Reversible Isolated Accessory Nerve Palsy due to a Large Thrombosed Vertebral Aneurysm

OBJECTIVE: Isolated accessory nerve palsy due to intracranial disorders is uncommon because intracranial accessory nerve injury usually occurs in case of a skull base tumor or trauma, resulting in one of multiple cranial nerve palsies. We report a very rare case of isolated accessory nerve palsy due to a large thrombosed aneurysm of the intracranial vertebral artery. Full recovery was achieved after surgery. CASE REPORT: A patient complaining of transient numbness in the right side was referred to our hospital. An MRI indicated a large thrombosed aneurysm of the right vertebral artery. The aneurysm severely compressed the medulla oblongata. First, the proximal vertebral artery (VA) was clipped with an aneurysm clip to reduce the pressure inside the aneurysm. However, cerebral angiography revealed a partial recanalization of the right VA. The patient then underwent coil embolization of the right VA just proximal to the aneurysm clip. Subsequently, the right VA was completely obliterated. The patient was discharged without any neurological deficit. Two weeks later, however, she complained of right shoulder pain. Physical and neurological examinations demonstrated atrophy of the right trapezius and sternocleidomastoid muscle, leading to a deepening of the right supraclavicular fossa. The symptoms were considered to result from the right isolated accessory nerve palsy. Follow-up MRI showed that the VA aneurysm gradually decreased in size over a period of several months. At the same time, her symptoms disappeared completely. CONCLUSION: We should keep in mind that isolated accessory nerve palsy can be caused by a large or giant vertebral aneurysm.