Adaptation to multi-focal contact lenses

What is the adaptation to aspheric multi-focal contact lenses based on?

This is just a random site found with Google:
http://www.3d-eye.com/3d-eye/contact_lens/multi_focal.html

The video shows how refractive power is divided in the lens. How can a
person see only through one portion at a time?

 

A person cannot see through one portion of a multifocal contact lens at a
time (unless they are tangent streak gas perms). The vision is
simultaneous-- i.e. some light is focused at distance while some is focused
at near. The resultant image is "somewhat" clear at distant and near with
peripheral blurring or ghosting. Some people can adapt to the resulting
image while others cannot.

In my experience, if the patient's expectations are managed appropriately,
and a little chair time is expended by the doctor and patient working
together to get the best correction possible, most patients are satisfied.

The most important part is not to expect perfect distant and near vision--
expect "good enough" acuity to accomplish everyday tasks.

===========

 

When I was trying multifocal RGPs some of them DID provide perfect distant and
near vision -- but only for 1 second out of 60 (against-the-rule astigmatism,
apparently). Crying shame, because it was like having real eyes again :-(

 

I guess it is a variation on the mono-vision theme - the brain is remarkably
good at learning to ignore things it does not want to see... As I posted in
another message on this group, I am part way through a trial of Bausch&Lomb
multi-focals and I am very pleased with them. I am moderately long sighted
and need a +1 add reading glasses for any significant period of close work.
From the minute I put the B&Ls in I had adequate vision at all distances.
For the first half hour or so, if I analysed what I was seeing objectively
there was something "strange" about it - not really possible to pin it down
and not unacceptable. Now, I simply do not notice it - my distance vision is
probably slightly better than I was getting with varifocal glasses, my
close-up vision is about the same as with varifocals. I suspect that
middle-distance vision (i.e. around ten to fifteen feet) is slightly worse,
though that is subjective and it is still acceptable - the television does
not seem quite as sharp as it does with glasses.

As others have said, it is probably very dependent on expectations - if my
son who is thirty years younger than me and has 20-20 vision without glasses
saw the world the way I am, I dare say he would complain. For me, following
years of living with varifocals or carrying round two pairs of glasses all
the time, this is wonderful - I have more than adequate vision at all
distances without anything hanging on my face! It is probably also dependent
on your prescription - I am moderately long sighted with just a +1 add - not
too much... Other people's experience could be very different, but it is not
particularly expensive to find out.

Martin Bradford

 

When I was trying multifocal RGPs some of them DID provide perfect

I'm not sure what you mean. Do you mean that you could adapt to the
uncorrected astigmatism momentarily or what?

By perfect distant and near vision do you mean 100% unblurred, absolutely
flawless vision in both sunshine and darkness, no halos?

 

I have fit some version of multi-focal contact lenses for 25 years, and
this is truly a golden age for the multifocal lens.
The Bausch & Lomb can be "stunted" a little by putting a "low add" on
your dominant eye and a "high add" on your non-dominant eye, and you
get a "modified monovision" that relies on simultaneous perception at
times, but allows great binocular fusion. In the hands of a skilled
optometrist who loves to "play" with lenses, these products can let a
good 90%+ of soft lens bifocal candidates be successful. Contrarily, a
"cookbook" optometrist who reads the fitting guide and doesn't go
"off-label" in his thinking might hit 50% success on his best day.

 

I've been reading this thread with interest and now I'll add my
comments. I have a great deal of experience in fitting aspheric
multifocal RGP and soft lenses. I wear them myself, mostly RGPs as they
provide, for me, outstanding vision.

Generalized statement about performance are not at all useful. I have
some patients who do indeed achieve uncompromisingly excellent vision at
all distances. (Definition: distance and near acuity equal to or better
than that achieved with spectacle lenses, no "ghost/halo" at distance
under any conditions.) I myself am in that group. Most of my patients
do not achieve that degree of quality of vision as there are some
optical compromises, but most achieve excellent distance and
intermediate vision and good to excellent near vision. (Definition: as
above, nearest near point of 20/20 vision equal to at least 20cm, which
may not be close enough for some people's needs.) Some people
experience some degree of "ghosts/halos" in reduced lighting at distance
which does not impact near acuity at all but may reduce distance visual
quality somewhat and generally be a little annoying. This is far more
common in soft lenses than RGP.

There are many optical and eye structural issues that affect what the
wearer's optical experience will be. In my practice, I'd estimate that
80% are completely satisfied, 15% live with less than optimum vision but
are still quite pleased and 5% do not stay with the lenses for either
comfort or optical reasons. With RGPs and carefully selected patients,
the issue is more often comfort than optics.

For soft multifocals, the " I think these are great!" numbers are much
lower, with less than 50% achieving a visual experience approaching RGPs
but 70-80% being quite satisfied. The "I give ups" are in the 20-30%
range. Still, many people enjoy the freedom from multiple pairs of
spectacles or the hassles of hanging readers around their necks or
stuffed in their pockets and purses. With careful patient selection and
doctor experience, soft multifocals are an excellent option for the
"over 40's" group. And they're getting better all the time.

I believe (hope, dream, loose sleep over it) that when they arrive,
silicone hydrogel multifocals, due to their more stable/stiff
construction may be much better. In theory, it can be done. They will
need to come in a few different optic zone diameters and toric
(astigmatism correcting) powers.

