[en] OBJECTIVE: We present a case of brachial plexus avulsion and reconstructive surgery with cerebrospinal fluid leak between the cervical subarachnoid space and the pleural cavity responsible for tonsillar herniation and syringomyelia. CLINICAL PRESENTATION: A 17-year-old man presented with headaches when he was positioned upright, simultaneously with a persistent right pleural effusion for about 4 months after reconstructive surgery for a right brachial plexus avulsion. In addition, the headaches had worsened considerably after two aspirations of the pleural effusion. Magnetic resonance imaging (MRI) demonstrated signs of chronic intracranial hypotension and tonsillar herniation with a presyrinx cavity from vertebral level C1 to C7. None of those abnormalities were seen on the MRI scan obtained a few days after the initial trauma 7 months previously. Plexus brachial MRI confirmed the presence of a cerebrospinal fluid leak between the avulsed root of C8 and the pulmonary apex. INTERVENTION: The leak was treated by surgical closure of the dural tear of the C8 root. Postoperatively, the patient's headaches immediately resolved, and MRI 4 months later showed resolution of cerebellar tonsil herniation and regression of the syrinx. CONCLUSION: Resolution of acquired tonsillar herniation and syringomyelia can be achieved by closure of the dural tear responsible of the cerebrospinal fluid leak.