Objective: Autoimmune dysautonomia has been described in patients with positive nicotinic ganglionic (alpha-3) acetylcholine receptor antibodies (α3-AchR Ab). We present a case of myoclonus in a patient with subacute cognitive decline and elevated α3-AchR Ab level.

Background: α3-AchR mediates signal transmission across the synapses in all autonomic ganglia. Positive antibodies against α3-AchR are found in autoimmune autonomic ganglionopathy which can lead to severe diffuse autonomic failure. While there have been rare cases of movement disorders such as ataxia and parkinsonism associated with this antibody, myoclonus has not been previously reported.

Method: A 79-year-old male with no prior history of movement disorder was evaluated for progressively worsening cognitive decline and jerking that caused gait instability and falls. Examination revealed reduced short-term memory, intact cranial nerves, full strength in all four extremities, and reduced ankle reflexes. He required significant assistance to stand and ambulate. There was negative myoclonus seen with outstretched arm posture, but also truncal myoclonus and positive myoclonus of bilateral upper and lower extremities while the patient was sitting in the chair.

Results: Initial blood work revealed mild uremia and hypercapnia. MRI brain showed mild generalized parenchymal atrophy but no evidence of restricted diffusion or abnormal contrast enhancement. Continuous video EEG was negative for seizures or epileptic discharges. CSF 14-3-3 protein testing could not be performed due to the presence of blood in the CSF sample, but CSF RT-QUIC was negative. Serum paraneoplastic antibody evaluation revealed that α3-AchR Ab level was 187 (negative: < 55 pmol/L). CT chest, abdomen, and pelvis initially raised suspicion for a complex renal cyst, but PET CT was negative for malignancy. He received a five-day course of IV methylprednisolone and myoclonus significantly improved.

Conclusion: Elevated α3-AchR Ab level has been commonly associated with severe autonomic insufficiency and cases of autoimmune encephalitis have been described. While there have been rare reports of ataxia and parkinsonism associated with this antibody, we describe a case of myoclonus as a rare phenomenology seen with α3-AchR Ab mediated syndrome.

References: https://www.neurology.org/doi/10.1212/01.wnl.0000903380.14508.43

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MDS Abstracts - https://www.mdsabstracts.org/abstract/rare-phenomenology-with-nicotinic-ganglionic-acetylcholine-receptor-antibodies/