Optical Coherence Tomography in Vigabatrin-Exposed Patients: Exploring the Structure-Function Relationship

Abstract

Purpose: : Vigabatrin-associated visual field loss (VAVFL) occurs in 25-50% of patients. VAVFL is routinely monitored using perimetry, which has inherent limitations. Around 20% of adult patients with epilepsy are unable to perform perimetry, and in many other patients results may be inconclusive or unreliable.Using optical coherence tomography (OCT), retinal nerve fibre layer (RNFL) thinning has been described in a small number of patients with VAVFL.The aim of this study was to explore the relationship between RNFL thickness and visual field size in vigabatrin-exposed patients, and to determine whether OCT is a suitable tool to use in patients who are unable to perform perimetry. Understanding the structure-function relationship may reveal more about the pathological mechanisms leading to VAVFL.

Methods: : 171 epilepsy patients with previous or current exposure to vigabatrin were invited to participate. Visual fields were obtained using Goldmann kinetic perimetry and quantified using mean radial degrees (MRD) for the I4e isopter. RNFL imaging was performed using Cirrus high-definition OCT or Stratus OCT, software version 4.0.

Results: : 34/171 (19.9%) patients were unable to perform perimetry reliably due to inability to cooperate with the task. Nine patients (5.3%) were unable to perform OCT.There was a strong correlation between MRD and average RNFL thickness (r=0.773, p<0.001)

Conclusions: : This study is the largest to date using OCT to measure RNFL thickness in vigabatrin-exposed patients. We have demonstrated a strong linear relationship between RNFL thickness and visual field size measured by MRD. This provides some evidence that VAVFL may be related to loss of retinal ganglion cell axons (as represented by RNFL thinning).OCT provides a useful tool to monitor and assess patients exposed to vigabatrin, and can provide an accurate estimate of the extent of VAVFL in the absence of a reliable direct measure of the visual field.

Keywords: visual fields • nerve fiber layer • imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound)