Treating coma aberration

With this recognition comes the need to treat HOAs to improve quality of vision in keratoconus patients. I have found that adapting the Keraring (Mediphacos, Minas Gerais, Brazil) intracorneal ring segment (ICRS) implantation technique to specifically target coma can effectively achieve this goal.

Evolution in ICRS implantation

However, at the time that these strategies were devised, the main method of examining the cornea was through the use of axial maps; but such maps provide limited information about the corneal surface. Newer techniques such as corneal aberrometry provide much greater detail about the cornea than axial maps. By means of aberrometry maps, ophthalmologists can measure not only the aberrometric and keratometric astigmatism axes, but they can also measure the axes of higher order aberrations.

This additional information, however, creates a new dilemma for the ophthalmologist - where to implant the ICRS if the astigmatism and coma axes differ?

Coma axis or astigmatism axis?

Clinical study

I performed a prospective study on the effect on HOAs of implanting the ICRS along the coma meridian. This study included 12 consecutive eyes of 10 patients with up to grade 3 keratoconus as determined by the Amsler scale. A single segment of the Keraring ICRS was implanted in all patients along the coma axis using a manual technique and patients were followed-up for 6 months. We found that there was a significant improvement in uncorrected distance visual acuity and best corrected visual acuity in these patients. Interestingly, however, we did not see a significant decrease in cylindrical and spherical error. Instead, we saw a significant decrease in coma (18% reduction) and total HOAs (21.1% reduction), suggesting that an improvement of HOAs can also improve vision in keratoconus patients.

How relevant is coma?

Several studies have now shown that HOAs, especially vertical coma, are highly prevalent in keratoconus. Indeed, clinical experience also attests to the same fact. Often during visual acuity examination, keratoconus patients show a chin up response to be able to see well. Essentially, this behaviour allows the inferior lid to cover the more deformed parts of the cornea. Thus HOAs should be suspected in patients who show this behaviour. Similarly, in patients with very low best corrected visual acuity, HOAs must be considered. Whereas, ICRS implantation has traditionally focused on correcting cylindrical error in keratoconus patients, I propose an approach focused on correcting coma aberrations instead, thus allowing us to improve the patient's overall quality of vision.

Reference

1. J. Bühren et al., Invest. Ophthalmol. Vis. Sci., 2010;51(7):3424–3432.