Paraneoplastic motor neuron disorders (MND) are rare conditions; their exact clinical and electrophysiological phenotype have not been exhaustively described yet. The purpose of this study is to depict the main characteristics of paraneoplastic MND to highlight the features that may allow its diagnosis. Based on the description of eight original cases, and on the revision of 21 patients identified from a systematic review of the literature, the main features of paraneoplastic MND can be summarized as follows: (1) subacute; (2) lower motor neuron syndrome, associated or not with upper motor neuron involvement; (3) predominant asymmetric upper limb involvement; (4) presence of other non-motor neurological manifestations, including sensory neuronopathy; (5) signs of inflammation in the cerebrospinal fluid (CSF); (6) neurological improvement or stabilization after immunotherapy and tumor treatment. The diagnosis of paraneoplastic MND may be difficult because of its rarity, the absence of pathognomonic clinical features, and the frequent absence of prior tumor history. However, it is of capital importance to correctly identify patients with paraneoplastic MND, as this represents a potentially treatable condition. In the presence of subacute lower motor neuron impairment, especially when atypical clinical features for degenerative MND or other non-motor neurological manifestations are present, we recommend testing for onconeural antibodies. In the case, the search for onconeural antibodies is negative, but it exists a strong clinical suspicion for a paraneoplastic etiology; CSF analysis and total-body 18FDG-PET/CT imaging should be performed to circumstantiate diagnosis.
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