Vascular dysfunction in children and young adults with autosomal dominant polycystic kidney disease

Abstract

Background: Adults with autosomal dominant polycystic kidney disease (ADPKD) exhibit vascular dysfunction, as evidenced by impaired endothelium-dependent dilation (EDD) and stiffening of the large elastic arteries. However, it is unknown whether vascular dysfunction begins earlier in the course of ADPKD. The aim of the study was to assess EDD and arterial stiffness in children and young adults with ADPKD.

Methods: Fifteen children and young adults 6–22 years of age with ADPKD and normal renal function were prospectively recruited for participation in a cross-sectional study. Fifteen healthy controls were enrolled to match cases for age and sex. The primary outcomes were EDD, measured as brachial artery flow-mediated dilation (FMD_BA), and arterial stiffness, measured as carotid-femoral pulse wave velocity (CFPWV).

Results: ADPKD cases were more likely to be taking an angiotensin-converting enzyme inhibitor, but otherwise did not differ from controls in clinical characteristics, including blood pressure. FMD_BA was 25% lower in children and young adults with ADPKD (7.7 ± 0.9%, mean ± SE) when compared with matched controls (10.2 ± 0.8%) (P < 0.05). CFPWV was 14% higher in children and young adults with ADPKD (544 ± 23 cm/s) when compared with matched controls (478 ± 17 cm/s) (P < 0.05). Secondary measures of arterial stiffness, carotid augmentation index and carotid systolic blood pressure were also increased in cases when compared with controls (P < 0.05).

Conclusions: Impaired EDD and increased arterial stiffness, important independent predictors of future cardiovascular events and mortality, are evident very early in the course of ADPKD in the presence of normal kidney function. Novel interventions to reduce vascular dysfunction in children and young adults with ADPKD should be evaluated, as childhood and young adulthood may represent a critical therapeutic window to reduce future cardiovascular risk in patients with ADPKD.

INTRODUCTION

Autosomal dominant polycystic kidney disease (ADPKD) is the most common life-threatening genetic disease, affecting more than 600 000 Americans [1, 2]. While the hallmark of ADPKD is the development and continued growth of multiple renal cysts that result in ultimate loss of kidney function [3], the leading cause of death among affected patients is cardiovascular in nature [1, 2]. As much as 80% of all cardiovascular diseases (CVDs) are associated with dysfunction and disorders of the arteries [4]. Two of the greatest contributors are vascular endothelial dysfunction, most commonly assessed as impaired endothelium-dependent dilation (EDD), and stiffening of the large elastic arteries (aorta and carotid arteries) [5]. Importantly, both measures are independent predictors of future cardiovascular events and mortality [6–9].

Adults with ADPKD demonstrate impaired EDD [10–12] as well as large elastic artery stiffening [13]. However, it is currently unknown whether these vascular changes are evident earlier in the course of ADPKD when kidney function is still preserved. Complications of ADPKD are known to begin in childhood [14]. Thus, vascular dysfunction may begin in childhood or young adulthood in patients with ADPKD.

Accordingly, the aim of this study was to assess EDD, as measured by brachial artery flow-mediated dilation (FMD_BA), and arterial stiffness, as measured by carotid-femoral pulse wave velocity (CFPWV), in children and young adults with ADPKD. Cases of ADPKD were matched for age and sex to healthy controls and vascular function was compared between groups. We hypothesized that vascular dysfunction would be evident in children and young adults with ADPKD.

MATERIALS AND METHODS

Participants

Fifteen children and young adults 6–22 years of age with a documented diagnosis of ADPKD based on prior renal imaging and normal renal function were prospectively recruited to participate in this cross-sectional study at the University of Colorado Anschutz Medical Campus assessing vascular function. Fifteen healthy controls were enrolled to match cases for age (±2 years), sex and, when possible, race/ethnicity. Six years of age was selected at the lower age limit because it would be challenging for younger children to remain still during the vascular measurements. All participants were free from active infection, immunosuppressive therapy in the last year, had not been hospitalized in the last month, had a body mass index (BMI) <95th percentile (<18 years of age) or <40 kg/m² (18–22 years of age) and were nonsmokers. Females were not pregnant or lactating. Cases were free from uncontrolled hypertension (blood pressure ≥140/90 mmHg in adults and ≥95th percentile for age, sex and height for children) and had normal renal function [estimated glomerular filtration rate (eGFR) >80 mL/min/1.73 m² using the Schwartz formula [15] if <18 years of age or Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) prediction equation [16] if ≥18 years of age]. All controls were non hypertensive and free from chronic disease.

All procedures were approved by the Colorado Multiple Institutional Review Board and conform with the Declaration of Helsinki. The nature, benefits and risks of the study were explained to all participants and participant or parental written informed consent was obtained prior to participation. Children <18 years of age provided assent.

