said to me the other day, if you have a patient with the above Rx who has only been wearing S/V distance all their lives, and you then give them a slab off bifocal to balance the prism at near, I bet you will have them back complaining that "It isn't quite right" at near. This is because they are used to having the prism imbalance at near ever since they have worn specs and if you change it, they will notice.

As most people say...."If it ain't broke, don't fix it!"

Another thought,.......What if the same person was wearing progressives?

[David Wilson]

Hi Corey,
Our posts must have passed in the ether. I'm not sure that your quote from me actually reflects what I meant. People will certainly notice a difference in prismatic effect and someone who has become accustomed to unwanted prism (due to an incorrectly made pair of specs) mayfind the lack of prism in the new (correctly made) pair strange and, quite possibly, uncomfortable. The anisometrope, though is a slightly different case, in that they have avoided differential prism by useing the paraxial region of the lens. When they get their bifocals or progressives they will find the differential prism uncomfortable. The slab-offs will actually improve their reading comfort.
Regards
David

[willsaake]

Can slab-off be done on a progressive lens? I've only seen it on ST-28/35's before? So I gather from everybodys response that the optician should be the one to make the slab off call unless of course the dr orders it.

[Michele]

I was trained with the theory that it is the opticians responsibility to fit a slab-off since the amount of vertical decentration creating the near prismatic effect is determined by the type of segment the patient is best suited. (ie; a progressive lens will create more vertical imbalance than a FT28.....Prentices Rule;
power x decentration / 10). I have been lucky enough to successfully fit bifocals, trifocals and progressive lenses with slab-off. One thing I have learned over the years is to consider visual acuity. Working in a ophthalmological setting, I have the advantage of reading the patients chart and obtaining more information including visual acuity. If a patients prescription is anisometropic, but one eye sees 20/20 and the other eye is only 20/50 or 20/60, I've found that the extra expense of a slab-off probably won't benefit the patient. And if I can't justify a benefit for a patient visually, I can't justify the expense and I don't sell it to them.

There's my two cents.

Michele

[Texas Ranger]

Michele, You bring up an interesting point, you, working in an MD practice have access to information that the dr's generally don't put on the rx, i.e. the pts' VA. So that lack of knowledge may cause the pt not using your services, to be put in the position of their optician recommending, with good reason, a slab-off, so why doesn't the doctor share that information on the RX? It would be most helpful to all, especially the patient. This only appears to be a lack of care for the best interest of their patient? We do slab offs on Variluxs quite often. Borderline cases of undetermined need, we'll advice the pt of the possible need of a slab-off and why it might be necessary, but make it up without it, then remake the lens with the slab-off at the additional charge if and when it proves to be necessary. Also, in the rx in question that started this thread, there are more considerations than whether to do a slab-off, base curves and lens materal and thickness also can be very important considerations, which would almost certainly rule out the "reverse" slab, since it would add much thicness to the already most plus lens.

[louise]

I have used different sized round segs with m.a.r to equal differential prism at minimal cost to the patient but the prism controlled bifocals available are great. P.S this might be really obvious but slab off an executive bif and the line isn't noticable.

[Jeff Trail]

Starting with the original post, slabbing is a weird animal, when someone has a natural imbalance I have seen upto a 4 or so degree of imbalance that was still tolerated, if it was due to say an IOL then I have seen as low as a 1.5^ difference cause fusion problems. I think alot depends on the pre-existing problem as much as the problem now. Some of the options are mixing segs and taking advantage of driving the imbalnce down by using the OC location in the seg's to reduce the imbalance ie. a RD22 (where the seg oc is 11 mm down) and a FT28 (where the seg oc is only 5 mm into the seg), which what you are doing with a slab, manipulating the oc in the reading only to help with fusion. Another option is to grind the distant oc right on the seg line and push that imbalance from power and the seg oc imbalance further down and past the point where they converge.
Now, can you slab a PAL? Sure, but you want to really be careful here and make sure that the slab is located in the correct area.. you do NOT want the slab at the 180 but down mid way through the intermediate, I mainly try to put it around 2 or so mm above the reading circle in a PAL. I have done it many times but alot of people tend to think it's kinda ugly. Since the intermediate has a "graduated" power zones you do not have much of a fusion problem out there past arms length to 8 or so feet (which the intermediate power is technically for) but that near (reading) is where you start to double (usually the 22 inches or so and inward..that's why I tend to try to keep it a couple a mm above the circle (reading) so it starts to kick in at around 25 to 30 inches (depending on the power of the add)
I also seen someone mention slabbing an E-Line..yikes, I do not think I would do that in most cases because the majority of slabs you do see are being done because of a cat. surgery induced imbalance and the slab usually ends up being a temp fix till the other cat gets removed (most of the time) .... Actually slabbing something like a 8x35 that has a fairly wide seg it is not as bad as say a 28 and if it's a photochromic it even hides that line more so...
I'm still a big fan of mixing segs and taking advantage of the lens designs to do the work for me or manipulate the distant oc and work the seg oc's that way...
I think a lot of people tend to over look that it is the SEG OC that is the actual culprit in combination with the imbalance of power.
I had an optician argue with me a few weeks back that there is only ONE oc in a lens and no way the seg had an oc that was seperate from the oc on the ground side of the lens ..go figure, and I'm supposed to be the "dumb" lab rat? :-) Oh well hope some of this information maybe helpful

