HYPEROPIC PATIENTS

Highly precise biometry and the correct IOL calculation formulas are essential ingredients to achieving good visual outcomes in hyperopic cataract patients, said Wolfgang Haigis PhD, University of Wuerzburg, Wuerzburg, Germany.

“Refractive outcomes in hyperopic eyes are more sensitive and less forgiving of measurement errors than are normal eyes,” Dr Haigis told the XXXI Congress of the ESCRS in Amsterdam. He noted that contact lenses have a distorting effect on the cornea which continues for some time after patients take the lenses off their eyes. For example, research has shown that an eye’s keratometry can change from 46.88 D to 46.04 D three hours after removal of a hard contact lens (Haigis W: unpublished data). Therefore, surgeons must insure that patients remove hard contact lenses at least two weeks before they undergo keratometry. Soft contact lenses should be removed at least three days prior to keratometry, he added.

He stressed that the amount of refractive error induced by mistakes in determining the effective lens position is magnified in short eyes compared to long or normal eyes. For example, when an IOL’s optic is in front of or behind its predicted position by 1.0mm, the difference from the intended refraction will be only 0.6 D in an eye with an axial length of 27mm, compared to nearly 2.0 D in an eye with an axial length of 21mm. Another potential source of error is the fact that IOL manufacturers’ maximum allowed deviation from the nominal dioptric power of an IOL increases as the IOL power increases. Thus, the maximum allowed deviation of an IOL with a nominal dioptric power below 15 D is only 0.3 D, compared to 1.0 D in an IOL with a nominal dioptric power above 30 D.

Optical biometry has become the preferred type of biometry instrumentation in Europe and the US because of its highly reproducible measurements. Immersion ultrasound can provide similarly consistent results, Dr Haigis said. “With immersion ultrasound and optical biometry you can expect a consistent result, but you have no idea what to expect from contact ultrasound.” He added that although research has yielded conflicting results regarding the interchangeability of the different OCT devices’ measurements of the anterior chamber depth and keratometry, the majority of studies show consistency between the instruments regarding axial length measurements.

Several of the IOL power calculation formulas, such as the SRK/T and the SRK II will provide inaccurate results in short eyes. The recommendation is that in eyes with an axial length below 22mm the most accurate formulas are the Hoffert Q or the Haigis or the Holladay 2. None of the usual formulas will work in extreme cases of hyperopia because the lenses required are so thick, Dr Haigis said. Instead, in those cases he provides the patients biometry information to an IOL manufacturer who will make a customised lens based on raytracing calculations.

Wolfgang Haigis: w.haigis@augenklinik.uni-wuerzburg.de