Clinical assessment

eGFR was calculated in ADPKD cases by obtaining serum creatinine levels from clinical records (available from 12 participants; all participants had normal renal function by self-report). BMI was determined using height and weight to the nearest tenth. Blood pressure was measured manually under quiet resting conditions in the supine position during the assessment of CFPWV, as described below.

Assessment of vascular function

Vascular measurements were performed following an overnight fast (water only), 48-h abstention form any non-prescription medications and 24-h abstention from vigorous exercise and alcohol (if applicable). Participants rested quietly supine for at least 15 minutes prior to beginning the vascular assessment. All analyses were performed by an investigator blinded to participant status.

FMD_BA

FMD_BA (occlusion cuff on the upper forearm) and peak shear rate during FMD_BA were assessed using duplex ultrasonography (Xario 200, Toshiba, Tustin, CA, USA) with a multifrequency wide-footprint 10-MHz linear transducer (5–14 MHz), as described previously by the investigators [17–20]. Baseline, occlusion (i.e. the average diameter for the 15 s immediately before rapid cuff deflation) and peak brachial artery diameters were analyzed using commercially available software (Vascular Research Tools 5.0, Medical Imaging Applications, Coralville, IA, USA) by the same investigator, who was blinded to the subject group assignment. Shear rate was calculated as described previously [17, 19].

CFPWV

CFPWV was measured using applanation tonometry [Noninvasive Hemodynamics Workstation (NIHem), Cardiovascular Engineering, Norwood, MA, USA], as described previously by the investigators [18, 21, 22]. A transcutaneous custom tonometer was positioned at the carotid, brachial, radial and femoral arteries to noninvasively assess CFPWV, as well as carotid-radial pulse wave velocity (CRPWV), as a measure of peripheral stiffness. The distance from the suprasternal notch to the carotid was subtracted from the distance between the two recording sites, and pulse wave velocity was calculated as distance divided by time between the foot of waveforms recorded at each site.

Secondary measures of arterial stiffness

Immediately following applanation tonometry, the right common carotid artery was imaged using high-resolution ultrasonography (Xario 200) for subsequent analysis of change in diameter (Vascular Research Tools 5.0). Carotid arterial compliance and carotid artery β-stiffness index were calculated using the change in carotid artery diameter and carotid waveforms and brachial artery blood pressure obtained during tonometry (Noninvasive Hemodynamics Workstation), as described previously [23]. Carotid intimal-medial thickness (IMT), carotid augmentation index (AIx) and carotid artery blood pressure were also assessed [23].

Statistics

The prespecified primary outcomes for this study were FMD_BA and CFPWV. Secondary outcomes were CRPWV, carotid AIx, carotid artery compliance, carotid artery β-stiffness index and carotid systolic blood pressure (SBP). Primary outcomes, secondary outcomes, potential covariates and clinical characteristics were compared between groups using an independent samples t-test for normally distributed data and a Mann–Whitney U-test for skewed data, as determined by the Shapiro–Wilk test of normality. Potential bivariate relations of interest between patient characteristics, including age, SBP (brachial and carotid) and eGFR, and CFPWV were assessed using Pearson product–moment correlation analyses. All analyses were performed in SPSS 23 (IBM, Armonk, NY, USA). All data are reported as mean ± SE or median (interquartile range) for skewed data. Statistical significance for all analyses was set at P < 0.05.

RESULTS

Clinical characteristics

Participants were successfully matched for age, sex and race/ethnicity (Table 1). Two children with ADPKD were ≤10 years of age and five were ≤16 years of age. ADPKD patients had an eGFR in the normal range. Cases and controls did not differ in height, weight, BMI category, blood pressure or heart rate. Seven ADPKD cases were taking an angiotensin-converting enzyme inhibitor (ACEi; P = 0.006 versus control), while controls were free from hypertension for study inclusion. However, no cases had uncontrolled (≥140/90 mmHg in adults or ≥95th percentile for age, sex and height in children) blood pressure. Cases and controls did not significantly differ in the use of other prescription medications.

Table 1.

Clinical characteristics

Variable	ADPKD (n = 15)	Healthy controls (n = 15)	P-value
Age, years	21 (15–22)	20 (16–20)	0.27
Female	11 (73%)	11 (73%)	1.00
Caucasian	12 (80%)	13 (87%)	0.60
eGFR, mL/min/1.73m2	125 ± 22	N/A	N/A
Height, cm	167.6 (162.6–180.0)	167.6 (160.0–179.2)	0.84
Mass, kg	64.5 ± 5.6	60.9 ± 4.3	0.62
BMI category			0.43
Normal weight or underweight	9 (60%)	12 (80%)
Overweight or obese	6 (40%)	3 (20%)
SBP, mmHg	114 ± 5	108 ± 3	0.33
DBP, mmHg	63 ± 3	59 ± 2	0.28
Heart rate, beats per min	63 ± 3	63 ± 2	0.97
Medications
ACEi	7 (46%)	0 (0%)	<0.01
Oral contraceptive or IUD	4 (26%)	2 (13%)	0.65
Psychostimulant for ADHD	1 (7%)	2 (13%)	1.00
Antiseizure for migraines	0 (0%)	1 (7%)	1.00
Albuterol	1 (7%)	0 (0%)	1.00

