Unilateral Mu Rhythm and Associated Cortical Lesions on Brain MRI

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The purpose of this study was to assess the frequency of occurrence of a unilateral mu rhythm and the associated neuroimaging findings on dedicated epilepsy protocol brain MRI.


We retrospectively reviewed the EEG reports database at the American University of Beirut Medical Center between 2011 and 2014 searching for the presence of a unilateral mu rhythm. For patients with a unilateral mu rhythm, we recorded the patients' demographics, number of EEGs performed, characteristics of the mu activity, and the findings on the epilepsy protocol brain MRIs.


A total of 7986 patients underwent 9,509 EEG between 2011 and 2014. Four patients (0.05%) aged between 19 and 55 years had evidence of a unilateral mu rhythm. Three patients were diagnosed with localization-related epilepsy and one with syncope. The brain MRIs showed cortical lesions involving the parietal cortex, ipsilateral to the unilateral mu rhythm in the three patients with epilepsy.


A unilateral mu rhythm is a rare phenomenon on the scalp EEG that should prompt a search for an ipsilateral lesion, even in the absence of additional EEG abnormalities.

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