Background and objective: To describe the process of the cultural and psychometric adaptation (validation) into Spanish of the Glaucoma Symptom Scale (GSS) in order to measure the impact of glaucoma symptoms, and inform about the psychometric properties.
Material and method: The study was supervised by a 6-member expert panel. After forward and backward translations in duplicate, a Spanish version was obtained which was administered to three samples: a 16-patient sample to check comprehenssion and face validity, a 100-patient sample to check structural validity (factor analysis and reliability) and, finally, a 387-patient sample for assessing construct and discrimination validity.
Results: The questionnaire yielded 2 dimensions, which are not exactly the same as the ones in the original study. Reliability was high (alpha=0.82), and correlation between even-odd items was high too (r=0.92). The scale discriminated between lower and upper quartile groups (p<0.001). The average on non visual symptoms was higher than on visual ones (p=0.001), showing more inconvenience of the former. There were differences between compliant and non-compliant patients (p=0.004), but a lack of agreement between self-reported measures was also found. We found differences between patients with one or 2 drops treatments (p=0.0014).
Conclusions: The results showed acceptable psychometric properties and they make available to health professionals a new instrument to assess glaucoma symptoms, taking into account the patient perspective.