Data are presented as n (%), median (interquartile range) or mean ± SE.

eGFR, estimated glomerular filtration rate (Schwartz formula if <18 years of age or CKD-EPI equation if ≥18 years of age; available from n = 12); ACEi, angiotensin-converting enzyme inhibitor; IUD, intrauterine device; ADHD, attention deficit hyperactivity disorder.

FMD_BA

FMD_BA was 25% lower in children and young adults with ADPKD (7.7 ± 0.9%) when compared with matched controls (10.2 ± 0.8%) (Figure 1). Groups did not differ in baseline diameter or peak shear rate (potential covariates, Table 2), thus statistical correction for these variables was not performed.

FIGURE 1.

(A) Mean ± SE and (B) individual participant values for brachial artery flow-mediated dilation (FMD) in children and young adults with ADPKD (white bars) and age- and sex-matched healthy controls (black bars). *P < 0.05.

Table 2.

Hemodynamic factors

Variable	ADPKD	Healthy controls	P-value
Baseline brachial artery diameter, mm	3.25 ± 0.10	3.21 ± 0.15	0.83
Peak shear rate, per second	829 ± 102	682 ± 75	0.26
Carotid AIx, %	0.33 ± 2.9	−8.1 ± 2.4	0.034
Carotid IMT, mm	0.41 ± 0.016	0.40 ± 0.009	0.86
Carotid artery compliance, mm/mmHg × 10−1	0.15 ± 0.014	0.18 ± 0.015	0.23
β-Stiffness index, AU	4.57 ± 0.32	3.86 ± 0.30	0.12
Carotid SBP, mmHg	107 ± 4	94 ± 2	0.012
CRPWV, cm/s	808 ± 45	702 ± 28	0.054

Data are presented as mean ± SE.

CFPWV

CFPWV was 14% higher in children and young adults with ADPKD (544 ± 23 cm/s) when compared with matched controls (478 ± 17 cm/s) (Figure 2). CFPWV was strongly associated with both brachial (r = 0.68, P < 0.01) and carotid SBP (r = 0.71, P < 0.01) in ADPKD, but not age or eGFR.

FIGURE 2.

(A) Mean ± SE and (B) individual participant values for CFPWV in children and young adults with ADPKD (white bars) and age- and sex-matched healthy controls (black bars). *P < 0.05.

Secondary measures of arterial stiffness

ADPKD cases had significantly greater carotid AIx and carotid SBP and tended to have greater CRPWV when compared with healthy controls (Table 2). Groups did not differ in carotid IMT, carotid artery compliance or carotid artery β-stiffness index.

DISCUSSION

In the first study evaluating vascular function in children and young adults with ADPKD, we found evidence of vascular dysfunction very early in the course of the disease. FMD_BA, a measure of EDD, was significantly impaired and CFPWV, the gold standard index of large elastic artery stiffness was increased in children and young adults with ADPKD when compared with matched healthy controls. Additionally, secondary endpoints of carotid AIx and carotid SBP were also increased in the ADPKD group. This is the first evidence that impairment in vascular function, a key antecedent of CVD, begins at a young age in patients with ADPKD.

Two previous studies of young to middle-aged adults evaluated FMD_BA in patients with ADPKD and normal renal function [10, 24]. FMD_BA was impaired when compared with healthy controls [10, 24], with an even greater impairment in APDKD patients who were also hypertensive [10]. Additionally, impaired EDD to acetylcholine has been demonstrated in subcutaneous resistance arterioles dissected from fat biopsy samples collected from middle-aged adults with ADPKD and normal renal function [12]. The present study extends these findings by demonstrating impaired EDD even earlier in the course of ADPKD.

In contrast, two previous studies assessing CFPWV in young to middle-aged adults with ADPKD and normal renal function found no significant increase in arterial stiffness when compared with healthy controls [13, 24]. It is unclear why an increase in CFPWV was not seen in these studies yet was in the present study; however, methodology to assess CFPWV or study inclusion criteria may have contributed to these differences. However, the present finding of increased carotid AIx, another index of arterial stiffness, is consistent with an increase in aortic AIx also shown previously in young adult ADPKD patients with normotension and normal renal function [13].

