Associations of Enlarged Perivascular Spaces With Brain Lesions, Brain Age, and Clinical Outcomes in Chronic Traumatic Brain Injury

Abstract

Background and Objectives

Enlarged perivascular spaces (ePVS) have been identified as a key signature of glymphatic system dysfunction in neurologic conditions. The incidence and clinical implications of ePVS after traumatic brain injury (TBI) are not yet understood. We investigated whether individuals with chronic moderate-to-severe TBI had an increased burden of ePVS and whether ePVS burden is modulated by the presence of focal lesions, older brain age, and poorer sleep quality. We examined whether an increased burden of ePVS was associated with poorer cognitive and emotional outcomes.

Methods

Using a cross-sectional design, participants with a single moderate-to-severe chronic TBI (sustained ≥10 years ago) were recruited from an inpatient rehabilitation program. Control participants were recruited from the community. Participants underwent 3T brain MRI, neuropsychological assessment, and clinical evaluations. ePVS burden in white matter was quantified using automated segmentation. The relationship between the number of ePVS, group membership, focal lesions, brain age, current sleep quality, and outcome was modeled using negative binomial and linear regressions.

Results

This study included 100 participants with TBI (70% male; mean age = 56.8 years) and 75 control participants (54.3% male; mean age = 59.8 years). The TBI group had a significantly greater burden of ePVS (prevalence ratio rate [PRR] = 1.29, p = 0.013, 95% CI 1.05–1.57). The presence of bilateral lesions was associated with greater ePVS burden (PRR = 1.41, p = 0.021, 95% CI 1.05–1.90). There was no association between ePVS burden, sleep quality (PRR = 1.01, p = 0.491, 95% CI 0.98–1.048), and sleep duration (PRR = 1.03, p = 0.556, 95% CI 0.92–1.16). ePVS was associated with verbal memory (β = −0.42, p = 0.006, 95% CI −0.72 to −0.12), but not with other cognitive domains. The burden of ePVS was not associated with emotional distress (β = −0.70, p = 0.461, 95% CI −2.57 to 1.17) or brain age (PRR = 1.00, p = 0.665, 95% CI 0.99–1.02).

Discussion

TBI is associated with a greater burden of ePVS, especially when there have been bilateral brain lesions. ePVS was associated with reduced verbal memory performance. ePVS may indicate ongoing impairments in glymphatic system function in the chronic postinjury period.

Perivascular spaces (PVS) are fluid-filled compartments surrounding cerebral blood vessels penetrating the brain parenchyma. Enlarged PVS (ePVS) have been implicated in neurologic conditions such as dementia,¹ Alzheimer disease,^2,3 cerebral amyloid angiopathy,⁴ and Parkinson disease.⁵ There is emerging evidence that ePVS may be a key indicator of glymphatic system dysfunction.^6,7 The glymphatic system promotes the clearance of proteins and interstitial waste solutes out of the brain to maintain brain homeostasis.^6,8 Within the glymphatic pathway, the CSF flows from the subarachnoid space into the PVS, where it passes into the brain parenchyma facilitated by perivascular aquaporin-4 (AQP4) water channels and mixes with interstitial fluid, and then drives removal of solutes and waste products through perivenous drainage.^8,9 Because PVS are critical components of the glymphatic system, alterations in the glymphatic flow mechanism, caused by factors such as impaired arterial pulsatility, abnormal protein aggregation in vessel walls, and mislocation of AQP4 channels, could lead to ePVS.^9-11

ePVS have been identified in acute and chronic periods after traumatic brain injury (TBI).^12-16 In controlled studies, a greater burden of ePVS has been found in both mild^13,17 and moderate-to-severe TBI samples.¹⁶ Evidence of ePVS in the chronic period is particularly notable because it raises the possibility of ongoing active glymphatic disruption many years after TBI. However, the characteristics and burden of ePVS are unknown in the chronic phase of TBI because many studies included participants across a broad range of time postinjury.^13,17,18 In addition, most work has been conducted with participants with mild TBI,^13-15,17,19 and thus the association between moderate-to-severe chronic TBI and ePVS burden remains to be comprehensively examined. This includes the investigation of how focal lesions and more generalized atrophy may be associated with ePVS burden. An association between focal lesions, brain atrophy, and ePVS could provide a key neuropathologic marker in moderate-to-severe TBI indicating an increased likelihood of chronic glymphatic system disruption and associated morbidity.

