It was estimated that the best start point for the cylindrical part of refraction in the subjective test could be an auto refractometer in keratoconic patients. Cyclorefraction and manifest refraction lead to significantly different J0 and J45 results, and this difference increases as corneal astigmatism and Q-value increases.
In the estimation of the agreement between an autorefractor (Topcon), cycloplegic autorefractor, anterior and posterior corneal astigmatism with the subjective refraction, It can be seen from data that the maximum estimations of the agreement were found between the cylindrical measurement of an auto refractometer and subjective manifest refraction in cylindrical power(Icc:0.703, P < 0.026, cylindrical axis(Icc:0.943, P < 0.0001) and astigmatism vector analysis ofJ0(Icc:0.972, P < 0.0001) and J45( Icc:827, P = 0.006). Excellent ICC coefficient between cycloplegic autorefractor and subjective refraction was obtained for J0(Icc:0.809, P < 0.0001) and J45(Icc:0.832, P < 0.0001). Cycloplegic refraction usually plays an important role in the control of accommodation in clinical practice. A previous study reported that higher-order aberrations and pupil size play a significant role in the Accommodation stimulus-response function. Some studies indicated that cycloplegic drops caused a statistically significant J0 vector astigmatic shift[7, 8]. The study showed that Cycloplegice cylindrical axis could not be a good starting point for refraction. It seems that corneal abnormality makes an effective Contribution to final astigmatism in keratonic patients. But according to our data, ICC coefficients for cylindrical power, axis, J0, and J45 between anterior corneal astigmatism and subjective refraction were not excellent. This could be caused by the sampling of keratoconus patients between possible candidates for intracorneal ring segment implantation. Most of them had asymmetrical cones, and the mean of their cone location was 0.82mm ± 0.32. Cyclorefraction and manifest refraction lead to significantly different J0 and J45 results.. The previous study showed that the repeatability of refraction in keratoconus patients was substantially worse than in normal myopes; this was probably due to further accommodative fluctuations or the additional blur resulting from corneal irregularities in keratoconic patients. Although accommodative fluctuations were controlled by cyclorefraction in our study, the difference between subjective manifest refraction and cyclorefration was significant for astigmatic components (J0 and J45). These differences could be associated with corneal and pupillary changes for cycloplegic effects. We investigated which factors were associated with the difference between cycloplegic refraction and manifest refraction for J0 and J45. Front corneal astigmatism was associated for both J0 difference (B coefficient = 0.68,P < 0.0001) and J45 different (B coefficient = 0.89,P < 0.0001). If front corneal astigmatism was more, the dioptric difference was more J0 and J45. The values of asphericity (Q-value) described the geometry of the cornea. The studies reported that the normal anterior corneal Q-value ranges from − 0.01 to − 0.80. Keratoconic patients usually showed more negative asphericity .We found negative association between Q-value and J0difference(B Queficient=-3.42,P = 0.004) and positive association between Q-value and J45difference(B Queficient = 0.81,P < 0.0001). Our study showed that a negative association between Kmax and J0 difference was found (B coefficient =-0.37, P = 0.025), meaning that J0 difference was decreased with keratoconus severity. The previous study measured a significant M difference between auto refractometer and manifest refraction in keratoconic patients and showed Kmax was the only variable associated with M dioptric difference. Positive association was found between best corrected visual acuity and J45 difference(B coefficient = 1.24 P < 0.0001).
corneal astigmatism(B Coefficient = 0.68) ,pupil dameter(B Coefficient = 1.64),Q-value(B Coefficient= -3.45) ,Kmax(B Coefficient=- 0.37) and IHA(B coefficient + 0.04) were found to be variables associated with J0-difference and Gender(B Coefficient = 0.89), corneal astigmatism(B Coefficient = 0.15),VAsc(B Coefficient = 1.24), Q-value(B Coefficient = 0.81)and IHA (B Coefficient= -0.02) were found to be factors associated with J45 difference. If front corneal astigmatism was more, the dioptric difference was more in J0 and J45.
Cone location (B Coefficient= -33.37) and corneal astigmatism (B coefficient= -0.1) were found to be variables associated with axis-difference between Cyclorefraction and subjective manifest refraction. It can be predicted that the closer the keratoconus cone location relative to the center of the cornea and the lower the keratoconus patient's corneal astigmatism, the more the probability of axis changes by cycloplegic drop.