Glaucoma symptom scale: is it a reliable measure of symptoms in glaucoma patients?

Glaucoma is the leading cause of irreversible blindness worldwide after cataract and accounts for 10% of the world's blind. 1 Although patients with acute angle closure glaucoma are symptomatic, those with other forms of glaucoma (at least in the early stages) usually do not experience many symptoms, thereby, resulting in the disease often going unnoticed with possible progression. In some cases, patients with fluctuating levels of intraocular pressure may experience blurred vision and see haloes around lights. The Glaucoma Symptom Scale (GSS) was developed to assess ophthalmic symptoms experienced by patients with glaucoma. 2 The GSS comprises 10 ocular symptoms, of which six are non-visual and four are visual. The non-visual symptoms include 'burning/ smarting/stinging', 'tearing', 'dryness', 'itching', 'soreness/tiredness', and 'feeling of something in the eye'. The visual symptoms include 'blurry/dim vision', 'hard to see in daylight', 'hard to see in darkness', and 'halos around lights'. The GSS is unique in that it assesses symptoms as compared with visual functioning by other glaucoma-specific questionnaires. Like most questionnaires in ophthalmology, the GSS was also developed using traditional psychometric methods, that is, the classical test theory (CTT). 3 The limitations of CTT have been well acknowledged. A major shortcoming of CTT pertains to its scoring assumptions: Likert or summary scoring in which the scores are calculated from simple additions of ordinal values assigned to response categories for each item. Such a method of scoring is erroneous as it assumes that response categories are equidistant on a measurement scale and that all questions are of equal difficulty , and thereby treating the whole questionnaire as interval scale based on ordinal level scoring. This limitation is overcome by the use of item response theory, in particular Rasch analysis, that makes use of interval-level data. A particularly important advantage is the fact that when questionnaire data fit the stringent requirements of the Rasch model, the ordinal scores generated by summing scores for all items (summary score) can be transformed into interval level or linear measurements. This means that by using a Rasch-based scale, symptoms can then be measured on a linear scale. Additionally, Rasch analysis helps improve sensitivity to change by reducing noise in measurement and so has advantages for outcomes research. 4 Recently, the GSS was subjected to Rasch analysis in an Australian Glaucoma population. 5 Although the investigators found the GSS to have good psychometric characteristics, an important property—targeting was suboptimal indicating that …