Off the Cuff: Our Student Problem

Based on clinical inferences, investigators theorized in situ soft lens hydration was linked to the precorneal tearfilm pH. Forty-one myopic subjects in Fort Rucker, Ala., were fitted with one of two types of extended-wear soft contact lenses, and were followed quarterly for a period of 33 months. The anterior soft contact lens surface pH was measured in situ, while the in vitro lens water content was measured immediately after lens removal, using a handheld refractometer on one lens and a gravimetric means of hydration measurement on the other lens.

The in situ pH increased logarithmically across extended wearing time, reaching an asymptote at approximately five days’ wearing time at a pH of 7.45 ± 0.03. Lens water content was shown to similarly decrease at an inversely logarithmic rate, leveling off at four to five days’ extended wearing time. Both means of hydration assessment correlated well with each other across days' extended wearing time.

A log-log dual conversion yielded significantly different linear slopes, based on a multifactorial analysis of both lens types, by pH and water content. The differing ionic status of each material accentuated their varied polar attraction characteristics. Two soft lenses, identical in all patient-based parameters, could provide differing oxygen availability, as well as differing physical fits in patients of identical physical characteristics, as a result of their unique tearfilm pH differences.

The in situ hydrogel lens water content is directly dependent on the precorneal tearfilm pH.

A cross-sectional study of central corneal thickness (CCT) measurement in 52 eyes of 52 healthy volunteers was conducted by a single examiner at the Ocular Surface and Contact Lens Laboratory to compare CCT measured by standard ultrasound pachymetry (USP) and three non-contact devices in healthy eyes. Three consecutive measurements were performed by standard USP, a non-contact tono-pachymeter, Pentacam corneal topographer,and Anterior Segment Optical Coherence Tomography (AS-OCT). The mean values were used for assessment. The results were compared using multivariate ANOVA, linear regression and Pearson’s correlation. Agreement among the devices was analyzed using mean differences and Bland-Altman analysis with 95% limits of agreement (LoA). Finally, reliability was analyzed using the intraclass correlation coefficient (ICC).

Mean CCT by ultrasound pachymeter, tono-pachymeter, corneal topographer and AS-OCT were 558.9µm ± 31.2µm, 525.8µm ± 43.1µm, 550.4µm ± 30.5µm and 545.9µm ± 30.5µm respectively. There was a significant positive correlation between AS-OCT and USP, corneal topography and USP, and corneal topography and AS-OCT. There was a lower correlation between CT-1P tono-pachymeter and the other three modalities. Intraclass correlation coefficients showed an excellent reliability between pairs except for CT-1P against the other three instruments that were found to be moderate.

Researchers wrote that CT-1P tono-pachymeter underestimated CCT measurements compared with the Scheimpflug system, AS-OCT and USP. They found that mean CCT among USP, Pentacam and AS-OCT were comparable and had significant linear correlations. Researchers concluded that, in clinical practice, these three modalities could be interchangeably used in healthy patients.

Literature searches were conducted in January 2018 in PubMed, Cochrane Library and ClinicalTrials.gov databases, with no date or language restrictions, to review the published literature of visual outcomes and adverse events associated with the two most commonly used contact lenses for treating aphakia in children: silicone elastomer (SE) and rigid gas permeable (RGP). These combined searches yielded 167 citations, 27 of which were reviewed in full text. Of these, 10 articles were deemed appropriate for inclusion in the investigators’ assessment and were subsequently assigned a level of evidence rating by the panel methodologist.

The literature search identified four level II studies and six level III studies. There were insufficient data to compare visual outcomes for eyes treated using SE lenses vs. RGP lenses. Silicone elastomer lenses had the advantage of being worn on an extended-wear basis, but they were associated with more adverse events than RGP lenses. These adverse events included microbial keratitis, corneal infiltrates, corneal edema, corneal scars, lenses adhering to the cornea, superficial punctate keratopathy, lid swelling and conjunctival hyperemia. The lens replacement rate was approximately 50% higher for RGP lenses in the only study that directly compared SE and RGP lenses. Limited evidence was found in the literature on this topic.

The authors determined that silicone elastomer and RGP contact lenses were effective for treating aphakia in children. They found silicone elastomer lenses to be easier to fit and able to be worn on an extended-wear basis. Rigid gas permeable lenses needed to be removed every night and required a more customized fit, but were associated with fewer adverse events. The authors suggested that the choice of which lens a practitioner prescribes should be based on the particular needs of each patient.