Indian Spine Journal

SYMPOSIUM - CERVICAL SPONDYLOMYELOPATHY
Year
: 2019  |  Volume : 2  |  Issue : 1  |  Page : 20--32

Imaging in cervical myelopathy


Rajavelu Rajesh, Shanmuganathan Rajasekaran, Sri Vijayanand 
 Ganga Medical Centre and Hospitals Pvt. Ltd., Coimbatore, Tamil Nadu, India

Correspondence Address:
Prof. Shanmuganathan Rajasekaran
Ganga Medical Centre and Hospitals Pvt. Ltd., Coimbatore, Tamil Nadu
India

Abstract

This is a narrative review. The objective of this study is to provide an overview on the imaging modalities and their utilization in cervical myelopathy (CM). Using PubMed, studies published on the “imaging modalities in CM,” “cervical spondylotic myelopathy (CSM) imaging,” “computed tomography (CT) and magnetic resonance imaging (MRI) in CM,” “imaging in ossified posterior longitudinal ligament (OPLL),” “dural ossification in OPLL,” “diffusion tensor imaging (DTI) in CSM,” and “dynamic MRI, functional MRI, and magnetic resonance spectroscopy (MRS) in CSM” were evaluated. The review addresses the evaluation of CM with various imaging modalities ranging from radiographs, CT, and MRI to advanced imaging techniques such as DTI and MRS. Each investigation contributes specific detail to the disease process in a different dimension. Specific parameters for CSM and OPLL, and their influence on outcome are discussed. Imaging in CM plays an important role in analyzing the cause of myelopathy, defining the level of the lesion, parameters to assess the time of intervention and to predict the outcome.



How to cite this article:
Rajesh R, Rajasekaran S, Vijayanand S. Imaging in cervical myelopathy.Indian Spine J 2019;2:20-32


How to cite this URL:
Rajesh R, Rajasekaran S, Vijayanand S. Imaging in cervical myelopathy. Indian Spine J [serial online] 2019 [cited 2019 Oct 23 ];2:20-32
Available from: http://www.isjonline.com/text.asp?2019/2/1/20/249903


Full Text



 Introduction



Degenerative cervical myelopathy (DCM) is defined as symptomatic cervical myelopathy (CM) associated with a broad variety of degenerative changes of the extradural spinal tissues in the cervical spine. The spectrum includes cervical spondylotic myelopathy (CSM), ossified posterior longitudinal ligament (OPLL), and ossification of the ligamentum flavum.[1] Radiological assessment in CM helps in establishing the diagnosis and preoperative planning by localizing the site, assessing the contributing factors and severity of the disease.[2],[3],[4] Various modalities of imaging with their unique benefit are available ranging from roentgenogram to the more advanced diffusion tensor imaging (DTI). Magnetic resonance imaging (MRI) provides accurate and greatest range of information including the disc, ligaments, and neural elements, as compared to all the other available imaging.

 Plain Radiographs in Cervical Myelopathy



Even in the presence of advanced diagnostic imaging, plain radiographs are valuable initial radiological evaluation tool of the cervical spine in CSM. Anteroposterior (AP), lateral, and oblique radiographs show changes in the facet and uncovertebral joints, vertebral body morphology, osteophytes, abnormal ossifications (diffuse idiopathic skeletal hyperostosis), cervical alignment, and disc space. Normal vertebral bodies are symmetric and rectangular. A loss of disc space height with nonbridging and nonmarginal osteophytes or syndesmophytes is a classic finding in patients with degenerative spinal disease.[5] A lateral radiograph must include from the base of the skull to cervicothoracic junction, and an AP view should include C3 to T1. Weight-bearing plain lateral films in CM are used to assess (a) sagittal canal diameter, (b) alignment, and (c) evaluation of OPLL.

