Iol Calculation Challenge Case Report: Keratoconus Patient With Intrastromal Segment And High Myopia

IOL calculation in keratoconus is quite a challenge, furthermore since most of them are myops or high myops. The main question is unreliable biometric measurements, and it gets worse the steeper keratometric values are.

The Kane keratoconus formula, a theoretical modification of the original Kane formula, uses a modified corneal power to better represent the true anterior/posterior ratio in keratoconic eyes and enables more accurate predictions.

Purpose: To present the importance of the modification of the Kane formula in the management of a case of a high myop (-12D) patient with keratoconus and cataract

A 70 y.o.  female patient, with keratoconus (KC), was submitted to intrastromal segment implant due to keratoconus and after 6 months a phacoemulsification and monofocal IOL implantation was performed. For the IOL calculation, the Kane Keratoconus formula was used, aiming to refraction of -1 D since previous K’s were higher than 53D.

Refraction before any surgery was -12,50 (-1,00 x 10) in her right eye (RE) and -7.00 (-1.75 x 180) in her left eye (LE), visual acuity with RG contact lenses was below 10/20 in the RE and 16/20 in the fellow eye. KC types were nipple on the RE and a duck type KC in LE. The RE was treated with a 210º Ferrara segment with an optic zone of 5 mm. After this surgery refraction switched to -5.50 (-3.00 x 12) with a better quality of vision although the same visual acuity. Left eye was not treated for the moment.

Once the right cornea was stabilized after 6 months, cataract surgery was to be performed in the RE. IOL Calculation was performed for a Johnson AR40 IOL, aiming to a -1 D postop refraction, different formulas were used, and results were quite variable proposing IOL powers of +2,00 to +4,00. Finally, the Kane Keratoconus formula for the IOL calculation was used with a result of an IOL of +7.00 D

Phacoemulsification of the RE was performed with the implantation of a capsular tension ring as well as a +7.00 D Johnson AR40 IOL. The result being a Visual acuity of 16/20 achieving emmetropia (although aiming to slight myopia) in a previous amblyopic 

Conclusion: IOL calculation is quite a challenge in high myopic eyes, and even worse with keratoconus. Hadn’t we used the modified Kane formula for keratoconus, the patient would have ended with hyperopia which is never intended in a high myopic patient.