Bilateral Third Nerve Palsy in Mirror Aneurysms of the Posterior Communicating Arteries

BACKGROUND: Bilateral third cranial nerve palsy has only been reported in a handful of conditions including some with inflammatory, tumoural and vascular causes. An urgent imaging study is mandatory to rule out vascular aetiology, mainly aneurysmal subarachnoid haemorrhage (aSAH). CASE PRESENTATION: A 28-year-old Hispanic woman presented to the emergency department with a 21-day history of a sudden-onset and severe headache that on three occasions was accompanied by transient loss of awareness, the last episode occurring a week previously. The simple CT image showed minimal bleeding at the level of the perimesencephalic cisterns, with evidence of SAH. An angioCT revealed a 5×6 mm bilobed saccular aneurysm of the right posterior communicating artery and a 2×2 mm saccular aneurysm in the posterior left communicating artery. CONCLUSIONS: A mirror aneurysm is found in 2–25% of aSAH cases. To date there is no consensus about the optimal management of patients with these findings. LEARNING POINTS: The presence of third cranial nerve palsy should always raise the suspicion of an aneurysm. Subarachnoid haemorrhage is the most common cause of a thunderclap headache. Aneurysmal subarachnoid haemorrhage requires surgical management in all cases.