Objective: N/a

Background: Myorhythmia is a hyperkinetic involuntary movement disorder characterized by slow rhythmic movements (1-4 Hz), typically affecting cranial and/or limb muscles. Recognizing the phenomenology of myorhythmia is crucial, as it often signifies an underlying etiology associated with lesions primarily in the brainstem and diencephalon. This condition has been linked to cerebrovascular, infectious and paraneoplastic factors. Notably myorhythmia has also been described in the context of several autoimmune processes including anti-NMDA encephalitis, von Economo’s encephalitis lethargica, and Hashimoto’s encephalopathy. However, to our knowledge, limb myorhythmia has not been documented in patients with Anti-LGI-1 encephalitis.

Method: N/a

Results: A 66-year-old woman presented with intractable seizures, cognitive decline, and hyponatremia. The combination of these symptoms and resistance to anti-seizure medications prompted further investigation, revealing positive serum and CSF anti-LGI-1 antibodies and bilateral hyperintensities in the hippocampi. Her seizure semiology consisted of focal unaware seizures with oral and limb automatisms arising from bitemporal regions. She was treated with intravenous methylprednisolone and IVIG, alongside Lamotrigine and Levetiracetam, resulting in seizure freedom. Several weeks later, the patient returned to the hospital due to ongoing cognitive decline and new involuntary left leg movements. These movements occurred mainly at rest, were uncomfortable, interfered with but did not persist during sleep, could not be suppressed and did not impact patient’s gait. Her neurological examination was normal, except for impaired immediate recall. Prolonged continuous video EEG monitoring showed slow (1-2 Hz) rhythmic oscillations in her left leg, primarily occurring while lying down and disappearing during sleep, with no corresponding EEG findings. A diagnosis of myorhythmia was established.

Conclusion: Although myorhythmia is frequently misdiagnosed as tremor or clonus, it is crucial to distinguish it from seizures (e.g epilepsia partialis continua), particularly in patients with a prior history of seizures secondary to encephalitis, as this distinction can have important treatment implications. Notably, a slower velocity and the disappearance of activity during sleep can be helpful distinguishing clues. This case highlights that limbic encephalitides can lead to atypical movement disorders in such patients.

References: Baizabal-Carvallo, J. F., Cardoso, F., & Jankovic, J. (2014). Myorhythmia: Phenomenology, etiology, and treatment. Movement Disorders, 30(2), 171–179. https://doi.org/10.1002/ mds.26093

Hoe, R. H. M., Yang, F., Shuit, S. K., Yong, G. K. W., Puah, S. H., Ting, J. S. J., Sadasiv, M. S., & Umapathi, T. (2023). Myorhythmia and other movement disorders in two patients with coronavirus disease 2019 encephalopathy. Journal of Movement Disorders, 16(2), 217–220. https://doi.org/10.14802/jmd.22215

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MDS Abstracts - https://www.mdsabstracts.org/abstract/limb-myorhythmia-in-a-patient-with-anti-lgi-1-encephalitis/