We read with great interest the article titled The Effect of Prior Trabeculectomy on Refractive Outcomes of Cataract Surgery by Zhang and associates. We applaud the authors for highlighting the issue of refractive surprises following cataract surgery in post-trabeculectomy eyes. We wish to highlight a few points in the study.
In the current study the major issue they faced was calculation of intraocular lens (IOL) power in the post-trabeculectomy group. They have calculated the axial length by either contact or immersion A-scan ultrasonography. The post-trabeculectomy group had a mean intraocular pressure of 8.7 ± 4.2 mm Hg. It is possible to get an erroneous axial length measurement using the contact method in eyes with low IOP. This is mainly because of exaggerated deformation of the globe due to low IOP. The authors did comment about this issue in the discussion but had not analyzed the data in relation to IOP. To avoid fallacies in IOL calculation, the authors suggested using the other eye’s axial length whenever available. Even though it is a good alternative, it may prove to be a wild guess if the axial lengths in the 2 eyes are different. We believe it is worthwhile to look at other options to minimize or eliminate these errors.
One such option is use of newer techniques for axial length measurements, such as partial coherence interferometry and optical low coherence reflectometry. IOL Master (Carl Zeiss Meditec, Jena, Germany) and LENSTAR LS900 (Haagstreit, Bern, Switzerland) are the commercially available devices. Beside the advantages of being noninvasive and noncontact, these devices are also more accurate in measuring axial length because the question of indentation does not arise. The devices also come with the disadvantages of inaccuracy in dense cataracts, corneal pathology and tear film abnormalities. In such situations, one must still depend on conventional ultrasound for the IOL power calculations, but the number may be lower.
The second option we would like to propose is a simple and probably an economical method. Anticipating a cataract surgery in the future, it may be advisable to document biometry readings before doing a trabeculectomy. This information can be used as and when required at a later date to maximize the surgical outcomes in post-trabeculectomy eyes and reduce the incidence of refractive surprises.