Multiday Anodal tDCS of the Left Frontal Eye Field has a Compounding Effect that Lasts Beyond the Period of Treatment in Symptomatic Convergence Insufficiency

Abstract

Purpose : Convergence insufficiency (CI) is a binocular vision disorder that can lead to eye strain, double vision, headaches, and difficulty concentrating during near work. Previously, we reported that 2 sessions of anodal transcranial direct current stimulation (a-tDCS), a form of non-invasive brain stimulation, applied over the left frontal eye field (FEF) showed an improvement in the clinical markers of CI. This prospective, double-masked study investigated whether multiday a-tDCS of the FEF imparts compounding and lasting effects in adults with symptomatic CI.

Methods : 6 adults (25 ± 1.5 y) with symptomatic CI were randomly assigned to a-tDCS (2 mA for 30 mins; n = 3) or sham-tDCS (s-tDCS; 0 mA for 30 mins; n = 3) stimulation of the left FEF for 5 consecutive sessions 24h apart. Near point of convergence (NPC) break and recovery and near positive fusional vergence (PFV) blur, break, and recovery were measured before and after each stimulation session and for 5 days post-treatment.

Results : After 5 days of tDCS, the active group (baseline NPC, break: 34 ± 3 cm, recovery: 38 ± 2 cm) showed a 63.05 ± 5.59% and 58.83 ± 3.51% improvement in NPC break and recovery, respectively, compared with 34.26 ± 14.31% and 32.93 ± 12.27% in the sham group (baseline NPC, break: 17 ± 6 cm, recovery: 21 ± 7 cm). After 5 days of treatment cessation, the active group continued to progress with an overall improvement in NPC of 64.29 ± 2.81% (break) and 60.17 ± 2.72% (recovery). In contrast, the measures returned to the pre-stimulation level in the sham group. For near PFV, the active group (baseline PFV in prism diopter (PD) blur: 8 ± 1, break: 11 ± 3, recovery: 7 ± 6) improved by 75.00 ± 43.30%, 92.86 ± 12.37%, and 315.87 ± 423.47% for blur, break, and recovery, respectively. Post-treatment, the improvements persisted, with an overall enhancement of 141.67 ± 62.92%, 129.76 ± 63.92%, and 419.84 ± 632.45%. Conversely, the sham group showed no measurable improvement in PFV (baseline PFV in PD; blur: 17 ± 4, break: 19 ± 1, recovery: 15 ± 1).

Conclusions : This study provides evidence of the sustainability and efficacy of tDCS as a promising treatment option for symptomatic CI in adults. These findings offer a foundation for future research investigating the role of tDCS as either a standalone or adjunctive therapy to vergence/accommodative therapy in treating CI.

This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.