Introduction
Normal pressure hydrocephalus (NPH) is a neurological disorder primarily affecting older adults, characterized (among other features) by the accumulation of cerebrospinal fluid (CSF) in the brain’s ventricles and associated with progressive cognitive and motor dysfunction. Diagnosis of NPH typically involves a combination of clinical evaluation, brain imaging, and invasive tests such as the lumbar tap test and infusion testing to evaluate CSF dynamics [
18,
31,
60]. According to the last guidelines for the management of idiopathic NPH in Japan [
52], more than one symptom in Hakim’s triad [
28] should be observed to suspect NPH. The incidence of the triad syndromes varies across studies: a gait disturbance exhibits 94–100% of NPH patients, cognitive impairment is present in 78–98% of NPH patients, and a urinary dysfunction affects 60–92% of NPH patients [
24,
29,
41,
50,
67]. It was reported that a full triad was observed in approximately 60% of NPH-diagnosed patients [
24,
35,
67], whereas a large-scale questionnaire study in Japan revealed that a complete triad was exhibited in only 12.1% of NPH patients [
41]. The identification of the triad symptoms is an initial step in the further NPH diagnosis procedure. The next step is usually the assessment of ventriculomegaly based on CT/MRI images which are also not unified. Several parameters related to the ventricle’s size or shape are used in NPH diagnosis. The most frequently reported parameters are Evans’ index and callosal angle. A recently published meta-analysis revealed that the diagnostic performance expressed as the area under the ROC curve (AUC) was 0.87 (95% CI: 0.84–0.90) for Evans’ index and 0.97 (95% CI: 0.95–0.98) for callosal angle [
55]. The threshold for both parameters is not unified and has been reported to be 0.3 [
30,
71] or 0.32 [
49] for Evans’ index and 90° [
30,
46,
57,
61,
71], 100° [
49], and 123° [
10] for callosal angle. Another two parameters useful in MRI image evaluation are the brain-to-ventricle ratio and the convexity cistern to ventricle ratio. Their accuracy in differentiation between NPH patients and healthy individuals reported as AUC was equal to 0.97 and 0.96 for the brain-to-ventricle ratio and convexity cistern to ventricle ratio, respectively [
72].
If Hakim’s triad and CT/MRI scan evaluation suggest the diagnosis of NPH, the CSF tap test or infusion study is often performed in order to assess the dynamics of CSF circulation and the probability of benefit from shunting [
18,
31,
52,
60]. The tap test is an invasive procedure in which typically 40–50 ml of CSF is drained from the lumbar space [
66]. According to a systematic review [
48], the CSF tap test has a sensitivity of 58% (range 26–87%) and a specificity of 75% (range 33–100%). The positive response to the CSF tap test is an improvement in clinical symptoms after the test. However, the test is evaluated using different scores around the world [
52]. Alternatively to (or together with) the CSF tap test, the infusion test is performed [
18,
31,
52,
60]. The infusion test is more invasive than the tap test because it requires the injection of physiological saline or artificial CSF into the CSF space. During the injection, the intracranial pressure (ICP) is monitored, and the resistance to CSF outflow (R
CSF) is calculated based on the pressure response to a controlled volume increase. The threshold for R
CSF is reported to be 13–18 mm Hg/ml/min with a positive predictive value between 80 and 92% [
52]. It was also reported that analysis of slow waves of ICP, recorded during overnight monitoring [
16,
59,
63,
64], and measurement of optic nerve sheath diameter [
23] may be helpful additional measures in NPH diagnosis. However, the pathophysiology of hydrocephalus includes not only impaired CSF circulation and poor pressure–volume compensation but also the interference of abnormal CSF with cerebral blood flow (CBF) [
19,
54]. A reduction in CBF associated with increased cerebrovascular resistance and decreased cerebrovascular compliance is frequently noted in NPH patients [
4,
7,
8,
14,
27,
38,
39,
42,
45,
53,
65,
68]. The decrease in CBF observed in NPH is thought to result from increased CSF pressure and increased ventricular volume [
26,
44,
51,
70], leading to cortical compression and stretching of blood vessels and white matter fibers [
20,
22]. Another study also points out the role of parallel changes in cardiac function and systemic blood flow in the decrease of CBF in chronic hydrocephalus [
21]. Moreover, underlying cerebrovascular disease is an important predictor of poor outcomes after the implantation of a hydrocephalus shunt [
7]. Patients with cerebrovascular disease that prevails over disturbance in CSF circulation and poor pressure–volume compensation may not exhibit clinical improvement after shunting [
15,
19].
Positron emission tomography (PET) and magnetic resonance imaging (MRI) can be used to assess alterations in cerebral blood circulation and cerebral blood volume; however, the downsides of these advanced imaging techniques are their high cost and low availability. In contrast, acoustic-based methods provide non-invasive, low-cost, real-time measures of cerebrovascular function. By transmitting short ultrasonic pulses from one side of the skull to another and dynamically measuring the time-of-flight of the pulses [
56,
58], altered shapes of the cerebral arterial blood volume (C
aBV) pulses have been observed in NPH-diagnosed patients. Following the successful treatment, the shape of the C
aBV pulses became similar to those observed in healthy volunteers, suggesting it is a possible indicator of effective NPH treatment [
12]. However, this method of measurement is still under development and is not yet available on the global market. We recently proposed an ultrasound-based method for assessing C
aBV changes based on the cerebral blood flow velocity (CBFV) signal measured with a commonly available transcranial Doppler (TCD) device and modeling global cerebrovascular dynamics [
37]. In the current study, we aim to analyze the shape of the pulse changes of C
aBV in healthy volunteers and probable NPH patients using this methodology. We hypothesize that the shapes of C
aBV pulses calculated from TCD measurements differ between healthy individuals and NPH patients and that a quantitative measure may help to non-invasively identify patients suffering from hydrocephalus.
