ICP monitoring, through invasive means, forms the crux of current guideline based management of moderate/severe TBI, with a current threshold for treatment of 22 mmHg (Sorrentino et al., 2012; Carney et al., 2017). Similarly, CPP the second element of current TBI care, which is actively targeted to a range of 60–70 mmHg, and is derived based on the simple formula: CPP = MAP – ICP (Carney et al., 2017). Time spent above the defined ICP thresholds, and outside of the suggested target CPP range, have both been documented to be associated with worse long-term outcome in moderate/severe TBI (Güiza et al., 2017; Donnelly et al., 2020). The major studies in these areas with high-frequency data show that older patients tended to have high ICPs and low CPPs and suffer undesirable outcomes (Czosnyka et al., 2005; Czosnyka et al., 2008). Males were predominant in these studies therefore leading in numbers for fatal outcomes.

Exploring age and ICP/CPP in more detail, it is clear that MMM based high-frequency ICP and CPP have not been directly evaluated in association with age and biological sex. Adams et al. compared the mean ICP and CPP of patients in functional survivors and fatal outcome groups (Adams et al., 2017). Patients in the functional survivors group were younger with mean age 38±16 years vs. 45±18 years (p < 0.001) for patients in fatal outcomes group (Adams et al., 2017). Patients in the functional survivors group compared to the fatal outcome group, had lower mean ICP and higher mean CPP (15.1±8.2 mmHg vs. 21.0±10.2 mmHg (p < 0.001); and 78.5±8.0 mmHg vs. 75.9± 8.3 mmHg (p = 0.009), respectively) (Adams et al., 2017). Nourallah et al. compared the mean ICP and CPP of patients in favourable outcome and unfavourable outcome groups as defined by GOS (Nourallah et al., 2018). Patients in the favourable outcome group were younger with mean age of 38.2±17.0 years vs. 43.0±17.0 years (p = 0.0091) for patients in unfavourable outcome group. Patients in the favourable outcome group compared to unfavourable outcome group, had lower mean ICP (12.6±4.6 mmHg vs. 15.4±19.5 mmHg, p = 0.0039) but no difference in CPP was found (77.6±6.7 mmHg vs. 77.6± 9.9, p = 0.31) (Nourallah et al., 2018). Czosnyka et al. showed in a 2005 study that mean ICP slightly decreased with age from 19±12 mmHg to 15±6 mmHg (p < 0.004) and mean CPP increased with age from 72±14 mmHg to 79±12 mmHg (p < 0.001) (Czosnyka et al., 2005). Though interpretation of these results must be cautioned, given the primary goal of the analysis in the parent manuscripts was outcome association. Many factors confound patient care trajectories and outcomes, such as institutional treatment guidelines/practices and goals of care discussions between health care providers and families. As such, one cannot directly presume that given younger patients had a higher rate of survival, and the survival groups had lower ICP (or higher CPP), that younger patients have lower ICP/higher CPP. Such inferences should be discouraged at this time.

With regards to biological sex, in the study by Sorrentino et al., it is seen that ICP threshold for survival was lower in males compared to females at 22 vs. 23 mmHg (p < 0.001), and lower in patients >55 years of age compared to young, at 21 vs. 23 mmHg respectively (p = 0.018) (Sorrentino et al., 2012). Furthermore, the CPP threshold for survival in that study was higher in older patients (>55 years) at 75 vs. 70 mmHg in young patients (p = 0.047), regardless of sex. The first study by Czosnyka et al. showed that females <50 years had more fatal outcomes than males, with a 29% death rate in females compared to a 17% death rate in males (p = 0.026) (Czosnyka et al., 2008).

It must be emphasized that most large studies on high-resolution (i.e., full waveform) ICP/CPP failed to comment on any detailed statistical association between age or biological sex, other than gross groupings of male vs. female, and arbitrary binary age cutoffs. Further to this, the multi-variable covariance patterns for ICP and CPP were not commented on, only simple comparative statistics have been employed. The studies outlined, are the largest representative studies found, but are by no means exhaustive of all studies documenting the link between ICP/CPP and age/sex. Table 1 provides a summary of selected large studies showing the relationship of these parameters with age and sex.