Assessment of sagittal canal diameter

Congenitally narrow spinal canal is proposed as a risk factor for myelopathy in cervical spondylosis[6] and is assessed in radiographs using Torg–Pavlov ratio (TPR). TPR [Figure 1] is calculated from the lateral cervical radiographs by the ratio between the distance between the midpoint of posterior surface of the vertebral body to the closest point on the spinolaminar line and the vertebral body diameter of the same level.[7] According to Pavlov et al.,[8] cervical stenosis is present if the ratio is ≤0.82.{Figure 1}

Studies have shown that TPR is lower in patients with CM than in normal people.[9],[10],[11] In contrast, some studies[12],[13] reported that the TPR was not necessarily associated with spinal stenosis in view of vertebral body size variability. However, the AP diameter assessment in radiographs has been largely replaced by computed tomography (CT) and MRI.[14]

Assessment of cervical alignment parameters

CSM has been associated with and potentially exacerbated by cervical sagittal mal-alignment. In CSM, loss of lordosis leads to forced draping of the spinal cord against the vertebral bodies and disc-osteophyte complexes and may predispose to myelopathy. With increase in kyphosis, tethering of the spinal cord may increase intramedullary pressure and result in neuronal loss and demyelination. Simultaneously, the smaller arterial feeders to the cord will be compressed and flattened, resulting in further cord injury.[4],[15] Surgical decompression alone may not reduce the cord tension due to kyphosis, if sagittal alignment is not taken into consideration. Poor spinal cord posterior migration and expansion, leading to poor neurological improvement after laminoplasty, have been reported in patients with kyphotic alignment.[16] Cervical curvature has been classified into lordosis and nonlordosis; the latter includes straight, sigmoid, reversed sigmoid, and kyphosis curvature[17] based on the relationship with the line drawn between the posteroinferior corner of the C2 body and posterosuperior corner of the C7 body (or the C6 body, if the C7 body was invisible behind the shadow of the shoulders) [Figure 2].{Figure 2}

Cervical alignment parameters are assessed in lateral cervical radiographs, and it includes (1) cervical lordosis, (2) sagittal plane translation, and (3) dynamic cervical parameters.

Cervical lordosis

The normal curvature of the cervical spine is lordosis, and this is the result of cervical vertebra shape and to compensate for the thoracic kyphosis. In normal volunteers, the mean total lordosis is approximately 40°. The largest percentage of cervical standing lordosis (75%–80%) is localized to C1–C2, and relatively, little lordosis exists in the lower cervical levels; only 15% of lordosis occurs at the lowest 3 cervical levels (C4–C7).[18]

There are three methods to assess cervical lordosis including Cobb angles [Figure 3]a, Jackson physiological stress lines [Figure 3]b, and the Harrison posterior tangent method [Figure 3]c.[19] The most commonly used method is Cobb angle, typically measured from C1 to C7 or C2 to C7. This is described in [Table 1].{Figure 3}{Table 1}

Lee et al.[20] analyzed the influence of cervicothoracic junction anatomy on cervical spine sagittal alignment in cervical sagittal radiographs. The authors introduced the concepts of neck tilt (NT), thoracic inlet angle (TIA), and T1 slope (T1S). NT was defined as an angle between 2 lines both originating from the upper end of the sternum with one being a vertical line and the other connecting to the center of the T1 endplate [Figure 4]a. The TIA has been defined as the angle between a line originating from the center of the T1 endplate and perpendicular to the T1 endplate and a line from the center of the T1 endplate and the upper end of the sternum. T1S is the angle between horizontal plane and T1 endplate. The authors found that large TIA increases T1 slope and finally increases cervical lordosis to obtain a horizontal gaze and vice versa [Figure 4]b. The normal calculated values in the study were as follows: TIA – 69.5 ± 8.6°, T1S – 25.7 ± 6.4°, and neck tilt – 43.7 ± 6.1°.{Figure 4}

C2 sagittal vertical axis

Sagittal plane translation is assessed using C2 sagittal vertical axis (CSVA) [Figure 5]. It is measured as the horizontal distance between a plumb line dropped from the center of the C2 vertebral body and the posterior-superior corner of C7. The CSVA ranges from 15 to 17 ± 11.2 mm. Tang et al.[21] reported that positive CSVA of >40 mm is correlated with poor health-related quality of life and neck disability index scores following multilevel cervical fusion.{Figure 5}

Dynamic cervical spine parameters

Lateral fl exion-extension views are used to assess the dynamic parameters such as (i) assessment of cervical instability, (ii) center of rotation, and (iii) cone of kinesis. Cervical instability is when translation of >3.5 mm and sagittal plane angulation of >11° are present [Figure 6].[22]{Figure 6}

