. 2023 Nov 1;81(1):151–161. doi:10.1161/HYPERTENSIONAHA.123.21628

Tissue Compliance and Intracranial Pressure Responses to Large Intracerebral Hemorrhage in Young and Aged Spontaneously Hypertensive Rats

Cassandra M Wilkinson¹,Anna CJ Kalisvaart^1,^✉,Tiffany FC Kung¹,Ashley H Abrahart¹,Elmira Khiabani¹,Frederick Colbourne^1,²

¹Department of Psychology (C.M.W., A.C.J.K., T.F.C.K., A.H.A., E.K., F.C.), University of Alberta, Edmonton, Canada.

²Neuroscience and Mental Health Institute (F.C.), University of Alberta, Edmonton, Canada.

^✉

Correspondence to: Anna C.J. Kalisvaart, Department of Psychology, University of Alberta, P217 Biological Sciences Bldg, Edmonton, AB, T6G 2E9, Canada. Emailakalisva@ualberta.ca

^✉

Corresponding author.

Received 2023 Jun 8; Accepted 2023 Oct 12; Issue date 2024 Jan.

Hypertension is published on behalf of the American Heart Association, Inc., by Wolters Kluwer Health, Inc. This is an open access article under the terms of theCreative Commons Attribution Non-Commercial-NoDerivs License, which permits use, distribution, and reproduction in any medium, provided that the original work is properly cited, the use is noncommercial, and no modifications or adaptations are made.

PMC Copyright notice

PMCID: PMC10734784 PMID:37909235

Abstract

BACKGROUND:

After a large intracerebral hemorrhage (ICH), the hematoma and swelling cause intracranial pressure (ICP) to increase, sometimes causing brain herniation and death. This is partly countered by widespread tissue compliance, an acute decrease in tissue volume distal to the stroke, at least in young healthy animals. Intracranial compensation dynamics seem to vary with age, but there is no data on old animals or those with hypertension, major factors influencing ICH risk and outcome.

METHODS:

We assessed hematoma volume, edema, ICP, and functional deficits in young and aged spontaneously hypertensive rats (SHRs) and young normotensive control strains after collagenase-induced ICH. Macroscopic and microscopic brain volume fractions, such as contralateral hemisphere volume, cortical thickness, and neuronal morphology, were assessed via histological and stereological techniques.

RESULTS:

Hematoma volume was 52% larger in young versus aged SHRs; surprisingly, aged SHRs still experienced proportionally worse outcomes following ICH, with 2× greater elevations in edema and ICP relative to bleed volume and 3× the degree of tissue compliance. Aged SHRs also experienced equivalent neurological deficits following ICH compared with their younger counterparts, despite the lack of significant age-related behavioral effects. Importantly, tissue compliance occurred across strains and age groups and was not impaired by hypertension or old age.

CONCLUSIONS:

Aged SHRs show considerable capacity for tissue compliance following ICH and seem to rely on such mechanisms more heavily in settings of elevated ICP. Therefore, the ICP compensation response to ICH mass effect varies across the lifespan according to risk factors such as chronic hypertension.

Keywords: aging, cerebral hemorrhage, edema, hypertension, intracranial pressure

NOVELTY AND RELEVANCE.

What Is New?

Using the collagenase model of intracerebral hemorrhage in rodents, we demonstrate that brain tissue volume is not immutable in the face of substantial intracranial mass effect, regardless of rodent strain, age, or comorbid hypertension; these findings challenge common interpretations arising from the centuries-old Monro-Kellie doctrine.

What Is Relevant?

Aged spontaneously hypertensive rats experience proportionally worse outcomes versus their younger counterparts following intracerebral hemorrhage, a highly translationally relevant finding. Compensatory strategies for intracranial mass effect seem to be differentially affected by age and comorbid hypertension with important implications on poststroke outcomes.

Clinical/Pathophysiological Implications?

The degree to which each intracranial compensatory mechanism can be engaged in response to severe mass effects may vary across the lifespan and comorbidity. This has therapeutic implications, as manipulating tissue compliance may be a novel treatment approach to intracranial pressure management following severe stroke or brain injury.

