A 74-year-old man presented to our hospital with involuntary and painful contractions in his right pectoralis muscles for the past 6 months. The muscle contractions persisted predominantly at rest, stopped during sleep, and could be relieved by active muscle forcing. Carbamazepine, gabapentin, clonazepam, levodopa, and topiramate therapy were ineffective for controlling symptoms. The patient denied a history of trauma, tumor, surgery, or prior exposure to any toxicant.

On admission, physical examination revealed rhythmic movements of his right pectoralis muscles. No other neurological signs were found. The involuntary contractions could be observed in both the pectoralis major and minor muscles on B-mode ultrasonography (Video). Electromyography of the pectoralis major showed 1.5-Hz rhythmic discharges of motor units, each lasting approximately 250 milliseconds (Figure). Electrolyte, creatine kinase, and thyroid hormone levels were normal. Antinuclear antibody test results were negative. Cancer screening, including the serum tumor markers and a whole-body computed tomography scan, revealed no evident abnormalities. Brain, cervical spinal cord, and brachial plexus magnetic resonance imaging results and the electroencephalogram results were also unremarkable. However, an elevated cerebrospinal fluid (CSF) protein level (1317.4 mg/L) with a normal cell count (3 × 10⁶/L) was observed. Furthermore, autoimmune encephalitis and paraneoplastic syndrome–related antibody profiles (MT333-16, MT389-16, and MT152-16 [MyBiotech Co, Ltd]) were screened. As a result, an anti-Tr or Delta/notchlike epidermal growth factor–related receptor (DNER) antibody was identified by immunoblot in the patient’s serum, but not in his CSF. Due to the elevated protein level in his CSF and the presence of anti-Tr antibody in his serum, methylprednisolone pulse therapy was administered, followed by oral prednisone, 1 mg/kg, daily for 1 month, tapering by 5 mg per week. After 3 months of treatment, the involuntary contractions in the patient’s right pectoralis muscles were effectively alleviated (Video).

Myorhythmia is defined as repetitive, rhythmic, slow (1-4 Hz), and involuntary muscle contractions with a characteristic duration of 200 to 300 milliseconds per burst. Myorhythmia mainly affects cranial and limb muscles, often presenting at rest and suppressed by sleep.¹ This differs from irregular and sudden myoclonic jerks and could be differentiated from classic tremors due to the absence of antagonist muscle movement.² Myorhythmia is believed to be a centrally mediated movement disorder arising from lesions involving the brainstem, thalamus, or other subcortical structures.¹ Symptomatic treatment with carbamazepine, benzodiazepines, or levodopa is usually ineffective. Unless the underlying cause is resolved, myorhythmia generally has a poor prognosis.² The Tr antigen is the extracellular domain of DNER, which is expressed almost exclusively by Purkinje cells.^3,4 Rapid, progressive cerebellar ataxia is the most common clinical manifestation in patients with anti-Tr antibodies in their serum or CSF, which is classically associated with a history of Hodgkin lymphoma.³ However, extracerebellar presentations, such as encephalopathy or peripheral neuropathy, could also be identified. As the Guillain-Mollaret triangle is believed to be an important anatomical substrate in myorhythmia,¹ we hypothesize that myorhythmia might be a rare clinical manifestation of the presence of anti-Tr antibodies in patients, which warrants further investigation.

Corresponding Authors: Cuiping Zhao, MD, PhD, Department of Neurology, Qilu Hospital of Shandong University (Qingdao), Cheeloo College of Medicine, Shandong University, 758 Hefei Rd, Qingdao, Shandong 266035, China (zhaocuipingzsu@126.com); Yuying Zhao, MD, PhD, Research Institute of Neuromuscular and Neurodegenerative Diseases and Department of Neurology, Qilu Hospital, Cheeloo College of Medicine, Shandong University, Jinan 250012, China (zyy72@126.com).

Published Online: July 1, 2024. doi:10.1001/jamaneurol.2024.2008

Conflict of Interest Disclosures: None reported.

Additional Contributions: We thank the patient for granting permission to publish this information.