Expect the Unexpected-Refitting from Conventional Hydrogels

Kathy Dumbleton. MSc MCOptom FAAO (Dip CL) - Senior Research Associate, CCLR Senior Researcher Centre for Contact Lens Research (CCLR) University of Waterloo, Ontario, Canada

One of the “unexpected” findings first reported in patients refitted into silicone hydrogel lenses was a change in refractive error, and in particular a clinically significant reduction in myopia. This change usually occurs within the first few months of commencing silicone hydrogel lens wear and although the majority of patients experience hyperopic shifts or myopic reductions of approximately 0.5D, some patients may experience as much as a 2 D change. Hyperopic shifts of 0.25 to 0.50D may sound small, but can be clinically significant, particularly for those patients on the verge of presbyopia. Asthenopia and difficulty with close work may be the only symptoms in these patients.

While the precise etiology of the hyperopic shifts with silicone hydrogel lens wear is not clear, it is likely that both increased oxygen supply compared to their previous hydrogel lenses and an “orthokeratology-like” effect are involved. A study of patients who wore low-Dk hydrogels on an extended wear basis when refitted with silicone hydrogel lenses (also worn on an extended wear basis) exhibited a mean hyperopic shift of 0.37D within a month (1). Initially, this change in refractive error was attributed to recovery from the myopic shift commonly reported to occur following chronic corneal hypoxia (2-4). However a subsequent study found similar but smaller hyperopic shifts in silicone hydrogel lens wearers who had no previous lens wear experience (5). This finding suggests that silicone hydrogels, which are relatively stiff compared to conventional soft lens materials, may flatten the central cornea leading to an “orthokeratology-like” effect and two studies have reported central corneal flattening during silicone hydrogel wear (1,5). It has also been reported in a previous editorial (see Vision and silicone hydrogels) that patients who have inadvertently worn their lenses inside out would suffer from central corneal flattening causing a decrease in myopia or increase in hyperopia by as much as 2D.

Managing shifts in refractive error

A number of measures are recommended in order to diagnose this condition promptly, if it occurs, and to limit any inconvenience which may be caused: 

Precise assessment of refractive error, using both objective (auto refraction or retinoscopy) and subjective techniques and corneal curvature (preferably corneal topography rather than keratometry) should be undertaken at the time of initial re-fitting and at all follow up visits. Also, lenses should be inspected on-eye initially and at all visits to determine if one or both are inside-out. Assessment of the lens fit using high molecular weight fluorescein can be useful, but for this, the lens should be removed and a drop of fluorescein placed on the lens (inside surface) and then the lens reinserted. Care should be taken not to over-minus patients and patients should be counselled that changes in their prescription may occur as a result of refitting with silicone hydrogel materials. Most myopes do not mind when their prescription decreases, but hyperopes may not be as happy to see their prescription increase!

Follow-up visits within the first month of refitting should be scheduled for all patients but particularly for those at higher risk (high myopes and hyperopes). In addition to the regularly scheduled assessments, all patients should be advised to return for a follow-up visit should they experience any unusual visual symptoms. Although hyperopic shifts are reported to occur relatively soon after refitting, changes may also occur in subsequent months and eye care practitioners are encouraged to remain attentive to these possible changes.

At each follow-up visit, a careful history must be taken to elucidate symptoms which may be associated with a shift in refractive error. Questions should be asked about specific near and distance visual tasks since the initial refit. A best sphere over-refraction should then be performed. A trial frame or loose trial lenses are recommended rather than a phoroptor, to reduce proximal effects. Increments of +0.25D are added to each eye in turn until the patient reports blurring at distance. The final over-refraction should be confirmed binocularly. The contact lenses must be removed for a full biomicroscopic assessment and at this time, refraction and corneal curvature should be measured.

If there is any indication of a hyperopic shift or corneal flattening, then a decrease in power (less minus or more plus) in the prescribed contact lens(es) should be considered. Large refractive shifts may result in required lens powers outside the range of the contact lenses initially fitted. Fortunately there are now a number of silicone hydrogel lenses available with powers up to +6.00D for overnight lens wear and up to +8.00D for daily wear. It is preferable for these checks and measures to be completed before a supply of lenses is ordered for the patient.

Conclusions

While refitting patients with lenses that provide the best physiological options and comfort, stability of refractive correction is imperative. Five different spherical silicone hydrogel lenses with a variety of oxygen transmissibilities and moduli are available that should satisfy the above criteria for all patients. A systematic examination strategy to establish the cause of the refractive change using the techniques described above should ensure correct management. In the words of the insightful Heraclitus: “Much learning does not teach understanding” and “There is nothing permanent except change”.

References

1. Dumbleton KA, Chalmers RL, Richter DB, Fonn D. Changes in myopic refractive error with nine months' extended wear of hydrogel lenses with high and low oxygen permeability. Optom.Vis.Sci. 1999;76:845-9.
2. Barnett WA, Rengstorff RH. Adaptation to hydrogel contact lenses: variations in myopia and corneal curvature measurements. Journal of the American Optometric Association 1977;48:363-6.
   3.     Binder PS. Myopic extended wear with the Hydrocurve II soft contact lens. Ophthalmology 1983;90:623-6.
3. Hill JF. A comparison of refractive and keratometric changes during adaptation to flexible and non-flexible contact lenses. Journal of the American Optometric Association 1975;46:290-4.
4. Jalbert I, Stretton S, Naduvilath T, Holden B, Keay L, Sweeney D. Changes in myopia with low-Dk hydrogel and high-Dk silicone hydrogel extended wear. Optom Vis Sci 2004;81:591-6.
   
5. Jalbert I, Stretton S, Naduvilath T, Holden B, Keay L, Sweeney D. Changes in myopia with low-Dk hydrogel and high-Dk silicone hydrogel extended wear. Optom Vis Sci 2004;81:591-6.