1. Introduction
Globally, glaucoma is one of the leading causes of irreversible blindness [
1]. The global number of glaucoma patients is projected to continue rising due to population aging [
2]. Increased intraocular pressure (IOP) is the strongest risk factor for glaucoma progression [
1,
3]. Additionally, vascular factors such as blood pressure (BP) and ocular perfusion pressure (OPP) may contribute to its development and progression [
4,
5,
6,
7]. Although the pathogenesis of glaucoma is thought to involve multiple factors, it remains incompletely understood.
Recent evidence suggests that glaucoma may be associated with autonomic nervous system (ANS) dysfunction [
8,
9,
10,
11,
12], which could contribute to its progression by altering IOP through the regulation of aqueous humor production and outflow [
13], and by influencing vascular risk factors such as BP and OPP [
13,
14,
15,
16]. The association between heart rate variability (HRV), a marker of autonomic function, and glaucoma has also been demonstrated in a population-based cohort study [
17]. In our previous studies, we compared HRV between primary open-angle glaucoma (PG) and exfoliation glaucoma (EG) and reported that sympathetic predominance was significantly greater in EG [
11,
18].
Moreover, systemic arterial stiffness and atherosclerosis may be associated with the pathogenesis of glaucoma. Several studies reported the association between increased systemic arterial stiffness and glaucoma [
19,
20,
21,
22,
23]. Additionally, our previous study demonstrated patients with EG may have greater arterial stiffness compared to those with PG or the controls [
23].
Despite these findings, the association between autonomic and hemodynamic function and systemic comorbidity burden in patients with glaucoma remains unclear. To address this gap, the present study aimed to examine the association between HRV and APG parameters and the Age-adjusted Charlson Comorbidity Index (ACCI) in patients with glaucoma. Furthermore, we investigated this association separately in patients with PG and those with EG.
2. Materials and Methods
2.1. Study Design and Participants
Ethics approval was obtained from the Institutional Review Board of Shimane University Hospital (Approval No. 20200228-2; revision dated 27 October 2024). All procedures complied with the Declaration of Helsinki. Informed consent was waived on the basis of public disclosure on the institutional website with an opt-out mechanism. All patient data were anonymized before analysis. Participants with a diagnosis of PG or EG were enrolled at Shimane University from June 2023 to July 2024. Ophthalmologists established the diagnosis of glaucoma using IOP, gonioscopy, optic nerve evaluation via fundus photography and optical coherence tomography (OCT), and visual field assessments. For analysis, one eye per participant was included. If disease was unilateral, we selected the diseased eye. If bilateral, we included the eye with the higher IOP; when equal, the right eye was chosen. Participants with a measured reliability score <95% or with ocular diseases other than PG, EG, or cataract were excluded from the study. Ophthalmic data obtained from patient records included the highest IOP recorded during follow-up, the medication score, and the mean deviation (MD) values derived from the central 30-2 perimetric protocol of the Humphrey Visual Field Analyzer (Carl Zeiss Meditec, Jena, Germany). IOP was measured by Goldmann applanation tonometry (Haag-Streit, Köniz, Switzerland). For the medication score, each topical drug component counted as 1 point, as did each tablet of oral acetazolamide.
2.2. Age-Adjusted Charlson Comorbidity Index (ACCI)
The Charlson Comorbidity Index (CCI) quantifies comorbidity burden by assigning weighted scores to 19 chronic conditions, such as cardiovascular, pulmonary, hepatic, renal, and malignant diseases, as well as diabetes and dementia [
24]. The total CCI score estimates the 10-year mortality risk, with higher scores reflecting greater disease burden [
24]. To enhance its prognostic accuracy in older individuals, ACCI incorporates age by adding 1 point for individuals aged 41–50, 2 points for those 51–60, 3 points for 61–70, 4 points for 71–80, and 5 points for those 81 and older, thereby accounting for both comorbidities and age-related vulnerability [
25].