PbtO₂ is measured using an intraparenchymal Clarke electrode, which assesses extracellular diffusible oxygen levels of the brain parenchyma. Threshold by which intervention is currently suggested by guideline-based approaches is 20 mmHg, values below this are linked to poor outcomes (Okonkwo et al., 2017; Chesnut et al., 2020). This association has triggered ongoing phase III trials in TBI. There are few studies in the literature examining the association between age/sex and PbtO₂ in moderate/severe TBI. Studies examined showed that older patients trended towards having lower PbtO₂ and tended to suffer undesirable outcomes (Stiefel et al., 2005; Martini et al., 2009; Zeiler et al., 2020d). Though, it must be acknowledged, that all of the identified relationships failed to reach statistical significance, and only showed a potential relationship based on raw differences in mean/median magnitudes. This lack of significance occurred in the absence of adjusting for multiple comparisons, highlighting that no definitive comments on the association between PbtO₂ and age/sex can truly be made at this time. Based on the Zeiler et al. study, it can be seen that older patients were part of the group with high ICP and low PbtO₂ (Zeiler et al., 2020d). In this study, median age for patients in the group with mean ICP below 15 mmHg was 51 (IQR: 31–62.3 years) vs. 54 (IQR: 35.3–68.3 years) (p = 0.311) in the group with mean ICP above 20 mmHg, though failed to reach significance (Zeiler et al., 2020d). Those patients in the group with mean ICP above 20 mmHg had lower median PbtO₂ of 22.1 (IQR: 18.2–26.2 mmHg) vs. 27 (IQR: 23.2–33.1 mmHg) (p = 0.183) in those with mean ICP below 15 mmHg, though this failed to reach significance. As can be seen from the identified studies, they both made some reference to age in relation to this PbtO_2, however it was somewhat vague. Males were predominant in these studies, but no major association was documented between the sexes. Table 1 provides a summary of the available studies showing the relationship of this parameter with age and sex.

NIRS is a non-invasive technique which utilizes light in the range 700–1,000 nm on the electromagnetic spectrum to monitor frontal cerebral oxy-hemoglobin and deoxy-hemoglobin concentration, as well as regional cerebral oxygen saturations (rSO₂) (Mathieu et al., 2020). Thresholds for outcome in TBI remain unclear at this time, though data suggests worse cerebral oxygen saturations, and duration of impairment, are linked to worse long-term outcome (Zweifel et al., 2010; Mathieu et al., 2020). Despite a recent systematic review on NIRS in TBI, there are limited studies available documenting the statistical association between age/biological sex and high-frequency NIRS measures in moderate/severe TBI. These limited studies demonstrated that older patients tended to have low rSO₂ and suffer undesirable outcomes, though this was only a trend which failed to reach statistical significance (Adatia et al., 2018; Adatia et al., 2020). Adatia et al. showed that the younger patients, with a median age of 54 (IQR: 31) years vs. 61 (IQR: 21) years (p = 0.38), had the highest mean rSO₂ of 66 (IQR: 12%) vs. 63 (IQR: 8%) (p = 0.13) (Adatia et al., 2018). While in follow-up, Adatia et al. demonstrated that patients without midline shift were older, with mean age of 62±16 years vs. 59±15 years (p = 0.36) (Adatia et al., 2020). In this follow-up work, older patients had lower median rSO₂ of 60.5 (IQR: 45.1–65.2%) compared to younger patients with rSO₂ values of 64.7 (IQR: 52.0–68.1%) (p = 0.39) (Adatia et al., 2020). There was not a clear breakdown of male vs. females. Table 2 provides a summary of selected large studies showing the relationship of this parameter with age and sex.

