Demographics
A total of 513 patients were included in this study. The baseline characteristics of the included patients with cervical SCI are shown in Table 1. Of these 513 patients (413 males and 100 females), 179 (34.9%) had a history of smoking. In terms of neurologic status, 174 (33.9%) patients were classified as having ASIA grade A, 116 (22.6%) ASIA grade B, 127 (24.8%) ASIA grade C, and 96 (18.7%) ASIA grade D. The most common NLI was C5 (n = 150, 29.2%), followed by C6 (n = 115, 22.4%), then C4 (n = 112, 21.8%). Among all patients, the SAS grade was as follows: 216 patients (42.1%) had grade 0, 67 (13.1%) grade 1, 26 (5.1%) grade 2, 101 (19.7%) grade 3, 47 (9.2%) grade 4, and 56 (10.9%) grade 5.
In the present study, 84 (16.4%) patients with TCSCI underwent tracheostomy. The comparison of patients who underwent tracheostomy and those who did not is shown in Table 2. The tracheostomy group had a significantly higher proportion of patients that were of advanced age (35.7% vs. 25.2%, p = 0.046). At neurological status, there were statistically significant differences in the NLI at C2-C5 (71.4% vs. 52.4%, p = 0.001), and ASIA grade A (61.9% vs. 28.4%, p = 0.001) between tracheostomy and non-tracheostomy group. The proportion of patients with the SAS grade 0–2 was significantly higher in the tracheostomy group than in non-tracheostomy group (88.1% vs. 54.8%, p = 0.001). There were also significantly more patients who have smoking history in the tracheostomy group (p = 0.001).
The SAS and tracheostomy rate
Regarding the SAS, 26.4%, 19.4%, and 15.4% of patients with grade 0, grade 1 and grade 2 muscle strength, respectively, underwent tracheostomy. The proportions of patients with the SAS of grades 3, 4, and 5 who underwent tracheostomy were 4.0%, 6.4%, and 5.4%, respectively. The percentage of tracheostomies in patients with TCSCI was correlated with the distribution of the SAS. Overall, the percentage of patients requiring tracheostomy decreased as the SAS grade increased (γ = -0.829, p = 0.042) (Table 3).
The SAS and other predictors in the nomogram
The results of the MLRA showed that ASIA A (OR = 11.344, p = 0.001), NLI at C2-C5 (OR = 4.533, p = 0.001), the SAS grade 0–2 (OR = 4.505, p = 0.001) and age>60 (OR = 1.898, p = 0.048) were significantly associated with predicting the tracheotomy (Table 4). These predictors and smoking history (OR = 1.798, p = 0.051) were included in the nomogram for visual analysis of the effects of the SAS on tracheostomy. In the nomogram, each factor was given a point, and the total number of points was calculated, which corresponded to the risk of tracheostomy. The important finding was that, the points corresponding to the SAS grade 0–2 were between 60 and 70, which suggested that the SAS had pretty good predictive capabilities of tracheotomy. ASIA A carried the most weight, and its corresponding number of points was 100 in the nomogram. The points corresponding to the NLI at C2-C5 were second only to ASIA A, which reflected the NLI had an important impact on the prediction of tracheotomy (Figure 1).
The Evaluation of the SAS
The nomogram was firstly evaluated. The C-index of the nomogram was 0.880 (SD = 0.039, p < 0.05). The ROC curve used to evaluate the performance of the nomogram is shown in Figure 2. The AUC value was 0.880 (sensitivity = 0.807, specificity = 0.798). The internal calibration curve is shown in Figure 3. The calibration curves revealed satisfactory consistency, indicating that the nomogram had excellent calibration capabilities. Therefore, the visual analysis of the predictive ability of the SAS for tracheostomy by nomogram was reliable.
Further, to examines the predictive value of the SAS for the tracheostomy, the ROC curve analysis was performed. As shown in Figure 4, the area under the curve for the SAS grade 0–2 was 0.692. The sensitivity of the SAS grade 0-2 was 0.239. The specificity of the SAS grade 0-2 was 0.951. These findings suggest that the SAS grade 0–2 could predict tracheostomy in patients with TCSCI, providing valuable information for the physicians to make treatment decisions. The comparison of the SAS with ASIA and NLI was also performed. The area under the curve for ASIA A was 0.735. The sensitivity of ASIA A was 0.299. The specificity of ASIA A was 0.906. The area under the curve for NLI at C2-C5 was 0.724. The sensitivity of NLI at C2-C5 was 0.211. The specificity of NLI at C2-C5 was 0.895.