You Missing Out on a Piece of Comanaging Cataract Surgery
A wealth of information about A-Scan
ultrasonography, including how to create a healthy balance
between O.D. and M.D. involvement.
Gupta, O.D., Stamford, Conn.
Many optometrists comanage
cataract surgery, including performing both the preoperative and
postoperative visits. However, many don't perform the A-scan as
part of their pre-op, even though they're qualified to do so.
This article will discuss the
basics and practice management aspects of A-scan ultrasonography,
including billing and political issues with ophthalmologists.
A-scan ultrasonography measures
the axial length of the eye through the use of an ultrasonic
transducer placed at the corneal surface. This probe emits and
receives sound waves along the patient's optical axis, ultimately
yielding a time-amplitude recording that's displayed on an
This recording highlights spikes
on a graph, each representing a specific ocular tissue area. The
amount of time required for a sound pulse to travel from the
cornea to the retina determines the eye's axial length.
Modern ultrasound units generally
employ two types of probes:
- Immersion probes
are coupled to the eye via a fluid-filled chamber.
probes are similar to those used in tonometry -- they
involve physical contact between the probe and the cornea.
Because both methods yield similar
results and the applanation technique is much easier to use, it's
the predominant probe used in most offices.
The probe tip has a small light-emitting
diode (LED) to assist patient fixation. It can be hand-held or
mounted to a slit lamp, and in either case it's connected to a
microprocessor that gathers, stores and calculates all of the
necessary intraocular lens (IOL) data.
When properly aligned with the
patient's optic axis, the screen should display approximately
equal spike heights for the cornea, anterior and posterior lens
surfaces, and retina. In many cases, you'll need to get multiple
readings and average them to ensure accuracy.
SRK II Modification
for Long and Short Eyes
Axial length 21 mm to 22 mm:
Add 1 to lens power for emmetropia
Axial length 20 mm to 21 mm:
Add 2 to calculated power
Axial length < 20 mm:
Add 3 to calculated power
Axial length > 24.5 mm:
Subtract 0.5 from calculated power.
When performing an A-scan, watch
out for the following:
- If values are less than 22.00
or greater than 26.00, repeat the scan to verify the
- Poor spikes are caused by
poor alignment of the probe with the patient's optic axis.
If this is the case, subtly realign the probe to get
better results. Sometimes with dense cataracts, it's
difficult to get even spike heights. You can often
improve results by dilating the patient's eyes.
- The difference in
measurements between the left and the right eyes should
be comparable unless significant anisometropia or some
other factor is present.
- Axial lengths should
generally correlate with the patient's refractive error.
For example, myopes generally have
axial lengths greater than 24.0 mm; hyperopes have lengths
shorter than 24.0 mm.
Beware of the 5.00D myope with an
axial length of 22.50. Make sure you didn't compress the cornea
with the probe during the A-scan. Doing so will create an
artificially shallow anterior chamber depth, which will
underestimate the true axial length.
IOL power calculation
Once you've completed the A-scan,
use one of the three most common methods for IOL power
- Linear regression (SRK and
To calculate the IOL power, you
need the average of each major keratometry meridian, the average
ultrasound axial length and the surgeon's anterior and posterior
IOL constants. After you enter all of the appropriate data into
the machine, the microprocessor will generate multiple IOL
dioptric powers with the projected postoperative prescription for
In most cases, two IOLs (one
anterior chamber and one posterior) with the desired powers are
brought to surgery because complications may arise during it,
necessitating the use of an anterior chamber design even though a
posterior chamber lens was originally planned.
To see how to calculate IOL power
regression, refer to "Original IOL Power" .
SRK regression was modified, based
on the results of numerous cataract surgeries, to enhance its
refractive predictability in abnormally long or short axial
The resulting modification (called
SRK II) basically works by adding or subtracting IOL power from
the original SRK value, depending on the length of the eye. (See
box at the top of the page for SRK II modification.)
Formulas such as Binkhorst take
the lens thickness into account, so theoretically, they should be
more accurate. The Colenbrander formula accounts for the minus
effect of the posterior corneal surface and adds a correction
factor of .05 mm to the estimated postoperative anterior chamber
|Original IOL Power
Regression Is Based on the Following Formula:
IOL power = A + (B x axial
length in millimeters) + (C x average corneal power diopters)
where A, B and C are constants that vary with IOL design.
This expression is then
simplified by combining constants, leaving only the constant A
changing according to the lens style. The resultant formula is
commonly known as the Sanders-Retzlaff-Kraff (SRK) formula:
IOL power = A - (2.5 x axial length) -- (0.9 x average
keratometry in diopters).
