The glymphatic system has been recently recognized as a pathway for waste clearance and maintaining fluid balance in the brain parenchymal interstitium (1). Cerebrospinal fluid (CSF) from the subarachnoid space flows into the brain parenchyma through periarterial spaces of the penetrating arteries and, under the influence of aquaporin-4 (AQP4) water channels, mixes with parenchymal interstitial fluid. The interstitial fluid and its solutes then move into the perivenous and perineuronal spaces, thereafter leaving the brain parenchyma. The discovery of the glymphatic system led to a new perspective into the pathogenesis of brain diseases such as neurodegeneration (2) and acute ischemic infarct (3). Recent studies have shown that the glymphatic system may play an essential role in brain tumor immunity and might be targeted in brain tumor immunotherapy (4).

Rodent experiments have provided some insights into the changes of glymphatic function associated with glioma growth. In rodent studies, reduced CSF efflux rate (5) and remodeling of the glymphatic pathway (4) were observed in glioma-bearing mice, and these changes may be associated with the formation of peritumoral brain edema. Despite substantial knowledge having been gained from animal studies, further research is necessary to confirm whether the findings regarding the glymphatic system of animals apply to humans. Noninvasive approaches such as the morphological assessment of intracranial perivascular spaces with structural magnetic resonance imaging (MRI) (6–9) and measurement of water diffusivity using diffusion MRI (10–12) have been proposed for the evaluation of human glymphatic function.

The analysis along the perivascular space (ALPS) index (10) is a diffusion metric derived from diffusion tensor imaging (DTI). It is thought to reflect the diffusivity along the perivascular spaces of medullary veins at the level of the lateral ventricle body and, thus, may serve as an estimate of human glymphatic function. Alterations of the ALPS index correlate with the Mini-Mental State Examination scores in patients with Alzheimer’s disease and are thought to reflect glymphatic dysfunction. The ALPS index is also significantly lower in normal pressure hydrocephalus patients and points toward glymphatic dysfunction, as indicated by delayed clearance of intrathecally injected gadobutrol (13, 14).

The ALPS index offers an opportunity for the noninvasive investigation of the human glymphatic system. Thus, we took advantage of this method to evaluate the glymphatic system in patients with glioma. We aimed to investigate the correlations of glymphatic function with the volumes of tumor and peritumoral brain edema, tumor grades, and isocitrate dehydrogenase 1 (IDH1) mutation status.

Table 1 summarizes the patients’ characteristics and all measurements. Among the 201 patients with gliomas, 86 had grade II, 52 had grade III, and 63 had grade IV. A total of 107 (53.2%) tumors were IDH1-mutant. Eighty-four (41.8%) patients had glioma involving the right cerebral hemisphere.

There were excellent interobserver agreements (ICC = 0.822–0.948, p < 0.001) in the measurements of peritumoral brain edema volumes, whole tumor volumes, enhancing tumor volumes, non-enhancing tumor volumes, necrotic portions volumes, hemorrhagic portions volumes, and the ALPS index. The mean volumes (in cubic centimeters) of the peritumoral brain edema, whole tumor, enhancing tumor, non-enhancing tumor, necrotic portion, and hemorrhagic portion were 26.09 ± 30.38, 53.47 ± 39.40, 16.34 ± 18.58, 29.84 ± 38.25, 5.78 ± 14.65, and 3.82 ± 11.91, respectively. The mean ALPS index for grade II, grade III, and grade IV gliomas were 1.454 ± 0.186, 1.421 ± 0.214, and 1.220 ± 0.112, respectively.

The results of ALPS index comparisons according to sex, tumor grade, and IDH1 mutation status with age adjustment are summarized in Table 2 . The ALPS index of women was not significantly different from that of men (p = 0.098). The ALPS index of grade II and grade III gliomas was significantly higher than that of grade IV gliomas (p < 0.001). However, the ALPS index was not different between grade II and III gliomas (p = 0.670). The ALPS index of IDH1 wild-type gliomas was significantly lower than that of IDH1 mutant gliomas (p < 0.001). Figure 2 shows the differences in the ALPS index according to sex, tumor grade, and IDH1 mutation status.