2.3. Heart Rate Variability (HRV)
HRV was measured in each participant to assess ANS activity using a sphygmograph device (TAS9 Pulse Analyzer Plus View; YKC Corp., Tokyo, Japan). All measurements were performed during daytime outpatient visits with participants seated, following a standardized 2.5 min protocol at a sampling frequency of 1 kHz. In this study, both time- and frequency-domain HRV parameters were used to assess autonomic function [
26,
27,
28,
29,
30]. The time-domain parameters included the standard deviation of normal-to-normal intervals (SDNN), reflecting overall autonomic activity; the square root of the mean of the sum of the squared differences between adjacent normal-to-normal intervals (RMSSD), an index of cardiac parasympathetic activity; and the coefficient of variation in R-R intervals (CVRR), indicative of parasympathetic function [
26]. The frequency-domain parameters included total power (TP), indicating overall autonomic activity; very-low-frequency (VLF, 0.0033–0.04 Hz), related to thermoregulation and long-term control; low-frequency (LF, 0.04–0.15 Hz), reflecting both sympathetic and parasympathetic activity with baroreflex influence; high-frequency (HF, 0.15–0.40 Hz), representing parasympathetic activity; and the LF/HF ratio, an index of sympathovagal balance, with higher values indicating sympathetic dominance [
27]. To approximate a normal distribution for statistical analysis, each parameter was transformed using the natural logarithm and analyzed as LnTP, LnVLF, LnLF, LnHF, and LnLF/LnHF.
2.4. Accelerated Plethysmography (APG)
As with HRV parameters, APG parameters were measured using the same device in APG measurement mode. APG measurements were also performed under the same conditions during daytime outpatient visits. The method for calculating APG parameters has been described in detail in our previous study [
23]. In this study, the APG parameters included the following five components: a peak, the initial systolic upstroke associated with left ventricular ejection; b peak, the early systolic reflection wave reflecting cardiac output; c peak, the late systolic component reflecting residual blood volume; d peak, the diastolic reflection wave indicating arterial elasticity; e peak, the end-diastolic waveform reflecting peripheral vascular resistance [
31,
32]. The APG waveform was categorized into the following vascular types: Type A, reflecting elastic and healthy arteries; Type B, reflecting moderate arterial stiffness; Type C, reflecting advanced arterial stiffness.
2.5. Statistical Analysis
For all variables, descriptive summaries are provided: continuous data are reported as mean ± standard deviation (SD), and categorical data as number (percentage). Comparisons of PG vs. EG used the independent samples t-test for continuous data and the chi-square test for categorical data. Spearman’s rank correlation was performed to examine the correlations between ACCI and HRV and APG parameters. The association between ACCI and HRV and APG parameters was evaluated using multivariate regression, adjusting for sex, body mass index (BMI), pulse rate, systolic and diastolic BP, IOP (highest recorded), medication score, MD, and glaucoma type. In this study, missing data were minimal. No missing values were observed for HRV parameters. For APG parameters, 7 cases (2.7%) had missing values, which were excluded from the corresponding analyses. All statistical analyses were conducted using R (version 4.5.1; R Core Team, Vienna, Austria). A p-value of <0.05 was considered indicative of statistical significance.
3. Results
This study included 260 subjects (260 eyes): 186 in the PG group and 74 in the EG group. As shown in
Table 1, the EG group was significantly older than the PG group (PG: 66.3 ± 12.2 years vs. EG: 74.8 ± 8.53 years,
p < 0.001). The EG group had a significantly higher ACCI compared to the PG group (PG: 3.25 ± 1.51 vs. EG: 4.00 ± 1.21,
p < 0.001). The IOP was significantly higher in the EG group compared with the PG group (PG: 20.3 ± 7.39 mmHg vs. EG: 28.0 ± 9.72 mmHg,
p < 0.001). Regarding time-domain parameters, SDNN was significantly lower in the EG group than in the PG group (PG: 35.3 ± 20.1 vs. EG: 32.0 ± 18.5,
p = 0.047). Among the frequency-domain parameters, the EG group showed significantly lower values than the PG group for LnVLF (PG: 5.65 ± 0.75 vs. EG: 5.48 ± 0.69,
p = 0.006), LnLF (PG: 4.16 ± 1.35 vs. EG: 3.78 ± 1.52,
p = 0.001), and LnHF (PG: 4.53 ± 1.36 vs. EG: 4.26 ± 1.47,
p = 0.022). In terms of the APG parameters, the c peak and d peak were significantly lower in the EG group for c peak (PG: −152.9 ± 110.1 vs. EG: −183.1 ± 101.0,
p = 0.045) and d peak (PG: −322.7 ± 127.6 vs. EG: −360.5 ± 103.5,
p = 0.026).
Figure 1 shows the correlations between ACCI and HRV parameters. Significant negative correlations were observed for LnLF (R = −0.17,
p = 0.0062) and LnLF/LnHF (R = −0.24,
p = 0.00012), while no significant associations were found for the other parameters. In patients with PG, ACCI was inversely correlated with LnLF (R = −0.17,
p = 0.018) and LnLF/LnHF (R = −0.24,
p < 0.001). In patients with EG, ACCI showed a significant negative correlation with LnLF/LnHF (R = −0.28,
p = 0.016) (
Figure 2).
