Atlantoaxial synovial cyst causing isolated unilateral hypoglossal nerve paralysis

Cervical synovial cysts are rare entities that are, most of the time, asymptomatic. They can cause nerve root or spinal cord compression, especially when acute haemorrhage or a marked increase in size occurs. Isolated unilateral hypoglossal nerve paralysis caused by compression of its cisternal segment is also an extremely rare condition. We report the case of a 51-year-old woman who presented with dysarthria and tongue fasciculation. MRI revealed an atlantoaxial synovial cyst that extended cranially through the hypoglossal canal and compressed the fibres of the left XII nerve on its cisternal segment. To our knowledge, this is the first case report of XII nerve paralysis being caused by an atlantoaxial synovial cyst.