Poor sleep, commonly reported in TBI,²⁰ may critically contribute to the association between TBI and burden of ePVS. The glymphatic system is believed to be twice as efficient during sleep.²¹ Sleep disturbance may reduce waste clearance from the brain and cause neurotoxic waste products to accumulate and dilate the PVS. In non-TBI samples, sleep quality and sleep efficiency have been associated with a higher burden of ePVS.²² There is preliminary evidence, largely from mild TBI samples, to suggest that poor sleep may affect the burden of ePVS.^15,18

ePVS may play a critical role in chronic morbidity after TBI, signaling glymphatic system dysfunction. Cognitive impairment and emotional distress are among the most debilitating and persistent TBI sequalae.²³ ePVS have been associated with cognitive impairment in a moderate-to-severe TBI sample.¹⁶ The association between ePVS and emotional distress has been examined only in mild TBI; finding ePVS in the acute postinjury period was uniquely associated with somatization symptoms at an average of 6 months postinjury.¹⁹

There is a critical gap in the evidence examining the burden of ePVS in chronic moderate-to-severe TBI and associated morbidity. In this study, we examined whether there was a higher burden of ePVS in white matter using MRI in individuals at least 10 years after a single moderate-to-severe TBI when compared with controls. We also examined whether ePVS burden was related to the presence of focal lesions and older brain age—to assess changes in brain atrophy, sleep, cognition, and emotional distress. We hypothesized that individuals with TBI would have a greater burden of ePVS and that this would be associated with the presence of focal lesions, poorer sleep quality and duration, greater cognitive impairment, and greater emotional distress.

Methods

Standard Protocol Approvals, Registrations, and Patient Consents

All individuals or their legal representatives gave written permission before any research activities in line with the Declaration of Helsinki. The research received approval from the Austin Health Human Research Ethics Committee (HREC/17/Austin/202).

Participants

The study recruited participants from a database of individuals who had been admitted to an inpatient rehabilitation program at Epworth HealthCare in Melbourne, Australia (Recruitment flowchart included in eMethods, links.lww.com/WNL/C798). Participants were contacted for the study if they met the inclusion criteria, which included being 40 years or older, having sustained a single moderate-to-severe TBI—determined using medical records, being at least 16 years of age during injury, and being more than 10 years postinjury. These inclusion criteria ensured we could examine chronic neuropathology in an adult aging cohort of moderate-to-severe individuals with TBI. Control participants were recruited from the general community through advertisements and social media. Controls were 40 years or older, had no history of TBI or loss of consciousness, and had sufficient English language skills and cognitive capacity to participate in the study. They were also required to not have chronic substance abuse or severe psychiatric disturbances, other neurologic conditions, or contraindications for MRI.

Potential recruitment bias in our study sample was examined by comparing key sample demographic and injury-related variables between the current study sample and participants from the longitudinal head injury outcome study meeting similar inclusion criteria and admitted to hospital over the same time frame (1985–2009; n = 2,044). The current sample did not differ with this larger longitudinal cohort on sex (p = 0.857), age at injury (p = 0.270), duration of posttraumatic amnesia (PTA; p = 0.738), Glasgow coma scale (GCS) score (p = 0.547), or mechanism of injury (p = 0.298). However, the participants in this study were more highly educated (p < 0.001). Refer to eTable 1 (links.lww.com/WNL/C798) for detailed sample comparisons.

