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Tools & Pearls to Perfect Biometry
BY RHONDA G. WALDRON, M.M.Sc., C.O.M.T., C.R.A., R.O.U.B., R.D.M.S.

With today's patient expectations, medical/legal climate and the advent of multifocal and accommodating intraocular lenses (IOLs), perfecting IOL power is more important than ever. While there are several facets requiring accuracy in order for this to happen, namely biometry, keratometry, formula selection and pristine cataract surgery, biometry is one of the easiest problems to solve.

Figure 1. The set up for immersion ultrasound.

The solution is to eliminate completely the practice of contact or applanation biometry. This technique produces an unavoidable error; that of corneal indentation from the probe tip. Even in the best of hands, this technique results in an erroneously short measurement. While most physicians either personalize their lens constant or use some sort of "fudge factor" to counteract this error, no biometrist can exert the exact same amount of indentation each time they applanate, so there is no way to correct for this error absolutely. Also, softer eyes will have more indentation, and each biometrist in the practice will have a different amount of indentation with their applanation. This technique simply results in too many variables for the outcomes expected today.

Non-contact biometry is of course the most accurate way to measure axial length, and today can be accomplished in one of two ways: with immersion ultrasound or with optical coherence combined with immersion ultrasound. Either way, you must incorporate immersion biometry into your practice.

A survey presented at ASCRS 2006 demonstrated that optical coherence was being used in about 50% of practices, immersion about 20%, and contact biometry in about 30%. While it is marvelous news that non-contact instrumentation is being used in about 70% of practices now, what the survey did not show was what method those practices using optical coherence were resorting to in that percentage of patients that could not be measured optically due to dense cataract, other media opacity or difficulty fixating.

To use the contact method of biometry on this population is completely unacceptable. All patients deserve the accuracy of non-contact instrumentation, regardless of cataract density and compliance. With immersion equipment so affordable, the technique so easy to learn and so quick to perform, there is no excuse for not incorporating it into all cataract practices.

Optical Coherence Biometry

Figure 2. The preferred spike pattern for the IOLMaster.

Optical coherence biometry was designed by Wolfgang Haigis, M.D., and introduced by Carl Zeiss Jena in 1999. This technique utilizes infrared laser in an interferometer setup, and because it measures to the retinal pigment epithelial layer of the retina rather than the internal limiting membrane, as does ultrasound biometry, and because by history our intraocular lens formulas have been based on ultrasonic measurements, Dr. Haigis calibrated the system to immersion ultrasound using a Grieshaber Biometric System at 10 MHz (with 40 MHz counters). He found an excellent correlation between optical and acoustical measurements and found its accuracy to be equivalent to high-precision immersion ultrasound, and superior to the commonly used applanation method.¹ In my own experience comparing the two techniques on the same eyes during a 6-week period, ultrasonography yielded a very slightly shorter eye than interferometry (mean 0.018 mm) and the correlation between the two methods was very high (r = 0.99).

The IOLMaster System

One of the advantages of the IOLMaster system, a non-contact optical coherence biometer from Carl Zeiss Meditec (Dublin, Calif.), is patient-friendliness, since nothing comes into contact with the patient's eye and no topical anesthetics are needed. The system is easy to use and quite efficient in regard to exam length. Another advantage is that the system measures eyes with posterior staphylomata with more ease than ultrasound, so long as the cataract is not too dense and the patient has the ability to fixate, particularly if the practice has no B-scan capability for this measurement. It also measures through silicone oil-filled eyes with more ease than ultrasound.

Disadvantages include the high cost of the instrument, and the need for a certain amount of transparency along the laser's path, which precludes measuring eyes that have dense cataracts or corneal or vitreal opacities. The patient does need the ability to fixate. Also, there is a loss of signals that could represent undisclosed ocular pathology as can be seen with the spike pattern of ultrasound biometry when they exist along the A-scan pathway, since only the retinal reflectors are seen with optical coherence biometry.