The role of age and biologic sex in cerebral oxygen saturation, as measured by NIRS, has been more extensively examined outside the context of TBI. A recent study examined how rSO2 values differed by age and sex in a cohort of 1,616 adults undergoing cardiac interventions. They found that younger patients (18–49 years old) had significantly higher rSO2 values than middle-aged (50–74 years of age; 67% [95%CI 59–74%] vs. 63% [95%CI 56–69%], p < 0.001). Similarly, middle-aged individuals had higher cerebral oxygen saturations than elderly patients (>75 years old; 63% [95%CI 56–69] vs. 60% [95%CI 55–66%], p < 0.001). Males were also found to have significantly higher rSO2 values than females (65% [95%CI 58–70%] vs. 58% [95%CI 52–63%]) and while males had significantly higher hemoglobin values and were younger in this cohort, the significant effect of biological sex remained following multiple linear regression analysis accounting for these differences (Robu et al., 2020). In contrast, a longitudinal study of 3,110 healthy individuals over the age of 50 found that cerebral oxygenation was lower in males than females, when measured by NIRS. However, cerebral saturation values were found to also decrease with age (Newman et al., 2020).

Thermal diffusion flowmetry (TDF) is a technique employed via the use of a cortical or parenchymal probe to monitor CBF, by evaluating the power needed to sustain a temperature difference between a proximal and distal thermistor (Rosenthal et al., 2011). Such devices have seen limited use in the moderate/severe TBI literature, given expertise and cost associated with their use (Mathieu et al., 2019a). As such, when objectively evaluating the link between age/biological sex and high-frequency TDF-based CBF, there are only a few studies available (Dickman et al., 1991; Dias et al., 2015). In the small studies evaluating its use in TBI, low CBF measures have been correlated with worse outcomes. Furthermore, these works demonstrated that older patients tended to have low CBF and suffer undesirable outcomes. Dias et al. showed that patients with preserved cerebrovascular reactivity (measured through pressure reactivity index value of <0.25) were younger with mean age of 40±16 years vs. 52±11 years (p = 0.173) in those with impaired cerebrovascular reactivity (Dias et al., 2015). In this study, older patients in the impaired cerebrovascular reactivity group had lower mean CBF of 36.3±22.2 ml/100 g/min, compared to 39.0±20.9 ml/100 g/min (p = 0.953) in the younger group of patients, though this failed to reach significance (Dias et al., 2015). The second study referenced gave specific patient examples without reporting p values (Dickman et al., 1991). No major association was made between the sexes.

Aside from direct CBF measurement, TCD can be utilized to provide surrogate assessment of flow. TCD employs the use of ultrasound waves to insonate the basal cerebral arteries and assess CBFV non-invasively through Doppler frequency shift in the reflected signal, based on blood flow velocity in the insonated artery (Aaslid et al., 1982). Again, as with TDF, the requirement for technical expertise has limited its use in routine TBI monitoring. Low CBFV has been documented to be associated with worse long-term outcome in moderate/severe TBI populations (Czosnyka et al., 1996a; Budohoski et al., 2012; Czosnyka et al., 2005). As with the other monitoring modalities described above, there exists limited data in moderate/severe TBI patients on the association between CBFV and age/biological sex. The main studies have demonstrated that older patients tended to have low CBFV and suffer undesirable outcomes (Czosnyka et al., 2005; Bouzat et al., 2011; Cardim et al., 2020; Hu et al., 2008). This was seen in the study by Bouzat et al., where older vs. younger patients (median age of 46 (IQR: 20–80 years) vs. 34 (IQR: 15–84 years), p = 0.04) had lower mean CBFV of 31 (range: 18–60 cm/s) vs. 49 (range: 31–80 cm/s) (p < 0.01), respectively (Bouzat et al., 2011). Cardim et al., showed that older males had the lowest median FVm of 63 (IQR: 57.59–74.83 cm/s) vs. 69.33 (IQR: 65.33–72.67 cm/s) for females (p ≤ 0.05) (Cardim et al., 2020). Furthermore, Hu et al. described that mean CBFV was higher in patients (e.g., 65.7±33.9 cm/s on left side of brain compared to 56.5±19.8 cm/s on left side of brain for controls) who were younger than in controls (mean age and SD of patients were 38±16 years and controls were 43.7±11.9 years) (Hu et al., 2008). In contrast, Czosnyka et al. commented that CBFV may be independent of age, but required further investigation (Czosnyka et al., 2005). Table 2 provides a summary of large representative studies showing the relationship between TDF-based CBF and TCD-based CBFV, with age and sex. Outside of TBI, trends in FV and PI as measured by TCD have been identified based on age and sex. One study of 1720 healthy participants found that FVm, FVs, and FVd all decreased significantly with age while PI increased with age. Over the entire cohort, FVm, FV,s and FVd were lower in males than females (Bakker et al., 2004). Similarly, in a cohort of 524 healthy subjects, FV in the MCA were found to decrease with age in both males and females. Notably, the rate of decline was significantly greater in females than males, indicating a sex-specific trajectory to the reduction in FV with age (Alwatban et al., 2021).