Procedures

This study used a cross-sectional design. Participants completed 2 study visits: (1) clinical evaluation and (2) MRI scan. All data collection occurred between 2018 and 2020.

Clinical Evaluations

Participants completed a research interview where they provided demographic information and medical history. Injury-related information (GCS scores, PTA duration, and CT results) was obtained from medical records. PTA was measured prospectively using the Westmead Posttraumatic Amnesia Scale. Participants completed a battery of neuropsychological tests assessing memory, processing speed, and cognitive control (see eTables 2 and 3, links.lww.com/WNL/C798, for a list of neuropsychological tests and summary statistics). The neuropsychological tests have previously been shown to be sensitive to TBI-related cognitive impairment in the chronic period.²⁴ Functional outcome was assessed with the Glasgow Outcome Scale–Extended (GOSE). Self-reported emotional distress was assessed using the Hospital Anxiety and Depression Scale. Sleep quality was assessed using the Pittsburgh Sleep Quality Index (PSQI).

Image Acquisition

Participants completed a T1-weighted sequence using a Siemens Magnetom Skyra 3T scanner (Erlangen, Germany) with the following parameters: resolution = 1 × 1 × 1 mm, repetition time = 2,300 milliseconds, echo time = 3.65 milliseconds, field of view = 208 × 240 × 256 mm, flip angle = 9°, number of slices = 208 slices, and slice thickness = 1 mm. A fluid-attenuated inversion recovery (FLAIR) image was acquired with the following parameters: resolution = 1 × 1 × 1 mm, repetition time = 6,000 milliseconds, echo time = 390 milliseconds, inversion time = 2,100 milliseconds, field of view = 176 × 256 × 256 mm, number of slices = 176 slices, and slice thickness = 1 mm.

Image Analysis

ePVS were detected on T1-weighted images using the T1-weighted and T2-weighted FLAIR Multimodal Autoidentification of Perivascular Spaces (MAPS-T1) algorithm (refer to Schwartz et al.²⁵ for detailed methods). The MAPS-T1 algorithm automatically segments ePVS using T1-weighted and FLAIR images using voxel intensity and morphology information from a white matter mask that was constructed from the T1 image using FreeSurfer.²⁶ For additional details, refer to the eMethods (links.lww.com/WNL/C798).

We validated the MAPS-T1 algorithm in our sample, given it has not previously been used in a population with moderate/severe TBI. The MAPS-T1 algorithm segmented ePVS for all controls and participants with TBI. Participants were allocated into deciles, based on the total number of segmented ePVS. One participant from each group (TBI and control) was randomly selected from each decile for further manual ePVS segmentation, resulting in an overall sample of 20 participants. Selecting participants from each decile allowed us to control for potential differences in MAPS-T1 segmentation performance that may have depended on the overall ePVS burden (i.e., an overestimation or underestimation of ePVS for those individuals with low/high overall number of ePVS). Manual segmentation of ePVS was conducted for all individuals within the validation sample (n = 20), while being blind to decile group membership. ePVS were manually segmented on a single image slice, deemed to be the slice with highest number of ePVS using visual detection. The accuracy of manually identified ePVS was confirmed by a neuroradiologist (author M.L.). The correlation between the number of manually defined and MAPS-T1 ePVS was used to determine the quality of the MAPS-T1 algorithm for participants with TBI and controls. Overall, the MAPS-T1 algorithm was determined to perform well in our cohort, demonstrating a high correlation between MAPS-T1 and manually segmented ePVS (Spearman r = 0.85, 95% CI 0.65–0.94; refer to eFigure 1, links.lww.com/WNL/C798). Focal lesions were identified as hypointense areas on T1-weighted images, which may have comprised both gray and white matter. Lesions were manually segmented.