I've found a very effective optical correction for folks with low add
needs ( like up to 1 or 1.25 D) who have less than .5D astigmatism is
plain old Focus Night&Day/Optix or Purevision lenses, which are somewhat
aspheric by design. The experience can be enhanced by .25 more plus
power in the near-dominant eye. Very, very little compromise there.

--LB. O.D.

 

Did you really mean 20 cm, or should that be 20 inches?

w.stacy, o.d.

 

Thanks for the post.

At low illumination, when the pupil is large, the rays have to enter the
retina at multiple focals, don't they. So, a probable case is something like
50% of the rays are passing refractive power A and 50% are passing power B
(if it's bifocal and transition is not considered). The rays A and B are
hitting the retina in at slightly different angle.

You said that many patients experience no ghost/halo whatsoever. Do you have
an explanation or what is your best guess, how exactly is this possible?

Is there a difference between typically spherically aberrated (ex-myopic)
laser operated eyes and myopic eyes with multi-focal contact lenses? Putting
together 1 + 1 (if the portions are as shown in the video I posted the link
to), both cases have near vision at peripheral cornea and distance vision in
the center of cornea.

 

They can't, except for translating (true) bifocals. All others result
in out of focus paraxial rays. The "adapatation" that occcurs is some
peoples' ability to ignore the blur.

w.stacy, o.d.

 

Whoops! Thanks, doc.

20"/50cm.

 

The "adapatation" that occcurs is some

Not noticing the blur in certain circumstances is a different thing than
noticing the absence of any blur at all (as halos around lamps for example).
According to LarryDoc:

In my opinion it's impossible to not see a halo around a lamp unless the
halo really is very vague, but, that can not the case if 50% of the rays are
contributing to the halo. Unless there is something blocking those rays.

 

That may be true, but many people don't look at lamps or such objects
that are likely to cause halos. Also, vision is probably 80% psychology
and maybe 20% physics, biochemistry, physiology and neurology combined.

w.stacy, o.d.

 

my clinical experience is that halos are indeed noticable but can
become adapted to by many patients.

i think it is an overstatement to say that uncompromising distance and
near acuity equivalent to spectacles without ghost/halo is achievable
with simultaneous vision multifocal contacts. it is doable however
with translating RGP bifocals, but many patients cannot tolerate the
discomfort of RGPs.

 

That may be true, but many people don't look at lamps or such

They don't have to be especially looked at. Unless 1) you really don't care
what and how you see or 2) the vision in fact is so good that it can be
called practically excellent, then you won't notice the imperfections.

In my opinion maybe something like 70% optics, 20% neural
filtering/optimization/adapting and 10% physical condition. Of course, there
are exceptions, but in general (or averagely), I think the "spiritual state"
can affect vision relatively little. I say this even I'm actually a
supporter of great possibilities of the brain. After the optics, the
sensation is heavily processed by the brain, but still, the optics are there
and there are tasks not even the brain can manage. With proper tests,
detailed visual qualities can be tested too, there is no need to be
uncertain how one sees, it can be tested (with relatively simple methods).

 

I agree with you regarding the relative importance of optics vs. neural
adaptation.

I vaguely recall some discussion of this topic during my PhD training in
physiological optics. The upshot of the discussion was that precision of
the optics of the retinal image is the single biggest contributor to the
reported visual acuity by an observer. Sorry I cannot provide details;
I've slept a few times since then.

On the other hand, clinically I have seen many patients who happily accept
less than optimal optics in return for convenience, etc. IMHO, it's all
about pre-educating patients about what they should expect from multifocal
contacts. They are not perfect but they can provide a lot of advantages
that can outweigh the optical disadvantages to some people.

---------------------

 

No. They moved/rotated out of position whenever I blinked or moved my eyes.
Every once in a while they would land in the right place purely by chance, but
would move away as soon as I blinked or moved. Absolutely useless for any
practical purpose.

Yes, exactly that. Close-up view of the veins on leaves, distant view of the
TV antennas up in the mountains. As good as I get with glasses.

 

And so I cite myself as a case in point.

My Tangent Streak RGP multifocals provide incredibly precise vision from
distance to about 16"/40cm. They are 50% of the time at least noticeable
and 10% of the time downright uncomfortable----but I persist in wearing
them because I really enjoy the convenience of great vision without
spectacles.

My soft multifocals (a couple of aspheric types of similar designs)
provide reasonable distance and intermediate vision and fair near point
optics. Sometimes I'll need +1.00 "readers" to help. They are perfectly
comfortable 95% of the time, yet I prefer to use the RGPs instead. I use
the soft primarily for sports and beach use mostly because critical
acuity is not that important (my tennis game isn't that great and
surfing/ocean swimming doesn't demand great vision) and I wouldn't want
to loose/damage my RGPs!

I have many patients who have very similar issues. And I will repeat
that I have quite a few patients who have excellent vision at all
distances---even a couple with toric soft multifocals, although I find
it that much easier to achieve with RGP aspheric or segmented lenses.

Sure, some people wont be satisfied with multifocal contact lenses and
some "adapt" to and enjoy the freedom and deal with the compromise. And
some have little or no compromise.

Personally, I really dislike spectacles. Even my progressives custom
fitted by the lab representative from one of the premier manufacturers.
But that's me. Everyone has different needs and expectations. I makes my
day interesting! Speaking of which......off to work!


--LB, O.D.