SILICONE HYDROGEL PRESCRIBING
SiHy Prescribing Trends
Understanding the prescribing pattern of silicone hydrogel lenses for daily wear in a hospital-based contact lens practice.
By Rajeswari Mahadevan, PhD Optometry; Rajni Rajan, MPhil Optometry; Sheeba Swarna, MPhil Optometry; and Prafulla Kuthe, BOptom
Silicone hydrogel (SiHy) contact lenses are a breakthrough development in the contact lens industry. Initially these lenses were invented for the extended wear or continuous wear modality, eliminating the hypoxic complications associated with extended use of traditional hydrogel lenses (Maldonado-Codina et al, 2004). Since then, different brands of SiHy lenses commonly prescribed for daily wear have come into the market from various manufacturers.
While SiHy lenses were invented in 1999, their use in the daily wear modality didn’t become prominent until 2003 (Efron and Morgan, 2008). In many countries worldwide, there has been increased interest in the benefits of SiHy materials for ocular health as well as in their impact on prescribing patterns.
In 2008, SiHy lenses represented 36% of all soft contact lenses prescribed internationally (Morgan et al, Aug. 2010). Globally, soft contact lenses account for about 90% of the contact lenses fit; of that, SiHy lenses represented more than half of soft lens fits in 2012 (Morgan et al, 2013). In India, a survey assessed the prescribing patterns of soft and hard contact lenses in 2011. This survey concluded that 98% of the lenses fit were soft lenses, with 28% of the users selecting SiHy as the material of choice (Thite et al, 2013). According to that survey, SiHy lenses were equally preferred by new fits and refits.
A retrospective analysis from 2006 to 2011 was also conducted at an optometry centre in South India in 2011. It concluded that the soft lens prescribing pattern was more or less similar to the current trend noted worldwide, but SiHy lenses represented only 20% of soft contact lens fits in 2011 (including bandage lens and pediatric fits). However, the percentage of SiHy lenses dispensed in the last year alone was 21% (Sanker and Noushad, 2013). This reflects the slow growth of SiHy lenses until 2010 in the Indian market.
Though silicone hydrogel contact lenses are becoming the lens of choice globally, the trends of prescribing patterns for SiHys have yet to be assessed extensively among new fits and refits in the Indian scenario. To the best of our knowledge, this is the first report on SiHy prescribing patterns in a hospital-based contact lens practice. The aim of this study was to understand the pattern of prescribing the SiHy lenses in a tertiary eyecare center.
Retrospective data of 32 subjects (53 eyes) prescribed with SiHy lenses from Jan. 2012 to March 2012 was collected in our contact lens clinic using the hospital’s electronic medical records. The total number of soft lens fits during the study period was recorded; we excluded prosthetic, cosmetic, multifocal, and bandage contact lenses. The age at presentation, gender, eye, type of SiHy material prescribed (i.e., lotrafilcon B, balafilcon A, or senofilcon A), type of lens (spherical or toric), and preferred lens replacement schedule by the patient (monthly or biweekly) were obtained from the records for both fits (new contact lens users) and refits (existing hydrogel contact lens users). In refits, information about previously used lens types, replacement frequency, and reasons for shifting to SiHy lenses was also obtained. Almost all of the subjects were advised to disinfect their lenses daily with a multipurpose solution, and every two weeks with a hydrogen peroxide disinfectant one. This study was approved by the institutional review board. Data analysis was done using Microsoft Excel and IBM SPSS 15.0 software. Pearson’s chi square test was used for calculating the proportion of various lens designs among new fits and refits. The Pearson correlation was used to calculate the r value between the flat keratometry (K) value and the amount of flattening from flat K that is required for optimal fit.
During the study period, 699 eyes were fit with soft contact lenses; 562 eyes (80.41%) were fit with hydrogel lenses, and 137 eyes (19.59%) were fit with SiHy lenses. Within the SiHy lens group, 27 eyes (19.70%) were new fits, 26 eyes (18.97%) were refits, and 84 eyes (61.31%) were existing SiHy wearers.