The study used a measure of brain age, which is a marker of overall changes in brain volume and atrophy. We obtained estimates of brain age for each participant using a preprocessing and analysis pipeline developed by Cole et al.²⁷ We used the brainageR package in R, which generates a brain-predicted age value from raw T1-weighted MRI scans. The key measure of brain aging used in the study was the brain age gap, which was calculated by subtracting each participant's chronological age from their corrected predicted “brain age” (brain age gap = corrected predicted brain age–chronological age). Positive values of brain age gap indicate a biologically older brain, while negative values suggest a biologically younger brain. The methodology used in the study is described in detail in the eMethods (links.lww.com/WNL/C798).

Statistical Analysis

Statistical analyses were conducted in R, version 4.0.3.²⁸ All statistical analyses were 2-tailed and used a significance level of p < 0.05. The burden (i.e., number) of ePVS detected using the MAPS-T1 algorithm was the primary measure of interest. The burden of ePVS was modeled using negative binomial regression, given this variable is a count variable, and it was found to be overdispersed—the variance was significantly greater than its mean. The prevalence rate ratios (PRRs) are presented for negative binomial regressions, allowing us to compare the prevalence of ePVS between 2 groups or in relation to another continuous variable, given other variables in the model are held constant. A PRR above 1 indicates the incidence rate is higher in the exposed group compared with that in the unexposed group. A PRR below 1 indicates the incidence rate is lower in the exposed group compared with that in the unexposed group.

All key analyses controlled for a number of potentially confounding variables that may be associated with ePVS: age at assessment, sex, white matter volume, time of the MRI scan (previously seen to alter PVS appearance),²⁹ and vascular risk. White matter volume was centered and scaled to aid in visual presentation and interpretation. This transformation does not alter statistical significance. Vascular risk was defined as the sum of present vascular risk factors: diabetes, high blood pressure, atrial fibrillation, high cholesterol, and current smoking. The Wechsler Test of Adult Reading (WTAR) was added as a covariate for all analyses involving cognition. To examine the association between focal lesions, brain age, overall sleep quality, sleep duration, and ePVS, ePVS were entered as the outcome variable, with sleep measures, focal lesions, and brain age gap entered as independent variables.

Exploratory factor analysis was used to examine the association between the number of ePVS and cognition to reduce the large number of neuropsychological measures into interpretable cognitive domains (refer to eMethods 1, links.lww.com/WNL/C798, for detailed factor analysis methodology). To examine the relationship between ePVS and cognition, factor scores for each cognitive domain were entered as the outcome and the log number of ePVS entered as an independent variable, controlling for age at assessment, sex, white matter volume, time of the MRI scan, vascular risk, and WTAR. The log ePVS was similarly entered as an independent variable when examining the association with emotional distress.

Data Availability

Data and scans from this study will be made available in deidentified format to researchers through the Federal Interagency Traumatic Brain Injury Research database (fitbir.nih.gov/).

Results

Participant Characteristics

This study included 100 participants with TBI aged between 40 and 85 years (70% male; mean = 56.8 years, SD = 11.3), who were an average of 22 years postinjury at study enrolment (SD = 6.3, range = 10–33 years). There were 75 control participants aged between 40 and 87 years (54.3% male; mean = 59.8 years, SD = 11.4; Table 1).

Table 1.

