From January 1, 2017 to August 1, 2019, a total of 39 CSF samples were analyzed from 42 suspected iNPH patients: 6 women and 33 men of mean age 71.44 ± 8.10 (range 52–85) years. Figure 1 outlines the study profile.
The levels of lumbar Aβ1–42, T-tau, and p-tau, and their relationship with tap test response or numbers of combined symptoms
Previous studies have shown that iNPH patients tended to exhibit low levels of CSF T-tau, and this can be a good predictor of post-operative outcome. Similarly, in our study, the levels of tau and its phosphorylated form were significantly decreased, especially in the tap test (+) group (p < 0.01 and p < 0.05), although the levels of Aβ1–42 did not change significantly (Fig. 2A). The levels of Aβ1–42 in the tap test (+) and (-) groups were 584.86 ± 303.55 pg/mL (n = 17) and 641.36 ± 255.38 pg/mL, respectively (n = 22) (Fig. 2A). The level of T-tau in the tap test (+) and (-) groups was 191.21 ± 80.52 pg/mL (n = 17) and 382.88 ± 25.35 pg/mL, respectively (n = 22) (Fig. 2A). The level of p-tau in the tap test (+) and (-) groups was 32.38 ± 12.63 pg/mL (n = 17) and 43.38 ± 14.02 pg/mL, respectively (n = 22) (Fig. 2A). In contrast, the ratio of p/T-tau in the tap test (+) group was 17.23 ± 0.59%, which was higher than that in tap test (-) group (12.81 ± 0.90%) (p < 0.001) (Fig. 2B). The results suggested that the magnitude of p-tau reduction is significantly lower than T-tau, resulting in a relatively higher ratio of p/T-tau. In addition, the ratio of p-tau/Aβ1–42 was 6.87 ± 0.95% and 7.97 ± 1.09% in the tap test (+) and (-) groups (12.81 ± 0.90%), respectively (p > 0.05) (Fig. 2C).
To analyze the relationship between CSF biomarkers and the number of combined symptoms of iNPH patients, a correlation analysis was performed. Although the p values of correlation analysis between the actual values of Aβ1–42, T-tau, p-tau or p/T-tau with the number of combined symptoms were 0.777, 0.076, 0.051 and 0.568, respectively, a significant difference in the number of combined symptoms with p-tau/Aβ1–42 was found (p = 0.030) (Fig. 2D-H).
A subgroup analysis was used to determine the accurate significant difference of each CSF biomarker in groups with one/two/three combined symptoms. Unfortunately, no difference was found between the subgroups with different numbers of combined symptoms and Aβ1–42, T-tau and p/T-tau (p > 0.05, p > 0.05 and p > 0.05) (Fig. 3A, B and D). The levels of Aβ1–42 in the subgroup with one/two/three combined symptoms were 554.31 ± 300.82, 692.15 ± 247.35 and 550.73 ± 290.23 pg/mL, respectively (Fig. 3A). The levels of T-tau in the subgroup with one/two/three combined symptoms were 192.01 ± 130.78, 311.21 ± 162.83 and 348.92 ± 231.75 pg/mL, respectively (Fig. 3B). The ratio of p/T-tau in the subgroup with one/two/three combined symptoms was 0.16 ± 0.03, 0.14 ± 0.04 and 0.15 ± 0.05, respectively (Fig. 3D). However, the levels of p-tau in the subgroup with one/two/three combined symptoms were 29.50 ± 17.89, 39.70 ± 13.34 and 42.65 ± 11.81 pg/mL, respectively, which was significantly different between the group with one and three combined symptoms (p < 0.05) (Fig. 3C). The ratio of p-tau/Aβ1–42 in the subgroup with one/two/three combined symptoms was 0.06 ± 0.03, 0.06 ± 0.03 and 0.10 ± 0.06, respectively, which was significantly different between the group with one/two and three combined symptoms (p < 0.05 and p < 0.05) (Fig. 3E). In addition, the responsiveness of the tap test was also related to the number of combined symptoms (Fig. 3F). The numbers of one/two/three combined symptoms in the tap test (+) and (-) were 7/6/4 and 1/12/9, respectively. Significantly more combined symptoms were obtained by the patients in the tap test (-) group (p < 0.01).
The distribution of combined symptoms and its correlated factors
We showed that patients with a fewer number of combined symptoms may have a positive response to the tap test. Further analysis revealed that the prevalence of hypertension, initial pressure and pressure difference of CSF were also related to the number of combined symptoms. There were significant differences in the distribution of combined symptoms of patients with hypertension or not (p < 0.01). The number of hypertensive patients with one/two/three combined symptoms was 0/14/8, which was 8/4/5 in non-hypertensive patients (Fig. 4A). This finding suggested that patients with hypertension would have more symptoms of iNPH than those without hypertension. The initial pressure of CSF in patients with two symptoms of iNPH was 167.22 ± 45.45 mmH2O, significantly higher than those with three symptoms (125.92 ± 18.67 mmH2O), which was similar to those with one symptom (151.50 ± 25.51 mmH2O) (Fig. 4B). The pressure difference of patients with two symptoms of iNPH was 82.22 ± 48.63 mmH2O, significantly higher than those with one symptom (49.88 ± 10.63 mmH2O) or three symptoms (49.77 ± 18.75 mmH2O) (p < 0.05 and p < 0.05), although the end pressure of CSF had no significant difference in iNPH patients with different numbers of combined symptoms (Fig. 4C and D).
The responsiveness of the tap test and its correlated factors
In addition to combined symptoms, a correlation was also found between the end pressure or pressure difference of CSF and tap test responsiveness (p < 0.05), although there was no significant correlation between tap test responsiveness and initial pressure of CSF (Fig. 5A and B). The initial pressure of CSF was 145.12 ± 22.49 mmH2O (n = 17) and 154.18 ± 47.73 mmH2O (n = 22), which was similar. The end pressure was 96.35 ± 27.74 mmH2O (n = 17) in the tap test (+) group, which was significantly higher than that in the tap test (-) group (77.05 ± 25.24 mmH2O, n = 22) (p < 0.05) (Fig. 5A). The median value of pressure difference of CSF was 50 mmH2O in the tap test (+) group, which was lower than 60 mmH2O in the tap test (-) group (p < 0.01) (Fig. 5B). The finding suggests better brain tissue compliance in patients who were tap test positive.
To clarify the contribution of these related factors to tap test responsiveness, we conducted further binary logistic regression analysis. The results showed that P (tap test responsiveness) = 1/1 + e ^ - (-5.505 + 55.314 * ratio of p/T-tau − 1.586 * numbers of combined symptoms), suggesting a predictive value of a higher ratio of p/T-tau and fewer combined symptoms. Inferential combined indicators were calculated equal to -5.505 + 0.553* percentage of p/T-tau − 1.586 * numbers of combined symptoms.
Results of the actual value or ratio of p-tau/T-tau and the combined indicators were further analyzed using ROC curves to determine the sensitivity, specificity, and area under the ROC curve (AUC). The combined indicators gave the highest AUC of 0.90, compared with T-tau (0.87), p-tau (0.63), and p/T-tau (0.82) (Fig. 5C and D). The cut-off value (-0.77) was selected involving maximizing Youden's Index (0.67), of which the sensitivity and specificity were 94.12% and 72.73%, respectively.