Primary intracerebral hemorrhage (ICH) is largely caused by amyloid angiopathy and hypertension, conditions that are increasingly common with age.^1–4 Estimates for the prevalence of hypertension are as high as 70% to 80% in those aged 65 years, defined as blood pressure over 140/90 mm Hg.⁵ Approximately 60% of ICHs are considered hypertensive in origin,⁶ and ≈35% of patients with ICH are >80 years of age.⁷ In moderate-to-severe cases of ICH, the added mass of an intraparenchymal bleed and the development of perihematomal edema elevate intracranial pressure (ICP) levels, worsening secondary injury and increasing risk of patient fatality.⁸ As the ICH mass effect grows, the physiological mechanisms that provide compensatory pressure-volume reserves can quickly be overwhelmed, allowing ICP to rise in a decompensated manner with potentially harmful consequences (eg, tissue ischemia, brain herniation).^9,10 This situation may be particularly exacerbated in aged patients with hypertensive ICH, as persistent increases in cerebral blood flow and volume could worsen ICP decompensation, though there is a lack of sufficient data in this regard.¹¹

The 1783 Monro-Kellie neurosurgical doctrine traditionally asserts that an added mass (such as a hematoma) within the limited volume of the intracranial vault must be compensated for by displacement of cerebral blood and cerebrospinal fluid (CSF), while brain tissue remains immutable, providing no measure of pressure-volume compliance.¹² Modern interpretations of this doctrine now incorporate the current understanding of cerebral autoregulation and CSF dynamics (eg, arterial inflow, venous outflow, and CSF production, circulation, outflow),^12,13 yet brain volume is often still held as a constant despite growing evidence about the capacity for brain cell mechanotransduction and dynamic cell volume regulation.^14,15 We have also demonstrated in vivo that following an injury with moderate to severe associated mass effect, such as ICH or middle cerebral artery occlusion, a brain tissue compliance effect occurs. This is an acute phenomenon in which brain cells within uninjured distal brain regions transiently reduce in volume and pack closer together, manifesting as a total reduction in brain parenchymal volume as an adaptive response to limit direct (mechanical) and indirect cellular damage (eg, ischemia caused by high ICP).^16,17 Exploring how tissue compliance and ICP dynamics unfold following ICH in the setting of old age and comorbid hypertension now remains an important translational step.

Age and chronic hypertension result in gradual remodeling of arteries and arterioles over time so that they have a smaller lumen diameter and increase in the wall-to-lumen ratio. These vessels become stiff and arteriosclerotic in the process, leaving aged patients with elevated stroke risk and reduced autoregulatory efficiency.^18,19 Accordingly, hypertension has been independently associated with cerebral atrophy, altered neural metabolism, and dysfunctional cerebral blood flow regulation, which may affect CSF dynamics and glymphatic transport.²⁰ In preclinical work, spontaneously hypertensive rats (SHRs) are commonly used to model chronic hypertension.²¹ Notably, SHRs also experience decreased cortical volume, reduced neuronal density, and decreased dendritic density across many brain regions (eg, early cerebral atrophy).^22–24 Likely, these factors associated with hypertensive aging would alter ICP compensation dynamics in response to stroke-related mass effect.

Mean ICP tends to decrease with age by ≈0.7 mm Hg per decade on average (average ICP=≈10 mm Hg in adults),²⁵ possibly due to age-related cerebral atrophy. However, ICP monitoring is not standard after ICH, so there is limited data on how ICP differs among patient subpopulations, such as with age or comorbid hypertension.^26,27 In adult rats, mean ICP is typically ≈5 mm Hg, which can vary widely among studies depending on methodology,⁹ and few have recorded ICP in aged rats following stroke.^28,29 Research using SHRs to model ICH has shown that young SHRs experience worse neurological deficits and neuronal death following both the autologous whole blood and collagenase models of ICH, while hematoma volume and brain edema remain consistent between SHRs and controls.³⁰ However, to our knowledge, no study has yet assessed collagenase-induced ICH in aged SHRs or recorded ICP in aged SHRs following ICH. One study measured ICP in aged naive SHRs.³¹ Another study assessed brain injury using the collagenase and whole blood models in SHRs, but these rats were only 3 months old.³⁰

