Using Subjective Refraction to Calculate Glasses Prescription and Fit a Contact Lens

[DR. DAVID MEYER] I'm
Dr. David Meyer.

I'm the director of Contact Lens
Services here

at the Moran Eye Center
in Salt Lake City, Utah,

and today we're gonna go through
how to calculate a glasses

prescription for a patient,
and then based on that,

how to properly fit
a contact lens to that patient.

One thing that's very important
to remember when your patient

comes in is case history.

Find out the last eye exam,
any medical issues in general,

any previous history of eye
surgeries or eye diseases

or any complications that
they may have had in the past.

To begin the exam
will start with calculating

a glasses prescription.

We're gonna start by
putting the phoropter

in front of the patient,
and what's really important

is, first of all, to make
sure that the pupil distance

in the phoropter matches the
pupil distance of the patient.

And in order to do that,
there's this knob here

at the top that adjusts
whether the pupil distance

is smaller or larger.

In this case,
this patient has

a pupil distance of 62,
and so I'm gonna set it up here

at this indicator line at 62.

Another important thing
to factor in is to make sure

that the phoropter
is level, and to do so,

you use this leveling window
here, and to adjust it,

you turn this dial above,
and you try to get it

so it's just level right there.

So the bubble and dot match up.

Now, in this case,
we're gonna start

with the right eye and then
go to the left eye.

In order to do so,
you turn this knob

so it reads OC.

That means to
occlude the left eye.

So currently, both of his eyes
are open, but he can only see

through his right eye because
the left eye's occluded.

Now, to begin this,
I'm going to blur the patient

so that the
letters there are very blurry,

and because
of previous readings,

I already have an idea of
roughly what the prescription

is based on his last glasses.

But I'm gonna start out by
making his vision very blurry

in the right eye, and I'm gonna
explain to the patient

to tell me as soon as
he can read these letters

even if they're blurry, but
as soon as he can read them.

Right now,
I have displayed the 20/30 line.

The 20/30 line is a good line to
begin refining your refraction.

So in this case,
I want you to tell me

as soon as you can start
to read those letters,

even if they're blurry.

And I'm slowly adding more
minus to his prescription.

Right now.

About there.

So right when they say,
'About there,' you know

that you need to add another
0.5 to 0.75 more minus,

and that's gonna be in the
ballpark of your prescription.

So in this case, I'm continuing
to refine the sphere.

So the sphere is on this
side of the phoropter,

and I'm refining it to get
as close as we can before

I start refining the cylinder
component of his prescription.

So I'm gonna ask him,
'Is this any better?'

[STUDENT] Yes.

[DR. DAVID MEYER] And
even better?

[STUDENT] Yes.

[DR. DAVID MEYER] And at this
point, once you get a good

ballpark of the spherical
component of the prescription.

Now you switch over to
do the cylinder portion

of his prescription.

And to do so, I turn this.

This lens is called the JCC.

The JCC is short for
Jackson Cross Cylinder,

and it helps us refine
how much astigmatism

he has and what the orientation
of that astigmatism is.

And in this case,
as I flip it, as you can tell,

I'm gonna be asking him what
looks more sharp and clear.

So in this case,
I'm gonna ask him

what is more sharp and clear,
Number 1 or Number 2?

[STUDENT] Number 1.

[DR. DAVID MEYER] So in
this case, he said Number 1.

And for a plus cylinder
phoropter, which this is,

we turn the dial in the
direction of the white dot

here on the JCC.

So in his case,
because he said Number 1

is better, I turn the dial.

Currently, it's at
about 85 degrees.

I turn it to about 75 in this
case, and then I ask him again.

And let me reiterate here
that right now in the JCC,

the two dots are straddling
this axis here.

So right here on this axis
refinement, where all these

numbers are zero through 180,
the line right now matches up

at axis 75, and these two
dots are on either side of that.

So that's important in order
to figure out where the axis

of his prescription is.

So in this case,
I'm gonna move it here,

and I'm gonna say,
'What's more clear,

Number 1 or Number 2?'

We'll do it again.

Number 1 or Number 2?"

[STUDENT] Number 2.

[DR. DAVID MEYER] Okay, good.

So I'm gonna move it toward
the direction of the white,

which is this way.

I'm gonna ask him again,
'Which is more clear,

Number 1 or Number 2?'

[STUDENT] They're
about the same.

[DR. DAVID MEYER] Pretty close.

Just to be safe,
I'm gonna move it

a little bit more in that
same direction and ask him again

what's more sharp and
clear, Number 1 or Number 2?