Cerebrovascular reactivity monitoring refers to the continuous assessment of cerebral autoregulation, through assessing the response in slow-wave vasogenic fluctuations of a measure of CBF/cerebral blood volume (CBV), to changes in a driving pressure for CBF, such as MAP or CPP (Czosnyka et al., 1997; Zeiler et al., 2017d). Pressure reactivity index (PRx) is the most widely described continuous cerebrovascular reactivity measure in TBI monitoring, and is derived from the moving Pearson correlation coefficient between slow-waves of ICP and MAP, with negative values representing “intact” cerebral autoregulation, and positive values denoting “impaired” autoregulation (Czosnyka et al., 1997; Sorrentino et al., 2012). Various single and multi-center studies have documented the strong association between impaired cerebrovascular reactivity and outcomes in moderate/severe TBI (Zeiler et al., 2019a; Donnelly et al., 2019; Bennis et al., 2020). Similarly, PRx is one of the few multi-modal monitoring metrics which has evidence to support its added prognostic value beyond that of ICP, in models adjusting for baseline admission characteristics (Zeiler et al., 2018a; Zeiler et al., 2019a; Bennis et al., 2020). Again, most studies on PRx do not comment directly on the association between cerebrovascular reactivity and age/biological sex. We have highlighted the main and largest such studies on PRx in Table 3.

These works have demonstrated that older patients tended to suffer undesirable outcomes, with trends towards worse cerebrovascular reactivity metrics (Czosnyka et al., 2005; Czosnyka et al., 2008; Sorrentino et al., 2012; Zeiler et al., 2018b; Zeiler et al., 2019a). Three studies showed that older patients with high PRx suffered fatal outcomes. Statistical significance was commented on in two of these PRx studies, one by Czosnyka et al. (2005) and one by Adams et al. (2017) In the Czosnyka et al. (2005) study, it is seen that PRx degraded with increasing age from a mean of 0.01±0.13 a. u. to 0.05±0.12 a. u. (p = 0.03) (Czosnyka et al., 2005). In the Adams et al. study, mean PRx of patients in the functional survivors group was lower compared to patients in the fatal outcome group, with results of 0.05±0.15 a. u. vs. 0.16±0.21 a. u. (p < 0.001), respectively (Adams et al., 2017). In this study, patients in the functional survivors group were younger with a mean age of 38±16 years vs. patients in the Fatal Outcome group who were older at 45±18 years (p < 0.001). Sorrentino et al. found that when evaluating PRx thresholds for survival, PRx was lower in females at 0.25 a. u. vs. males at 0.3 a. u. (p = 0.002) (Sorrentino et al., 2012). Further to this, the early study by Czosnyka et al. showed that females <50 years suffered worse cerebrovascular reactivity compared to males, as evidenced by mean PRx for females of 0.11±0.047 a. u. vs. 0.044±0.031 a. u. for males (p < 0.05) (Czosnyka et al., 2008). Patients >50 years old in this study did not show comparable differences between males and females (Czosnyka et al., 2008). Of note, we have only commented on those studies on PRx. There exists an extensive and ever-growing literature in moderate/severe TBI on other cerebrovascular reactivity indices, derived from various invasive/non-invasive cerebral monitoring devices (Zeiler et al., 2017d). Figures 1, 2 provide patient examples of high-resolution physiology, documenting differences in ICP, CPP, PRx and rSO₂ for a young and elderly patient, respectively.

