This study shows lasting improvement on a self-assessed modified Rankin Scale in around 40% of shunted iNPH patients, and lasting subjective improvement in about 60% at follow-up after 2–6 years. The reoperation rate within 10 years was 26%, of which 58% were performed within the first year of surgery. Reoperation did not impede improvement up to 6 years after primary surgery.
Diabetes, heart disease, and a history of stroke were each associated with higher mRS scores at baseline, and the latter also after 3 years. However, neither of these three, nor hypertension, had a negative influence on the degree of postoperative improvement until more than 5 years after surgery.
Complications
The reoperation rate of 26% for any cause in our data was rather high. Fourteen percent of the patients had revisions due to mechanical problems. In the systematic review by Toma et al., the revision rate, constituting the need for revision due to shunt failure or “mechanical causes”, was 13% in 30 articles published after 2006 [
3]. One explanation for a higher reoperation rate in the current material is the longer follow-up period. Most studies have follow-up periods of up to 1 year, while 42% of the revisions in the present study were performed later. Another explanation could be that patients included in other studies aiming to investigate, e.g., specified measurements may have been carefully selected, whereas the present study includes all operated iNPH patients from the participating centers.
SDH was reported in 4.5% of the patients in the review [
3], which is in the same range as in our study (3.7%). However, our figure shows only those requiring surgical treatment, and the total number of SDH is probably higher. The infection rate of 6.4% is higher than in many other studies (average 3%, but ranging from 0 to 10%) [
3]. Similarly, the operation-related mortality of 0.5% is somewhat higher than in contemporary studies. This was calculated in the cited review to be 0.2%, for the years 2006–2010, with a range between 0 and 0.8%, for the 13 studies reporting numbers on shunt surgery-related mortality [
3]. Again, the unselected patient material in the SHQR compared with the less extensive single-center studies with stricter inclusion criteria reviewed by Toma et al. [
3] may have influenced the results.
One of the important findings in this study is that the complications leading to reoperation had no negative effects on the long-term outcome. This knowledge is essential for clinicians, well aware of the risk of problems with shunt surgery, when advising patients. Of course, complications cause a temporary decline in physical and mental functions but it appears that identification and treatment prevent major decline.
Comorbidities
There is increasing evidence that risk factors for cerebrovascular disease contribute to the pathophysiology of iNPH, and a recently published study calculated that multiple vascular risk factors could explain 24% of iNPH cases (population attributable risk, PAR, 24%) [
27]. Our study could not show that the specific vascular risk factors of hypertension or diabetes hampered the effects of shunting, either in the short- or long-term perspective, even if patients with diabetes performed worse in the baseline evaluation. This is in line with earlier studies concerning the influence of vascular risk factors on short-term outcome [
4,
16,
31], while their influence on the long-term outcome in iNPH patients has not previously been described.
Macro- and microvascular changes, where both hypertension and diabetes are thought to play a role, with stiffening of the arterial walls and development of endothelial dysfunction in small vessels, causing increased permeability [
38], could hypothetically contribute also to the disturbed hydrodynamics in iNPH, with increased CSF pulsations [
39] and diminished perfusion in small vessels in the periventricular region [
40,
41]. Shunting immediately changes the absorption mechanism and thereby “eliminates” both the vascular and hydrocephalic component of the disturbed CSF dynamics, as well as improves periventricular perfusion [
42]. As the effect of the shunt is permanent it may even compensate for the deterioration known to be associated with hypertension and diabetes and thereby explain the lack of negative long-term effects.
Earlier studies showed a less favorable outcome for patients with established cerebrovascular disease (CVD), defined as a history of stroke, infarctions on radiological imaging, or moderate to severe white matter lesions on a CT scan [
16,
31]. Boon et al. found 52% of the patients with CVD to be improved, as opposed to 79% of those without [
31], and Spagnoli et al. showed similar results in the long-term perspective (mean 52 months), with 49 vs. 79% improved patients [
16]. Our study included only a history of stroke, but the negative impact of CVD, in this narrower sense, regarding the proportion of improved patients in the short or long term could not be confirmed. Patients with a history of stroke performed worse with regard to the smRS score only after 3 years, but the proportion of patients still reporting “better” was not affected at any time point.
The main limitation to this study is the low letter response rates, ranging from 17% of available patients in the 6-year group to 33% in the 2-year group (Table
3). During some periods, letters were not sent from all centers as intended, which explains the lack of data in long-term follow-ups for 258 patients (26%), and why not all patients were followed up at each pre-specified time point.
However, the incomplete mailings were not influenced by patient characteristics. When comparing patients in each of the long-term follow-up groups to those without replies, there were no significant differences regarding the mRS at baseline or age at surgery, and the frequencies of the four reported comorbidities were equal. Furthermore, the prevalence of reoperation before follow-up in the 3-, 4-, 5- and 6-year groups was similar in repliers and non-repliers. Only in the 2-year group, which was closest in time relative to the majority of the reoperations, repliers had undergone fewer reoperations than non-repliers: 18% compared to 27% (p = .004). Altogether, this should indicate that there are no systematic errors in the distribution of follow-up letters, arguing for the representativity and validity of the samples.
The benefits of mailed follow-up questionnaires are that they allowed for follow-up of a large number of patients, and that the responses in the questionnaires are in no way influenced by an examiner.
Another limitation is the use of subjective outcome measures. The smRS and the patients’ comparisons between their present and their preoperative health condition as measures of outcome are evaluations whose correspondence to objective measures has not been studied. Still, we consider these measures valuable, representing outcome as experienced by the affected patient.
The major strength of the study is that it is a quality registry-based study, which allowed for a very large sample size of 979 patients with data collected prospectively during a long follow-up period of up to 10 years. Patients were diagnosed in five different centers, covering approximately 80% of the referral areas in Sweden [
34], according to standardized and clinically applied routines representing everyday clinical practice. There were no specific exclusion criteria, such as those often applied in other scientific studies, why the study cohort probably reflects the target population of iNPH patients more accurately.
Based on these results, physicians can inform patients about the risk of complications with greater certainty—stating that it is fairly high, but that complications are treatable and that no negative effect on the long-term outcome has been shown. These findings also give support to decisions regarding patients with hypertension, diabetes, heart disease or stroke, as these extensive data did not show a less favorable outcome for these patients, meaning that they should not be excluded from shunt surgery.