[STUDENT] Number 1.

[DR. DAVID MEYER] Number 1.

So we're gonna go back
to where we began here,

which is at axis 55,
and based on his previous

glasses that he walked in
with, or previous refraction,

or whatever previous
prescription we have,

we know that that's pretty
close to what he had before,

so we know we're on
the right track.

Now, in order to refine how
much cylinder he needs in his

prescription, we have to turn
the JCC so now the dots

line up with this
little arrow here.

So right now,
the two white dots here

match up at axis 55 that
you can see there,

and that is how you begin
your power refinements.

And so in this case,
it's the same thing.

If he likes it when
the white is lined up,

that means you want to
add more cylinder.

If he likes it when the red
is lined up, he wants less.

So in this case,
let's ask him.

What's more sharp and clear,
Number 1 or Number 2?

[STUDENT] Can
I see them again?

[DR. DAVID MEYER] Yeah,
Number 1 or Number 2?

[STUDENT] Two.

[DR. DAVID MEYER] Two, so
he liked it better with the red,

and currently,
he has one diopter

of cylinder in there,
so I'm gonna take out

a quarter, and then I'm gonna
ask him again

what's more
clear, Number 1 or Number 2?

[STUDENT] About the same.

[DR. DAVID MEYER] Pretty close.

And I wanna be a
little more sure,

so I'm gonna take it
down to 0.5 cylinder,

and because I've added 0.5
cylinder of essentially minus,

because I'm taking away some
plus, to compensate for that

I have to add a quarter of
plus here in the spherical

component of the phoropter
to maintain

a spherical equivalent
of the prescription.

So we'll ask him again,
what's more clear,

Number 1 or Number 2?

[STUDENT] About the same.

[DR. DAVID MEYER] About
the same.

So in this case,
since it's about the same,

you know that you're very
close to what we need.

Just to check one more time,
I'm gonna bring this down to

0.25 cylinder and ask him again,
Number 1 or Number 2?

[STUDENT] Definitely
Number 1.

[DR. DAVID MEYER] Definitely
Number 1.

And we'll do one more time,
what do you like better?

This is Number 1 or Number 2?

[STUDENT] They're
still pretty close.

[DR. DAVID MEYER] They're
still pretty close.

So at that point,
we've now refined

the axis of the cylinder,
and now we've refined the degree

or the power of the cylinder.

So as you can tell
here, right now,

it's a negative 575
plus 50, axis 45.

And then to go back,
I'm gonna go back

to the spherical component
and refine that a little bit.

I'm gonna ask him
a few more questions.

What looks more sharp and
clear, Number 1 or Number 2.

[STUDENT] Two.

[DR. DAVID MEYER]
Number 1 or Number 2?

[STUDENT] One.

[DR. DAVID MEYER] One.

Now, there are other means
by which you can make sure

that you haven't overminused
or overplused the patient.

You can balance the eyes if
we're working on both eyes

and that's something we can
cover in a different segment.

But for now,
based on his

previous findings,
his last glasses prescription

and what we
found on an autorefractor,

for example,
we know we're pretty darn close,

and then you're gonna have the
patient read those letters.

[STUDENT] O, F, L, C, T.

[DR. DAVID MEYER] Okay.

And we can even make it
down to the 20/20 line.

Go ahead and read that.

[STUDENT] T, Z, V, E, C, L.

[DR. DAVID MEYER] Excellent.

So now that we've got
the glasses prescription

calculated, at that point,
we need to work on fitting

the patient with a contact lens.

Now, when the patient expresses
desire to wear a contact lens,

there are a number of factors
that you need to consider.

You need to consider how
much the patient wants to

wear the contact lens.

Every day or once in a while.

You need to find out about
past history of his eyes.

Does he have dry eye problems?

Does he have keratoconus or
another corneal abnormality?

Those are all very important
questions to ask to help

fit the lifestyle
of the patient.

For today's purposes,
we're gonna be fitting

the patient with a monthly lens.

This particular one is
called a Biofinity Toric.

It's a lens where you take it
out at night and put it in

the morning, and after
one month, you throw it away.

But we're gonna go
through and show

how to put it on a patient's
eyes, how to assess it,

and how to get the best
vision out of it.

Now, one thing that's very
important is when you calculate

a glasses prescription,
remember that that is

almost always different than
a contact lens prescription.

The reason why is because the
optical surface of glasses

sits out, on average,
about 12 millimeters from

the front surface of an eye,
whereas a contact lens rests

directly on the eye.

