A Case of Paraneoplastic Neurological Syndrome Associated with Breast Cancer Detected while Searching for the Cause of Involuntary Movement.

Our case involved a 66-year-old woman who noticed progressive asymmetric involuntary movement, difficulty speaking, and difficulty swallowing. The patient fractured her femur due to a lower extremity involuntary movement while walking. During the course of her treatment for the fracture, her neurological symptoms worsened. Approximately 2 months after becoming aware of her symptoms, she visited our clinic for evaluation of difficulty with unassisted walking and weight loss due to dysphagia. To identify the cause of her neurological symptoms, hematological examination, brain magnetic resonance imaging, single-photon emission computed tomography for cerebral blood flow, electroencephalography, and a somatosensory evoked potential test were conducted. Although the cause of her neurological symptoms could not be determined, computed tomography revealed the presence of breast cancer, which led us to suspect paraneoplastic neurological syndrome (PNS). After breast cancer treatment, her neurological symptoms improved simultaneously. Therefore, the patient was retrospectively diagnosed with PNS. We report a case of PNS whose neurological symptoms followed a subacute course and were relieved after breast cancer treatment.