[A case of post-surgical inflammatory neuropathy with lower-extremity weakness after surgery for anorectal malignant melanoma that showed the effectiveness of immunotherapy].

A 59-year-old man with past histories of bronchial asthma and chronic sinusitis underwent transanal resection of anorectal malignant melanoma with general anesthesia. On the third day after surgery, he presented with subacute weakness with right dominant hypoesthesia in the bilateral lower limbs. Tendon reflexes were diminished without pathological reflexes. Blood examination showed increased eosinophils (2,058/μl) and elevated serum immunoglobulin E (675.0 IU/ml). Cerebrospinal fluid examination showed elevated protein (200 mg/dl) without pleocytosis (<5/μl). A nerve conduction study suggested multiple mononeuropathy with motor and axonal dominance in the right tibial, peroneal, and sural nerves. Because of eosinophilia and his past medical history (i.e., bronchial asthma and chronic sinusitis), we initially suspected eosinophilic polyangiitis granulomatosis (EGPA) as the cause of postoperative polyneuropathy. However, his neurological symptoms did not improve despite the decreased eosinophil count after tumor resection, which was inconsistent with EGPA. We biopsied the left sural nerve to exclude EGPA and make a diagnosis. Pathological findings revealed no demyelination, axonal degeneration, or eosinophil infiltration with granuloma formation; however, lymphocyte-dominated inflammation was observed around the epineurial small vessels. Thus, the patient was diagnosed with early onset post-surgical inflammatory neuropathy (PIN) based on his clinical course and the pathological findings. On post-surgery day 48, oral administration of prednisolone (40 mg/day) was started. His neurological symptoms improved quickly and remarkably. Our case suggests that, when multiple mononeuropathy develops early after surgery, PIN should be considered as a differential diagnosis to initiate appropriate treatment based on the pathological condition of neuropathy.