Selecting lens power
When deciding on the IOL power to
predict the desired postoperative refractive error, assess the
patient's clinical situation to maximize overall visual function.
The decision-making factors to consider in the clinical lens
power selection include the following:
- Refractive error and visual
acuity in the opposite eye
- Presence of significant
cataract in the fellow eye and projected time for surgery
- Patient's lifestyle and
Let's compare a patient's
preoperative prescription (-6.00D OU) with a visually significant
cataract in the right eye only.
In this case, you wouldn't aim for
plano for the post-operative refraction because that would leave
this patient anisometropic. Of course, you could always fit his
left eye with a contact lens, but what if the patient is contact
lens intolerant? In this case, it may be better to aim for
something around a -3.00.
Then, after surgery he'd only have
a difference of 3.00 between his two eyes, which might be
tolerated in glasses. This scenario changes entirely if the left
eye has a cataract as well, which would be operated on shortly
after the right eye. In this case, plano might be a good goal.
In terms of billing, Medicare has
two separate payment policies for A-scans -- one for the
technical component and one for the professional component.
- The technical
component: The actual measuring of the
axial length of the eye is the technical portion of 76519.
Medicare defines the technical component as a bilateral
code and only allows billing once a year unless there's
"medical necessity" to do it again in less time
When you bill 76519, payment for the technical component
for both eyes is included in the Medicare fee schedule (see
- The professional
component: Calculating the IOL power and
selecting the lens style is the professional portion of
76519. The professional component is defined by a
unilateral code so it can be billed for each cataract
surgery regardless of the time interval between
operations --whether it's 6 weeks, 6 months or 6 years.
Political issues with M.D.s
Although many optometrists do the
pre-op and post-op work for cataract surgery, the A-scan has been
a gray area.
The main argument I've heard from
ophthalmologists is that they aren't comfortable accepting the
IOL power from an O.D.'s A-scan because the M.D. is medically
responsible for the prescription's accuracy. If we perform the A-scan
and the power is incorrect, then the M.D. is still liable.
I have several comments about this:
- First of all, the
ophthalmologist should trust you enough to realize that
you'll only perform procedures that you're comfortable
with. If you feel unqualified to perform the A-scan
because of lack of experience, don't do it.
- In terms of liability, even
though the M.D. may not see the patient for each post-op
visit, he retains medical and legal responsibility for
the surgical outcome. So, if the ophthalmologist trusts
you to do post-op work, which he's still legally
responsible for, then why shouldn't he trust you to do an
- Most ophthalmologists have
technicians perform A-scans. Very rarely do you find an M.D.
who does the scan himself. I doubt these technicians have
had the formal training and education that we've had.
The Three Codes Listed
in Medicare's Fee Schedule for A-scans
- 76519 (with no modifier).
Includes payment for a technical component for both eyes
and a professional component for one eye.
- 76519-26. Includes
payment for the professional component for one eye.
- 76519-TC. Includes
payment for the technical component for both eyes because
both eyes are always measured to ensure the accuracy of
the measurement of the eye to be operated on.
For example, a patient has
cataracts in both eyes, with the right worse than the left. When
the right eye is scheduled for cataract surgery, you code 76519-RT
for the A-scan. The reimbursement for this will include payment
for the technical component for both eyes and the professional
component for the right eye.
Two months later, when the
left eye is operated on, you can bill 76519-26-LT for the A-scan
on that eye. In this case, the -26 modifier serves to subtract
the technical component of the service, so that only the IOL
calculation and determination of the IOL power and style for one
eye will be reimbursed.
Clearly, A-scan billing is more of
a financial issue than one based on merit. We should learn to
stick up for ourselves and what we know.
The best way I've found to counter
this mentality is to enclose a copy of the A-scan so the
ophthalmologist can see the quality of the peaks. I enclose at
least five readings -- all with a standard deviation of <.10
so the M.D. can see the validity and reliability of the readings.
After all, how can he argue the accuracy of the readings when he
can view the A-scans?
What I've found to be a reasonable
compromise in this area is to let the O.D. perform (and bill for)
the technical component of the A-scan (76519-TC) and to let the M.D.
bill for the professional component (76519-26). By doing this,
both doctors get to bill their portion of the A-scan.
Finding that middle road
If you're going to perform A-scans
as part of your pre-op, make sure you're aware of the basic
issues discussed here. Find that middle of the road and play the
game of give and take. Know your rights and what you're capable
of, and you'll be fine.
Comanagement ultimately comes down
to this: When both O.D.s and M.D.s understand their relationship
and see each other as equals, everyone wins -- including the
Dr. Gupta works for Stamford
Ophthalmology in Stamford, Conn. He has no financial interest in
any of the products or companies mentioned in the article.
Optometric Management, Issue: February 2001