What is the recurrence risk of subarachnoid hemorrhage after a benign perimesencephalic non‑aneurysmal subarachnoid hemorrhage?

Recurrence Risk of Perimesencephalic Non-Aneurysmal SAH

The recurrence risk after benign perimesencephalic non-aneurysmal subarachnoid hemorrhage (PNSAH) is extremely low, approaching zero but not absolute zero—patients can be counseled that while recurrence is exceptionally rare, it remains theoretically possible.

Understanding the Benign Nature of PNSAH

Perimesencephalic non-aneurysmal SAH represents a distinct clinical entity that differs fundamentally from aneurysmal SAH in both prognosis and recurrence risk. The condition is characterized by blood centered around the midbrain on CT imaging with consistently negative angiography 1, 2.

Key distinguishing features include:

• Favorable clinical presentation: 93% of PNSAH patients present in Hunt-Hess grade I-II, compared to only 70.8% of patients with aneurysmal SAH 3
• Uncomplicated clinical course: Patients experience minimal complications and excellent functional outcomes 3
• Benign natural history: The condition carries a fundamentally different prognosis than aneurysmal SAH 4, 5

Quantifying the Recurrence Risk

The recurrence risk is extraordinarily low but not zero. While earlier literature suggested no risk of recurrence, case reports have documented rare instances of rebleeding 4, 5, 6.

Documented Recurrence Cases

• First reported case: A 62-year-old man experienced recurrent PNSAH within 5 months, representing the first conclusive report of idiopathic recurrent PNSAH 5
• Delayed recurrence: One case documented ultra-late recurrence after 12 years in a patient not on antithrombotic therapy 6
• Early recurrence: A 56-year-old hypertensive patient experienced rebleeding 1 month after initial ictus 4

Statistical Perspective

If the two hemorrhagic events are assumed to be random and independent, binomial statistics suggest approximately a 79 per billion chance of two or more episodes occurring over an 80-year lifetime 6. This underscores the exceptional rarity of recurrence.

Contrast with Aneurysmal SAH

The recurrence risk in PNSAH is dramatically lower than aneurysmal SAH:

• Untreated aneurysmal SAH: 20-30% rebleeding risk in the first month, then 3% per year long-term 1
• Ultraearly aneurysmal rebleeding: Up to 15% within 24 hours, with 70% of rebleeds occurring within 2 hours 1, 2
• PNSAH: Near-zero recurrence risk with only isolated case reports over decades 4, 5, 6

Clinical Management Implications

Diagnostic Confirmation

Ensure true PNSAH diagnosis before counseling on recurrence risk:

• Blood pattern must be centered ventral to the midbrain/pons or in the quadrigeminal cistern 1, 7
• Negative four-vessel digital subtraction angiography is mandatory 1, 2
• Quadrigeminal variant comprises up to 22% of PNSAH cases and carries similar benign prognosis 7

Patient Counseling

Patients should be informed that:

• The risk of recurrence is close to zero but not absolute zero 6
• Recurrence, while possible, is exceptionally rare with only isolated case reports in the literature 4, 5, 6
• The prognosis remains excellent even in the rare event of recurrence 4, 5

Follow-Up Strategy

A conservative approach with close follow-up is warranted:

• No aggressive intervention or aneurysm treatment is required 4
• Patients should be monitored for complications during the acute phase 3
• Long-term surveillance imaging is not routinely indicated given the benign natural history 3

Risk Factors for Recurrence

Potential contributing factors in documented recurrence cases include:

• Hypertension (present in at least one recurrence case) 4
• Exertional activities preceding hemorrhagic events 5
• Possible occult tiny vascular lesions not visible on imaging 6
• Particular venous anatomy that may predispose to recurrent hemorrhage 6

Common Pitfalls to Avoid

Do not confuse PNSAH with aneurysmal SAH management:

• PNSAH does not require aneurysm repair or nimodipine administration 8
• The aggressive monitoring and treatment protocols for aneurysmal SAH are not indicated 2

Ensure complete diagnostic workup:

• Inadequate angiography may miss small aneurysms, leading to misclassification 1, 2
• Repeat angiography should be considered if initial study quality is suboptimal 5, 6

Avoid overly reassuring patients:

• While the risk is near-zero, telling patients there is "no risk" is inaccurate given documented recurrence cases 6