Monitoring and counselling
Based on the relatively high incidence of hypertension and proteinuria detailed above, two recent clinical guidelines recommend to examine urine and blood pressure in all at-risk children at regular intervals [
58,
74]. Because of generally slow disease progression, large intervals of one to several years are usually sufficient and should not be overly burdensome for those children who turn out not to be affected. For school-aged children, it is worth investing the extra effort of 24-h ambulatory rather than just clinic blood pressure measurements, due to the high incidence of isolated nighttime hypertension [
15], and to take the time to educate about lifestyle factors which influence blood pressure and progression of ADPKD (see Fig.
1 and section on “Non-pharmacological management”).
Even in children with an established diagnosis of ADPKD, repeated ultrasound examinations and “cyst counting” are of limited value for clinical management yet may cause significant anxiety for patients and families. Although the number of cysts and kidney volume have some correlation with hypertension, ultrasound does not replace blood pressure measurements. However, ultrasound examinations are useful in the work-up of clinical events such as urinary tract infections, hematuria, or abdominal pain. There is no need to screen children or adolescents for cerebral aneurysms or hepatic involvement, due to the extremely low incidence in young people with ADPKD.
Testing for hypertension and proteinuria can be done without revealing transmission status in children who have neither signs nor symptoms of ADPKD while children with positive findings can undergo focused work-up. It also provides repeated opportunities for families to communicate with their children about the possibility of disease transmission. Many families find this challenging, e.g., because of parental feelings of guilt or helplessness and traumatic experiences with the disease in older relatives, and may benefit from external help [
75,
76]. Initiating age-appropriate communication about the risk of disease early on helps to improve family communication and improves long-term coping strategies [
77,
78]. If the parents have chosen not to communicate the risk of disease transmission with their offspring during childhood, they should still be encouraged to address this issue when their children reach the age of maturity, as the ability to choose presymptomatic testing is perceived by many patients as an important and valuable opportunity to take ownership of their health [
79].
Antihypertensive and antiproteinuric treatment
Antihypertensive and antiproteinuric treatment have the dual aims of lowering cardiovascular mortality and preserving kidney function, because blood pressure is one of the important modifiable risk factors. There are universal recommendations to lower blood pressure in children with hypertension and especially children with CKD [
80‐
83] with a host of evidence, but we will focus on reviewing ADPKD-specific evidence here.
Adults with CKD already have a large increase in cardiovascular mortality, however, patients with ADPKD are at an even higher risk of cardiovascular disease than matched patients with other causes of CKD [
84,
85]. This may be due to intrarenal activation of the renin-angiotensin system by local cystic destruction and a high incidence of hypertension [
13,
20]. Placebo-controlled trials are not available for patients with ADPKD, but the decrease over time of the very high incidence of left ventricular hypertrophy in this group may be attributable to the more intensive treatment approaches to hypertension [
86]. In the HALT-PKD A study, more intensive lowering of blood pressure (but not single vs. dual RAS blockade) resulted in a greater decline of left ventricular mass index in adults with early ADPKD [
87]. This is in line with other studies which confirm a benefit of intensive blood pressure control on cardiovascular events, especially in high risk groups [
88].
Generally, proteinuric patients appear to be at greater risk of complications of hypertension and accordingly benefit more from intensive blood pressure lowering [
89], although due to the combined antihypertensive and antiproteinuric effect of RAS blockade this effect is difficult to separate in many clinical trials. In the HALT-PKD B trial an ACE inhibitor alone was not different to dual RAS blockade in preventing a composite outcome of death and loss of kidney function in later stages of ADPKD, with similar blood pressure levels achieved in both groups [
90].
Intensive blood pressure lowering appears to have greater benefits on cardiovascular mortality than on protection of kidney function in adults [
88], and this is also true for ADPKD, where intensive antihypertensive treatment reduced growth of kidney volume but not loss of GFR [
87,
91,
92]. In children, strict blood pressure control could further slow the decline of GFR in a large cohort with mixed-origin CKD [
93]. The only randomized controlled trial in children with ADPKD was much smaller and stratified into three groups of children with normal (< 75th percentile), high-normal (75th–95th percentile) and high blood pressure (> 95th percentile). Children starting from a high-normal blood pressure experienced a significant decrease in GFR and increase in serum creatinine and left ventricular mass index over 5 years, which did not occur in the group treated with ACE inhibitor [
16]. Children with established hypertension all received ACE inhibition and did not experience a significant benefit from intensified versus conventional blood pressure control, but a high drop-out rate and difficulty to obtain target-blood pressure make analysis of this group difficult.
