Refraction in Keratoconous

Keratoconus is an asymmetric and progressive thinning, steepening and centrally scarring of the cornea. Keratoconus patient often complain of photophobia, squinting to see better, halos or flare around lights particularly during night driving, asthenopia.Some patients also report ghost images and

monocular diplopia. They are seldom satisfied with their vision. Objective and subjective both the refraction procedures are significantly different from those performed on normal patients, and are really difficult and time taking procedure. Retinoscopic image is distorted, still it gives an approximate estimate of the refractive error. The retinoscopy end point is difficult to define accurately, while performing retinoscopy, attention has to be paid to the reflex appearing in the central pupillary area. Keratometry may give the practitioner a starting point with regard to cylinder
power and axis.


Keratoconic patient mostly need myopic correction with significant amount of astigmatic correction. The degree of myopia and astigmatism increases with the progression of the
condition. Subjective refraction is mostly performed by trial and error method, starting with minus spherical lens to be followed by minus cylinder lens. In case where objective refraction is not feasible, the refraction should begin with monocular subjective refraction by the addition of minus sph lenses at an increment of 2.00 Dsph or 3.00 Dsph to determine if there is any improvement
in acuity. If the visual acuity is not improved to the expectation, the examiner may take –2.00 Dcyl or –3.00 Dcyl and rotate it to determine if there is any meridional position with improved acuity. High cylinder lens may be used to find the meridian which provides improved acuity. A great deal of patience is required because patient’s ability to distinguish between the incremental changes will be lessened according to the severity of disease. Stenopaic slit may be used to isolate the meridians
which provide the clearer vision. The refraction may be carried on different meridians and the results of which may be used as the basis of the full refractive error. Binocular equalization has little meaning.

Fig:- RGP Lens

A more dynamic approach may be tried by putting the RGP lens on the patient’s eyes and performing the objective and subjective refraction over RGP lens. The examiner should select the suitable base curve on the basis of K reading or corneal topography for RGP lens and do the approximate correction over it. Nearly all corneal irregularity and distortion is masked, monocular diplopia is reduced and the patient is able to respond better to different lens changes.