Demographic and Clinical Characteristics of Participants With TBI and Controls

ePVS in Participants With Chronic TBI Compared With Those in Controls

We first examined the distribution of ePVS between participants with TBI and control. ePVS segmented using MAPS-T1 are presented in Figure 1. In our key analysis, we modeled the number of ePVS as a function of group (TBI vs control), age at assessment, sex, normalized white matter volume, time of MRI scan, and vascular risk score. In line with our hypothesis, individuals with TBI had a greater incidence of ePVS compared with controls (PRR = 1.29, SE = 1.11, p = 0.013, 95% CI 1.05–1.57, residual deviance/df = 184.2/168; Figure 2A). That is, TBI was associated with a 29% increase in the number of ePVS compared with controls, with other covariates held constant. As expected, greater age (PRR = 1.03, SE = 0.005, p < 0.001, 95% CI 1.02–1.04, residual deviance/df = 184.2/168; Figure 2B) and white matter volume (normalized) increased the number of ePVS (PRR = 1.27, SE = 0.07, p < 0.001, 95% CI 1.13–1.42, residual deviance/df = 184.2/168; Figure 2C). No association, however, was present between sex (PRR = 1.09, SE = 0.12, p = 0.437, 95% CI 0.88–1.34, residual deviance/df = 184.2/168), time of MRI scan (PRR = 0.99, SE = 0.01, p = 0.643, 95% CI 0.97–1.02, residual deviance/df = 184.2/168), and vascular risk (PRR = 1.00, SE = 0.06, p = 0.942, 95% CI 0.89–1.13, residual deviance/df = 184.2/168) and the number of ePVS.

Figure 1. Perivascular Spaces in Controls and Participants With TBI.

(A) ePVS overlap image for control participants (n = 75). (B) ePVS overlap map for Participants With TBI (n = 100). (C) Case example of ePVS in a participant with TBI. Red circles denote a region of ePVS. ePVS = enlarged perivascular space; TBI = traumatic brain injury.

Figure 2. Enlarged PVS in Chronic TBI.

(A) There are a greater number of enlarged PVS in chronic TBI. (B) Number of enlarged PVS increase with age, regardless of group. (C) Number of enlarged PVS increase with white matter volume, regardless of group. PVS = perivascular space; TBI = traumatic brain injury.

Association Between ePVS and Focal Lesions

We examined the association between the presence of a focal lesion on T1-weighted images and the number of ePVS in the TBI group. Individuals with TBI who had a focal lesion had a greater number of ePVS compared with those patients with TBI without lesions (PRR = 1.36, SE = 0.17, p = 0.013, 95% CI 1.06–1.73, residual deviance/df = 105.5/93; Figure 3A). That is, the presence of a focal lesion was associated with a 36% increase in the number of ePVS. We examined whether the greater number of ePVS depended on whether the lesion was unilateral or bilateral (Figure 3B). Participants with TBI with bilateral lesions had a 41% increase in the number of ePVS compared with those with no focal lesions (PRR = 1.41, SE = 0.21, p = 0.021, 95% CI 1.05–1.90, residual deviance/df = 105.4/92). No difference was detected between those with a unilateral lesion and those with no lesion (PRR = 1.30, SE = 0.20, p = 0.092, 95% CI 0.96–1.77, residual deviance/df = 105.4/92) or between those with a unilateral lesion and those with a bilateral lesion (PRR = 1.08, SE = 0.20, p = 0.657, 95% CI 0.76–1.54, residual deviance/df = 105.4/92).

Figure 3. Association Between Focal Lesions and Enlarged PVS.

(A) Comparison of Patients With TBI with and without a focal lesion present on a T1-weighted scan. (B) Comparison of individuals without a lesion vs those with either a unilateral or bilateral lesion. PVS = perivascular space; TBI = traumatic brain injury.

We further interrogated the association between focal lesions and the number of ePVS, considering the influence of lesion volume. Indeed, individuals with bilateral lesions had significantly greater lesions volumes, compared with those with only unilateral lesions (β = 4.96, SE = 2.10, p < 0.001, 95% CI 2.11–11.68). Lesion volume, however, was not associated with the number of ePVS (PRR = 1.01, SE = 0.06, p = 0.862, 95% CI 0.90–1.13, residual deviance/df = 55.6/41). In addition, we examined whether the relationship between focal lesions and increased number of ePVS was driven by greater injury severity. However, the relationship between bilateral lesions and the number of ePVS was still present once the duration of PTA was entered as an additional covariate (PRR = 1.40, SE = 0.21, p = 0.023, 95% CI 1.05–1.88, residual deviance/df = 103.2/89).