Cerebral compensatory reserve monitoring through continuously updating bedside metrics has recently been described in the TBI literature (Kim et al., 2009; Calviello et al., 2018; Zeiler et al., 2018c; Zeiler et al., 2019b). Such measures help with approximating how “tight” or “relaxed” the brain may be post-TBI. RAP (R – correlation, A – pulse amplitude of ICP, P – intracranial pressure) is an index which is derived from the relationship between slow-wave vasogenic fluctuations in ICP and pulse amplitude of ICP (Kim et al., 2009). This gives a constant bedside evaluation of cerebral compensatory reserve where impairment is linked to increased mortality (Calviello et al., 2018; Zeiler et al., 2018c). Further work into RAP is needed in order to employ its use in TBI patients, though preliminary data supports an association between abnormal RAP measures and worse outcomes. As such, there exists a limited literature body documenting the association between RAP and age/biological sex in moderate/severe TBI, with those in existence demonstrating that older patients tended to have undesirable outcomes (Czosnyka et al., 1996b; Zeiler et al., 2018c; Zeiler et al., 2019b). The first study by Zeiler et al. had no difference in mean RAP values between males and females (p > 0.05) and no relationship with age (p > 0.05) (Zeiler et al., 2018c). The follow-up study by Zeiler et al. demonstrated that increased age and compensatory-reserve-weighted intracranial pressure (wICP) was linked to poor outcomes (Zeiler et al., 2019b). Males were predominant in these studies and no major association was made between the sexes. Table 3 provides a summary of the available RAP literature, documenting any link to age and sex.

Advanced age is known to be associated with impaired cerebrovascular response through cerebrovascular reactivity testing in healthy populations during pCO₂ challenges in concert with advanced functional neuroimaging (McKetton et al., 2018; Miller et al., 2019; Taneja et al., 2020), and TCD assessments of MCA CBFV (Mariappan et al., 2015; Aguilar-Navarro et al., 2019; Klein et al., 2020; Tomoto et al., 2020). This has been confirmed in both populations of awake and anesthetized patients. We have also seen that advanced age is associated with worse cerebrovascular reactivity in moderate/severe TBI patients in our above review of MMM (Czosnyka et al., 2005; Zeiler et al., 2019a; Zeiler et al., 2019c).

There are a variety of pathological changes that could be associated with impaired cerebral vascular performance during the aging process (Camici et al., 2015). First, accumulation of amyloid protein precursor and beta amyloid in the wall of cerebral vessels has long been known to occur with advancing age, as seen in both human literature and experimental models (Grinberg et al., 2012; Weller et al., 2015; Qi and Ma, 2017). Such accumulation has been linked to reduced endothelial nitric oxide (NO) activity secondary to shuttling to the formation of peroxynitrite, and an increase in reactive oxygen species (ROS) (Hamel et al., 2008; Carvalho and Moreira, 2018). This leads to impaired resting tone, as well as attenuated vasodilatory responses of the cerebral vessels. In addition, the general response of the cerebral vasculature to free radicals and ROS becomes impaired. The angiotensin II (AGT) pathway is known to be involved in ROS vascular responses. With advanced aging, it appears that there is an uninhibited angiotensin II mediated generation of ROS, further potentiating the impacts on the NO vasodilatory pathways (Hamel et al., 2008; Camici et al., 2015; Carvalho and Moreira, 2018; Wang et al., 2019).