Role of plain radiographs in ossified posterior longitudinal ligament

In OPLL, radiographs help in assessing the alignment and measurement of K-line and thus planning surgical procedures. Fujiyoshi et al.[23] described the K-line [Figure 7], which connects the midpoints of the spinal canal at C2 and C7 on neutral lateral radiographs, as a guide for making decisions regarding the surgical approach for cervical OPLL patients. Based on the position with that of OPLL, it may be K-line (+) or K-line (−). K-line (+) group has OPLL that does not exceed this line while K-line (−) group does. In K-line (−) after posterior decompression, lack of sufficient posterior shift of cord and neurological improvement was noted due to anterior compression. Hence, if OPLL mass falls behind the K-line, then anterior approach is preferred.[24]{Figure 7}

Lee et al.[25] described kappa line [Figure 8] to assess the postoperative neurological recovery and residual cord compression in ≤4-level laminoplasty. Kappa line is a straight line connecting the midpoints of the spinal canal at 1 level above and 1 level below the decompressed segments on the plain lateral radiograph in neutral position. Kappa (+) meant that the OPLL mass did not pass the kappa line, while kappa (−) meant that OPLL had grown posteriorly beyond the kappa line. The authors reported that the kappa line predicted neurologic recovery and remaining cord compression following ≤4-level laminoplasty more correctly than the K-line.{Figure 8}

K-line was originally measured in cervical lateral standing radiographs taken in neutral position. However, measurement of the K-line in CT images with sagittal reconstruction is being used, stating that it can clearly reveal the precise size and morphology of the ossification.[26] Ijima et al.[27] compared the K-line measured in standing lateral radiographs versus CT in lying position. About 11% showed a change from K-line (+) to (−) when CT was used to assess K-line, and the authors stated that CT-based measurement is an over-indication for anterior or posterior decompression and fusion instead of laminoplasty which is optimal for K-line (+) patients and hence concluded that K-line should be measured with plain radiographs of standing patients.

 Computed Tomography in Cervical Myelopathy



(a) Cervical spondylotic myelopathy

CT provides excellent information about the dimensions of the spinal canal and helps in differentiating the various causes for narrowing of the canal such as posterior osteophytes or hypertrophied facet joints and ossified ligaments. It plays a vital role in assessing the vertebral artery variations and cervical pedicle morphology and its diameter and thus helps to choose the surgical fixation modality. Anterior cervical compression secondary to disc osteophyte complexes and their extent, whether confined to the disc interspace or extension behind the vertebral body as depicted by CT, is important to choose the surgical procedure to address the pathology by either discectomy and fusion or corpectomy and reconstruction, respectively.[28]

(b) Ossified posterior longitudinal ligament (OPLL)

CT is essential in identifying the OPLL in lower cervical levels, less densely calcified OPLL mass, associated dural ossification (DO), and other ligamentous ossifications such as ossified ligamentum flavum and to differentiate between osteophytes and segmental OPLL.[29],[30],[31] Sagittal image in CT helps in measurement of K-line and occupancy ratio, and to classify the type of OPLL. The axial sequences help to assess the thickness, shape, and extension and to localize the OPLL (central or paracentral).

Based on longitudinal extent, OPLL is classified into four types [Figure 9]:{Figure 9}

Continuous OPLL [Figure 9]a: a long lesion extending over several vertebral bodies; Segmental OPLL [Figure 9]b: one or several separate lesions behind the vertebral bodies; Mixed OPLL [Figure 9]c: a combination of the continuous and segmental types; and Circumscribed OPLL [Figure 9]d: mainly located posterior to a disc space.

Continuous or mixed types constrict the spinal cord more severely.[32] In order of frequency, OPLL is found at levels C4, C5, and C6.[32],[33]

Space available for cord

The space available for cord (SAC) is measured by subtracting the anterior to posterior distance of the OPLL from the spinal canal. The risk of CM is high in patients with a SAC of <6 mm, and the risk is low in patients with a SAC of >14 mm.[34] A reduction of spinal canal AP diameter from 40% to 60%[35],[36] has been claimed as a high-risk factor for the development of myelopathy.

