Cervical nerve root to foraminal size ratio correlates with post-surgical patient-reported outcomes
Yeni YN, Lindquist M, Oravec D, Baumer T, Bey MJ, Bartol S, and Chang V. Cervical nerve root to foraminal size ratio correlates with post-surgical patient-reported outcomes. J Orthop Res 2017; 35
Journal of orthopaedic research
INTRODUCTION: Changes in cervical neural foraminal (CNF) dimensions are considered a key factor in development of cervical radiculopathy due to their role in nerve root compression. However, cadaveric and in vivo studies indicate considerable variation in the size of nerve roots as well [1, 2], which, intuitively, should affect their risk of being compressed by the foramina. Recent studies reported a small but significant difference in nerve size between asymptomatic and symptomatic patients ; however, the relationship of nerve root size with clinical outcomes has not been studied in patients who have received surgical treatment of degenerative disease in their cervical spine and are at a time point significant for development of adjacent segment disease. This pilot study examined such relationships. We hypothesize that nerve root size (alone or relative to foraminal size) is associated with clinical symptoms. METHODS: Under local IRB approval, clinical MRI images obtained, using a 3D BTFE sequence (1.5 T, 0.65x0.65x1.5 mm voxel size) from 11 patients (7F, 35-75 years; 4M, 44-66 years; a convenience group) who had previously underwent either arthroplasty with an artificial disc (n=1) or anterior cervical discectomy and fusion (n=10) at the C5-6 level were used. At the time of imaging, average time post-surgery was 6.5 years (sd ±1.7 years). Resliced anterolateral views were prepared from axial images using multiplanar reconstruction (MPR) tools in Synedra View (v. 220.127.116.11, Innsbruck, Austria). Views were constructed parallel to and passing through the left and right nerve roots (approximately 50 degrees oblique to the sagittal plane) at C3-4, C4-5, C5-6 and C6-7 levels. Freehand selection tool was used to delineate the nerve and the foramen (Fig. 1). Foraminal area (FA) and width (FW), nerve root area (NA) and width (NW) and nerve root to foramen ratios of these variables (N/F.A and N/F.W) were calculated. All foraminal, nerve root and foramen to nerve root ratio variables measured within a patient were averaged over left and right sides and spine levels to obtain a single composite variable of each type for each patient. Each patient was assessed using the following validated patient reported outcome measures : the visual analogue scale (VAS) for neck and arm pain, the modified Japanese Orthopedic Association (mJOA) score , the Neck Disability Index (NDI), and the EuroQol EQ-5D score. The VAS and NDI assess for pain and functional disability directly related to neck and arm symptoms (higher worse). The mJOA is a disease specific scale for assessing myelopathy (higher better) and the EQ-5D is a general health assessment (higher worse). The relationship between survey and image parameters were examined using correlation and regression analyses. For survey variables with a binary outcome, the correlations were examined using logistic regression. RESULTS: The average (±sd) composite NA, NW, N/F.A and N/F.W were 6.31 ± 0.72 mm2, 1.71 ± 0.14 mm, 0.103 ± 0.022 and 0.297 ± 0.044, respectively. No spine level was significantly more represented than others in composite variables. Increased overall NDI score was associated with decreased FW (R= -0.65, p<0.05) and increased N/F.W (R= 0.68, p<0.05) (Fig. 2). Increased FW was also associated with decreased scores in sections of NDI related to pain intensity (R= -0.88, p<0.001), driving (R= -0.67, p<0.04) and recreation (R= -0.75, p<0.02) as well as neck and arm pain VAS (R= - 0.82, p<0.004 and R= -0.81, p<0.005, respectively), while N/F.W was associated with increased scores in sections of NDI related to pain intensity (R= 0.68, p<0.05), reading (R= 0.74, p<0.04), driving (R= 0.74, p<0.03) and recreation (R= -0.75, p<0.02). Additionally, increased N/F.A was associated with increased neck pain VAS (R= 0.79, p<0.02). None of the image variables were significantly associated with the overall EQ-5D scores. However, increased FW was associated with decreased usual activities subscore (R= -0.76, p<0.02) and increased N/F.A was associated with increased pain and discom ort subscore (R= 0.72, p<0.03) of the survey. None of the image variables were significantly associated with the overall mJOA scores, except for a marginally significant positive correlation for FW (p=0.055). However, increased FW was associated with increased leg numbness subscore (R= 0.73, p<0.02), increased NA was associated with increased urination subscore (R= 0.71, p<0.04) and increased N/F.W was associated with decreased trunk numbness subscore (R= -0.67, p<0.05) of the survey. DISCUSSION: To our knowledge, this is the first data on nerve root dimensions obtained over 6 years after cervical spine surgery, offering a connection between clinical outcomes and dimensions of nerve roots relative to foramina. The post-operative time of the examinations is significant in that it represents the onset of adjacent segment disease after initial surgery . Our nerve root width and area measurements are generally in agreement with those of others measured in vivo via ultrasonography [2, 3, 7, 8] or from cadavers via dissection [1, 9]. The size of nerve roots relative to foramina were associated with worsening overall NDI and specific pain subscores of NDI and EQ-5D surveys. As expected, this is partially attributable to smaller foramina alone. However, foraminal size did not correlate with all variables that nerve to foramen ratio variables did, indicating that nerve size has value independently from foraminal size. Width variables were associated with survey variables more frequently than did area variables. While it is possible that foraminal and nerve root width are indeed more important than their respective height or area, this may be a result of anisotropic resolution in the images providing a higher resolution in the width direction. When nerve size was considered alone, greater nerve width was associated with worse personal care, driving and work subscores of the NDI survey, consistent with the idea that a larger nerve may be associated with an increased risk of compression. Interestingly, however, greater nerve area was associated with a better bladder function in the mJOA survey. This, and correlations to subscores such as leg numbness may be coincidental or indicative of underlying neurodegenerative disease or response to injury  rather than a causal association to mechanical stress. These results are considered preliminary due to the low number of patients examined, particularly those who are significantly symptomatic. Also, the measurements could not be performed at all locations of the cervical spine due to image artifacts caused by implants . Future work is needed to further optimize imaging protocols for measurement of neuro-foraminal dimension variables and determine cut-off values for prediction of clinical outcomes in a prospective cohort. (Figure Presented).