Paraneoplastic Brachial Amyotrophic Diplegia With Favorable Outcome and Anti-Ank3 Antibodies: A Case Report.

[OBJECTIVES] To report a case of paraneoplastic motor neuron disease (MND) with a favorable outcome after cancer treatment.

[METHODS] Clinical, electrophysiologic, radiologic, and serum/CSF study of a patient with brachial amyotrophic diplegia/flail-arm syndrome (BAS/FAS).

[RESULTS] A 68-year-old man presented with a subacute-onset upper limb weakness and atrophy evocative of BAD/FAS. Electrodiagnostic study (EDX) confirmed MND; CSF examination found oligoclonal bands and elevated neurofilament light chains (NfL). A prostate adenocarcinoma was concomitantly diagnosed in a context of dysuria and elevated prostate-specific antigen level. One month after onset of cancer treatment (double hormonotherapy), the patient's impairment started to improve. Subsequently, the muscle denervation signs on EDX disappeared and CSF NfL levels decreased. At last follow-up visit, 2 years after onset, he was asymptomatic and unimpaired. Anti-ankyrin-3 (Ank3) autoantibodies were detected in the CSF by tissue-based immunofluorescence and confirmed by cell-based assay.

[DISCUSSION] We herein describe an original case of subacute-onset BAD/FAS with evidence of CSF inflammatory changes, autoantibodies of unknown significance, and simultaneous diagnosis of prostate cancer suggesting a possible paraneoplastic neurological syndrome (PNS). The outcome was remarkably favorable after hormonotherapy. Even in the absence of usual onconeural antibodies, MND may be encountered among PNS, and their prognosis is not always poor.