Electroencephalographic findings in Hashimoto's encephalopathy

Hashimoto's encephalopathy (HE) is a steroid-responsive relapsing disorder that frequently presents with seizures and diffuse EEG abnormalities. We report the EEG findings in seven patients with encephalopathy, seizures, or both, associated with elevated antithyroid antibodies. There were several combinations of findings within the same patient and between patients. The EEGs of five patients had generalized slowing or frontal rhythmic slowing; two also had triphasic waves and one had periodic sharp waves. Three had focal left temporal slowing. HE is heterogeneous clinically and electrographically. A high level of suspicion is necessary to establish the diagnosis.