Therefore, we conducted 3 experiments exploring how age and hypertension affect tissue compliance and ICP following severe ICH. For experiment 1, we used the SHR strain and 2 control strains: Sprague-Dawley rats (SDRs) that undergo tissue compliance^16,17 and Wistar-Kyoto rats (WKRs), the strain from which the SHR strain was bred.³² Our primary objective for experiment 1 was to assess differences in brain morphology across strains and investigate whether tissue compliance is an ICH response unique to SDRs (by assessment of macroscopic and microscopic brain volume fractions in comparison to shams). For experiments 2 and 3, we used young (2–3 months) and aged (20–24 months) SHRs. Our primary objectives for experiments 2 and 3 were to compare how edema, tissue compliance, and ICP responses following ICH vary with age and comorbid hypertension, while our secondary objective was to compare functional deficits. For all experiments, we used the well-established collagenase model of ICH, as it causes true cerebral bleeding^33,34 which we expected to be worsened by hypertension.

METHODS

Data Availability

The authors declare that all data are available within supportingSupplemental Material, which also contains detailed methods and material sources. Timelines and end points collected for each of the 3 experiments are shown in Figure1.

Figure 1. — **Experimental timelines. A**, In experiment 1, young adult Sprague-Dawley rats (SDRs), Wistar-Kyoto rats (WKRs), and spontaneously hypertensive rats (SHRs) were randomized into intracerebral hemorrhage (ICH) or sham groups with a random subset of animals within each group implanted with core temperature probes. Following a 4-day recovery period, baseline neurological deficit scale (NDS) data were collected, and animals were given either ICH or sham procedure, followed by euthanization at 24 hours following collection of post-ICH NDS data. Brain tissue was then taken for morphological assessment via histology and stereology.B, In experiment 2, young adult or aged SHRs were randomized into ICH or sham groups, and baseline NDS data were collected. Following the ICH or sham procedure, animals were euthanized 24 hours later, following the collection of post-ICH NDS scores. Brain tissue was again taken for morphological assessment via histology and stereology.C, In experiment 3, young adult and aged SHRs were randomized to ICH or sham procedure, and baseline NDS data were collected. At the time of ICH or sham procedure, intracranial pressure (ICP) probes were also implanted, followed by a 72-hour survival period and euthanasia for other experimental end points. Arterial blood was collected via cardiac puncture at euthanasia for blood gases assessment, and brain tissue was divided by hemisphere for assessment of edema (as approximated by brain water content percentage [BWC%]).

Subjects

All procedures complied with the Canadian Council on Animal Care Guidelines and were approved by a University of Alberta Animal Care and Use Committee (Protocol 960). All experiments were also conducted in compliance with Animal Research: Reporting of In Vivo Experiments (ARRIVE) guidelines.³⁵ We used 160 male rats, randomly assigned within each experiment usingrandom.org (experiment 1: 20 SDRs, 20 WKRs, and 20 SHRs, plus an additional 12 SHRs for pilot work; experiments 2 and 3: 44 young SHRs and 44 aged SHRs).

Statistical Analysis

The co-first authors have full access to all of the data and take responsibility for its integrity and data analysis. Power and effect sizes (Cohen f) were calculated using GPower (v. 3.1.9.6). An a priori sample size calculation determined that 9 rats per group would give ≈80% power to detect a 35% change in cell volume based on an SD of 443 μm³ and a mean of 1791 μm³ in SHRs (pilot data). We increased this to 10 per group in experiment 1 and 12 per group in experiment 2 to mitigate data loss from mortality or exclusions. For experiment 3, we used 10 per group to have 80% a priori power to detect a 1% change in edema. Data were analyzed using GraphPad Prism (v.9.0) and are presented as mean±95% CI, with the exception of neurological deficit scale (NDS) scores, which are presented as median and interquartile range. NDS scores were assessed using Kruskal-Wallis and Mann-WhitneyU test. Hematoma volume was assessed using either an unpaired student’st test or 1-way ANOVA test. Data collected at a single time (eg, brain volume, muscle water content) were assessed using a 2-way between-subjects ANOVA with Sidak’s multiple comparisons test to assess effects of strain/age and injury (ICH versus sham). Data collected at multiple times (eg, food consumption and body weight) were assessed using a mixed effects analysis with the Tukey’s multiple comparisons test. Time was a within-subjects factor; strain/age and injury were between-subject factors. Mortality among groups was assessed using the Pearson χ² test. AP value of <0.05 was considered statistically significant.

