Article Date: 2/1/2001

A-Scans:Are You Missing Out on a Piece of Comanaging Cataract Surgery Patients?
A wealth of information about A-Scan ultrasonography, including how to create a healthy balance between O.D. and M.D. involvement.
By Deepak 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 basics

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 oscilloscope screen.

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:

  1. Immersion probes are coupled to the eye via a fluid-filled chamber.
  2. Applanation 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.

Scanning tips

When performing an A-scan, watch out for the following:

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 calculation:

  1. Linear regression (SRK and SRK II)
  2. Binkhorst
  3. Colenbrander.

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 each power.

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 lengths.

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 depth.


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:

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.

Billing issues

In terms of billing, Medicare has two separate payment policies for A-scans -- one for the technical component and one for the professional component.

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:

The Three Codes Listed in Medicare's Fee Schedule for A-scans

  1. 76519 (with no modifier). Includes payment for a technical component for both eyes and a professional component for one eye.
  2. 76519-26. Includes payment for the professional component for one eye.
  3. 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 patient.

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