Discussion
The results of this study support the hypothesis that the shape of the TCD-based pulse of CaBV differs between patients with NPH and healthy individuals. Our analysis revealed that the rising slope of the CaBV pulse in NPH patients was less convex and more like a straight line, resembling a triangle arm, while the pulse in healthy individuals had a more pronounced convexity.
The results are consistent with previous studies that have reported alterations in cerebral hemodynamics in NPH patients, including decreased cerebral blood flow, increased cerebral vascular resistance [
8,
14,
27,
38,
39,
42,
45,
53,
65,
68], and decreased vascular compliance [
4]. The mechanism underlying these changes is not fully understood, but it has been suggested that impaired drainage of CSF from the brain may result in the compression of small cerebral vessels [
5,
25], leading to decreased CBF, which may also influence the shape of the C
aBV pulse. Therefore, the possible mechanism for the observed changes in C
aBV pulse shape can be explained that in healthy individuals, the cerebral vessels are able to rapidly accommodate changes in blood flow demand, resulting in a more pronounced convexity of the C
aBV pulse. In contrast, in NPH patients, impaired cerebral venous drainage may lead to reduced vascular compliance and increased vascular resistance, which may limit the ability of cerebral vessels to rapidly accommodate changes in blood flow demand, resulting in a less convex ascending slope of the C
aBV pulse.
Although the alterations in the shape of the C
aBV pulse in NPH patients were previously reported by Chambers et al. [
12], we cannot provide a direct comparison between their results and ours. Chambers et al. used a method based on the transmission of short ultrasonic pulses from one side of the skull to another and dynamic measurement of the time-of-flight of the pulses [
56,
58]. This technique is not widely available. Whereas we used a global model of cerebral blood circulation and estimated C
aBV pulses based on TCD measurement [
36]. The shapes of the C
aBV pulses differ between these two methods—the pulses assessed by Chambers et al. have three clearly distinguishable peaks (see Fig. 1 in [
12]), whereas the C
aBV pulses obtained with our method have barely visible peaks (see, for example, Fig.
2 or [
11,
17,
34,
36,
69]). Therefore, Chambers et al. analyzed the heights of the detected peaks, and we proposed the quantitative similarity parameters. Nevertheless, our results are consistent with the results obtained by Chambers et al. [
12] in the context of alterations in the C
aBV pulse shape in NPH patients.
The proposed CaBV pulse analysis has several advantages. First, it uses a commonly available TCD device. The method is fully non-invasive and does not require the use of contrast agents or ionizing radiation, making it safe for repeated use. It has the potential to provide an objective and quantitative measure, which can improve the accuracy and reliability of diagnostic tests and may be especially useful in lower-income countries where the availability of MRI or computer tomography scanners is reduced.
Several limitations should be considered when interpreting the results of the present study. First, the groups of subjects were relatively small. In particular, signals from only 31 probable NPH patients and from 23 healthy individuals were analyzed. Thus, the findings should be interpreted with caution and confirmed in larger database. Second, the brain blood circulation model used in the study for the calculation of cerebral arterial blood volume was not directly compared with imaging modalities. Therefore, it is necessary to conduct further research to validate this model. Third, the custom-written algorithm for distorted pulse removal could exclude parts of reliable pulses from the analysis, but only a small percentage of pulses (10.5%) were rejected from the analysis as distorted. Fourth, the CBFV signals were up-sampled from 50 to 200 Hz to increase their temporal resolution, and a low-pass filtering with a cut-off frequency of 12 Hz was performed prior to analysis. It may have had a minor impact on the pulse shape—pulses became more smoothed (reduction of high-frequency noise), and both the pulse onset and the pulse maximum can be detected with width-augmented precision. It is possible that due to filtering, we lose important information from the high signal component, but both ICP and CBFV pulses are similar to some extent, and it was reported that the power of the ICP signal is mostly contained in the range below 8 Hz [
9]. However, if this influence exists, it is systematic and fully reproducible. We have successfully applied the same up-sampling procedure and filter to CBFV recordings in our previous studies related to the shape of CBFV pulses [
73,
74]. Fifth, we analyzed the diagnostic accuracy of NPH classification for only one, the most prominent C
aBV shape-related parameter in our dataset. Studies conducted on larger cohorts are required to evaluate the diagnostic accuracy of this parameter and combinations of the proposed parameters. Sixth, we did not define the minimum length of the CBFV signal sufficient to evaluate the C
aBV pulse shape-related parameters, which should be done in prospective studies. Finally, we did not find any significant correlation between the proposed parameters and CSF compensatory parameters. However, NPH is a heterogeneous disease often associated with changes in CBF, and CSF compensatory parameters themselves do not reflect the full picture of this complex disorder [
47]
.
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