Assessing children's visual acuity with steady state evoked potentials
The majority of children attending ophthalmology clinics require a visual acuity assessment. The optimal technique depends on age as well as the ability to cooperate with testing. Most acuity assessments are performed subjectively by an orthoptist. Objective acuity assessment by Visual Evoked Potential (VEP) provides a complementary assessment in those subjects who cannot complete subjective tests. The aim of this study was to develop and evaluate a rapid, objective visual acuity assessment. The technique was named the step_ VEP and is based on the real-time analysis of steady-state VEPs (ssVEP). It presents high contrast checkerboard stimuli of sizes 0.4 to 3.0 LogMAR with a successive approximation algorithm. Speed of response detection, specificity and sensitivity were optimised by investigation of recording montage and analysis techniques in a group of normal children and adults (N=102). The success, duration and outcome of step_ VEP acuity assessment was compared to transient VEP (t-VEP) acuity assessment and subjective acuity assessment in a group of paediatric patients (N=218). I-D Laplacian analysis of three occipital electrodes was significantly faster than conventional recording and analysis (Oz-Fz) at detecting ssVEP responses near visual acuity threshold (3' checks) from three years upwards, and at detecting responses to 6' and 9' checks in the 7-9 year age group. A lateral electrode site at 15% of the half-head circumference was fastest most often in adults. Step_ VEPs were 16% more successful than t-VEPs and 9% more successful than subjective tests in providing a complete acuity assessment. Subjective acuity scores were systematically higher than VEP acuity scores in subjects who successfully completed both assessments. A closer agreement with subjective acuity scores was found for step_ VEPs than t-VEPs. The disparity between step_ VEP acuity score and subjective acuity score was shown to reduce with age.