Immersion Ultrasound

Immersion ultrasound is much less expensive and can be used no matter the cataract density, other media opacity or patient fixation. There is a slightly longer learning curve with this method, since the biometrist will need to learn to insert a scleral shell, and will also need to learn to interpret spike patterns and be able to recognize proper alignment patterns for various eye types. There are different scleral shells available for this technique, with the Prager Shell recommended for the novice because the probe is locked into a central position within the shell and thus alignment achieved much faster. And while not required for this shell, it is much easier to perform immersion with the patient reclined, which is quite comfortable for the vast majority of patients.

The Hansen shell is also available for immersion biometry, but the learning curve is slightly longer with this shell since the probe is hand-held within the saline and alignment takes a few seconds longer. Also the patient must be reclined to prevent loss of saline from the shell. Both shells are easy to insert, and come in small enough diameters so that even with smaller fissures they can be inserted without difficulty.

Figure 3. The preferred spike pattern for immersion ultrasound.

Importance of Having the Proper Equipment and Setup

When using the immersion method, it is important that the equipment and setup be optimal. It is no longer acceptable to use substandard equipment demonstrating crooked spikes on the display screen and incorporating average sound velocity rather than independent velocities for each section of the eye. Nor is it acceptable to have a substandard room setup. The room should have a reclining exam chair, with the ultrasound machine on a cart to one side of the patient's head and the examiner seated to the other side on an adjustable stool with the patient's head essentially in their lap (Figure 1). Using this setup positions the screen and buttons on the machine within easy reach, and the patient in a comfortable position with the head firmly braced against the headrest.

A fixation target should be placed on the ceiling in the appropriate position so that the fellow eye is directed into primary gaze. The anesthetized eye to be examined is opened and the shell gently placed on the limbus with the lids released over the lip of the shell. Then saline is expressed into the shell. Once it reaches the probe tip, the spike pattern will appear on the display. If the spike pattern demonstrated is not optimal, then the entire shell should be manipulated until the required pattern appears. Both eyes can easily be measured within 5 minutes with this technique, so efficiency as well as accuracy is at maximum when compared to the contact method.

What to Look for to Get the Desired Results

No matter which method is used, there are specific "spike patterns" that are desired, and specific pearls with which to be familiar. With optical coherence, the primary and secondary maxima should be visible, and the primary maxima should have a thin peak (Figure 2). With immersion ultrasound, five tall, straight spikes representing the cornea, anterior and posterior lens, retina and sclera should be visible, with a series of shorter spikes thereafter representing the orbital fat to ensure alignment (Figure 3).

With both techniques, it is recommended that multiple readings be obtained per eye, and that all measurements be within 0.1 mm. Any unusual findings should be rechecked, and any imperfect patterns deleted. It is also required that all measurements be verified with the patient's records. The pre-cataract refractive error is demonstrated in the axial length and keratometry readings and should be verified by both the biometrist and surgeon.

Recommended Formulas

Once measurements are obtained and verified, it is critical that they be placed in a recommended formula. The two most accurate for any eye length have been shown to be the Holladay IOL Consultant formula and the optimized Haigis. If practices don't wish to purchase the IOL Consultant, nor invest the time into optimizing the Haigis formula, it has been recommended by most experts that the Hoffer Q formula be used for eyes shorter than 22.0 mm, the SRK/T formula for eyes 26.0 mm or longer, and either the Hoffer Q, SRK/T, or Holladay I for anything in between, assuming there is no history of refractive surgery.

For those patients who have had refractive surgery, it is recommended that the Holladay IOL Consultant or optimized Haigis formula be used since the preop anterior chamber depth is a factor in those formulas. With these patients, the extremely flat K-readings can fool other formulas that do not consider ACD into assuming it is more shallow than normal and not accurately "place" the IOL within the formula.

The precision of the IOL power is directly related to the precision of the pre-operative measurements. Contact biometry is of historical interest only, and can no longer be used in today's practices. With immersion ultrasound, or with optical coherence biometry combined with immersion ultrasound, all patients receive the biometric precision they so deserve.

Rhonda Waldron, M.M.Sc., C.O.M.T., C.R.A., R.O.U.B., R.D.M.S. is the diagnostic echographer and senior associate in ophthalmology at Emory Eye Center in Atlanta, Ga. She also owns Eye Scan Consulting, which holds ophthalmic ultrasound training courses across the United States and Europe.

Reference

1. Haigis, W. Optical coherence biometry. Dev Ophthalmol. 2002;34:119-130.