RESULTS

For detailed information about statistics, refer toTables S1-S4.

Experimental Exclusions and Mortality

In experiment 1, 3 SHRs in the ICH group died prematurely (strain difference where SDR and WKR groups had no mortality;P=0.036). In experiment 2, 1 animal died prematurely from the young ICH group. In experiment 3, 5 animals died prematurely, all from the young ICH group (P=0.001). Further information about individual end point exclusions (based on a priori exclusion criteria) can be found inSupplemental Material.

Experiment 1 Results

NDS Scores

ICH caused neurological impairments (Figure2A), but there were no significant differences across strains.

Hematoma Volume

The young SHRs and WKRs had larger hematomas compared with the SDRs, but these differences were not statistically significant (Figure2B). The SHR and WKR groups had significantly higher rates of ventricular extension (Figure2B). Refer toSupplemental Material for representative images of coronal sections from each strain (Figure S1A and S1B)

Histology and Stereology

Overall, while WKRs had significantly larger contralateral hemisphere volumes compared with both SHRs and SDRs, animals with ICH experienced significant shrinkage of the contralateral hemisphere (parenchymal tissue only) compared with sham groups (Figure2C), regardless of strain. In SHR shams, contralateral ventricle volumes were significantly larger compared with other strains, and SHRs were also the only group to experience a significant reduction in ventricle volume following ICH (Figure2D). Ventricle volume in SDRs and WKRs following ICH did not differ significantly across individuals who experienced ventricular extension versus those who did not (Table S3). Overall, SHRs had significantly thinner cortices compared with SDRs and WKRs, but all strains had a significant reduction in cortical thickness following ICH compared with shams (Figure2E).

Contralateral CA1 (hippocampal subfield cornu ammonis layer 1) neuron volumes were significantly smaller following ICH across strains, though SDRs had significantly larger cell volumes on average across groups compared with WKRs and SDRs (Figure3A). Though contralateral CA1 neuron density differed across strains, with WKRs possessing a significantly higher CA1 neuron density compared with SHRs, all strains had significantly higher CA1 neuronal densities following ICH (Figure3B). Similarly, in contralateral S1 (primary somatosensory cortex), neuron volumes following ICH were significantly lower overall compared with shams, despite differences among strains within sham groups where SDRs had the largest S1 neuron volumes, followed by WKRs, and then SHRs (Figure3C). Contralateral S1 neuronal density also increased in ICH groups compared with shams, across all strains (Figure3D). There was no effect of strain or ICH on the volume and density of lobe VI cerebellar neurons (Table S3). This finding helps exclude possible systemic confounds that could impact brain volume regulation and demonstrates that there is a spatial limit to tissue compliance. Overall, despite some preexisting morphological differences, all strains exhibited some degree of tissue compliance following ICH, as evidenced by both macroscopic and microscopic assessments of parenchymal brain volume fractions.

Figure 3. — **Experiment stereology (strain comparison). A**, Contralateral CA1 (hippocampal subfield cornu ammonis layer 1) cell volume was significantly smaller across all strains following intracerebral hemorrhage (ICH).B, ICH animals had a significantly higher CA1 cell density compared with shams.C, Cell volume in contralateral S1 (primary somatosensory cortex) was significantly decreased in ICH animals compared with shams.D, S1 cell density was also significantly higher in ICH animals compared with shams. SDR indicates Sprague-Dawley rat; SHR, spontaneously hypertensive rat; and WKR, Wistar-Kyoto rat. *P<0.05, †P<0.001, ‡P<0.0001, and §P<0.01 for direct comparisons or those completed after significant interactions in the ANOVA.