The results of linear regression analyses of the factors associated with the ALPS index are summarized in Table 3 . On univariable linear regression analysis, the ALPS index correlated with age (β = −0.147, p = 0.038), sex (β = −0.139, p = 0.049), tumor grade (β = −0.361, p < 0.001), IDH1 mutation (β = 0.384, p < 0.001), peritumoral brain edema volume (β = −0.439, p < 0.001), enhancing tumor volume (β = −0.348, p < 0.001), and non-enhancing tumor volume (β = 0.307, p < 0.001). The associations of the ALPS index with whole tumor volume (β = 0.101, p = 0.155), necrotic portion volume (β = −0.058, p = 0.410), and hemorrhagic portion volume (β = 0.002, p = 0.983) were not statistically significant. Figure 3 illustrates the correlations of the ALPS index with age, peritumoral brain edema volume, enhancing tumor volume, and non-enhancing tumor volume.

On stepwise multivariable linear regression analysis, IDH1 mutation (β = 0.308; p < 0.001) and peritumoral brain edema volume (β = −0.353; p < 0.001) were the two independent factors associated with the ALPS index. A lower ALPS index was associated with IDH1 wild-type gliomas and larger peritumoral brain edema volume.

In the present study, the IDH1 mutation status of gliomas and the peritumoral brain edema volume were two independent factors associated with the ALPS index. As reflected by a lower ALPS index, glymphatic function was impaired in IDH1 wild-type gliomas and gliomas associated with larger peritumoral brain edema volumes. To our knowledge, this is the first study of human glymphatic function in patients with glioma.

The inverse correlation between the ALPS index and peritumoral brain edema volume was observed in meningiomas (17). It was proposed that tumor growth disrupts the balance between periarterial CSF influx and perivenous interstitial fluid efflux and results in the accumulation of interstitial fluid, i.e., peritumoral brain edema. A higher glymphatic function may facilitate interstitial fluid clearance and reduce or even prevent edema. In contrast, insufficient glymphatic function for interstitial fluid clearance may contribute to edema formation. Therefore, glymphatic dysfunction may explain the pathogenesis of peritumoral brain edema in meningiomas, which are extra-axial and have no direct contact with the brain interstitium.

The pathogenesis of peritumoral brain edema in gliomas is traditionally thought to represent the net transport of fluid from the intravascular compartment into the brain interstitium due to the proliferation of microvessels that have defects in their inter-endothelial tight junctions (18). However, this theory does not explain the formation of peritumoral brain edema in low-grade gliomas with intact tight junctions (19, 20). In our study, we also observed an inverse relationship between the ALPS index and peritumoral brain edema volume. This finding suggests that the peritumoral brain edema associated with intra-axial tumors (i.e., gliomas) may also be related to glymphatic dysfunction.

A recent study has shown that changes in the supporting structures of the blood–brain barrier, such as astrocytes, pericytes, and microglial cells, may also be associated with the influx of fluid into the brain interstitium (21). The astrocyte covering of brain microvessels seems to be rate limiting to water movement (21), and AQP4 water channels located on astrocytic foot processes may play a significant role in peritumoral brain edema formation. A strong correlation between peritumoral brain edema and upregulated astrocyte AQP4 expression in human gliomas suggests that increased AQP4 expression may be essential to the pathogenesis of peritumoral brain edema (22). Since AQP4 water channels are part of the glymphatic system, we speculate that there may an association between the ALPS index and the expression of AQP4 water channels. Further studies are needed to establish the role of the ALPS index as an imaging marker of AQP4 expression in gliomas.

The changes of glymphatic function associated with glioma growth have only been investigated in a few animal studies. In a study using an orthotopic xenograft glioma model (5), glioma growth caused glymphatic dysfunction with reduced CSF flow to the extracranial space. In our study, the glymphatic function of IDH1 wild-type gliomas, measured with the ALPS index, was significantly lower than that of IDH1 mutant gliomas. To the best of our knowledge, no human studies have reported the association between glymphatic function and the IDH1 mutation status of gliomas.

