Subjective and objective measures of corneal staining related to multipurpose care systems

Lakshman N Subbaraman BS Optom, MSc After graduating from the the Elite School of Optometry, Chennai, India in 2002, Lakshman N Subbaraman practiced as a Clinical Optometrist in Sankara Nethralaya Eye Hospital. In 2004 he completed a Masters at the University of Waterloo under the joint supervision of Drs Lyndon Jones and Michelle Senchyna. Currently he is enrolled as a PhD student under the supervision of Dr Lyndon Jones, investigating the influence of protein deposition on in vitro and ex vivo wettability associated with silicone hydrogel contact lens materials.

Corneal fluorescein staining is one of the most common contact lens related complications, and lens care products have been directly or indirectly implicated as one of the major causes [1,2]. Currently practitioners measure corneal staining subjectively, using photographic and pictorial clinical grading scales [3,4]. These grading systems allow quantification of clinical signs using a generic ordinal scale [5]. Typically, the absence of a sign (no staining) is given a grade of zero, and numbers up to four are used to describe increasing levels of the sign (grade 4 staining is extreme and requires immediate intervention) [5]. While a small expansion of the five level scale may be effective[6,7], other reports suggest that inter-observer variability may be a limiting factor [8,9]. Hence, an objective method of measuring staining would eliminate or reduce interpretation by the investigator, thus improving the reliability of the measure.

In the March 2003 issue of Contact Lens & Anterior Eye, Pritchard and co-workers evaluate an objective, digital-imaging method of measuring corneal staining in soft contact lens wearers using three common multipurpose care systems (MPS) and compare this with traditional methods of corneal staining.

This study was a prospective, cross-over trial of three different MPS that involved 24 participants. All participants were fitted with Group II soft contact lens (alphafilcon A) on a daily-wear basis. One of the MPS contained polyquaternium-1 – Opti-Free Express®; Alcon Laboratories, Inc. (PQ), while the other two systems contained polyhexanide – ReNu Multi-Purpose Solution; Bausch & Lomb, Inc. (PX1) and ReNu MultiPlus; Bausch & Lomb, Inc. (PX2). Participants used each of the three MPS for two weeks. Subjective comfort was rated using a visual analogue scale. Subjective symptoms were recorded and conjunctival hyperemia and papillae were evaluated using a photographic grading scale [3]. Corneal staining with each MPS was measured objectively using an image-processing system.

The results from this study revealed that corneal staining was lower with the MPS containing polyquaternium-1 (PQ) than with the polyhexanide systems (PX1 and PX2). There were no significant differences between the three MPS with respect to comfort or in the symptoms reported. More than double the number of subjects expressed an overall preference for PQ (50%) compared to those expressing a preference for PX1 (23%) and PX2 (23%), however this difference was not statistically significant. Other studies have also shown a direct relationship between epithelial response and concentration of polyhexanide [10,11]. Even though there was a difference in biocompatibility between the three MPS used in this study, the subjects with the worst staining did not show evidence of poorer comfort. These results were consistent with the findings of Begley and co-workers, in that no correlation was found between staining and comfort [1]. Thus, the fact that corneal response was not accompanied by symptoms, indicates the need for using other measures to assess care product compatibility.

This study also found that the objective image-analysis technique was an effective method of evaluating different corneal staining responses and a high correlation was seen between this method and the investigator grading. One of the advantages of the image-analysis technique was that a permanent record of the clinical results was possible. The authors experienced several limitations with the image-analysis technique. It was not possible to capture and analyze an image of the entire cornea. The method used was also not entirely objective in that there was some subjective variability in capturing the video image and in highlighting the staining area. This method was also more time consuming than the simple grading system.

Although this image-analysis technique is a useful research tool for detecting subtle differences in corneal signs, it is unlikely to be adopted routinely in clinical practice. Subjective grading scales are a useful adjunct to practitioners in measuring the severity and monitoring the progression of ocular complications of contact lens wear. In view of differences in precision afforded by the use of various grading systems [8], clinicians are advised to consistently use the same grading system. This consideration applies to individual practitioners, who are advised to choose one grading system and to use only that system over time. It also applies to practitioner groups (e.g. many clinicians working in single or multi-location practices) whereby practitioners in the group may be expected to frequently exchange grading information with colleagues within or between practice locations.

References

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