Feeding, Drinking, Body Weight, and Muscle Water

Following ICH, rats decreased their food and water consumption, regardless of strain; however, SDRs drank and ate more on average across experimental groups (Figure S2A and S2B). This difference in feeding behavior for SDRs is also reflected by their weight. All strains assigned to sham groups maintained a consistent weight, although all strains in the ICH experienced a similar degree of weight loss compared with baseline after stroke (Figure S2C). Note that rodent allometry studies demonstrate a small increase in brain size in rodents who gain weight, whereas those who lose weight tend to maintain steady-state brain volume.³⁶

There was a significant interaction between the experimental group and strain for abdominal muscle water content (Figure S2D); however, there were no significant posthoc comparisons. This significant interaction was likely driven by slightly increased muscle water content in the SDR sham group. Importantly, the lack of experimental group effects suggests that ICH animals did not experience systemic dehydration.

Temperature and Activity

There was no effect of strain on temperature (Figure S2E) or activity (Figure S2F) post-ICH. There was a significant effect of ICH on temperature, but ICH animals were only warmer from 0 to 12 hours post-ICH by ≈0.1 °C relative to baseline, a trivial difference. There was a significant effect of time on temperature and activity, likely due to anesthesia causing a temporary (<1 hour) drop in temperature and activity in all groups after surgery.

Experiment 2 Results

Hematoma Volume

The hematoma volume in young SHRs was significantly larger (by 52%) versus aged SHRs (Figure4A and4B) despite using the same dose and batch of collagenase.

Neurological Deficits

The young sham SHRs all scored 0 on NDS (no deficits), whereas 7/10 aged sham animals scored ≥1, indicative of possible minor deficits, but we were not powered to statistically detect these small differences. Deficits present in the aged sham group were largely noted for beam walking (minor slips), potentially indicating decreased coordination typical of aging. There was a significant effect of ICH on neurological deficits in both young and aged animals (Figure4C). However, when accounting for baseline scores, there were no significant differences when comparing young versus aged ICH or sham groups, meaning that our observations are likely not attributable to an age-related response to surgery or anesthetic effects.

Histology and Stereology

Despite a difference in hematoma volume, both ICH groups displayed a similar decrease in contralateral hemisphere volume (Figure4D). Aged animals had significantly larger lateral ventricle volumes, a known effect of aging (Figure4E). Regardless of age, ICH groups had decreased cortical thickness compared with shams, indicating tissue compliance; aged SHRs had decreased cortical thickness compared with young animals, likely due to age-related atrophy (Figure4F).

Contralateral CA1 neurons decreased in volume following ICH compared with shams, with no effect of age (Figure5A). ICH animals also had significant increases in CA1 neuronal density (Figure5B). There was a trend, however, for aged animals to have a slightly lower neuronal density versus young animals. A blinded qualitative examination looking for signs of CA1 cell death was conducted by an experienced investigator (F.C.) with no obvious signs of pathology noted on light microscopy (3 sections examining both hemispheres per animal). Similarly, in contralateral S1, ICH groups had decreased neuronal volume versus shams (Figure5C). There was also a significant effect of age on S1 neuron volume, where aged SHRs had decreased neuron volumes compared with young SHRs, regardless of the experimental group. There was no effect of ICH on neuronal density in S1 (Figure5D), but aged SHRs had significantly higher S1 neuronal density compared with young SHRs. Therefore, despite evidence of age-related atrophy and differences in hematoma volume, both aged and young SHRs demonstrate tissue compliance within macroscopic and microscopic brain volume fractions.

Experiment 3 Results

Neurological Deficits

The ICH caused significant neurological impairments (Figure6A). However, similar to experiment 2, there was no effect of age across ICH and sham groups.