Relationship Between ePVS and Brain Age

We examined whether brain age gap, in addition to focal lesions, was associated with ePVS. There was no significant association between the number of ePVS and brain age gap (PRR = 1.00, SE = 0.009, p = 0.665, 95% CI 0.99–1.02, residual deviance/df = 105.8/93).

Relationship Between ePVS and Sleep

Previous studies suggest that sleep quality is associated with ePVS. However, we found no significant association between the number of ePVS and overall sleep quality (PRR = 1.01, SE = 0.01, p = 0.491, 95% CI 0.98–1.048, residual deviance/df = 104.6/92) or sleep duration (PRR = 1.03, SE = 0.06, p = 0.556, 95% CI 0.92–1.16, residual deviance/df = 105.8/93).

Association Between ePVS and Outcomes in Chronic TBI

We examined the relationship between ePVS and 2 domains of outcome shown to be impaired after TBI—cognition and emotional functioning. Factor analysis resulted in 4 cognitive domains: episodic memory, visual memory, processing speed and cognitive control, and verbal memory. Participants with TBI displayed poorer performance on episodic memory (β = −0.38, SE = 0.15, p = 0.012, 95% CI −0.67 to −0.08), processing speed and cognitive control (β = −0.45, SE = 0.13, p < 0.001, 95% CI −0.72 to −0.19), and verbal memory (β = −0.42, SE = 0.15, p = 0.006, 95% CI −0.72 to −0.13) domains compared with controls (eFigure 2, links.lww.com/WNL/C798). There was no significant difference between groups in visual memory (β = −0.15, SE = 0.16, p = 0.370, 95% CI −0.47 to 0.17).

We next examined whether the number of ePVS was associated with cognitive performance after TBI (Figure 4). A greater number of ePVS was associated with poorer verbal memory (β = −0.42, SE = 0.15, p = 0.006, 95% CI −0.72 to −0.12). There was no association between the number of ePVS and episodic memory (β = −0.10, SE = 0.15, p = 0.511, 95% CI −0.39 to 0.19), processing speed and cognitive control (β = −0.20, SE = 0.13, p = 0.139, 95% CI −0.46 to 0.07), and visual memory (β = 0.08, SE = 0.15, p = 0.576, 95% CI −0.21 to 0.38). We found no association between the number of ePVS and cognition in healthy controls.

Figure 4. Association Between ePVS and Cognition After TBI.

The figure displays the associations between ePVS and scores on cognitive domains. A greater number of ePVS was associated with poorer verbal memory (A). However, no association was found between ePVS episodic memory (B), processing speed/cognitive control (C), or visual memory (D). ePVS = enlarged perivascular space; TBI = traumatic brain injury.

Participants with TBI had greater emotional distress compared with controls (β = 3.22, SE = 0.81, p < 0.001, 95% CI 1.61–4.82). However, there was no statistically significant association between the number of ePVS and emotional distress (β = −0.70, SE = 0.94, p = 0.461, 95% CI −2.57 to 1.17).

Discussion

This study examined ePVS in the chronic period (≥10 years) after a single moderate-to-severe TBI. The burden of ePVS was elevated in our TBI group compared with that in the control group. The presence of bilateral lesions was associated with greater ePVS burden, and these findings were not changed after accounting for lesion volume and injury severity. We found no evidence that poor sleep quality or brain age affected the burden of ePVS. The burden of ePVS was associated with chronic impairment in verbal memory but not other cognitive domains or emotional distress. These findings highlight that ePVS may be a clinically significant neuroimaging marker decades after a single moderate-to-severe TBI and suggest bilateral focal lesions may indicate those at risk of chronic ePVS and perivascular clearance dysfunction.