Based on ossified foci in transverse plane, three types have been described [Figure 10]: (1) square, lines tangential to the bilateral margin of the ossified mass are parallel; (2) mushroom shaped, the two lines cross ventrally; and (3) hill shaped, the lines cross dorsally.[37]{Figure 10}

Dural ossification (DO) in ossified posterior longitudinal ligament

DO increases the chance of dural tear and CSF leak in anterior surgeries, and the DO can be detected using a bone window on CT scans. Based on the appearance of DO in OPLL, three signs have been described: (1) single-layer sign, (2) double-layer sign[38] [Figure 11], and (3) unilateral or bilateral C signs.[39]{Figure 11}

The single-layer sign: An irregular but continuous ventral OPLL massDouble-layer sign: Ventral hyperdense mass and dorsal hyperdense intradural mass along with an intervening hypodense areaUnilateral or bilateral “C signs” [Figure 12]: OPLL presenting laterally in the spinal canal with an angular C-shaped configuration; this sign indicated that the lateral dura had become imbricated in the OPLL mass.{Figure 12}

Chances of dural tear in DO with OPLL is in the following order of frequency double-layer, single-layer, and C signs[39] and if width >40% or thickness >50% [Figure 13].{Figure 13}

Occupancy ratio

Occupancy ratio [Figure 14] was defined as distance between largest width of OPLL and posterior spinal canal divided by spinal canal diameter and multiplying by 100. A ratio of 30%–60% is predictive of the development of myelopathy, and in those with an occupancy ratio >60%, all of them develop myelopathy,[40] and an anterior approach gives better neurological recovery as compared to posterior decompression.[24],[41]{Figure 14}

One other extended use of CT in the cervical spine is T1S and helps in the measurement of cervical sagittal alignment. T1S changes with various positions and age; hence, it cannot be used as an absolute parameter for cervical lordosis; however, TIA can be taken as a fixed reference value.[42]

 Role of Magnetic Resonance Imaging in Cervical Myelopathy



MRI in CM provides better anatomical details of disc, vertebral bone, ligamentous changes, alignment, and spinal cord pathologies. Detailed MRI assessment is usually done in conventional T1-weighted (T1W) and T2-weighted (T2W) sagittal and axial sequences.[43] A minimum of 1.5-Tesla imaging is essential for spine imaging, and screening of the whole spine in single mid-sagittal plane of T2W MRI is a routine to rule out the coexisting tandem lesions, the prevalence of which is about 11%.[43],[44],[45] Whole-spine screening also helps in identifying other pathologies, the incidence of which is reported to be about 15.8%.[46] MRI gives a comprehensive detail about the site, location and degree of compression, tandem stenosis, and cord changes, thereby predicting prognosis and planning appropriate intervention. However, controversies exist around the prediction of neurological recovery through signal intensity (SI) changes.[47],[48],[49],[50],[51],[52],[53],[54] The recent advancements in MRI such as DTI and magnetic resonance spectroscopy (MRS) attempt to address these issues by providing micro-architectural details and function of the spinal cord.[43] In OPLL, MRI is less sensitive and specific in diagnosis, but it is useful to assess the associated cord compression and intramedullary cord lesions, such as cord edema and myelomalacia.

Spinal cord compression and cervical spondylotic myelopathy

The severity of cord compression can be evaluated by various methods such as maximum spinal cord compression, compression ratio (CR)[55] (CR ratio between the AP and transverse diameter), or cross-sectional area[56] at the compression site [Figure 15]. These parameters are useful in assessing severity of compression as well as to assess the cord expansion postsurgically.{Figure 15}

Signal intensity (SI) change in spinal cord and cervical spondylotic myelopathy

In CM, SI change appears as hyperintense on T2 sequences and hypointense on T1 sequences. Hypointensity in T1 sequences indicates ischemic death of nervous tissue, while hyperintensity in T2 is due to edema of the spinal cord[47],[48],[57],[58] [Figure 16]. However, T1 hypointense signal change will not occur without T2 hyperintensity signals, unless there is a hemorrhage. In the early and intermediate stages of myelopathy, cord shows a high SI on T2-weighted image (T2WI), whereas in the late stages, hypointensity on T1-weighted images (T1WIs) and a hyperintensity on T2WIs occur.[59]{Figure 16}

