Introduction. Keratoconus and related corneal ectatic disorders are conditions characterised by a misshapen cornea. Keratoconus is typically managed with corneal rigid gas permeable contact lenses (CRGPcl) and when these are unsuccessful patients may be fitted with the much larger scleral rigid gas permeable contact lenses (SRGPcl). It has been hypothesised that due to their superior performance, SRGPcl might be considered as the first option for management of keratoconus and the present research investigates this hypothesis.
Purpose. To assess the visual performance, vision related quality of life (Qol) and subjective perception of vision (SPV) and the subjective perception of comfort (SPC) in two contact lens types: CRGPcl and SRGPcl, in successful CRGPcl wearers with keratoconus (and related ectatic corneal disorders).
Methods. Thirty-four successful CRGPcl wearers, with keratoconus or related disorders, participated in a crossover randomised control trial (RCT). This research was approved by the National Research Ethics Service (NRES) of London-Camden and King’s Cross as well as the research ethics committees of London South Bank University (LSBU) and the Institute of Optometry. Participants were randomised into two groups, group 1 (sequence AB) were fitted with new CRGPcl and after a washout period, in which habitual CRGPcl were worn, were fitted with and crossed-over to SRGPcl. Group 2 were first fitted with SRGPcl and after a washout period were fitted with and crossed-over to new CRGPcl (sequence BA). Data for experimental outcome measures were collected three times: first on recruitment in habitual CRGPcl, and once after each period in experimental CRGPcl and SRGPcl. The outcome measures were: The Early Treatment Diabetic Retinopathy (ETDRS) log of minimum angle of resolution (logMAR) best corrected visual acuity (BCVA); the VectorVision 1000E contrast sensitivity function (CSF), expressed in both numeric and log contrast sensitivity (logCS); the National Eye Institute Visual Function Questioannaire-25 (NEI-VFQ) to assess the visual Qol; and the reported SPV and SPC, recorded on a Likert-like scale from 1–10. The final measure was at the end of the second period, each participant selected the preferred lens type, out of the two experimental lenses, for future habitual use.
Results. Thirty participants completed the trial, 13 in group 1 and 17 in group 2. Randomisation demographics revealed no significant differences between the two randomised groups except in corneal pachymetry (thickness): group 1[Mean 423.2 (±45.1)], group 2 [Mean 462.8 (±44.7)] (p= 0.002).
The SPC in the experimental lenses and the SPC in the experimental CRGPcl in participants who selected CRGPcl as the habitual lens for future use, were the only measures, which exhibited significant differences. The SPC was not-normally distributed in SRGPcl, [Median=9.0, IQR=2.0, Mean=8.85, (±1.10)] and normally distributed in CRGPcl [Mean=7.78, (±1.45), Median=8.0, IQR=2.0]. The intra-subject period differences in SPC between group 1 (Median=1.0) and group 2 (Median=-1.0), revealed significantly higher scores in SRGPcl (p=0.002), rejecting H0. The preferred habitual lens choice outcome was: 14 participants (47%) chose SRGPcl and 16 (52%) chose CRGPcl. Higher SPC scores in the experimental CRGPcl, were found in participants who chose CRGPcl, (p=0.006) and (p=0.009) by independent samples t-test and Mann-Whitney U test respectively, rejecting H0. The only significant carryover effect was found in the logCS scores (p=0.019), no other outcome was found to have significant carryover or period effects.
No other outcome was found to have significant differences between the two lens types, supporting H0, with respect to: the ETDRS logMAR BCVA, the CSF numeric and logCS, the specific logCS at 6 cycles per degree (CPD), the 12 domains of the NEI-VFQ, the specific ocular pain domain of the NEI-VFQ and the SPV.
Conclusion. The research population exhibited significantly better comfort in SRGPcl compared with CRGPcl, as measured by the Levit Subjective Comfort Scale (LSCS). Furthermore, participants who chose to remain in CRGPcl had significantly higher LSCS scores in CRGPcl than those who chose SRGPcl. Successful CRGPcl wearers whose LSCS in CRGPcl is < 7 are likely to achieve better comfort / tolerance with SRGPcl. No significant differences were found in this research population between the two experimental lens types, in the visual outcomes of logMAR, logCS and SPV and no significant difference was found in the visual Qol outcomes in the 12 domains of the NIE-VFQ. This research indicates that on average, successful CRGPcl wearers find SRGPcl more comfortable and there should be no visual and visual Qol advantage or disadvantage in refitting successful keratoconic CRGPcl wearers with SRGPcl and vice versa.