Investigating central visual field loss and its effects on how patients read the ETDRS chart

Abstract

Purpose : Choroideremia is an inherited retinal degeneration in which asymmetric retinal deterioration occurs with greater temporal preservation. This correlates with higher retinal sensitivity nasal to central fixation, which we have previously documented. A key question is whether visual acuity (VA) in constricted visual fields measured with the Early Treatment Diabetic Retinopathy Study (ETDRS) chart is impacted by a neglecting of letters temporally compared to nasally. To investigate this further, we assessed whether choroideremia patients made errors more on the temporal than nasal side of the ETDRS chart in each eye under test conditions.

Methods : Monocular VA for right (RE) and left (LE) eyes was measured with a standard ETDRS chart at 4 metres. Letter columns were labelled from nasal (N) to temporal (T) visual field: N2, N1, C (central), T1 and T2 (Figure 2). Total errors per column in the chart were counted until 1 row below the “threshold” row (lowest row with no more than two errors). The distribution of column errors for each group was analysed with the Χ² test.

Results : Sixty eyes from 30 choroideremia patients (median age 44.9 years [IQR 39.1-58.9]) and 86 eyes from 43 healthy controls (median age 20 years [IQR 20-21]) were examined. Median VA was 72 for patients and 93 for controls. Results showed greater errors in T2 compared to N2 in the RE of choroideremia patients (p=0.045). However, there was no significant difference in the LE of patients (p=0.26), or in either eye in controls (RE p=0.77, LE p=0.32).

Conclusions : Choroideremia patients seem less able to read the right compared to the left side of the EDTRS chart with the right but not left eye. Since the left side is read first with each row, this may represent difficulties in tracking saccades in the right eye since the more sensitive temporal retina is not aligned with any letters along the row being read. Conversely, this surviving temporal retina in the left eye may provide a clearer view of the entire row, which may facilitate correct identification of subsequent letters. The ETDRS chart may not therefore be optimal for patients with asymetrically constricted visual fields so alternative approaches, such as reading rows backwards, may provide more consistent results.

This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.

 

Figure 1: Image of right fundus in choroideremia showing preservation of the retina temporal to fovea

 

Figure 2: Example ETDRS chart for RE with labelled columns