Our findings highlight that the burden of ePVS is elevated after a single moderate-to-severe TBI greater than 10 years and up to 33 years postinjury. Our findings are consistent with previous work examining ePVS in a mixed mild-to-severe chronic TBI sample.¹⁶ We critically extend this work, which included 47.3% repeated TBI, by showing that ePVS burden is also elevated in a cohort that has sustained a single moderate-to-severe TBI. This suggests that persistent and clinically significant ePVS can occur even after a single TBI and does not require repeated injuries to remain enlarged. Two studies that are discordant with our findings, both failing to find significant group differences in ePVS burden, may have been underpowered to identify an association with TBI (TBI group, n = 15³⁰ and n = 13¹⁸).

Greater burden of ePVS may suggest ongoing and active glymphatic system dysfunction in the chronic postinjury period. It is also possible that chronic ePVS may reflect persistent but stable changes from time of injury. However, given that PVS are known to be dynamic, for example, showing changes in volume with time of day,²⁹ it is not clear why dilation of PVS in the immediate postinjury period would not recover over time in the absence of an ongoing active process. Glymphatic dysfunction after TBI, currently confirmed only in animal models,^31,32 is believed to be the result of multiple factors including shear strain injury stretching PVS and reducing glymphatic flow, inflammation, loss of AQP4 polarization or perivascular localization, and buildup of proteinaceous waste products including β-amyloid and tau.^6,8,12,32

Glymphatic system dysfunction could be maintained into the chronic period by ongoing axonal degeneration, self-propagation and accumulation of toxic proteins, and chronic neuroinflammation.^33-35 Buildup of β-amyloid and tau may also be implicated more broadly in post-TBI development of Alzheimer disease.³¹ However, this contention needs to be considered in light of recent work from this team that failed to find a greater burden of β-amyloid or tau in a chronic moderate-to-severe TBI cohort.³⁶

We did not find an association between sleep quality and ePVS burden. Our findings are discordant with previous TBI work that found poor sleep was associated with ePVS.^15,18 Examination of this prior work suggests that poor sleep may affect ePVS volume as opposed to burden (number of ePVS) and that this be more evident in repeated mild TBI.¹⁵ Glymphatic system dysfunction is believed to be exacerbated by poor sleep, given sleep is a critical period for glymphatic system activity.²¹ It is possible that within the context of more significant alterations to the glymphatic system function and PVS dilation caused by moderate-to-severe TBI, additional influences caused by variability in sleep quality cannot be easily discerned. Larger studies would be better powered to differentiate possible synergistic or additive effects of poor sleep on ePVS burden. Polysomnography may also provide a more objective and sensitive measure of sleep quality. Indeed, work in non-TBI populations has found associations between ePVS and sleep were evident when using polysomnography but not the PSQI (i.e., subjective sleep measure used in the current study).²²

A novel observation from this study is that bilateral focal lesions, but not general atrophy as captured by brain age, were associated with ePVS burden. It is of importance that this association was not accounted for by lesion size or injury severity, suggesting that the association may be driven by the bilateral nature of the lesions. This is an intriguing possibility for which a rationale remains to be elucidated. One potential theory is that this finding may indicate the importance of hemispheric preservation. That is, for individuals with unilateral lesions, their preserved hemisphere may take on a compensatory role resulting in overall better functioning of brain systems including the glymphatic system. This finding has important clinical implications because it suggests that the presence of bilateral focal lesions, commonly visible on routine clinical CT scan during injury, may be a useful indicator of chronic ePVS.

The burden of ePVS is clinically meaningful in chronic moderate-to-severe TBI. The unique association with verbal memory, identified only in our TBI group, mirrors previous work with a mixed mild-to-severe chronic TBI sample that found an association between ePVS burden and verbal memory, but no significant association with processing speed, executive function, or learning.¹⁶ Our team also found a unique association between verbal memory and brain age in chronic moderate-to-severe TBI sample.³⁷ Taken together, greater verbal memory impairments in chronic TBI may suggest an underlying neuropathologic profile that includes greater ePVS burden and greater brain age (i.e., a biologically “older” brain). A specific association between ePVS burden and verbal memory has not been identified in other clinical populations, and the broader evidence of an association between ePVS and cognitive function is mixed.¹¹ There seems to be stronger evidence for an association between ePVS and longitudinal changes in cognition when compared with cognitive function measured at a single time point.^1,38,39 Further work is required to replicate our cross-sectional findings, examine how ePVS burden may be associated with longitudinal changes in cognitive, and explore possible mechanisms for the unique association between ePVS burden and chronic verbal memory impairment in TBI.