In terms of drug choice, RAS inhibition with ACE inhibitor or ARB blocker is generally preferred for adults with CKD due to the additional renoprotective effects in proteinuric patients [
89]. For patients with ADPKD and hypertensive children, these drugs also have a much greater body of evidence, but direct evidence of their superiority over other antihypertensives is sparse [
92]. However, diuretics should probably be avoided as they are associated with a larger loss of GFR in a small retrospective study [
94]. Calcium channel blockers increase cyst growth in an animal model of ADPKD [
95], but studies in humans have not shown a large effect [
96‐
98]. In adults with ADPKD, there is no convincing benefit of double RAS control versus treatment with ACE inhibition or ARB blockade alone [
87].
Pharmacological treatment to delay disease progression
This has been a very active area of research in the last decade. Many potential drugs have been identified from in vitro, animal and genetic studies, partly with extensive bioinformatic analysis [
99], but we will focus here on those with pediatric data or which have been licensed in adults.
From a pediatric perspective, the fact that loss of GFR is a very gradual, continuous process, has both upsides and downsides: on the one hand, there is a theoretical benefit of starting to delay disease progression as early as possible in order to maximize the achieved delay of kidney failure [
100]. Mathematical models of ADPKD progression support this idea; however, they necessarily extrapolate medium-term data to much longer time intervals [
101]. On the other hand, starting pharmacological treatment early in life requires drugs with very good safety profiles. Additionally, there are several principal dilemmas for interventional pediatric ADPKD studies: firstly, GFR is not a good marker of progression in early disease (see ‘Complications of ADPKD’ above); however, even though kidney volume correlates well with kidney function and progression in adults [
49], some drugs have had discrepant effects on the two and therefore kidney volume is not sufficient as a sole outcome measure [
102]. Secondly, even a small reduction in the rate of disease progression would have a significant long-term effect but would require very large or very long-term studies, which are both difficult. Thirdly, in “real life” treatment of adolescents is often hampered with serious compliance problems and is more difficult in patients who feel subjectively well.
Nonetheless, in children with ADPKD who already received ACE inhibition, a prospective randomized controlled trial of pravastatin could show a significant reduction of height-adjusted kidney growth over 3 years [
103]. There was also a non-significant reduction in left ventricular mass index, and kidney function was unchanged, as could be expected in a group with a mean creatinine clearance of 135 ml/min per 1.73 m
2 over this time span. Despite this promising effect, and a plausible mode of action via the anti-proliferative, anti-inflammatory and anti-oxidant effects of statins (HMG-CoA reductase inhibitors), which improve endothelial function and increase kidney blood flow, no larger confirmative studies have been initiated. This is probably due to lack of financial incentives and because the results are less relevant to the much larger adult ADPKD population, where statins are frequently used for their well-documented positive effect on the rate of death and cardiovascular events in CKD [
104]. With regard to kidney function in adults with ADPKD, two small randomized trials of statins show conflicting results [
105,
106] and a recent post hoc analysis of large adult treatment trials did not show positive effects of ongoing (non-standardized) statin treatments [
107]. In terms of safety of statins in children, there were no safety concerns in the 3-year ADPKD study with around 50 children who received pravastatin, but longer-term safety data are only available from a variety of small studies in children with hyperlipidemia (e.g., [
108]). At least recent post hoc analyses are reassuring about the safety of combined use of tolvaptan and statins in adults [
109].
The vasopressin 2 receptor antagonist tolvaptan is thought to reduce cyst growth by blocking the effect of arginine vasopressin on collecting duct cells, which otherwise transmits proliferative stimuli via cyclic AMP and promotes secretion of fluids into the cyst lumen via the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel. Clinically, tolvaptan causes the picture of renal diabetes insipidus with polyuria due to loss of free water. In a large randomized trial in adults (TEMPO 3:4), tolvaptan has been shown to reduce the rate of GFR loss as well as growth of total kidney volume in adults with ADPKD [
90,
110] and has consequently become the first licensed drug for delaying disease progression, but only for adults with rapidly progressive disease. Difficulties of treatment include aquaretic side effects such as large polyuria, thirst and nocturia, which were experienced by over 65% of patients and led 7% to discontinue the drug [
111]. Idiosyncratic liver damage is another issue, affecting about 1.2% of patients in TEMPO 3:4 and 3.7% in the extension study TEMPO 4:4. It appears to be a specific but reversible side effect in ADPKD which warrants regular control of liver function enzymes [
112]. Another important unresolved question, which is particularly relevant to patients with early ADPKD, is whether the effect of tolvaptan really is sustained in the long-term. While it was continuous over the 3-year period of TEMPO 3:4, the follow-up study up to 5 years was not totally conclusive on this [
113‐
115].
