The measurement of axial length constitutes the largest source of error in intraocular lens (IOL) power calculation.1 Most clinics perform contact A-scan biometry for estimation of axial length using a handheld probe that makes contact with the anterior surface. The problem with this technique is a source of error known as corneal compression, that is, inadvertent shortening of axial length as a result of pressure from the A-scan probe. This shortening varies depending on the skill of the biometrist. A 0.4-mm compression of the cornea will result in 1.0 diopter (D) error in the calculated IOL power for a normal-length eye (24 mm).2 A stronger (more positive) IOL is predicted than is actually required, producing myopia. This can be a problem, because patients are increasingly demanding precision in their final refractive outcome.
Corneal compression can be controlled to a degree by eliminating measurements with reduced anterior chamber depth as displayed on the A-scan monitor or in the printout. Attention to this source of error requires multiple A-scan readings so that anterior chamber depth (ACD) comparisons can be made, which can be time-consuming. If one technician in the clinic performs the A-scan, it would be possible to make an adjustment in the A-constant/surgeon factor/ACD constant through retrospective analysis, assuming that the technician is consistent with the degree of his or her corneal compression. The lower the intraocular pressure, however, the more likely it is that significant corneal compression will occur, increasing the chances that the “one biometrist” policy may nonetheless produce postoperative refractive surprises.
Using an immersion A-scan technique can solve the corneal compression problem.3 A water bath is placed on the eye to provide a medium through which the probe can measure the axial length without touching the cornea. A topical anesthetic agent is used, and the patient does not have to struggle to keep the eyelids open.
The immersion A-scan technique, although more accurate, reproducible, and more efficient, has not been welcomed effusively because of some inherent problems associated with it. The apparatus was considered more expensive, messy to use, and more time consuming. The water bath is perceived to be uncomfortable for both the biometrist and the patient, causing spillage and interrupting the smooth flow of patients. The problem was partially solved by the availability of scleral shells,4 which are custom made to fit the particular probe of the A-scan unit. The shell locks the probe into place, and balanced salt solution (BSS®) flows into the shell through a tube after the shell is in place on the eye.
The problem of inadequate control of BSS flow and spillage has been solved in our clinic by devising an elegant yet simple “drip controller” (Figure 1). The BSS bottle is connected to an infusion tube, which fits on to the shell (Figure 2). The infusion tube passes through the drip controller (Figure 3), which is activated by a foot pedal. On pressing the foot pedal, a small pinch valve disengages the tube, allowing the BSS to flow in. On releasing the foot pedal, the pinch valve occludes the tube, stopping the flow of BSS. This gives an accurate degree of control over the influx of BSS, circumventing the problem of leakage and spillage. With the addition of this device, we have the advantage of immersion biometry without the attendant inconveniences. Not only do we improve the accuracy of axial eye length measurement, this technique takes less time because fewer scans are needed to confirm an accurate reading. The introduction of the drip controller has enabled us to have the best of both worlds.
Noshir M Shroff MBBS, MS
Saswat Ray MBBS, MS
Ranjan Dutta MBBS, MD
New Delhi, India
1. Olsen T. Sources of error in intraocular lens power calculation. J Cataract Refract Surg 1992; 18:125-129
2. Hoffer KJ. Preoperative cataract evaluation. In: Caldwell DR, ed, Transactions of the New Orleans Academy of Ophthalmology. New York, NY, Raven Press, 1988; 24
3. Ossoinig KC. Standardized echography: basic principles, clinical applications and results. Int Ophthalmol Clin 1979; 19:127
4. Byrne SF, Green RL. Ultrasound of the Eye and Orbit. St Louis, MO, Mosby, 1992