The Visual Impairment Instrument (IVI) and the Glaucoma Symptom Scale (GSS) are suboptimal scales for assessing patients with glaucoma but relatively good vision.
The Visual Impairment Instrument (IVI) and the Glaucoma Symptom Scale (GSS) are suboptimal scales for assessing patients with glaucoma but relatively good vision, according to a report published in the August issue of Optometry & Vision Science.
Ecosse Lamoureux and colleagues from the University of Melbourne, Australia recruited glaucoma patients from private and public clinics to complete the IVI and GSS questionnaires. The two scales were assessed for fit to the Rasch model; specifically they were assessed for unidimensionality, individual item and person fit to the model, response category performance, differential item functioning and targeting of items to patients.
A total of 175 subjects were recruited, the majority of whom (65%) had primary open-angle glaucoma (POAG) and a visual acuity ≥6/9 in the better eye (87%). Only one third of the subjects had severe visual field loss in both eyes. Disordered thresholds were evident across all GSS items, indicating that the categories were difficult to discriminate and required category collapsing (five to three categories). Both scales demonstrated ineffective person-item targeting.
The authors believe that both IVI and GSS are ineffective scales for assessing glaucomatous patients who have good visual acuity.