The laminar subarachnoid hemorrhage of infant venous stroke: Evidence of a 4 th 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); 10/24 thin (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.