ICP and Activity in Young and Aged SHRs

When averaging hourly across the 72-hour recording period, both young and aged ICH groups experienced significant elevations in mean ICP relative to shams (Figure6B). There was no effect of age on hourly mean ICP, though young ICH animals experienced higher ICP elevations on average. Peak ICP was also significantly greater in the ICH groups (Figure6C) relative to shams, but similarly, there was no effect of age on peak ICP. The variability in minute-to-minute ICP (as measured by SD over the 72-hour epoch) was significantly greater in ICH versus sham animals (Table S3), indicative of challenged intracranial compensatory reserves, but again, there was no significant effect of age.

Instances of prolonged ICP elevations >20 mm Hg (raised ICP events) were significantly higher in ICH groups and were significantly longer in duration, with no effect of age (Table S3). Instances of disproportionate increases in ICP (eg, sudden spikes in ICP>10 mm Hg) were significantly higher in ICH animals though their duration was not significantly longer across groups, with no effect of age in either case (Table S3). In summary, despite the lower hematoma volumes observed in aged SHRs, they experienced comparable elevations in mean and peak ICP compared with their younger counterparts following ICH, along with equivalently variable ICP, and equivalent frequency of ICP spiking and elevation events; aged animals did experience lower (nonsignificant) ICP levels on average across measures, possibly due to brain atrophy.

As expected, average movement activity declined after ICH, but there was no effect of age (Table S4). Thus, physical activity levels were not obviously related to overall ICP scores.

Edema

Aging caused a modest but nonsignificant reduction in brain water content. There was a significant interaction between the brain hemisphere (ipsilateral versus contralateral) and the experimental group on edema levels (Figure6D). When collapsing across ages, there was a significant interaction between the experimental group and the brain hemisphere, where ICH animals had significantly higher ipsilateral edema values (Table S4).

Blood Gases

Serum sodium was slightly but significantly increased in ICH animals, regardless of age (Figure6E). There were no group differences in blood oxygen or blood carbon dioxide (Figure S3A and S3B). Serum sodium levels did not correlate significantly with ipsilateral brain water content or peak ICP (data not shown).

DISCUSSION

Advancing age and comorbidities substantially affect stroke outcomes, yet the vast majority of studies use young healthy animals.³⁷ Here, our primary objective was to examine how hypertension and age influence acute ICH mass effect (as measured by bleeding and edema) in rodents. In turn, we examined how these factors shape ICP compensation (as measured by the degree of tissue compliance, mean/peak ICP, and ICP spiking behavior). We found that the impact of hypertension on bleeding varied by age, which has important implications for modeling ICH in aged SHRs. Despite this age-related difference in bleeding, where aged SHRs had 52% smaller bleeds compared with young SHRs, we found that aged SHRs still experienced proportionally worse outcomes relative to hematoma volume (eg, edema, ICP elevations/spiking behavior, neurological deficits) compared with their younger counterparts. Finally, we found that young and aged SHRs had an equivalent degree of tissue compliance in response to ICH, aligning with their comparable ICP profiles following stroke. Taken together, our experiments emphasize the interactive effects of age and hypertension on ICP compensation and ICH outcomes.

In experiment 1, across young animal strains examined (SDR, WKR, and SHR), we observed no differences in hematoma volume, neurological deficit, or degree of tissue compliance following ICH. Although there were only small differences in hematoma volume among strains (SHRs≈WKRs>SDRs), there were notable differences in ventricular extension (SHRs>WKRs>SDRs). It is likely that elevated mean arterial pressure in SHRs predisposes them to greater stroke-related mechanical damage as the hematoma forms, in turn conferring a higher likelihood of ventricular extension.³⁸ Although young SHRs had the largest hematoma volumes on average (nonsignificant) compared with SDRs and WKRs, they experienced the smallest effect of ICH on brain volume measures. Our histology and stereology results also indicate that young SHRs undergo early cerebral atrophy and ventriculomegaly, aligning with previous findings.²⁴ Perhaps these morphological differences in young SHRs somewhat reduce reliance on last resort compensatory mechanisms to equalize ICP in the face of mass effect, such as tissue compliance. Effects related to hypertensive aging, which could adversely impact other ICP compensatory mechanisms (eg, cerebral autoregulation and glymphatic/CSF outflow), may have not yet had the time to fully manifest in young SHRs.²⁰

