Changes in the background EEG in response to activation procedures in strabismic amblyopes depending on type of fixation in the amblyopic eye

Abstract

Background: Analysis of electroencephalogram (EEG) visual-evoked potentials (VEPs) is important for assessing the general integrity of the visual pathway. Pathology in any portion of the visual pathway causes changes in VEP. Percentages of alpha, beta, delta and theta rhythms in the EEG record, frequencies and amplitudes of these waves, and the level of alpha-rhythm suppression are essential for characterizing the state of the visual system. Purpose: To indentify the features of the state of the visual system and changes in the background EEG in response to activation procedures in strabismic amblyopes depending on type of fixation in the amblyopic eye.   Material and Methods: Fifty-two strabismic amblyopic patients aged 5-8 years and 15 healthy children of the same age underwent an examination. Of the amblyopic children, 32 had unilateral amblyopia with esotropia (6-15 degrees). Of these 32 children, 20 had eccentric fixation and 12 children, central fixation. The 10–20 system of electrode placement was employed to perform EEG in all children. Rhythms of the background EEG, interhemispheric asymmetry and alpha, theta and delta wave percentages were assessed. Brain potentials were recorded using standard activation procedures to activate deep brain structures.   Results: Alpha rhythm was not discernable in the background EEG in the occipital areas in 3.0 ± 2.6 % of cases, and the alpha index was below the normal range in 48.6 ± 8.2 % of patients with a visual acuity worse than 0.3. Delta index was above the normal range (25%-45%) in 48.6 ± 8.2 %, and theta index was above the normal range (with a mean value of 48.4 ± 8.2 %) in 48.6 ± 8.2 % of children. Reduced percentage of alpha activity was found in almost half of strabismic amblyopic children, indicating immaturity of the synchronizing system and impaired oculomotor tuning in the eyes of these children. Abnormal bilateral EEG response to eye-opening stimulation and abnormal bilateral EEG rhythmic driving response (RDR) were seen in most children with eccentric fixation, but not in most children with central fixation. Photic stimulation in strabismic amblyopic children resulted in bilaterally asymmetrical and synchronous theta and delta waves in symmetric frontal (theta and delta percent time of 46.0 ± 8.4%) and occipital (theta and delta percent time of 50.0 ± 8.4%) responses.   Conclusion: Increased theta and delta wave percentages in frontal and occipital derivations were found in strabysmic amblyopes (irrespective of type of fixation in the amblyopic eye) compared to healthy controls. Abnormal EEG response to eye opening and abnormal EEG RDR were found in strabysmic amblyopes. This indicates the presence of functional changes in the midline brain structures including the corpus callosum, which impedes the development of binocular vision in strabismic amblyopes.