Six versus thirty night extended wear with silicone hydrogel contact lenses

Judith Stern - B. Optom, UNSW, Sydney, Australia Following the completion of her degree in 1997, Judith worked in private practice in Sydney, Australia. She is currently a senior research optometrist at Vision Cooperative Research Centre (Vision CRC) where she has been working with silicone hydrogels for nearly 3 years.

Introduction

A recent online poll conducted by the CL Spectrum asked ‘will you prescribe new high Dk contact lenses?’ Out of 1292 respondents 34% said that they would for up to 30N EW, and 19% said they would for up to 6N EW . Practitioners consider safety the main concern when fitting silicone hydrogel lenses. Other concerns include adverse responses, cost to patient, the company, fit and initial discomfort.

Patients have indicated they would like to wear their lenses for continuous wear due to the convenience and permanence of vision correction . Nearly three quarters of all myopic patients have indicated they would try a high Dk EW lens especially if the lens was recommended for 30 days . So, what is the appropriate wear schedule, 6N or 30N?

The Vision Cooperative Research Centre (VisionCRC) has looked at the issue of 6N vs 30N wear schedules of high Dk silicone hydrogel lenses to determine if there are any performance differences between the lenses worn on these schedules. In our studies the 6N group removed, cleaned and disinfected their lenses overnight after 6N lens wear. The 30N group wore their lenses for 30N. Both groups discarded their lenses after one month. Approximately 50% of patients used saline rinses with the lenses. It is hoped that this study will aid practitioners in deciding the appropriate wear schedule for their patients.

Lens related discontinuations

Discontinuations directly attributable to lens wear, including lens fit, adverse events, discomfort and poor vision can be an indication of how successful a wear schedule is. The discontinuation rates seen at the Vision Cooperative Research Centre (VisionCRC) were similar for the 6N and 30N group over three years in the study (Figure 1). A similar result was seen in a 12 month study conducted by Nilsson where lens related discontinuations in the 30 day group were 7.6% and in the 7 day group were 13.2% . These discontinuation rates are higher than those seen in daily wear (0.8%) but are dramatically reduced from those seen in extended wear of low Dk soft contact lenses .

Figure 1: Lens related discontinuations

In the CCLRU study adverse responses, in particular CLPC, were responsible for a large proportion of the discontinuations.

Lens characteristics

As we would expect there was little difference in the lens fit variables (centration, primary gaze movement, lag and tightness) between the 6N and 30N groups. When we looked at the surface performance characteristics the 30N group had consistently lower front surface deposits, lower back surface debris and better wettability than the 6N group . Figure 2 illustrates the comparison of front surface deposits between the 6N and 30N groups over time. Deposit levels remain low (0.5-1.3) and similar to those observed with conventional lenses over three years of lens wear.

Figure 2: Front surface deposits

There was no deterioration in the lens surface with either the 6N or 30N wear schedule. The similarities in lens surface and fit characteristics with 6N and 30N wear schedules indicates that the removal and cleaning of the 6N groups’ lenses three times a month had minimal effect on the lens characteristics.

Ocular physiology

Bulbar and limbal hyperemia, numbers of microcysts and corneal staining were minimal and similar for both the 6N and 30N wear schedules. It is interesting to note that these low levels remained constant over three years (Figure 3) showing that the effects of hypoxia were not affecting the cornea. Levels of hyperemia and microcysts were higher at baseline (figure 3) due to the wearers who had changed from low Dk extended wear.

Figure 3: Microcysts

Symptoms

Overall Comfort was rated highly for both groups as displayed in figure 4. Weekly removal of the lens in the 6N group did not have an effect on the level of comfort compared to the 30N group.

Figure 4: Overall comfort

End of the day comfort was 4-7 %, which was lower than overall comfort but still remaining high for both groups. Comfort for the 6N and 30N group remains stable over the three years.

Vision was rated highly and similarly by both groups (85 -94). As with comfort, the vision ratings remained stable over the three year study.

Patient satisfaction

Ultimately a patient has to be satisfied with their prescribed lens wear schedule. Both groups were satisfied with their assigned wear schedule (figure 5). This demonstrates that patients can be happy to wear lenses within either wear schedule. The practitioner should take this into account when determining the appropriate wear schedule for the patient.

Figure 5: Patient satisfaction

Adherence to wear schedules

To look at how successful the wear schedules are, it is important to ascertain if patients are able to complete the assigned wear schedule. The percentage of patients who did not have any unscheduled removals was high (Figure 6) for both groups. This shows that either schedule can be worn successfully if appropriate for the patient.

Figure 6: Number of unscheduled removals

Patient preferences

Patients would prefer to wear their lenses on a CW schedule for 30N or more, as seen in Figure 7. The main reason given for this is the convenience of this wear mode. Convenience, seeing on waking and permanence of correction have continually been cited as reasons for preferring a 30N schedule. There are a small number of patients that still prefer the 6N EW schedule for reasons of perceived safety.

Figure 7: Patient preferred wear mode

Adverse Responses

The rate of inflammatory adverse events (CLPU, CLARE and IK) was identical (7.7%) in both wear schedules. The rate of inflammatory events was low and these events continued to occur over the three years of the study. However these events are all manageable and rarely cause permanent discontinuation from lens wear.

Mechanical events (SEAL, CLPC, corneal erosion) occur in both wear schedules. The 6N or 30N wear schedule does not seem to affect the rate at which these events occur. SEAL events were most predominant in the first year of the study. The rate of CLPC peaked in the second year for both the 6N and 30N group. Small numbers of corneal erosions occurred during the first two years of the study in both lens wear schedules. SEAL and CLPC events generally lead to discontinuation from the silicone hydrogel lenses.

Nilsson’s one year study also found the complication rate was low. There were no significant differences seen in the complication rates between the 30 day and 7 day group?.

Conclusions

Six and thirty night wear of silicone hydrogel contact lenses showed little difference in terms of lens performance, avoiding hypoxia, discontinuation rates, subjective responses and inflammatory adverse responses. The 6N schedule does not seem to offer any advantages over the 30N schedule, in terms of reducing lens spoilage or improving corneal physiology.

The high Dk silicone hydrogel contact lenses perform well for up to 30N wear. The 30N CW schedule is preferred by patients. Therefore flexibility is the key when determining the appropriate wear schedule for the patient. The practitioner should use their discretion to determine a wear schedule that suits the patient, ensures patients have appropriate instructions for their care regime and follow proper procedures if they have any trouble with the lenses.