Figure 3 demonstrates the correlations between ACCI and APG parameters. Significant negative correlations were observed for d peak (R = −0.17,
p = 0.0072) and e peak (R = −0.15,
p = 0.015). Additionally, a significant positive correlation was found for b peak (R = 0.14,
p = 0.031). As shown in
Figure 4, ACCI showed significant inverse correlations with bPeak (R = −0.16,
p = 0.035), dPeak (R = −0.22,
p = 0.003), and ePeak (R = −0.17,
p = 0.022) in patient with PG.
In
Table 2, multivariable linear regression analysis revealed a significant positive association between ACCI and RMSSD (coefficient: 2.860; 95% CI: 0.130 to 5.589). In addition, pulse rate was significantly negatively associated with all three time-domain parameters.
Table 3 shows the results of the multivariable linear regression analysis for HRV frequency-domain parameters. ACCI was significantly negatively associated with LnLF (coefficient: −0.147; 95% CI: −0.274 to −0.019) and LnLF/LnHF (coefficient: −0.037; 95% CI: −0.062 to −0.011). In addition, pulse rate was significantly negatively associated with all frequency-domain parameters.
Table 4 shows the results of the multivariable linear regression analysis for APG parameters. ACCI was significantly negatively associated with e peak (coefficient: −5.89; 95% CI: −11.7 to −0.120). BMI was significantly associated with a peak and b peak. Pulse rate was significantly associated with a peak, b peak, and c peak. With regard to sBP, the significant associations with b peak and d peak were observed. Additionally, there was a significant association between MD and d peak.
Table 5 shows the results of the multivariable logistic regression analysis for vascular types. ACCI was significantly associated with Type B (coefficient: 0.305; 95% CI: 0.057 to 0.552). Type C were significantly associated with pulse rate and sBP.
Table 6 shows the association between HRV and APG parameters and ACCI, stratified by glaucoma type. In the PG group, ACCI was significantly positively associated with RMSSD (coefficient = 3.734, 95% CI: 0.505 to 6.963) and significantly negatively associated with LnLF (coefficient = −0.148, 95% CI: −0.286 to −0.009) and LF/HF (coefficient = −0.035, 95% CI: −0.066 to −0.005). In terms of the APG parameters, c peak was associated with ACCI in the PG group (coefficient = −16.3, 95% CI: −29.2 to −3.42), whereas a peak was associated with ACCI in the EG group (coefficient = −27.7, 95% CI: −45.1 to −10.4).
4. Discussion
This cross-sectional study examined the association between ACCI and HRV/APG parameters. We found a significant positive association between ACCI and RMSSD and significant negative associations between ACCI and both LnLF and LnLF/LnHF. Additionally, the negative association between ACCI and e peak was observed in this study. These findings suggest that systemic comorbidity burden may be associated with ANS activity and hemodynamics in patients with glaucoma.
This study suggests that ANS function was lower in patients with advancing age and increasing comorbidity burden. As shown in
Table 2 and
Table 3, parameters reflecting the balance between sympathetic and parasympathetic activity, such as LnLF and LnLF/LnHF, significantly decreased, whereas parasympathetic parameters, including RMSSD, significantly increased. In several previous studies, a similar trend was observed with increasing age, supporting the present findings [
11,
33,
34,
35,
36]. Moreover, a previous study of elderly inpatients reported that higher CCI scores were associated with reduced HRV [
37], suggesting a link between comorbidity burden and impaired ANS function. These findings are consistent with our results and imply that similar mechanisms may operate in patients with glaucoma.
Several hypotheses can be proposed to explain these findings. First, in glaucoma patients with advanced age and a higher burden of comorbidities, sympathetic activity may be markedly reduced, resulting in a state of relative parasympathetic predominance. Second, in elderly patients with multiple comorbidities, the use of antihypertensive agents or β-blockers may suppress sympathetic activity and enhance parasympathetic dominance. A previous study has reported that β-blocker use is associated with increased RMSSD and HF, as well as decreased LF/HF [
38]. Furthermore, Zaliunas et al. demonstrated that during amlodipine use, HF were maintained while LF/HF decreased, indicating a shift toward parasympathetic predominance [
39]. Given these findings, medication use may influence autonomic function. However, in this study, medication use was not accounted for as a confounding factor; therefore, future studies should evaluate the impact of medications on autonomic function.
