Opposing Sex Patterns in Hemorrhagic Stroke Cancel Out at the Population Level
Japan’s hemorrhagic stroke burden splits into two biologically and epidemiologically distinct diseases. In this integrated population-based study, intracerebral hemorrhage (ICH) was more common in men, while subarachnoid hemorrhage (SAH) was more common in women, yet the overall incidence of hemorrhagic stroke ended up nearly the same between sexes.
Among 4,517 patients with hemorrhagic stroke captured in the Shiga Stroke and Heart Attack Registry from 2011 through 2015, 3,461 cases were ICH and 1,056 were SAH. After age adjustment, ICH incidence measured 61.2 per 100,000 person-years in men and 49.8 in women. SAH moved in the opposite direction, with an incidence of 22.4 per 100,000 person-years in women and 10.4 in men. Those subtype-specific gaps are wide enough to matter on their own, but they do not translate into a sex gap for the combined endpoint: overall age-adjusted hemorrhagic stroke incidence was 71.6 per 100,000 person-years in men and 72.2 in women.
The result is a reminder that the brain does not present one uniform hemorrhagic phenotype. Deep parenchymal bleeding and aneurysmal or cisternal bleeding are driven by different vascular substrates, different age profiles, and likely different hormonal, hemodynamic, and structural influences. When those two entities are analyzed separately, sex disparity appears pronounced; when they are combined, the divergence largely disappears.
Why the Combined Endpoint Looks So Similar
The numerical balance in the combined incidence rate comes from the way the two subtypes distribute across sex. Men carried a larger share of ICH, the more common hemorrhagic stroke subtype overall. Women carried a larger share of SAH, which is less frequent than ICH but still substantial enough to offset the male excess in ICH when the categories are pooled.
This matters because clinicians and epidemiologists often think about hemorrhagic stroke as a single outcome. It is not. ICH and SAH occupy different ends of the cerebrovascular spectrum: ICH is typically tied to small-vessel disease, hypertensive arteriopathy, cerebral amyloid angiopathy in older patients, and other chronic vascular injury, while SAH reflects rupture of aneurysms or related vascular lesions. Sex differences may emerge through distinct pathways in each disorder, including vascular remodeling, arterial stiffness, blood pressure exposure, connective tissue biology, and differences in aneurysm formation or rupture risk.
By integrating the two subtypes rather than reporting them in isolation, the study exposes a simple epidemiologic truth: a population can show large sex differences inside each disease category while appearing nearly equal overall. For stroke systems planning, that means subtype-specific prevention strategies remain necessary even when broad hemorrhagic stroke rates look symmetric.
Registry-Based Incidence and a Large Regional Cohort
The analysis drew on first-ever ICH and SAH events recorded in a population-based registry covering roughly 1.4 million residents in Japan. Incidence estimates were calculated by sex and subtype, and age adjustment used the 2015 Japanese Census population. That design strengthens the value of the findings because it captures community-level disease occurrence rather than a narrow hospital sample.
The registry-based approach also gives the results practical credibility for cerebrovascular surveillance. Regional stroke registries are well suited to detect patterns that can be blurred in tertiary-care cohorts, where referral bias, age mix, and case severity may distort incidence estimates. Here, the data reflect what actually happened in a defined population, which is exactly the setting where sex-specific prevention policy has to work.
A particularly important feature is the study’s focus on first-ever events. Recurrent hemorrhage would complicate interpretation by layering prior disease burden onto incident vascular risk. By restricting to incident cases, the investigators isolated the initial population risk signal more cleanly.
What the Sex Split Suggests About Vascular Biology
The subtype pattern is consistent with different biological pressures acting on cerebral vessels in men and women. The higher ICH incidence in men fits with a burden of chronic vascular injury that may be amplified by higher lifetime exposure to hypertension or other vessel-damaging risk factors. The higher SAH incidence in women fits with prior epidemiologic observations that aneurysmal disease is not distributed evenly by sex and may involve sex-linked differences in vessel wall integrity, hormonal influences, or aneurysm susceptibility.
The striking point is not simply that the patterns differ. The point is that they move in opposite directions with enough magnitude to neutralize one another in the combined outcome. For hospitalists, neurologists, and stroke prevention teams, that means a patient-facing discussion of hemorrhagic stroke risk should not stop at the umbrella diagnosis. The likely hemorrhagic pathway matters.
There is also a prevention message hidden inside the incidence rates. If ICH prevention is more relevant to men in this setting, tighter attention to hypertension control and other small-vessel risk modifiers may have the largest yield there. If SAH prevention carries a larger sex-specific burden in women, aneurysm awareness and vascular risk assessment may deserve more emphasis in the populations where that pathology clusters.
Limits of Interpretation and the Need for Subtype-Specific Surveillance
This is a preprint, and the findings have not yet undergone peer review. The article also does not provide a deeper breakdown of mechanistic covariates, individual-level risk factors, treatment patterns, or aneurysm and hypertension history, so the observed sex differences cannot be traced to a single cause. The data availability statement further notes that the underlying data are not publicly available on ethical grounds.
Even with those limits, the epidemiologic message is durable. Broad hemorrhagic stroke totals can conceal opposing subtype forces, and sex analyses that ignore ICH and SAH separately may miss the true structure of risk. In a field where prevention depends on identifying which vessel bed is failing and in whom, that is a consequential oversight.
The next step is not to ask whether men or women have more hemorrhagic stroke in the abstract. It is to ask which hemorrhagic stroke, at what age, and through what vascular pathway.