So, that changes the optics,
so be sure you use

a vertex calculator to determine
the correct prescription

for a contact lens.

For example, for this patient,
the spherical equivalent of his

prescription is a negative 5.75.

If you put that in a vertex
calculator what that would give

you is that the proper contact
lens would be a negative 5.5.

Now, in today's purposes,
we're gonna put a toric

contact lens on his eye for
demonstration purposes only.

And before we do that,
it's very important that you

get some keratometry reading,
whether manual keratometry,

automated, or even a topography.

Any of those will help
determine which base curve

and diameter would be best
suited for the patient.

So, for today's purposes, like
I said, we're gonna be fitting,

fitting the patient with a
Biofinity Toric contact lens,

and when you take it out
of the blister pack,

it's important to make sure
that the contact lens

is not inside out.

And the best way to tell is to
hold it up against the light,

and if you look at the edge
of the contact lens,

the edges should point
straight up toward the

ceiling like a bowl.

If you look at it,
and the edges are kind of

flared out or splayed out
or they're flattened out,

that usually means
it's inside out.

Now, that usually doesn't
cause a lot of problems,

but it can cause some
discomfort for the patient,

so make sure that the contact
lens is not inside out.

Now, I've put the contact lens
on the edge of my finger, and

contact lenses typically stick
to the surface that is wetter.

And so try to make sure that
your finger is nice and dry

so that the contact lens
will stick on the patient's eye

instead of your finger.

Now, to do so,
I'm gonna have

the patient lean back
and look up high

toward the ceiling,
and very gently,

I'm gonna pull down on
the lower lid

and pull up on the upper lid.

And as he looks up high,
I'm gonna first place it on,

close to the lower limbus
of his cornea, like so.

Now, have the patient look
down toward the floor,

because when you look
down toward the floor,

it has a tendency to help
center the contact lens.

And make sure that the patient
blinks normally

to help center the contact
lens even more.

Now, when you place the contact
lens on the patient's eye,

be sure to give the contact
lens a couple of minutes

to settle down, especially
if it's a toric lens.

A toric lens has a tendency to
orient in the correct position,

but it takes a few minutes for
the patient's natural blinks

to put it in the
correct position.

In this case,
we've given the patient

a good solid
five minutes

to let the contact lens
settle down.

At this point,
I'm gonna pull

the slit lamp over here
and take a close-up look

at the lens to make sure
it's fitting properly.

Come on forward, put your
chin down there,

and your forehead
resting forward.

Now, there are several factors
when assessing a contact lens.

Number 1, what we can do
is we can pull down on

the lower lid just gently, and
we can see the toric marking.

In this particular
brand of contact lens,

the toric marking should
be ideally at 6 o'clock.

As you can see here,
the contact lens is about

ten degrees to the left,
so it's rotating

a little bit temporally.

As you assess this,
have the patient blink

normally, and as he blinks,
you can see the movement

of the contact lens.

Now, ideally,
you would like

the contact lens to
move a little bit,

maybe 0.1 to 0.2
millimeters, but not too much,

because if it's too much,
then it'll be uncomfortable.

Blink.

Another important thing is to
make sure there's full limbal

coverage and that the contact
lens extends beyond the limbus

onto the conjunctiva, and it's
not rubbing against the limbus.

In this case,
the contact lens

is rotating about
ten degrees temporal,

or as you're looking
at his right eye,

it's rotating to my left.

In that case,
whatever the axis of

the glasses prescription,
you add 10 degrees and put

that in the contact lens if you
decide to put another one on.

That rule is commonly
referred to as the Lars rule.

So, if it's to
the left, you add.

If it's to the
right, you subtract.

So, in this case, it was
to the left ten degrees,

so we added ten degrees to
the glasses prescription,

and then we place that on
the contact we use that

in the final contact
lens prescription.

Now, after the contact lens
has settled, then you check

the visual acuity, and then
you will do what's commonly

called an overrefraction.

Now, an overrefraction
is to make sure that

the contact lens is not too
weak or too strong,

or that you've got
the axis wrong.

And for soft contact lenses,
most of the time, as long as

you vertexed correctly,
put the correct contact

lens on, the overrefraction
should be close to plano.

But behind the phoropter,
it's always good to make sure,

so you do a spherical refinement
to make sure the patient is not

accepting more plus or more
minus, or in some cases,

some astigmatism.

Thank you for
watching this video.

Again, I'm Dr. David Meyer,
and today's video was about

fitting a patient with a soft
contact lens after doing

a basic subjective refraction.

Thank you.