Jeff "grind IF you can find'em" Trail

[Bev Heishman]

I also work in an ophthalmology office and have access to records. However I was trained in college to solve for the amount of vertical imbalance (effective power in the 90th meridian) present between the two lenses. If I beleived there was a potential problem a simple call to the refractionist ( doctor) to ask if this would be of benefit to the patient. This act of courtesy adds to respect between the professions. It also sets you apart from the rest.

The optoms that do our refracting sometimes forget the availability of slab offs. They say that is our responsibility since location of OC's factor significantly. We also do outside Rx's. In every case my staff and I have been thanked for thinking ahead. The status of the eye, stereopsis, VA, and eye dominance, and how close an individual may be to a surgery date are factors.

When to correct varies between doctors most of the MD's in this area say between 2 & 3D of imbalance is when to start. Some OD's and old schoolers start at 1.5D (because years ago we couldn't slab anything but glass and to do so was a fete.) Today reverse slab in plastic can take us to .75 in some lens designs. How sensitive a patient is to image (prismatic effect, magnification) is an individual thing. This is mostly due to differences of widths in Panum's area of the brain. This is where our all of our senses and learned visual clues are encoded. (Dartmouth studies on Aniseikonia)

Will it benefit? One way to test is to add a trial lens with a close prism equivalent over the reading area of the existing glasses. Scraps of Fresnel's prisms in lower powers on the bottom half of a plano trial lens also works well in trial framing a patient.

My optics instructor required as part of our final in geometric and mechanical optics II was to surface a slab-off, deblock it and not have it break. This was when we used balsam pitch to mount the 2 glass lenses and pan polishing (1978). Terrific learning experience.

[Homer]

Willsaake, this gal knows what she is talking about - it was her Master's thesis!

There are many reasons why you might not slab-off, but generally MD and OD that I have known rearely if ever order it on the Rx.

Opticians; here's your sign!!!


Let's make a difference,

Homer

[bblack]

I am anismetropic and most of my life I was OD +. 50 OS+4.00. My left eye functioned but did not quite have the acuity of my right eye. When I did go into a multifocal I did not have any prismatic problems at close. At 40 years old avoiding the poor esthetics of a slab off was more important to me than optimizing my correction theoretically.
The reasons that slab offs where so prevalent in the 60's and 70's is that cataract surgery results had been less accurate than the present procedures are. An imbalance you acquire later in life is more difficult to adapt to than one you grew up with. Slab offs are for the doctor to determine the necessity for. Of course when the optician sees a situation that clearly indicates slab off a call to the doctor is in order.

You know what they say about opinions


Bill

[Alan W]

Reread Bevs posting three or four times.
It's as complete as one can get and 100% accurate.
We went to separate schools together and had the same job separately!

[Homer]

First, aren't you also amblyopic? If so, it would explain why the slab-off was not necessary in your case; diminished visual accuity in OS.

Secondly, when I have called doctors about slab-off there is usually a lot of either silence, him/hawing, or simply "do what you think is best"

Even have one MD practice where they were in the process of rechecking the RX before I remade a lens with a slab-off. They called me and said, we don't think she needs it because when we put her reading Rx in the trial frame she has no problem!

DUH !

Opticians are usually the only ones who can figure out what the patient really needs to live "20/happy"

Homer

[Texas Ranger]

Homer, the last time I talked to a doctor about doing a slab off, we had done a reverse slab off on an rx with a -7.00 OD, and a +1.25 OS, the dr's tech told the pts daughter that she could see clearly in the "distance" because we had the slab off on the wrong lens (they had prescribed a slab); anyway, in an attempt to salvage our reputation with the dr., I went and talked with him, he had no clue what reverse slabs were and how long had they been doing that, and the tech was clueless too. and what does that have to do with their "distance" acuity? my normal experience with phone calls to doctors offices. like going from a
-2.00 rx on a 6.00 base curve to a -5.00 rx on a 4.00 base, pt. complains adjusting to new rx, we get blamed for not "maintaining same base curves". duh. same doc gives pt. an increase of -.50 in the distance, increases the add by +.25, pt. can't read with new rx, well he's got -.25 less near power, but he was told his glasses were a 1/4 stronger, which is what the pt went in for the exam for! the pt. is a dr. also. why is this concept so hard for drs. to grasp?

