Repetitive miniature spikes – An underreported EEG pattern

Objectives: Low-voltage repetitive spikes are mainly described with invasive recordings and considered highly suggestive for focal cortical dysplasia (FCD). This EEG pattern has received less attention in routine scalp EEG. Methods: Prospective collection of EEGs with low-voltage (<50 µV) repetitive spikes (repetitive miniature spikes – RMS) between July 1982 and July 2017 at the EEG laboratory of the Medical University of Innsbruck. We analyzed patterns of RMS on routine scalp EEG recordings and examined the relationship to clinical and brain imaging data. Results: Overall, RMS were seen in 38 patients representing zero to four observations out of 5000 records per year. RMS occurred rhythmically in 14, periodically in 17 and irregularly in seven patients. The EEG pattern appeared with a frontal and central predominance. All but five patients had epilepsies; eleven patients had non-convulsive status epilepticus. Cerebral magnetic resonance imaging (cMRI) detected malformations of cortical development in eleven patients, including six patients with focal cortical dysplasias. Conclusions: RMS are rare EEG patterns indicating focal epilepsy. Their observation on routine scalp EEGs should prompt further clinic-radiologic investigation. Significance: RMS resemble a clearly recognizable pattern in routine EEG, which is highly associated with focal epilepsy. The term is descriptive and can be added to the red flags, which can be found on routine EEG indicating underlying structural brain pathology, often in form of focal cortical dysplasia.