The role of SI changes in recovery was analyzed in detail by many with varied results. Patients with altered SI on both T1WI and T2WI have a relatively poor outcome compared with the patients with signal changes only on T2WI.[49],[57] Most studies show poor prognosis related to the presence of T1 (low) signal change.[49],[50] High SI changes on T2WIs indicated both pathologically reversible and irreversible changes in the spinal cord, while low SI changes on T1WI were considered to reflect pathologically irreversible changes.[50] Better recovery rate was noted if the SI decreased after surgery as compared to those who had increased or persistence of signal change postoperatively.[47],[48],[49],[50],[60] In contrast, some studies[51],[52],[53],[54],[61] reported that the altered SI on T2WIs was not related to the poor outcome. The size (sagittal extent and area) of T2WI signal hyperintensity is significantly associated with surgical outcome, indicating that the larger the signal change, the lower the odds of achieving an Modified Japanese Orthopedic Association (mJOA) ≥16.[62]

In a retrospective study to analyze the SI changes and its role in prognostication of outcome in patients who underwent cervical laminectomy,[63] the authors observed that resolution of signal changes in T2WI was noted in most patients postsurgically and found that there is no significant difference in the recovery rate of patients with different grades of SI change in the T2WI or with focal or multisegmental SI changes when pre- and postoperative MRI images were compared; however, patients with low SI changes in T1WI had poor surgical outcome and concluded that out of all SI changes, low SI in T1WI is the best predictor of surgical outcome.

Role of dynamic magnetic resonance imaging in cervical spondylotic myelopathy

In CSM, the cord compression is due to both static and dynamic factors. Dynamic MRI (dMRI) taken in both flexion and extension separately allows to assess the complex interaction between disc, spinal canal, neural foramina, and segmental instability. dMRI is done typically using 1.5-T MRI with special functional positioning device. The ideal angles for flexion and extension are 40°–45°.[64] Studies have demonstrated more levels of compression in extension position as compared to flexion and neutral position.[64],[65],[66],[67] The hyperintense intramedullary lesions are visualized more often in flexion MRI.[68] CM patients with increased intramedullary signals during flexion had a greater degree of neurological recovery.[69]

Role of diffusion tensor imaging (DTI) in cervical spondylotic myelopathy

MRI is able to delineate the extent of anatomic lesion of the cervical cord, but it is difficult to infer the degree of cervical cord dysfunction from MRI findings. DTI is a recent advancement in MRI that solves this physiological lacuna.[70] DTI assesses the microstructural architecture by measuring the three-dimensional shape and direction of diffusion via the apparent diffusion coefficient (ADC) and fractional anisotropy (FA). It helps in identifying the compression earlier as it shows the abnormalities well before the development of SI changes in MRI.[70],[71]

DTI is based on the principle of anisotropic diffusion of molecules in biological tissues. Anisotropic diffusion refers to the diffusion of molecules in a medium where the molecular movement is restricted to one particular direction because of physical barriers. Most biological tissues such as the muscle fibers and neurons are anisotropic, because the molecular diffusion in them is restricted by physical barriers such as the myelin sheath. In these tissues, the direction of molecular diffusion is preferential along their longitudinal axis, and the DTI characteristics are often quantified by FA and ADC. FA is a diffusion index that indicates the directional preference of the diffusion where 0 represents isotropic diffusion (along all directions) and 1 represents completely anisotropic diffusion (along single longitudinal direction). ADC represents the mean diffusivity that is measured from 3 orthogonal directions within the imaged structure. In neuronal axons, where horizontal diffusion is restricted, diffusion of water molecules primarily occurs along the length (parallel) of the axon, and the FA values are closer to 1 [Figure 17].[70]{Figure 17}

Acute compression of spinal cord tissue may result in a focal decrease in ADC as well as a focal increase in FA. Progressive, chronic compression of the spinal cord results in a significant increase in ADC and decrease in FA.[70],[71],[72],[73],[74] Patients with a higher preoperative FA at the site of compression have better functional recovery, implicating mean FA at the site of compression as a biomarker for determining favorable surgical candidates.[73] DTI parameters and ratios help in the prediction of functional recovery after surgery.[75],[76],[77] However, in contrast, Rajasekaran et al. showed that the preoperative DTI datametrics are not predictive of postoperative recovery;[70] the difference might have been due to the magnitude (1.5 vs. 3 Tesla) of the MRI used.