In experiment 2, aged SHRs displayed some signs of cerebral atrophy relative to young SHRs, but it was not as exaggerated as the atrophy observed in young SHRs compared with nonhypertensive strains. Additionally, aged SHRs had considerably smaller hematoma volumes (by ≈52%) compared with young SHRs. Vascular stiffness induces basal lamina hypertrophy,³⁹ perhaps more rapidly exhausting the action of exogenous collagenase. In fact, aging alone is associated with a 25% to 30% increase in collagen type-IV within the composition of cerebral vasculature,⁴⁰ and the structure of the vascular extracellular matrix is altered by hypertension in numerous ways.⁴¹ Additionally, past studies have demonstrated that SHRs develop age-related coagulopathies⁴²; each of these factors may have played a role in producing the smaller bleed size observed here in aged SHRs. These hypotheses warrant further consideration for the use of this model in preclinical ICH work. The unexpected discrepancy in hematoma volume between young and aged SHRs in our experiments did provide a key insight, however; despite substantially lower hematoma volumes, aged SHRs had equivalently severe outcomes following ICH compared with young SHRs in both experiments 2 and 3.

When expressed relative to hematoma volume, aged SHRs experienced a 2-fold increase in both edema and ICP and a 3-fold increase in the degree of tissue compliance compared with their younger counterparts. This suggests that (1) aged SHRs experience proportionally worse mass effect per microliter of hematoma; (2) aged SHRs are less able to comply with resulting ICP elevations via atrophy or compensatory mechanisms such as diversion of cerebral blood and CSF from the cranium, given their inefficiency in the setting of hypertensive aging; and (3) aged SHRs must then rely more heavily on tissue compliance to equalize ICP. The former point may also account for the fact that aged SHRs experienced equivalent neurological deficits following ICH versus young SHRs despite no (significant) differences in the performance of young and aged sham groups. Indeed, we have observed sublethal cellular injury in association with tissue compliance.¹⁶ The relative severity of edema experienced by aged SHRs following ICH may be mechanistically attributable to age-related differences in the poststroke immune response,⁴³ setting off a cascade of events ultimately resulting in proportionally worse outcomes. Though further investigation of these hypotheses is required, taken together, our evidence indicates that hypertensive aging is associated with worse ICH outcomes in this model, as observed clinically.^44,45

Regardless of strain and age, we observed widespread tissue compliance occurring in all ICH groups in our study. Indeed, this tissue compliance, estimated from contralateral hemisphere volume, accommodates at least 35.3 µL (pooled estimate with 95% CI, 17.7–52.9 μL) of mass effect, and this does not include the compliance response within intact regions of the affected ipsilateral hemisphere. Importantly, our work demonstrates that significant tissue compliance develops as ICP rises, even across a range of bleed volumes that result in no to modest mortality, and does not seem to be impeded by chronic hypertension. As with earlier work, the present findings help exclude systemic complications (eg, changes in temperature, feeding, drinking, or tissue hydration).¹⁶ Additionally, we found that the cerebellum did not undergo tissue compliance, an internal negative control that helps rule out methodological confounds or systemic complications. Regional differences in compliance dynamics are likely best explained by pressure gradients and conferred by structural factors, warranting further study. Our blood sodium data indicate that some ICH animals were hypernatremic (blood sodium levels above 145 mEq/L; normal range=135–145 mEq/L).⁴⁶ High blood sodium levels could induce changes in cellular volume,⁴⁷ but normal water content in the contralateral hemisphere, cerebellum, and muscle tissue suggests that the uninjured tissue, despite having reduced volume, is osmotically normal. The hypernatremia instead may arise from ongoing tissue compliance, as cells must extrude sodium along with other osmolytes to reduce in volume (eg, K⁺ and taurine),⁴⁸ detectable as higher blood sodium levels. As our data are correlational, future studies should investigate the causative effects of more severe hypernatremia on tissue compliance.