Aside from NO/ROS mediated changes seen with aging, chronic inflammation has also emerged as a potentially important pathological driver of cerebrovascular dysfunction (Gu et al., 2019; Low et al., 2019; Mészáros et al., 2020).Advanced age has demonstrated increased expression of various systemic pro-inflammatory cytokines, such as interleukin (IL)-1B and IL-6, in both experimental and human literature (Gu et al., 2019). This response is believed to be a maladaptive autoimmune action to cellular breakdown products found during the normal aging process. Furthermore, recent literature also suggests some of the chronic cerebral vascular changes seen with aging, such as amyloid deposition, lead to increased leukocyte adhesion, further potentiating the immune response (Gu et al., 2019; Low et al., 2019; Mészáros et al., 2020). This chronic immune response may be linked to ongoing dysfunction in the central nervous system, and degeneration. Increased circulating IL-6 levels have even been associated with magnetic resonance imaging (MRI) defined white matter ischemic volumes in elderly populations (Gu et al., 2019; Low et al., 2019). The exact mechanism of cerebrovascular dysfunction related to chronic neuroinflammation remains unclear at this time. However, of interest, the acute TBI literature has also found strong associations between serum and cerebrospinal fluid (CSF) pro-inflammatory cytokines, such as IL-6, with poor long-term outcome (Thelin et al., 2017a; Zeiler et al., 2017c; Thelin et al., 2018). Furthermore, the impact of the immune response post-TBI has also been supported by the association between various single nucleotide polymorphisms (SNP) in genes regulating pro-inflammatory cytokines, and patient outcomes (Zeiler et al., 2019e; Zeiler et al., 2019f). Genetic variation in immune response has also been highlighted as an important consideration in the genetics of biological aging, where SNP’s in IL-6 has been underlined as an important future area to consider in aging research (Cardoso et al., 2018).

Finally, neural modulation and repair mechanisms are also impaired as a result of aging. Decreased expression of brain derived neurotropic factor (BDNF) has been recently associated with advancing age. SNP’s in BDNF have also received note as potential areas of importance for future research in aging (Cardoso et al., 2018). Of note, BDNF SNP’s have also been linked to long-term outcomes in adult TBI literature (Zeiler et al., 2019e; Zeiler et al., 2019f). It may be that specific BDNF SNP’s potentiate impaired BDNF expression associated with aging, which leads to reduction in downstream regenerative mechanisms post-injury. Such impaired recovery mechanisms in the central nervous system may lead to increase in the pro-inflammatory process described previously, leading to worse cerebral vascular function.

As an additional layer of complexity, biological sex has some potentially major roles to play in cerebral vascular responses both in healthy and disease states. It impacts cerebrovascular responses both independent of, but also potentially mixed effects with, advancing age. With respect to moderate/severe TBI, the incidence of such injury overwhelmingly favors males. The majority of studies to date on TBI have had male predominance in their cohorts, limiting proper assessment of the impact of sex on secondary injury patterns and outcome (Maas et al., 2017; Steyerberg et al., 2019). This is also the case with all of the above-mentioned studies on MMM and their association with biological sex. However, biologically, sex differences leave important considerations regarding disparities in treatment responses, secondary injury severity, and outcomes (Mikolić et al., 2021).