There may be differences in ANS function depending on the type of glaucoma [
8,
10,
11,
40,
41]. In our previous studies, the EG group exhibited greater sympathetic predominance compared to the PG group [
11,
18]. In the present study, a significant positive association between ACCI and RMSSD and a significant negative association between ACCI and LnLF and LF/HF were observed in the PG group, whereas these associations were not significant in the EG group. As shown in
Table 1, the PG group consisted of younger patients with a significantly lower systemic comorbidity burden compared to the EG group (Age: 65.6 ± 12.7 vs. 74.9 ± 9.45 years,
p < 0.001; ACCI: 3.25 ± 1.51 vs. 4.00 ± 1.21,
p < 0.001). A systematic review demonstrated that frail older adults exhibit reduced adaptability of heart rate dynamics compared to their non-frail counterparts [
42]. Given their younger age and lower ACCI, the PG group may have retained relatively preserved adaptability of ANS, making the influence of systemic comorbidities on ANS function more apparent in this group. In contrast, in the EG group, impaired tissue integrity and vascular sclerosis may have diminished ANS responsiveness, making the effect of ACCI on HRV less apparent. Exfoliation syndrome (XFS), which underpins EG, is characterized by vascular dysregulations, including endothelial dysfunction [
43], oxidative stress [
44,
45], and abnormalities of coagulation [
46]. Additionally, a recent study has shown that systemic arteriosclerosis is more pronounced in patients with EG than in those with PG [
23]. These vascular changes may indirectly contribute to dysregulation of ANS. Our findings suggest that ANS dysfunction in PG may primarily result from systemic comorbidities, whereas in EG it may arise from disease-specific mechanisms. Future studies should further evaluate ANS adaptability according to glaucoma subtype.
Furthermore, this study suggests systemic arterial stiffness and atherosclerosis may be associated with both aging and increasing comorbidity burden in patients with glaucoma. As shown in
Figure 2, significant correlations were observed between ACCI and several parameters, including the b, d, and e peaks. After adjusting for confounding factors, there was a significant association between ACCI and the e peak, as presented in
Table 4. In addition,
Table 5 represents that ACCI was significantly associated with Type B. Our previous study demonstrated that all APG components, except for the a peak, were significantly associated with age [
23]. Several studies reported that arterial stiffness and atherosclerosis were significantly correlated with higher CCI scores [
47,
48,
49]. Therefore, the findings of our study are consistent with those of previous studies. One possible explanation for the present findings involves the following pathophysiological mechanism. Systemic arterial stiffness and atherosclerosis represent a shared pathophysiological basis among several conditions represented in the ACCI. Arterial wall stiffening associated with aging may increase the risk of developing multiple vascular-related comorbidities. Conversely, the presence of various chronic comorbidities may promote the progression of arterial stiffening through tissue remodeling processes. This study provides new insights into the relationship between arterial stiffness and comorbidity burden in glaucoma patients. Future studies are needed to further examine these relationships in patients with glaucoma.
There may be differences in the relationship between arterial stiffness and comorbidity burden among glaucoma subtypes. In this study, a significant association between a peak and ACCI was observed in the EG group. Our previous study demonstrated a significant association between a peak and pseudoexfoliation material (PEM) [
23]. In EG, the accumulation of PEM in vascular walls may impair arterial compliance, leading to reduced early systolic acceleration as reflected by a lower a peak. This vascular dysfunction may underlie the observed association between a peak and comorbidity burden in patients with EG. Additionally, a significant association between c Peak and ACCI was observed only in PG in this study. patients with PG may retain relatively preserved vascular function, allowing c Peak to more sensitively reflect systemic vascular burden. Further studies are needed to clarify the relationship between ACCI and APG parameters in each glaucoma subtype.
Visual field impairment may be related to arterial stiffness. As shown in
Table 4, there was a significant association between MD and d peak in glaucoma patients (coefficient = 2.66, 95% CI: 0.160 to 5.16). Several studies reported that systemic arterial stiffness and atherosclerosis are linked to glaucoma progression and visual field loss [
50,
51]. These associations may be influenced by various factors, including reduced blood flow to the optic nerve [
4,
7] and oxidative stress [
44].
This study has several limitations. First, this cross-sectional study was unable to clarify the causal relationship between ACCI and HRV/APG changes. Second, the timing of HRV and APG measurement was not controlled which may have impacted the results. Third, medications with potential effects on autonomic and vascular function (e.g., β-blockers, calcium channel blockers, ACE inhibitors) were not considered as confounders as mentioned in the Discussion. Future studies should include detailed information on systemic and topical medications. Fourth, no a priori sample size calculation was performed, because no prior studies were available to estimate the expected effect size. In this study, all consecutive eligible patients during the study period were included. Fifth, autonomic and vascular changes in glaucoma may be affected by the timing of measurements. Future studies will need to take the timing of measurements into account. Sixth, this single-center study conducted in a university hospital may limit the generalizability of the findings.