Demographics In the new user group, the mean age of the subjects fit with SiHy lenses was 23 ± 8 years, with a range of 6 to 42 years. In refits, the mean age was 27 ± 6.5 years, with a range of 21 to 48 years. No statistically significant age difference was noted between new fits and refits (p = 0.07). Almost 89% and 92% of subjects were less than 35 years of age in new fits and refits, respectively. The ratio of male to female among those fit with SiHy lenses was 55 to 45. There was no significant difference in the preference of SiHy contact lenses among males and females in new fits or refits (p = 0.90).
K Readings and Base Curve The mean flat and steep K reading in refits was 43.14D ± 1.44D and 44.36D ± 1.44D, respectively. The mean flat and steep K reading in new fits was 42.97D ± 1.40D and 44.30D ± 1.97D, respectively. Single base curve lenses were fit in 73% of refits and 48% of new fits. Among the flat/steep base curve lenses present in the study, flatter base curves (8.8mm) were prescribed more than steeper base curves were in both new fits and refits. Figure 1 shows the distribution of flat corneal K values versus the prescribed base curve of contact lenses in new fits (a) and refits (b) among different materials.
Figure 1. The flat corneal keratometric values versus contact lens base curve selection among (a) new fits and (b) refits.
The average K values prescribed with single base curve lenses ranged from 42.00D to 46.88D and from 41.63D to 46.25D in refits and new fits, respectively. Among the flat/steep base curve lenses, the average K values fit with flat base curve lenses ranged from 40.88D to 44.63D in refits and from 40.25D to 45.75D in new fits. On the other hand, in the steep base curve lenses, the average K values ranged from 45.75D to 45.90D in refits and from 44.00D to 45.75D in new fits.
The average amount of flattening from flat corneal K values for the base curve of the prescribed lens was 0.83mm ± 0.23mm in both new fits and refits. There was significant negative correlation noted between the amount of flattening and the base curve of flat K values (Pearson correlation coefficient r = –0.876; p = 0.0001) (Figure 2).
Figure 2. The correlation between corneal flat keratometric (K) value and the amount of flattening of contact lens base curve.
Lens Design Based on the refractive error, the lenses were categorized into spherical and toric designs. Fourteen out of 53 eyes (26%) were fit with SiHy toric lenses during the study period. Spherical lenses were more commonly prescribed in new fits compared to toric lenses (93% versus 7%, respectively).
In refits, the trend was to prescribe an almost equal proportion of spherical (54%) and toric (46%) lenses. A statistically significant difference in the lens designs between new fits and refits (p = 0.0001) was noted. Among the refits, 23% of eyes shifted from a spherical equivalent lens to a toric contact lenses (Figure 3).
Figure 3. The change in trend in lens design among refits.
Figure 4 shows the proportion of SiHy lenses prescribed in new fits and refits based on the manufacturers’ design (aspheric/spherical) of the lens. No significant association was found among manufacturer designs among new fits and refits (p = 0.23).
Figure 4. The preference of manufacturer design of lens among new fits and refits.
Replacement Frequency Figure 5 explains the change in the replacement frequency schedule among refits. The prescribed lens replacement schedule was most commonly monthly replacement, followed by biweekly replacement among refits (monthly: 92%; biweekly: 8%). In new fits, most of the subjects preferred a monthly replacement schedule. The modality of choice followed by subjects in new fits and refits was daily wear. Except for a nap in the afternoon or during travel, neither extended wear nor continuous wear was recommended among refits.
Figure 5. The prescribed replacement frequency among refits.
Reasons for Shifting Most of the hydrogel contact lens users preferred being able to wear lenses for longer hours and were willing to try the newer lens materials available (62%). In the previous hydrogel contact lens users, 36 percent of the subjects had complaints with their lens material. The most common complaints were dryness, discomfort, and blurred vision with the previous lenses (Figure 6). The average wearing time noted with the previous hydrogel lenses was 11.5 ± 1.8 hours.
Figure 6. The distribution of symptoms among previous hydrogel lens users in refits.
