Objective: We aim to report a case of myoclonus as an adverse event mRNA COVID-19 vaccine.

Background: There have been multiple reports of neurological adverse event possibly caused by the COVID-19 vaccine, but establishing a clear connection in numerous instances is challenging. Herein, we report a case with myoclonus manifested following the COVID-19 vaccine with evident encephalitis on the brain imaging.

Method: We retrospectively reviewed medical charts and brain magnetic resonance imaging(MRI).

Results: A 54-year-old woman visited the hospital, complaining of myoclonus and numbness in her left arm. The patient had bronchiectasis two years ago and it was controlled well. She received the fourth Pfizer mRNA Covid-19 vaccination 3 weeks ago. About a week after vaccination, a tingling sensation occurred on the lateral side of the left elbow, and three days later, there was intermittent clonic movement at approximately 1Hz in the left upper arm. On the next day, numbness occurred in the left ankle and hip. The clonic movement was also developed in the right limbs. On the neurological examination, left shoulder myoclonus with slow velocity was observed. Twitching of the both shoulders and left leg occurred when the patient lifted the left arm.

Her brain MRI showed a high signal intensity on T2 imaging with enhancement in the right insula and frontal cortex. [Figure 1] The electroencephalogram and laboratory findings including COVID-19, paraneoplastic antibodies, herpes simplex virus polymerase chain reaction in cerebrospinal fluid were normal.

She was treated with levetiracetam 500mg twice a day and her myoclonus improved in part. Intravenous methylprednisolone was followed for the next 5 days, and her symptoms were completely resolved. Three weeks after discharge, her brain MRI showed that the right insula and frontal cortex lesion had almost disappeared.[Figure 2]

Conclusion: Since the outbreak of COVID-19, movement disorders related to COVID-19 or the COVID-19 vaccine have been reported. In most reported cases, they improved spontaneously or a result of immunotherapy such as steroid, IVIG, or plasmapheresis. Although the patient in the recent case did not have any symptoms suggestive of encephalitis, such as decreased consciousness or disorientation, we concluded her symptom was triggered by the COVID-19 vaccine. Her paresthesia and myoclonus developed 1 week after vaccination, and the encephalitis found on brain MRI.

References: HR Shun et al. Autoimmune Encephalitis as an Adverse Event of COVID-19 Vaccination. J Clin Neurol. 2022 Jan;18(1):114-116.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/myoclonus-and-encephalitis-as-an-adverse-event-of-mrna-covid-19-vaccine/