PERSPECTIVES

Ultimately, the interactive effects of age and hypertension seem to differentially affect ICH outcomes in aged SHRs, with proportionally worse ICP compensation, edema, and neurological deficits relative to bleed volume. Future studies should further investigate ICP compensation in young versus aged SHRs following ICH with a consistent degree of injury between groups. Regardless, our results indicate that tissue compliance itself does not arise as a result of a strain- or age-related difference following ICH although these factors do seem to play an important role in the relative magnitude of the effect. We have correlational data in patients with ICH indicating that tissue compliance also occurs with large bleeds (Khiabani et al. unpublished data). There are clear structural signs of mass effect in humans after severe strokes, including midline shift and loss of sulcal space, but whether these reflect tissue compliance (true volume reduction) must be further clarified by clinical stroke studies. By demonstrating that brain tissue volume decreases in settings where ICP is pathologically elevated, our findings challenge the assumption that brain tissue volume is an immutable intracranial component, arising from the centuries-old Monro-Kellie doctrine¹⁶; this distinction may one day be leveraged for therapeutic purposes.

ARTICLE INFORMATION

Acknowledgments

The authors thank the Colbourne Laboratory for their feedback on this article.

Sources of Funding

This research was funded by a Canadian Institutes of Health Research (CIHR) project grant to C.M.W., A.C.J.K., and F.C. (PI). C.M.W. and A.C.J.K. were both supported by a CIHR doctoral scholarship and the Isaak Walton Killam Memorial scholarship. T.F.C.K. was supported by a CIHR doctoral scholarship.

Disclosures

None.

Supplementary Material

hyp-81-151-s001.xlsx^{(145.3KB, xlsx)}

hyp-81-151-s002.docx^{(10.9MB, docx)}

hyp-81-151-s003.pdf^{(22.6KB, pdf)}

hyp-81-151-s004.pdf^{(28.2KB, pdf)}

Nonstandard Abbreviations and Acronyms

BWC: brain water content
CA1: hippocampal subfield cornu ammonis layer 1
CSF: cerebrospinal fluid
ICH: intracerebral hemorrhage
ICP: intracranial pressure
NDS: neurological deficit scale
S1: primary somatosensory cortex
SDR: Sprague-Dawley rat
SHR: spontaneously hypertensive rat
WKR: Wistar-Kyoto rat

C.M. Wilkinson and A.C.J. Kalisvaart share co-first authorship.

For Sources of Funding and Disclosures, see page 160.

Supplemental Material is available athttps://www.ahajournals.org/doi/suppl/10.1161/HYPERTENSIONAHA.123.21628.

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Associated Data

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Supplementary Materials

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Tissue Compliance and Intracranial Pressure Responses to Large Intracerebral Hemorrhage in Young and Aged Spontaneously Hypertensive Rats

Cassandra M Wilkinson

Anna CJ Kalisvaart

Tiffany FC Kung

Ashley H Abrahart

Elmira Khiabani

Frederick Colbourne

Abstract

BACKGROUND:

METHODS:

RESULTS:

CONCLUSIONS:

NOVELTY AND RELEVANCE.

What Is New?

What Is Relevant?

Clinical/Pathophysiological Implications?

METHODS

Data Availability

Figure 1.

Subjects

Statistical Analysis

RESULTS

Experimental Exclusions and Mortality

Experiment 1 Results

NDS Scores

Figure 2.

Hematoma Volume

Histology and Stereology

Figure 3.

Feeding, Drinking, Body Weight, and Muscle Water

Temperature and Activity

Experiment 2 Results

Hematoma Volume

Figure 4.

Neurological Deficits

Histology and Stereology

Figure 5.

Experiment 3 Results

Neurological Deficits

Figure 6.

ICP and Activity in Young and Aged SHRs

Edema

Blood Gases

DISCUSSION

PERSPECTIVES

ARTICLE INFORMATION

Acknowledgments

Sources of Funding

Disclosures

Supplementary Material

Nonstandard Abbreviations and Acronyms

REFERENCES

Associated Data

Supplementary Materials

Data Availability Statement

Figure 1.

ACTIONS

PERMALINK

RESOURCES

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