Tractography gives a visual representation of fiber tracts, which is generated by placing initial seed at C1–C2 level covering the same region of interest that was used for obtaining DTI datametrics. The criteria for cessation of tracking are to be set at FA values >0.2. Four patterns are described as completely intact, waisted, partially interrupted, or completely interrupted [Figure 18]. Fiber indentation at the compression level without interruption was taken as waisted pattern. Fiber interruption across parts of the axial plane was taken as partially interrupted. Fiber interruption across the entire axial plane was taken as complete interruption.[70] Tractography helps in prognostic prediction in CSM,[70],[72],[77] and neurological improvement is more common in those with intact tractography.[72]{Figure 18}

Rajasekaran et al. analyzed DTI datametrics and tractography and observed an increasing trend of preoperative DTI values between control, ambulant, and nonambulant patients with CSM.[70] The authors analyzed that the efficacy of DTI anisotropy indices in predicting the postoperative recovery in CSM patients and postoperative changes in the DTI indices based on neurological recovery after surgery and found that there was a significant improvement in postoperative DTI indices in patients who showed neurological recovery; however, the preoperative DTI evaluation could not predict postoperative recovery for patients with CSM.[78] In tractography, none of the patients had a normal tractography whereas all the controls had normal pattern. However, the distribution of abnormal patterns was variable between the patients without any specific trend, and there was no significant association with myelopathy severity,[70] while in multilevel compressions, tractography helps in identifying the most severely compressed level.[70] On comparing eigenvalues (E1 is the longest vector along the long axis of the axonal cylinder and represents the longitudinal diffusion in the white matter, and E2 and E3 are minor eigenvectors representing the transverse diffusion within the spinal cord), the authors observed that minor eigenvectors had significantly higher values, denoting the increased transverse diffusion and loss of anisotropy, similar to the results shown by Kang et al.[79]

Magnetic resonance spectroscopy in cervical spondylotic myelopathy

MRS helps in understanding the progressive cellular alteration and provides metabolic information such as cellular biochemistry and function of the neural structures.[80] It assays a series of biochemical markers such as N-acetylaspartate (NAA), lactate, choline (Cho), myo-inositol (Myo-I), glutamine-glutamate complex (Glx), and creatinine (Cr), with particular sensitivity to NAA and lactate.[81] NAA is found only in axons and neurons and is considered as a marker for neuronal integrity.[82] Salamon et al.[83] reported that the early MRS changes in patients with cervical stenosis and without spinal cord signal changes had higher Myo-I and Glx compared to that of the control group and suggested Myo-I as a potential early marker for spinal cord inflammation and early-stage demyelination. Increased Cho levels appear later as cervical spondylosis progresses to a symptomatic state, and significantly, higher Cho/Cr ratio is found in patients with spinal cord signal changes than the control. Higher Cho/NAA ratio is significantly associated with poorer neurological function, and Cho/NAA had a significant correlation with the mJOA score, providing a useful radiographic biomarker in the assessment of cervical spondylosis.

Imaging in postoperative period

Cervical spine imaging after surgery plays a vital role in the evaluation of level of surgery, adequacy of decompression, position of implants, restoration of cervical alignment, cervical fusion assessment, diagnosis of pseudoarthrosis, spinal cord signal changes and to correlate with clinical improvement from myelopathy. Cervical fusion can be assessed by either radiographs or CT. Multiple radiological fusion criteria have been described. The four most commonly used are the presence of bridging trabecular bone between the endplates, absence of a radiolucent gap between the graft and endplate, absence of or minimal motion between adjacent vertebral bodies on flexion-extension radiographs, and absence of or minimal motion between the spinous processes on flexion-extension radiographs. However, <1 mm of motion between spinous process on lateral flexion and extension radiographs is considered as the most reliable indicator for fusion.[84] Although assessment through CT is accurate, it overestimates the fusion rate as it is taken in static position, while dynamic analysis is done in radiographs.[85],[86] Exact assessment of fusion requires dynamic information obtained from fl exion-extension x-ray in association with high-resolution static information from CT.

 Conclusion



Cervical spine imaging in CM plays an important role in analyzing the cause of myelopathy and defining the level of the lesion and parameters to assess the time of intervention and to predict the outcome preoperatively.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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