The laminar subarachnoid hemorrhage of infant venous stroke: Evidence of a 4th meningeal layer?

Background and Purpose: Subarachnoid hemorrhage is commonly associated with a venous stroke. In infants with a chronic subdural hemorrhage a laminar SAH is produced by blood leakage from thrombosed cortical veins into a potential subarachnoid space previously unrecognized. This paper discusses how to distinguish a laminar SAH from a subdural hemorrhage. Methods: A retrospective review of 34 selected cases of infant venous stroke in the period 2014-2025 is based on neuroimaging studies. The cases selected are from unsolicited referrals from parents, attorneys, and physicians where caregivers are accused of abusive head trauma. Results: The median age is 3 months old. All 34 infants exhibit a venous stroke defined as the presence of a blood clot(s) in a venous sinus or cortical vein or SAH from a leaking thrombosed cortical vein. 33/34 cases feature an associated chronic subdural hemorrhage/hygroma. Thrombosed veins are located subjacent to the arachnoid membrane. 28/34 cases show multifocal (<2cm diameter) SAHs. 24/34 cases show laminar SAHs (>2cm diameter); 10/24 thin (<3mm) and 14/24 thick (>3mm). Intrasulcal SAH is infrequent. Conclusion: The laminar SAH is a non-gyral SAH distinguished from a subdural hemorrhage by its location and rapid resolution. Its combination with the adjacent chronic subdural hemorrhage is commonly and incorrectly described as a mixed density or layered subdural hemorrhage. The laminar SAH is hypothetically located between the arachnoid membrane and a trabecular coat investing the cortical veins. Alternately, it may suggest existence of a 4th meningeal layer recently discovered by anatomists in animal models.