With regards to progesterone in moderate/severe TBI, as mentioned in the introduction, substantial prior work has been conducted. Pre-clinical models of TBI have provided evidence that progesterone carries a neuroprotective effect, with exogenous supplementation in animals leading to reduced neural loss, improved neurophysiologic and functional outcomes (Clevenger et al., 2018; Khaksari et al., 2018). In humans, sex-related disparities in outcome after moderate/severe TBI have been well documented, despite the overwhelming majority of patients being male (Maas et al., 2017; Steyerberg et al., 2019; Mikolić et al., 2021). Supplementation with progesterone has been explored in such TBI cohorts in numerous randomized control trials evaluating the impact on long-term global outcomes, including the Progesterone for Traumatic Brain Injury, Experimental Clinical Treatment (ProTECT) (Wright et al., 2007402) and Efficacy and Safety Study of Intravenous Progesterone in Patients With Severe Traumatic Brain Injury (SyNAPSe) Trials (Skolnick et al., 2014). However, despite early evidence suggesting a potential impact on mortality and functional outcome (Pan et al., 2019), all trials have failed to demonstrate long-term impacts on mortality and morbidity (Lu et al., 2016; Ma et al., 2016; Schumacher et al., 2016; Pan et al., 2019). Though, functional outcomes assessments in trials were limited to course outcomes metrics, standard in moderate/severe TBI, such as the Glasgow Outcome Scale (Lu et al., 2016; Ma et al., 2016; Pan et al., 2019). Similarly, progesterone supplementation has also failed to lead to reproducible measurable differences in biomarkers of neural injury in such populations (Korley et al., 2021). To date no assessments of sex steroid supplementation and its impact on high-frequency cerebral physiology in adult moderate/severe TBI have been conducted.

Aside from progesterone, circulating estrogen levels have a well-documented impact on cerebral vasculature (Krause et al., 1985). Estrogen is known to promote prostaglandin I2 (PGI2) release, an important vasodilator (Krause et al., 1985; Deer and Stallone, 2014). Similarly, in males, or instances of low estrogen production, thromboxane A2 (TXA2) predominates, leading to cerebral vasoconstriction and impaired vasodilatory responses of the cerebral vessels (Deer and Stallone, 2014). Furthermore, in young female mice with higher circulating estrogen levels, there appears to be an attenuated angiotensin II/ROS response, with reduced vasoconstriction responses seen in female models to ROS exposures (De Silva et al., 2009). Finally, experimental literature points to high endothelial nitric oxide synthase (eNOS) vasodilation in female versus male rats, under exercise conditions (Arrick et al., 2016).

Aside from the endothelial and myogenic impacts of estrogen on the cerebral vasculature, there is a known anti-inflammatory effect. Data suggests that higher circulating estrogen levels are linked to decrease in leukocyte adhesion to the cerebral vasculature (Krause et al., 1985). Furthermore, estrogen has been shown to decrease inducible nitric oxide synthase (iNOS) in response to IL-1B, which is linked to upregulation of cyclooxygenase-2 (COX-2) and prostaglandin E2 (PGE2). As COX-2 and PGE2 are known cerebral vasoconstrictors, circulating estrogen levels subsequently attenuate their responses in the setting of injury (Krause et al., 1985). These anti-inflammatory properties of estrogen have triggered neuroprotective studies in both experimental and human settings (Brotfain et al., 2016; Späni et al., 2018; Martin-Jiménez et al., 2019; Yilmaz et al., 2019). However, their impact on global outcome post-TBI in humans is still uncertain.

Finally, aside from estrogenic endothelial/myogenic and anti-inflammatory effects, there have been documented differences in potassium channel expression in males vs. females, with a direct link to estrogen levels (Reed et al., 2020). In rats, large potassium channel beta-1 subunit (BK-B1) displays decreased expression in both males and female rats with removed ovaries. This BK-B1 reduction leads to increased MCA reactivity, as the large potassium channels are known to regulate cerebrovascular myogenic activity. This has yet to be substantiated in humans, but carries potential to explain discrepancies in cerebrovascular physiology seen in health and disease.

