Case Presentation: A 74-year-old Hispanic male patient with a history of monoclonal gammopathy of undetermined significance presented with worsening right upper extremity (RUE) motor weakness for the last few months. This was present in an ulnar nerve distribution with muscle wasting in the hypothenar and fourth and fifth metacarpal areas. Blood work was notable for an increased Kappa (845 mg)/Lamda (13 mg) ratio of 65, as well for an M-spike of 2.2 g and total IgG of 2.5 g. The subsequent bone marrow biopsy showed 20-30% plasma cells; however, full-body PET and CT scans showed no evidence of malignancy or lytic skeletal disease. X-ray and MRI of the RUE were negative, but EMG showed an 80% decrease in motor amplitude. A fat pad biopsy testing for amyloidosis was negative, and his paraneoplastic antibody panel was also negative. This primarily negative work-up led to a diagnosis of Parsonage-Turner syndrome. His symptoms resolved following treatment with four months of daratumumab/hyaluronidase, lenalidomide, bortezomib, and dexamethasone therapy. Normalization of his lab values and remission of his myeloma on bone marrow biopsy also occurred.

Discussion: While paraneoplastic neurological syndromes (PNS) are exceedingly rare, they can be the presenting symptoms of malignancies such as myeloma [1]. The cancers most often associated with PNS are myeloma, lung, B-cell lymphoma, and Waldenstrom’s macroglobulinemia [1]. These syndromes result from malignant cells producing hormones, cytokines, peptides, or antibodies that can affect multiple organ systems and disrupt various neurological, dermatological, and vascular processes [2]. Possible symptoms of PNS include sensory loss, neuropathic pain, muscle weakness, myoclonus, and sensory ataxia [3]. When peripheral nerves are affected, the resulting neuropathies can be primarily sensory, motor, or mixed, with purely sensory neuropathies being the most common [3]. Parsonage-Turner syndrome, or brachial plexus neuritis, is a PNS characterized by the rapid onset of severe pain in the shoulder and arm, followed by wasting and weakness of the involved muscles [4]. These symptoms can occur throughout the distribution of the brachial plexus, affecting movement and sensation in the shoulders, arms, elbows, hands, and wrists. The etiology of Parsonage-Turner syndrome is poorly understood but is thought to be an immune-mediated inflammatory response to an infectious, malignant, or environmental trigger [4]. Treatment typically involves physical therapy, pain management, and lifestyle modifications, which can result in the spontaneous resolution of symptoms [4]. However, resolution may require treatment of the underlying malignancy if it’s responsible for the causative inflammation.

Conclusions: Paraneoplastic neurological syndromes should be considered in the differential for neurological symptoms in the setting of underlying malignancy, especially when imaging fails to reveal other causes. These syndromes often resolve with treatment of the underlying malignancy.