As additional indices of local arterial stiffness, we also evaluated carotid artery compliance, carotid artery β-stiffness index and carotid SBP, which have not been measured previously in an ADPKD population. Carotid SBP was significantly increased in the ADPKD patients, even with similar brachial SBP when compared with healthy controls. Central blood pressure may be a stronger predictor of cardiovascular outcomes than brachial artery blood pressure [25, 26]. Both carotid and brachial SBP had a strong positive association with CFPWV in cases, consistent with the known relation between blood pressure and arterial stiffness. In contrast, there was no difference in carotid artery compliance and carotid artery β-stiffness index, indicating a lack of increase in local arterial stiffness, despite the increase in aortic stiffness reflected by CFPWV. Additionally, there was no increase in carotid IMT, unlike in a previous study in middle-aged adults with ADPKD and normal renal function [10], indicating that the increase in carotid IMT in ADPKD may be delayed until middle age. Last, there was a surprising trend towards an increase in CRPWV, an index of peripheral arterial stiffness, in ADPKD patients in the present study. In addition to central stiffness, stiffening of the peripheral muscular arteries is also evident with declining renal function [27], but peripheral stiffness has only been evaluated in one other study of patients with ADPKD. In this study, there was also a non-significant trend towards increased peripheral stiffness in adults with ADPKD [13].

A contributing mechanism to the observed impairment in EDD and increased arterial stiffness is likely reduced nitric oxide (NO) bioavailability. FMD_BA is mediated, albeit not exclusively, by the production of NO [28]. Additionally, in the study that found impaired resistance arteriole dilation to acetylcholine in patients with ADPKD, coinfusion of the NO synthase inhibitor LG-nitro-l-arginine methyl ester demonstrated that this impairment was mediated by reduced NO bioavailability [12]. Impaired NO bioavailability is a common mechanism in both impaired EDD and increased arterial stiffness [5, 29]. In ADPKD, oxidative stress and inflammation are both increased and contribute to the decline in NO bioavailability [30–32]. However, it is possible that in children and young adults with ADPKD, vascular smooth muscle cell impairment also contributed, at least in part, to the impaired brachial artery dilation and forearm ischemia, as we did not assess endothelium-independent dilation to nitroglycerin to exclude this possibility. Given that the population studied included children <18 years of age, we felt this procedure posed unnecessary risk to a vulnerable population. In a previous study of middle-aged adults with ADPKD and normal renal function, endothelium-independent dilation was also impaired when compared with healthy controls [10].

Several important strengths of the present study merit emphasis. This is the first study to evaluate vascular function in a novel population including children and young adults with ADPKD. Patients with ADPKD were matched to healthy controls for age, sex and, when possible, race/ethnicity, and did not differ in other assessed clinical characteristics, thus minimizing the likelihood that group differences were due to factors other than ADPKD. Although approximately half of the cases (and no controls, as they were all normotensive) were taking an ACEi, if anything this would be expected to minimize the differences between groups, as an ACEi has been shown to both improve FMD_BA [33–35] and reduce aortic pulse wave velocity [36–38] in various populations. Last, an additional strength of the study is comprehensive assessment of arterial stiffness, including secondary indices in addition to CFPWV.

A limitation of the study is the lack of blood samples collected, in order to minimize the burden in a pediatric population and keep the study completely noninvasive. Thus, it was not possible to assess circulating clinical factors or markers of physiological mechanisms. However, we did verify normal renal function in ADPKD cases using clinical assessment of serum creatinine based on medical records. We also did not have a recent assessment of total kidney volume or cyst volume available to determine the association of vascular function with these variables. Additionally, there may have been residual differences between groups not accounted for by matching or in the assessed clinical characteristics that contributed to group differences in vascular function beyond the primary effect of ADPKD. Last, the sample size was small, but consistent with previous vascular studies in adults with ADPKD, and adequately powered for the purpose of providing initial insight into vascular function in children and young adults with ADPKD.

In conclusion, impaired EDD and increased arterial stiffness, important independent predictors of future cardiovascular events and mortality [6–9], are evident very early in the course of ADPKD. Future research is needed to delineate the mechanisms contributing to the observed impairment. Additionally, novel interventions to reduce vascular dysfunction in children and young adults with ADPKD should be evaluated, as childhood and young adulthood may represent a critical therapeutic window to reduce future cardiovascular risk in patients with ADPKD. For example, an ongoing randomized controlled trial is being conducted to assess whether early intervention with the polyphenol curcumin, which suppresses oxidative stress and inflammation, may reduce vascular dysfunction in this population (NCT02494141). While the time course to improve vascular endothelial function and reduce arterial stiffness is currently unknown, each participant will receive curcumin or placebo for 1 year. An additional intervention that may be of interest is treatment with a statin, as therapy has been shown to slow kidney growth in children and young adults with ADPKD [39], as well as improve vascular function in other populations [40–42].