Accounting of the biological impacts of aging and sex highlighted above, it’s clear that there is a potential for interaction on cerebrovascular function in the setting of TBI. This is particularly the case for aging females (Krause et al., 1985). During youth, females do benefit from the protective effects of higher circulating estrogen on cerebral vasculature. However, during menopause or post oophorectomy the risk profile may change. Menopause in particular, occurring with advancing age may now dramatically increase the risk of abnormal vascular responses in the setting of TBI. Such changes may be reflected in the emerging literature suggesting worse long-term outcomes for females suffering from TBI. It must be noted that in reviewing the above-outlined literature on MMM and its association between age and biological sex, there were no studies identified which adjusted for both age and sex in a multi-variable model. This is despite having age and sex information available in their core data sets. As such, it is clear that such future works in this area will need to evaluate such age/sex interactions in a more comprehensive manner.

As outlined in the review on MMM provided and overview of the cerebrovascular biological considerations of aging and sex, there are some key areas for future investigation. First, the initial future works on MMM and its links with age and sex, require the use of high-resolution measures (Smieleweski and Czosnyka, 2000-2021). Relying on grand mean summary metrics for MMM physiology is insufficient, as signal variance is essentially eliminated, and the ability to discern sub-classes based on age and sex near impossible. As such, future works in the area require the use of time-series methodologies (Thelin et al., 2019; Zeiler et al., 2020e; Zeiler et al., 2020f), including autoregressive integrative moving average (ARIMA) and vector autoregressive integrative moving average (VARIMA) techniques. These should be applied based on large populations of TBI patients with high-frequency physiology, using sub-groups of age and sex. Further, sub-class evaluation can take place using semi-supervised or unsupervised machine learning techniques, and latent class analytics. Furthermore, more complex multi-variable models adjusting for age and sex are needed, as such literature is lacking. Such analyses may facilitate answering some of the preliminary questions regarding age/sex interactions, which currently remain unanswered in moderate/severe TBI. This work is the focus of various multi-center groups focused on high-frequency physiology data processing in both Europe (Howells et al., 2012; Maas et al., 2015; Güiza et al., 2017; Bennis et al., 2020) and Canada (Zeiler et al., 2018d; Bernard et al., 2020), and requires multi-disciplinary integrated teams of clinicians, engineers, physiologists and data scientists.

Aside from the initial more detailed analytic pathways for MMM physiology, the second phase of such inquiry necessitates integration of the physiome data with proteomic and genomic information. In general, genome wide association data (Zeiler et al., 2019e; Zeiler et al., 2019f), combined with proteomic information (Timofeev et al., 2011; Maas et al., 2015; Thelin et al., 2017b), may shed light on particular pathways driving cerebral physiologic dysfunction in TBI. However, with respect to aging and sex, integrating blood/serum/CSF/microdialysate sampling during the acute phase of TBI care, with high-resolution MMM data is key. First, blood can be utilized for genome wide association studies (GWAS), identifying various SNP’s of interest, including those related to pro-inflammatory cytokines, neural repair and angiotensin pathways that are of interest in advanced age (Cardoso et al., 2018; Zeiler et al., 2019f). Second, serum/CSF/microdialysate can be exploited to quantify circulating levels of cytokines (Helmy et al., 2011a; Helmy et al., 2011b; Zeiler et al., 2017c; Thelin et al., 2018), such as IL-6, nitric oxide synthase activity (through nitrate/nitrite levels), prostaglandin levels, free radical analysis, and assessment of sex hormone concentrations. Finally, blood can also be exploited to provide proper quantification of biological age, using current epigenetic techniques (Ishikawa et al., 2016; Bisset and Howlett, 2019; Zarzour et al., 2019). It is well known the chronological age is a poor marker of biological age and function. Epigenetic techniques evaluating widespread genome methylation status, histone modifications, or telomere length of circulating white cells, can be used to quantify biological age. Such epigenetic information may allow us to more accurately determine biological age and its link to MMM based physiologic dysfunction in TBI. However, such work is easy to describe, but much more complicated to execute. As with the advanced signal analytics, such work requires large multi-center data sets for reasons of statistical power and multi-disciplinary expertise. Some preliminary work in this area is being done using the CENTER-TBI high-resolution data set (Maas et al., 2015), but will only set the stage for future larger multi-center works in the area of advanced omics research in TBI.