Perimesencephalic subarachnoid hemorrhage (PSAH): This condition is characterized by bleeding into the subarachnoid space surrounding the midbrain, which is a common location for such bleeds. PSAH often has a more benign course compared to other types of subarachnoid hemorrhages and typically does not result from aneurysmal rupture, making it a likely diagnosis for a small bleed in this area.

Traumatic subarachnoid hemorrhage: Head trauma can cause bleeding into the subarachnoid space, including the perimesencephalic region. The history of trauma would support this diagnosis.
Arteriovenous malformation (AVM): Although less common, AVMs can cause bleeding in the perimesencephalic region. The presence of an AVM could be suggested by specific imaging findings or a history of similar events.
Cavernous malformation: These are vascular lesions that can bleed, and while less common, they should be considered in the differential diagnosis for a perimesencephalic bleed.

Aneurysmal subarachnoid hemorrhage: Although less likely than PSAH in the perimesencephalic region, aneurysmal rupture is a critical diagnosis that must not be missed due to its high morbidity and mortality. Ruptured aneurysms require urgent intervention.
Hypertensive intracerebral hemorrhage: While typically located in deeper brain structures, hypertensive hemorrhages can occasionally present in atypical locations, including the perimesencephalic region, especially in the context of severely elevated blood pressure.
Vasculitis: Inflammatory diseases affecting the blood vessels, such as vasculitis, can lead to bleeding and should be considered, especially if there are systemic symptoms or findings suggestive of an inflammatory process.

Moyamoya disease: A rare condition characterized by narrowing or blockage of the internal carotid artery and its branches, leading to the formation of a network of small, fragile blood vessels that can bleed.
Amyloid angiopathy: Typically associated with lobar hemorrhages in older adults, amyloid angiopathy can rarely cause bleeding in other locations, including the perimesencephalic region, though this would be uncommon.
Tumor-related bleeding: Certain brain tumors can bleed, and while rare, this possibility should be considered, especially if there are other symptoms or findings suggestive of a tumor.