Age/Gender The mean age observed in this study (new fits: 23 ± 8 years; refits: 27 ± 6.5 years) was younger than that observed in international surveys that were conducted in seven countries (32.1 ± 13.0 years) (Morgan et al, Aug 2010) and in 36 markets (31.7 ± 13.9 years) (Morgan et al, 2013).
The mean age from our study was comparable to studies done by Thite et al (2013) (26.4 ± 7.4 years) and Sanker et al (2013) (24.9 ± 9.4 years). Similar to our study, Morgan et al (Feb. 2010) postulated that the median age of lens wearers was higher for refits versus new fits.
Due to the non-availability of SiHy multifocal lenses during the study period, only two subjects older than 40 years of age with presbyopia were prescribed with single-vision SiHy lenses for distance and glasses for the presbyopia. In our study, the majority of the subjects were younger than 35 years of age, which could possibly be explained by the above reason. Older age groups fit with contact lenses were mostly in the more established markets due to the wider array of lenses available there.
Females outnumbered males in the previous surveys conducted nationally and internationally (Efron and Morgan, 2008; Morgan et al, Aug 2010; Morgan et al, 2013; Thite et al, 2013; and others. Full list available at www.clspectrum.com/references.), but in our study, males outnumbered females. Males are commonly the wage earners in developing countries, and as such, they have the privilege to try new (and sometimes expensive) options. Some reasons that explain the predominance of male subjects (as compared with females in our study) could be due to the following traits observed in males: 1) bigger role in making decisions, 2) increased literacy levels, 3) lifestyle (i.e., more outdoor activities), and 4) a greater awareness of contact lenses.
Lens Design When considering lens design, spherical lenses were fit more commonly among soft lens users both in India and globally (Efron and Morgan, 2008; Morgan et al, Aug 2010; Morgan et al, 2013; Thite et al, 2013; and others). In our study, spherical lenses were prescribed more commonly in new fits, while the proportion of spherical versus torics in refits was almost the same.
The range of spherical error was from –1.50D to –10.00D among refits and from +8.00D to –11.00D in new fits. In a recent Indian survey conducted in 2011, toric lenses represented nearly 24% of the soft lenses fit (Thite et al, 2013). In our study, among SiHy lens users, the toric lens fitting showed a slightly higher percentage (26%) when compared with the Indian survey. Among the refits, 23% of eyes using spherical equivalent lenses shifted to toric lenses.
The cylinder error ranged from –0.75D to –2.25D in refits and from –0.75 to –1.75D in new fits. Based on the distribution of astigmatism in prospective contact lens wearers, Holden (1975) determined that if all astigmatism of 1.00D or more was corrected, 34.8% of lens wearers would need toric lenses.
Most of the developed contact lens markets fall below the threshold for correcting greater than 1.00D of astigmatism (Morgan et al, 2011; Morgan and Efron, 2009). Our study also falls below the threshold of correcting greater than 1.00D of astigmatism. In our study, patients who had a cylinder error less than –0.75D were fit with spherical contact lenses, and those who had an error greater than –0.75D were fit with a toric lens; 0.75D cylindrical power is the entry point for marketed toric contact lenses.
Contact Lens Base Curve Almost all soft contact lenses are available either as a single base curve or in the flat/steep range of base curves. In our study, single base curve lenses (8.6mm) fit most of the eyes in both new fits and refits. Among the flat/steep range of base curves, flatter base curves (8.8mm) were fit on most of the eyes in both new fits and refits.
The average flattening of the prescribed base curve of lenses from flat corneal K values was around 0.83mm in both the groups, which almost follows the theoretical rule-of-thumb for the selection of initial base curve. The amount of flattening required for contact lens base curve selection in flatter corneas was less when compared with steeper corneas in both new fits and refits (Figure 2).
This could be because as the corneal curvature becomes steeper, the sagittal depth increases. Douthwaite (2002) theorized that sagittal depth is influenced most by corneal asphericity, eccentricity, and diameter. But, the diameter plays a more significant role in determining the fit compared to the other variables (Rajan and Mahadevan, 2013). In our study, lenses were fit based on K values.