IDH1 wild-type gliomas are known to behave more aggressively than IDH1 mutant gliomas (23). We speculate that the spread of IDH1 wild-type gliomas is associated with greater disruption to the flow of interstitial fluid and, consequently, lower glymphatic function. Alternatively, tumor-associated glymphatic pathway remodeling may be responsible for the differences in glymphatic function between IDH1 wild-type and IDH1 mutant gliomas. In mice harboring gliomas, glymphatic function is increased for waste and fluid clearance by extensive growth of meningeal lymphatic vessels, which are downstream of the glymphatic pathway (4). A previous study showed that the growth of glioblastomas (mostly IDH1 wild-type) from small to large tumors could take as short as 1.5 months (24). In contrast, IDH1 mutant gliomas are known for longer survival and may be stable for years (25). We therefore speculate that the lower glymphatic function of IDH1 wild-type gliomas may be related to their short duration of tumor growth, which does not allow extensive remodeling of the glymphatic pathway.

Although the mechanism for the association between glymphatic function and IDH1 mutation status is unclear, the relationship has revealed some potential utilities for the ALPS index. Many MR imaging markers have been found to be useful in predicting the IDH1 mutation status in gliomas (26–29). However, most of them are measured in the tumors, but may not be feasible when a large hemorrhage or necrosis is present. In contrast, the ALPS index is measured in the periventricular region outside of the tumor and, thus, may serve as an alternative marker for IDH1 mutation prediction. Besides, the ALPS index may also be a prognostic factor for patient survival due to its correlation with IDH1 mutation status. Further studies are needed to establish the predictive and prognostic roles of the ALPS index.

The anatomical relationship of the perivascular spaces, projection fibers, and association fibers in the periventricular region is the theoretical basis of the DTI-ALPS method. Although this method is based on a deductive model, it is frequently used to investigate human glymphatic function. Its potential to serve as an imaging marker of glymphatic activity has been reported in many clinical conditions such as Alzheimer’s disease (10, 30), normal pressure hydrocephalus (31, 32), Parkinson disease (33, 34), age-related iron deposition (15), diabetic cognitive impairment (35), and meningioma-associated brain edema (17). However, there are limitations to this method and, consequently, our study. Firstly, the diffusion signal measured in clinical settings reflects overall changes in water mobility associated with many processes occurring at scales much smaller than typical MRI voxels. Therefore, we cannot definitely state that the ALPS index is a measure of glymphatic function. Secondly, the ALPS index could only measure the perivascular water flow that runs in a right-to-left direction and in the periventricular region. Further studies are needed to establish its correlation with overall glymphatic function. Finally, our study is a snapshot in time and does not include longitudinal data on temporal changes of the ALPS index and peritumoral brain edema volume following treatment. These pieces of information would be helpful to further establish the role of the glymphatic system in brain edema formation.

In conclusion, IDH1 wild-type gliomas and gliomas with larger peritumoral brain edema volumes were associated with a lower ALPS index, which may reflect an impaired glymphatic function. The lower ALPS index in IDH1 wild-type gliomas may suggest an association between glymphatic dysfunction and tumor aggressiveness. In addition, the correlation between larger peritumoral brain edema volumes and a lower ALPS index suggests that the formation of peritumoral brain edema may be related to glymphatic dysfunction.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The studies involving human participants were reviewed and approved by Chang Gung Medical Foundation Institutional Review Board. Written informed consent for participation was not required for this study in accordance with the national legislation and the institutional requirements.

CT and TS contributed to the conception and design of the study and to the acquisition and analysis of data. CT drafted the text, prepared the figures, and wrote the first draft of the manuscript. All authors read the final manuscript. All authors contributed to the article and approved the submitted version.

This study was funded by the National Science Council, Taiwan (NSC98-2314-B-182A-051-MY3 to CT).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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