A number of caveats should be considered when interpreting the results. First, our ePVS quantification was restricted to the white matter and did not include other areas associated with ePVS such as the basal ganglia. However, it is worthwhile noting that evidence from previous TBI research that included participants with moderate-to-severe TBI did not find a significantly greater burden of ePVS in the basal ganglia compared with controls, with group differences found only in the centrum semiovale.¹⁶ Second, our study focused on ePVS burden, defined as the number of ePVS, and did not measure other morphological features of PVS such as volume and size. The strength of the MAPS-T1 algorithm is that it automates the detection of ePVS, but at the sacrifice of precise tracing of the detected clusters. Third, the sizes of our TBI and control groups were uneven and not matched for age, sex, or vascular risk factors. We did, however, control for these variables in our analyses. Fourth, our participants had only a single moderate-to-severe TBI and were required to be at least 40 years at study entry. Thus, future studies should examine whether the study findings generalize to younger individuals and those with milder and repeated injuries. Moreover, although not directly quantified in the current study, admissions to the acquired brain injury care ward are largely of individuals who identify as Caucasian/ White Australians. Therefore, caution is required when generalizing these results to other cohorts. It is also important to acknowledge that the relationship between ePVS and glymphatic system functioning has not been directly established. In addition, there are unresolved controversies regarding the directionality and anatomical pathways of clearance, with other models also proposed including the intramural periarterial drainage model.⁴⁰ Nevertheless, although many questions about PVS remain, it is well established that normal PVS function is important for maintaining brain health.¹¹

Our study has several strengths, including the use of a robust quantification method for identifying ePVS and controlling for key covariates in all analyses. Our findings were statistically independent of key factors previously associated with ePVS: age at assessment, sex, white matter volume, time of the MRI scan, and vascular risk.^29,41 We used an established automatic segmentation algorithm²⁵ to quantify ePVS. This was validated by manual segmentation of a subsample (n = 20) of participants, which was independently reviewed by a neuroradiologist. This is a significant advance in the ePVS TBI evidence to date, which has mostly relied on subjective visual rating systems.^12-14,16-18

In conclusion, a single moderate-to-severe TBI is associated with a greater burden of ePVS, which may indicate chronic and ongoing glymphatic system dysfunction. The burden of ePVS is elevated in the presence of bilateral focal lesions and is associated with poorer verbal memory performance.

Glossary

AQP4

aquaporin-4

ePVS

enlarged PVS

FLAIR

fluid-attenuated inversion recovery

GCS

Glasgow coma scale

GOSE

Glasgow Outcome Scale–Extended

MAPS

Multimodal Autoidentification of Perivascular Spaces

PRR

prevalence rate ratio

PSQI

Pittsburgh Sleep Quality Index

PTA

posttraumatic amnesia

PVS

perivascular space

TBI

traumatic brain injury

WTAR

Wechsler Test of Adult Reading

Appendix. Authors

Study Funding

This work was supported by the National Health and Medical Research Council (NHMRC) under Grant APP 1127007. S.R. Shultz received funding support from Michael Smith Health Research BC. G. Spitz was funded by a NHMRC Early Career Fellowship (APP1104692). L.C. Silbert was funded by the NIH-funded Oregon Alzheimer's Disease Center (NIH-NIA P30AG066518).

Disclosure

The authors report no disclosures relevant to the manuscript. Go to Neurology.org/N for full disclosures.