Replacement Frequency and Modality As reported by Morgan et al (2013) in the international prescribing trends survey from 2012, 95% of soft lens wearers replace their lenses monthly. Similar results were reported in our study as well. Almost all of the subjects in new fits preferred a monthly disposable replacement schedule; in refits, 92% of the subjects preferred monthly replacement, followed by biweekly replacement schedule lenses.
The shift in replacement frequency among the refits could be explained by factors such as: 1) promotion of frequent replacement lenses by practitioners explaining the possible risk of developing adverse events; 2) patient preference for better maintenance of ocular health; and 3) increased availability of monthly replacement lenses in the market influenced by global studies. A higher frequency of monthly replacement contact lenses was also observed in other studies (Morgan et al, Aug 2010; Thite et al, 2013; Sanker and Noushad, 2013; Efron et al, 2010; and others.).
Nowadays, it is common to see SiHy lenses prescribed as a daily wear modality rather than an extended/continuous wear modality (Efron and Morgan, 2008). This has been observed in many countries, including Australia (Efron et al, 2010), Portugal (González-Méijome et al, 2007), and Hong Kong (Charm et al, 2010).
In India, the extended wear or continuous wear modality was not preferred by Indian practitioners due to the possible risk factors of developing adverse events (Thite et al, 2013). In our study, all the subjects in both new fits and refits were prescribed with the daily wear modality.
Reasons for Shifting Most of the patients in the refit category wanted to upgrade to the new material to avail themselves of the ocular benefits of silicone hydrogels. The main reasons for shifting to SiHy lenses were the need to wear lenses for more hours and complaints with their previous hydrogel lenses, such as dryness, discomfort, and blurred vision. Similar to our study, dryness was considered the main reason for the majority of the refits into SiHy lenses in the study done by González-Méijome et al (2007).
Knowing the pros and cons of retrospective studies, this study was conducted as a pilot study for a shorter period to evaluate the need for and design of prospective studies in the same area. Prospective studies about quality of life, lifestyle, occupation needs, environmental factors, detailed follow-up records, solution compatibility, and ocular physiologic responses are currently being considered in new fit and refit SiHy wearers.
We require a wider range of marketed SiHy lenses that are available for higher refractive errors to satisfy the patients who are either dropouts or have discontinued hydrogel lens wear due to non-availability of SiHy lenses in this category. The options available in SiHy daily disposable lenses and SiHy multifocal lenses are also minimal, which is probably an added reason for the lag in trends as compared to the international market.
Retrospective analysis of the subjects fit with silicone hydrogel lenses shows that male subjects outnumbered female subjects. Spherical lenses are prescribed more in our study, which follows the global trend. In addition, the percentage of toric lenses prescribed has slightly increased when compared with a recent survey conducted in India.
Monthly replacement schedules dominate the other replacement schedules. Single base curves and aspheric lenses fit better compared with the flat/steep range of base curve lenses.
And, finally, the desire to take advantage of the newer materials is the most common reason for patients wanting to shift to SiHy contact lenses.
SiHy prescribing trends in a hospital-based contact lens practice follow the global trend in the prescription of spherical lenses, the daily wear modality, and the monthly replacement schedule. CLS
For references, please visit www.clspectrum.com/references and click on document #233.
Acknowledgements: We thank Mr. Abdul Majid, Amudha Oli Arumugam, Madhumathi Subramanian, Sakunthala P, Karpagavalli Subramanian, and Biman Das for assisting in collection of data.
Dr. Mahadevan is head of the Department of Contact lenses at Medical Research Foundation, India. She is also an associate professor at the Elite School of Optometry.
Ms. Rajan is a senior optometrist at the contact lens department of Medical Research Foundation, India and is also on the faculty at Elite School of Optometry.
Ms. Swarna is a post-graduate student at Elite School of Optometry. Her current research work is in silicone hydrogel contact lenses.
Ms. Kuthe is a contact lens fellow at Medical Research Foundation, India.