[rfish777]

I agree with most of you slab should be used. But most OD's and MD's I believe either fell asleep in class during this lecture or just disregard it. We as trained Opticians in school or out have the responsibility to 1. Call the Doctor in question and tell him or her from your professional opinion that slab off is warranted. 2. We
should inform the patient and give them our professional opinion
whether to go or not with the slab. Then it is up to the patient
whether to go or not with slab. This way they can make an informed choice.:bbg:

It is not ours to reason why
but just to do or die.

It's been the best of times and its been the worst of times

[Alan W]

My sense is that we are expected to deliver clear, comfortable, single, simultaneous, binocular vision.
The doctor writes an Rx with the understanding mandate that we will deliver those elements. If we produce glasses that create an imbalance, and there is no indication the doctor is correcting for a visual condition calling for prism, we need to get rid of it. Power, positioning of the glasses, and location of the optical centers influence the vertical imbalance potential.
Using Bev Heischmans hypothesis (actual fact), that placing prism in front of glasses that have a known but unprescribed prism that cause discomfort, and we can reduce the cause, we need to let the doctor know the patient sees more comfortably or is free of multiple images as a result. There is no law or guideline to my knowledge that dictates where an optical center has to be. I believe it states what the limits are of where it cannot be.

[Mike Fretto]

One other possibility for not supplying a slab-off is if the patient has had cataract surgery on one eye and is scheduled for the other eye shortly. We are also in an Opthamology setting and see a lot of this, we normally don't assume the patient will need it, we inform them of the possibility and try lenses without.

[Bev Heishman]

I agree with this especially if we are between surgeries. What is a factor for us is how long till the sugery and the acuities.

Again checking with the doc is important especially if the patients acuities or pathology are unknown.

VA's of 20/30 and better ou factor in many decisions. We also are talking about binocular sighted individuals when we consider slab off.

Image magnification percentage also needs to be considered in patients who are binocularly sighted. The eye tolerates generally 1-3% difference and at 5% and above supression occurs. The eye is most sensitive with binocular vision in the vertical meridian as the eye ability to maintain fusion is lower than in the lateral meridians. The range vertically if I recall correctly is generally between 2 & 4 D of power. Like wise we don't patch a person who has significant imbalance between surgeries because the muscles can drift.

Eye dominance also plays a role.

Always check with the prescribing doc unless you now otherwise.

[Darryl Meister]

Hi Bev,

Excellent and well articulated points, as usual. I thought I would add to a couple of your points, though...

Panum's fusional area is not really a part of the brain or visual pathways (such as the LGN, visual cortex, etcetera), but rather a theoretical area surrounding corresponding points on each retina of the eye. It can also be thought of as the "thickness" of the horopter, which is an imaginary surface of single binocular vision at some given fixation distance.

Also, significant differences in refractive power between the two eyes are often the result of axial ametropia. In this situation, magnification differences produced by spectacle lenses are less likely to cause aniseikonia (because of Knapp's law). This is undoubtedly why so many patients with high anisometropia seem to function nicely without iseikonic lenses.

It was also good to note the role of visual acuity in the prescribing of slab-offs. If binocular vision is absent (or significantly impaired) because of poor visual acuity, binocular fusion may give way to suppression.

Best regards,
Darryl

[Pete Hanlin]

This little tip may have already been noted somewhere above (if it is, sorry for the repetition), but I like to keep a set of Fresnel Prisms handy in the dispensary to help the patient "test drive" the concept of slab...

I enjoy spotting the need for slab and having it work out as much as the next guy (or gal), but honestly I try every trick in the book to avoid it if I can. After all, I have yet to find a patient who is overjoyed that they have slab on their glasses...

My favorite method is to have the oc placed on the seg and fit the seg as snug (high) as possible. I've found few anisometropes who haven't been pretty satisfied with this approach.

Slab on PALs? I've done it five times. Worked once and was "tolerable" in one other case. Thing to remember is that slab only resolves vertical imbalance at a single reading point- not the whole way down the channel.

Pete

[Homer]

..... and often the moving of distance OC's in not considered. I specify many oc hights on multifocals for various reasons other than anisometropia.