Isolated Abducens Nerve Palsy as the Sentinel Sign of Clival Metastasis in High-Grade Neuroendocrine Carcinoma: A Case Report

We report a rare case of clival metastasis from a high-grade neuroendocrine carcinoma (NEC) of unknown primary origin. A 71-year-old man, previously cured of prostate adenocarcinoma, presented with abdominal pain leading to the diagnosis of metastatic NEC (Ki67 >90%) involving the liver and bones. After three lines of chemotherapy and immunotherapy, he developed sudden diplopia and right abducens nerve palsy. A brain contrast-enhanced MRI revealed a clival mass compressing the sixth cranial nerve, which was undetectable on prior imaging. The patient received palliative radiotherapy and corticosteroids without a significant clinical response. While neuroendocrine neoplasms (NENs) frequently metastasize to visceral organs, clival involvement is exceptionally rare, particularly as a delayed complication. To our knowledge, this is the first reported case of NEC with clival metastasis, emphasizing the tumor's aggressiveness. This case underscores the critical role of advanced neuroimaging in detecting atypical presentations in patients with new neurological deficits, even under systemic therapy, and highlights the challenges of managing skull base metastases in aggressive malignancies. Radiotherapy may stabilize symptoms, but functional recovery remains limited in cases of prolonged nerve compression.