From TEMPO 3:4, there is reason to believe that young patients benefit from tolvaptan too. Two post-hoc analyses in the subgroups of patients with CKD Stage 1 and in 18–24 year olds showed significantly slower growth of kidney volume with treatment compared to placebo [
116,
117]. However, decrease of GFR decline was not significant in both. Patients with CKD 1 and 2 had less hypernatremia than those with CKD 3, and were not more likely to suffer liver damage [
116]. Patients with CKD stage 1 had the highest rates of polyuria or study withdrawal due to adverse events [
111,
116]. Results of the ongoing first pediatric study of tolvaptan in adolescents are therefore eagerly awaited and can be expected soon [
118] (
ClinicalTrials.gov identifier: NCT02964273). However, as an initial study, the primary endpoints have been defined as urine osmolarity changes and change in kidney volume is only a secondary endpoint. Further study will also be required to define which children are likely to experience rapid progression and would therefore benefit from treatment. The difficulty of this task is illustrated by that fact that for adults with ADPKD different definitions/ recommendations across the US, Europe, and Japan result in widely varying groups of eligible patients [
119].
A case report has documented the use of tolvaptan in a neonate with severe, recessive-like ADPKD, edema, hyponatremia, and compression of the inferior vena cava [
120] with successful resolution of edema and hyponatremia and lack of further kidney growth without significant side-effects over 17 months.
Seeing as tolvaptan therapy is unlikely to be tolerated lifelong and treatments with fewer side-effects would be preferable, it is promising that there are currently 5–10 other ongoing interventional clinical studies for drugs as diverse as metformin, tyrosine kinase inhibitors, proglitazone, venglustat (oral glucosyl-ceramide synthase inhibitor), lixivaptan (non-peptide vasopressin 2 receptor antagonist) as well as prescribed water intake and caloric restriction diet [
121]. Previous experience with drugs which appeared promising, such as mTOR inhibitors and somatostatin analogues, but were not found to preserve kidney function in humans (despite effects on kidney growth and liver cysts respectively [
51,
52]) cautions against too much enthusiasm, but there is hope that further treatments will become available in due course.
Non-pharmacological management
Higher salt intake is prospectively associated with worse kidney outcomes in the general population [
122], as well as with higher kidney volume and growth in ADPKD [
123,
124]. A recent study could also demonstrate a negative effect of salt intake on prospective GFR decline in adults with ADPKD with a wide range of eGFR [
125], while in patients with advanced ADPKD it has been shown to increase the risk for a composite renal endpoint including death [
124]. While in the general CKD population the negative effects of sodium intake are probably mediated via increased blood pressure and activation of the renin-angiotensin system, in ADPKD sodium-induced increase of vasopressin is a likely additional mechanism leading to cyst growth [
125]. Unfortunately, there is a lack of prospective interventional trials [
126], but counselling patients with ADPKD to adhere to recommendations on salt intake appears wise, as especially younger patients often have much higher salt intake than recommended for the general population [
125].
Similarly, deliberate high water intake can suppress endogenous vasopressin (and its surrogate marker copeptin) and can be assumed to be beneficial because the efficacy of tolvaptan therapy correlates with copeptin levels [
51]. However, a small interventional trial suggests the opposite clinical effect [
127], so results of an ongoing larger randomized trial are eagerly awaited [
128]. Others have shown that a low osmolar diet (low sodium, low protein and adjusted water intake) can decrease copeptin in adults with ADPKD [
129], but protein intake itself did not correlate with GFR decline in ADPKD [
125]. Also, protein restriction has potential side effects in growing children and could not be shown to slow GFR decline in a pediatric group of mixed-cause CKD [
130]; therefore, low osmolar diet and protein restriction should not be recommended for children with ADPKD.
In addition to counseling families on lifestyle factors that affect cardiovascular risk, such as exercise and avoiding obesity and smoking, it is useful to point out patient organizations, which exist both internationally and in several countries. The interactive self-management tool “ADPKD Patient route map” [
131,
132] is based on a multidisciplinary position statement and available in several languages.