Abdominal Segmental Myoclonus Mimicking Belly Dancer Dyskinesias in CASPR2 Antibody Encephalomyelitis.

Abstract

An 86-year-old man presented with continuous movements of the abdominal wall (abdominal segmental myoclonus mimicking a belly dancer), gait unsteadiness, progressive cognitive deficit, episodic symptomatic hypotension, and diaphoresis for 2 months. Clinical examination confirmed cognitive dysfunction, and showed bilateral proximal muscle weakness of the upper and lower limbs and absent deep tendon reflexes. The Video shows the presence of spontaneous rhythmic, continuous, sinuous movements involving the abdominal muscles (Video 1). MRI of the spinal cord and the brain were normal. Electroencephalography showed generalized background slowing in the absence of epileptiform discharges. Polymyographic studies revealed rhythmic (about 2 Hz) electromyography bursts with variable duration of 500 to 1000 ms present with synchronous onset in the rectus abdominis and paravertebral muscles (Fig. 1). Detailed blood analysis was normal except for the presence of contactin-associated protein-like 2 (CASPR2) antibodies (IgG ≥1:40960). Cerebrospinal fluid analyses showed the presence of oligoclonal bands and CASPR2 antibody (IgG ≥1:160). Other anti-neuronal antibodies were absent, both on blood and cerebrospinal fluid analyses. Thoraco-abdominal-pelvic CT and whole-body FDG-PET did not show thymoma or other neoplasia. A diagnosis of CASPR2 antibody-related encephalomyelitis was made. Treatment with intravenous immunoglobulin and oral corticosteroids was started. Because of the absence of clinical improvement, plasma exchange and rituximab treatment were started leading to dramatic improvement of abdominal myoclonus but moderate effect on gait unsteadiness and cognitive deficit. CASPR2 is a membrane protein expressed in the central and peripheral nervous system, with its cytoplasmic domain essential for the function of potassium channel Kv1.1 clustering at the juxtaparanodes of myelinated axons. It is involved in neuronal excitability (i.e., stabilize conduction at nodes of Ranvier, and maintain resting potential) and synaptic transmission.1 CASPR2 antibodies induce a functional blocking of the potassium channel. CASPR2 antibody–associated disorders include acquired neuromyotonia, Morvan syndrome, limbic encephalitis, seizures and cerebellar ataxia. Diagnosis of CASPR2-related nervous system disorders is based on the clinical presentation and the presence of specific autoantibodies. Based on the earlier reported cases of CASPR2-associated encephalomyelitis, cerebral and spinal MRI is unremarkable in the majority of cases.2 T2/FLAIR hyperintensities can be observed in the medial temporal lobes especially in patients with cognitive deficit and seizures, and brainstem hyperintensities or cerebellar atrophy in patients presenting with ataxia. Focal (especially in the temporal regions) or diffuse slowing and sharp waves may be observed on electroencephalography. CASPR2 antibody-related encephalomyelitis are frequently associated with other autoimmune affections (including thyroiditis or myasthenia gravis), and paraneoplastic in a minority of cases (mainly related to thymoma).2 The most striking clinical feature in our case was the presence of rhythmic abdominal myoclonus resembling belly dance. Belly dancer myoclonus is a segmental myoclonus considered to be of spinal origin involving sometimes also diaphragm muscles. Lower limb myoclonus while standing or walking has been earlier reported as a movement disorder phenotype in CASPR2 antibody-related encephalomyelitis.3, 4 One patient with spinal myoclonus has been reported with clinical lower limb myoclonus associated with abdominal myoclonus only observed by electromyography.5 To the best of our knowledge, this is the first case of clinical abdominal myoclonus associated with CASPR2 antibodies. We would like to thank Drs Anne-Laurie Pinto and Sergio Muniz-Castrillo for the analysis of CSF samples. We would like to thank Dr Sarah Coudray for the attainment and interpretation of the electrophysiological data. (1) Research Project: A. Conception, B. Organization, C. Execution; (2) Statistical Analysis: A. Design, B. Execution, C. Review and Critique; (3) Manuscript: A. Writing of the first draft, B. Review and Critique. L.K.: 1A, 2C, 3A D.R.: 2C, 3B J.H.: 1C, 2B G.C.: 1A, 2C, 3A, 3B The present work respects ethical guidelines. The authors confirm that the approval of an institutional review board was not required for this work. Written consent was obtained for showing the patients' video. We confirm that we have read the Journal's position on issues involved in ethical publication and affirm that this work is consistent with those guidelines. The authors declare that there are no funding sources or conflicts of interest relevant to this work. The authors declare that there are no additional disclosures to report.