DSSEP N13_E and N13_F are sensitive parameters for evaluating the CSM
Demographic data was shown in Table 1. The preoperative mJOA score and Nurick grade were 13.95 ± 2.01 (range 9-17) and 2.61 ± 1.18 (range 1-4) respectively. There were 20 (52.6%) patients presented ataxia preoperatively. The two-year postoperative mJOA score and recovery rate were 16.89 ± 2.44 (range 12-20) and 45.6% ± 21.9% (range 0-80%) respectively (Table 1). We recorded latencies for N9, N13 and N20 and amplitudes for N13 and N20 during cervical flexion and extension. Four (10.5%) patients’ N13 or N20 waves were lost at all three positions, three (7.9%) were lost at both dynamic positions, and six (15.8%) were lost at either the extension or flexion position. We found that the DSSEP N13 amplitudes were significantly higher at the neutral position than that at both the extension (t-test, p<0.001) and flexion (t-test, p<0.01) positions (Table 2). We calculated the amplitude ratios, the N13_E and N13_F were 0.72±0.34 and 0.73±0.43 respectively, and the N20_E and N20_F were 0.97±0.29 and 0.97±0.29 respectively. The DSSEP N20 amplitudes, and N13 and N20 latencies did not change significantly during extension or flexion (Table 2).
Table 1. Summary of demographics and clinical data of 38 CSM patients
Variable
|
Measurement
|
No. males
|
21 (55.3%)
|
Mean age (yr)
|
53.6 (range 22-82)
|
Preoperative clinical assessment
|
mJOA score
|
13.95 ± 2.01 (range 9-17)
|
Nurick grades
|
2.61 ± 1.18 (range 1-4)
|
No. Hoffman sign
|
24 (64.9%)
|
No. Leg hyperreflexia
|
13 (35.1%)
|
No. Ataxia
|
20 (52.6%)
|
2-Year Postoperative clinical assessment
|
|
mJOA score
|
16.89 ± 2.44 (range 12-20)
|
Recovery rate
|
45.6% ± 21.9% (range 0-80%)
|
Compression levels (n) at MRI
|
Stenotic levels
|
C3/4 (15)
C4/5 (21)
C5/6 (30)
C6/7 (12)
C7/T1 (1)
|
Most stenotic level
|
C3/4 (4)
C4/5 (8)
C5/6 (22)
C6/7 (4)
|
No. (%) undergoing each procedure
|
Anterior
|
27 (71.1%)
|
Posterior
|
7 (18.4%)
|
Combined anterior-posterior
|
4 (10.5%)
|
Table 2. Results of DSSEP findings in neutral and dynamic positions †
|
Neutral
|
Extension
|
Flexion
|
|
N13
|
N20
|
N13
|
N20
|
N13
|
N20
|
Latency (ms)
|
13.2±1.1
|
18.88±1.09
|
13.25±1.01
|
19.06±1.01
|
13.14±1.07
|
18.95±1.07
|
p_value ‡
|
|
|
0.15
|
0.09
|
0.43
|
0.3
|
Amplitude (uV)
|
2.66±1.42
|
2.7±1.59
|
2.17±1.29
|
2.53±1.3
|
2.18±1.59
|
2.56±1.54
|
p_value ‡
|
|
|
0.000017 ***
|
0.29
|
0.0026 **
|
0.25
|
Amplitude Ratio §
|
|
|
0.72±0.34
|
0.97±0.29
|
0.73±0.43
|
0.97±0.29
|
Absent waves (Left, Right) ¶
|
4(4,4)
|
0
|
8(7,6)
|
2(1,2)
|
10(7,9)
|
1(1,1)
|
** p_value<0.01; *** p_value<0.001
† For absent N13 or N20 waves, their latencies were excluded and amplitudes were set as the baseline value (0mV) for statistical analysis.
‡ The p_values were calculated with the student's t-test by comparing the DSSEP value at dynamic (extension or flexion) position with neutral position.
§ Each patients' amplitude ratios were calculated with the following method: Amplitude Ratio = amplitude at dynamic (extension or flexion) position ÷ amplitude at neutral position.
