Upper motor neuron-predominant motor neuron disease: a novel immunotherapy-responsive association of GAD65 autoimmunity.

backgroundAutoimmune disorders can present as motor neuronopathies and need to be excluded prior to the diagnosis of amyotrophic lateral sclerosis (ALS). We aimed to characterize the clinical phenotypes of patients with motor neuron disease (MND) in the context of high-titer serum/CSF GAD65 antibodies (radioimmunoassay).

methodsA retrospective review of all Mayo patients (between 1/1/2003 and 12/31/2023) with motor neuronopathy and co-existing high-titer GAD65 antibodies (≥ 20 nmol/L in serum [equivalent to > 10,000 IU, ELISA] or detection in CSF) was performed. Clinical phenotypes and outcomes were compared with ALS patients diagnosed in the last 5 years (1/1/2019-12/31/2023) who tested negative for GAD65 IgG.

resultsWe identified 12 patients with high-titer GAD65 IgG and motor neuronopathy, who often had lower back spasms, history of an exaggerated startle response with immunotherapy responsiveness as compared to ALS patients. On further analysis, a subgroup of these patients with neurogenic changes on EMG, had an upper motor neuron (UMN) predominant syndrome (58%), with history of exaggerated startle (57%), lower back spasms (43%), tandem gait impairment (86%) and UMN bladder symptoms (71%) that were significantly different from the ALS controls. The UMN predominant GAD65 MN responded favorably to immunotherapy with stable electromyography; significantly lesser worsening in mRS and mortality on long-term follow-up. DISCUSSION: An upper motor neuron predominant motor neuronopathy is a distinct manifestation of GAD65 autoimmunity. Co-existing symptoms like exaggerated startle response, lower back spasms, impaired tandem gait, and UMN bladder signs might warrant consideration of an immunotherapy trial, which could yield favorable results.