¶ The number of patients whose waves at either left or right side at specific position was lost. The number of absent waves at left and right sides are respectively listed in the bracket.
We compared the DSSEP results with our clinical assessments. We found that the N13 DSSEP amplitude ratios at both extension and flexion positions positively correlated with the baseline clinical symptoms and postoperative outcomes: results having positive correlation with mJOA scores (Spearman, N13_E: R=0.40, p=0.014; N13_F: R=0.35, p=0.031), negative correlation with Nurick grades (Spearman, N13_E: R=-0.34, p=0.039; N13_F: R=-0.39, p=0.015) and positive correlation with recovery rates (Pearson, N13_E: R=0.37, p=0.021; N13_F: R=0.54, p=0.0004). However, only the N13_E in ataxic patients was significantly lower than that in patients without ataxia (0.61±0.39, 0.85±0.22, t-test, p=0.023). The N13_F did not show significant difference between the ataxia and non-ataxia groups (0.61±0.43, 0.82±0.34, t-test, p=0.11).
The radiographic findings and their correlations with N13_E and N13_F
The preoperative transverse area of the spinal cord and spinal canal obtained with MRI were 69.95±19.01 and 111.79±38.64 mm2 respectively, and the Cord/Canal Area Ratio was 0.66±0.16. The sagittal diameter of the spinal cord was 4.77±1.09 mm. The Compression_Ratio, Central_Ratio, and 1/4-Lateral_Compression_Ratio were 0.35±0.08, 0.44±0.1, and 0.34±0.08 respectively.
Both the N13_E and N13_F positively correlated with Compression_Ratio (Pearson, N13_E: R=0.33, p=0.042; N13_F: R=0.47, p=0.003), Central_Ratio (Pearson, N13_E: R=0.43, p=0.007; N13_F: R=0.52, p=0.0008) and 1/4-Lateral_Compression_Ratio (Pearson, N13_E: R=0.47, p=0.003; N13_F: R=0.51, p=0.001) measured in MRI axial images. There was no correlation between DSSEP N13 amplitude ratios and spinal cord area, Cord/Canal Area Ratio or Sagittal Diameter. (Figure. 3)
The number of 8, 13, 11 and 6 patients were classified as the Ax-CCM Type 0, Type 1, Type 2 and Type 3 respectively. The mean N13_E for each Ax-CCM group was 0.93±0.05, 0.48±0.34, 0.66±0.30, 1.08±0.07 respectively, and mean N13_F was 1.05±0.12, 0.63±0.43, 0.42±0.33, 0.96±0.14 respectively for each Ax-CCM group. Both the N13_E and N13_F were statistically different among Ax-CCM groups. (ANOVA, p<0.001) A post hoc test showed that the N13_E for the type 1 pattern was significantly lower than that for the type 2 pattern (p<0.05). (Figure. 4 A)
19 patients were identified as cervical segmental instability in dynamic X-rays, and the rest 19 patients were identified as cervical stable. The N13_F varied significantly between patients in Stable and Unstable groups. (T-test, p=0.007), but not for N13_E (T-test, p=0.2). (Figure. 4 B)
The radiographic findings' correlation with clinical data
Both the Compression_Ratio and Central_Ratio measured in MRI were significantly correlated with the baseline mJOA scores (Spearman, p<0.05), Nurick grades (Spearman, p<0.01) and recovery rates (Pearson, p<0.01). The 1/4-Lateral_Compression_Ratio was significantly correlated with the recovery rates (Pearson, p<0.05), but not with baseline mJOA scores and Nurick grades. Patients' mJOA scores, Nurick grades and recovery rates were all significantly different among different Ax-CCM groups (ANOVA, p<0.05). The mJOA scores, Nurick grades and recovery rates in patients with or without cervical segmental instability were also significantly different (t-test, p<0.05). (Figure 5). All radiographic findings above did not vary between ataxic patients and patients without ataxia.
The original DSSEP, MRI measurement and clinical data of all 38 patients in the study can be found in the supplementary file.