Roots trajectory abnormalities and stenosis severity

Transcription

1 Roots trajectory abnormalities and stenosis severity Xavier Banse 1*, Olivier Manil 1, Frédéric Lecouvet 2, Thierry Duprez 2 Department of orthopaedic surgery 1 and Radiology 2 Cliniques Universitaires St. Luc, Université catholique de Louvain, av. Mounier, Bruxelles (Belgium) *xavier.banse@uclouvain.be Abstract Diagnosis of lumbar spinal stenosis (LSS) mainly relies on clinical symptoms and signs, and is confirmed by CT or MR imaging in the frame of the pre-surgical work-up. This present retrospective work investigates the frequency of loop -like or serpentine -like morphotype of nerve roots above or below the stenosis at preoperative magnetic resonance imaging (MRI) work-up of 102 patients presenting with clinically-definite LSS. Loops were found in 36% of the cases, serpentine roots in 24%, and normal straight roots in 40%. Loops or serpentines were never observed in the absence of absolute stenosis (disappearance of cerebrospinal fluid (CSF) around the roots at the most severely impinged level). Mean values for dural sac cross-sectional area (DSCA) were 45, 46, and 61 mm² for loops, serpentine and straight nerve roots. Straight roots were found in larger canals (p < 0.01). Buckling of roots above critically impinged thecal sac has been previously described on conventional opaque plain myelograms and was coined as redundant nerve root syndrome. The present study enhances the fact that root trajectory abnormalities (either loop or serpentine) is a sign of anatomical severity of the stenosis in clinically-definite LSS. These abnormalities are clearly defined and illustrated. They are commonly seen in clinical practice and easy to point out on conventional MRI. Fig. 1. Roots presenting trajectory abnormalities (loops, horizontalization) in a case of L3-L4 stenosis (sagittal and coronal T2 weighed MRI). Such observation was found in 36% of the operated cases.

2 Introduction Lumbar spine stenosis (LSS) is an anatomical condition responsible for neurologic claudiction, leg pain and lumbar pain [32,3]. This syndrome is defined by a combination of these three symptoms together with significant narrowing of the lumbar canal at medical imaging. In the vast majority of the cases degenerative processes including disc protrusion, ligamentum flavum bulging and zygapophyseal joints hypertrophy are responsible for LSS [1]. Impingement on thecal sac by degenerative efflorescence results in nerve root entrapment, mainly when standing or walking. Till the early 80 s radiological work-up of LSS was made by conventional opaque myelography. Initially, only extradural mass effect resulting in thecal sac impingement was observed. In most severe case, caudal-cranial progression of the contrast agent was stopped either partially or completely resulting in a socalled block. In the late 60 s, an additional sign was described which was changes in the normal course of the roots above the block [6,15,27]. Cases of serpiginous defects within contrast agent corresponding to tortuous nerve roots were observed above the block because of a mechanical entrapment of the roots at the site of stenosis. Nerve roots formed loops and had a serpentine aspect (Fig. 2). In the year 1980, a total of 27 cases were reported and coined the term of redundant nerve root syndrome to describe this condition [12]. Fig. 2: Appearance of the redundant nerve root syndrome as described un myelography, compare to figure 1 right. The mechanism of forming redundant nerve roots (permission of Hacker et al [12]) The sign of redundant nerve root syndrome has nowadays been shaded by the emergence of magnetic resonance imaging (MRI) in clinical practice. MRI has widely replaced conventional opaque myelography as best suited, non invasive (no lumbar puncture) and safe (no threat of CSF leak or infection) imaging modality in the routine work-up of LSS. Even though patients are examined by MRI in a resting supine position which may lower thecal sac compression because of diminished mechanical constraints on lumbar joints when compared to upright standing position, the value and significant value the sign of redundant nerve root syndrome could be preserved. The present work was designed to investigate the incidence of the sign in a large cohort of consecutive patients with LSS undergoing presurgical MRI, and to investigate the relationships the serpentine and the looping pattern (Fig. 1) with the degree of severity of the spinal canal stenosis.

3 Materials and Methods Between September 2006 and February 2009, 143 LSS decompression procedures were performed in the department of orthopedics surgery of our institution. Only cases requiring decompression at two or more levels were included into the analysis. Patients with a previous history of lumbar spine surgery, congenital, infectious, traumatic, or neoplastic disorders of the lumbar spine were excluded. Availability of a good quality pre-surgical MR examination, including sagittal T1- and T2-weighted images and axial transverse T2-weighted images in the picture archive and communication system (PACS) of the institution was the other sine qua non criterion for inclusion. Patient s age at the time of surgery was recorded. All MR examinations were reviewed on an independent workstation equipped with the Carestream Healthcare software, release 10.2 (Carestream Health, inc., Rochester, NY, USA). Cases were classified in a consensus way according to the features of the nerve roots in the central part of the canal (neither in lateral recesses, nor in foramina), above or below the level of maximal stenosis. Nerve roots were considered as making loops when, at least one root had fully horizontalized course. This was assessed in the sagittal plane either by linear straight horizontal course of the root or by a dot sign corresponding to a orthogonal cut in the root having a left-right course. In the axial transverse plane, a straight line instead of a dot was pathognomonic for the condition (Fig. 3c). Multiple looping roots were commonly observed. Roots were considered as having a serpentine course when sinusoidal deformation was observed on sagittal T2-weighted images without horizontalization (Fig. 3b). To differentiate abnormal serpentine course from normal bending of a root extruding the canal, the angle between longitudinal axis of the cauda equina and serpentine root was thresholded at 30 (or more) degree. Remainders without loops or serpentine course were classified straight roots with normal course (Fig. 3a).

4 Fig. 3: The three patterns of nerve roots course distortion on sagittal T2-weighted MR images. Left, 3a: straight pattern: roots have a normal straight course with smooth anterior aspect of the cauda equina (black arrows) in spite of L4-L5 stenosis (white arrows). Center, 3b: serpentine pattern: roots have sinusoidal course at some places (black arrows) above and below but without horizontalization above and below severe L2-L3 and L3-L4 stenoses (white arrows). Right, 3c: loop pattern: severe root distortion with horizontalization of roots course in a patient with L3-L4 and L4-L5 stenoses (white arrows). The level of maximal stenosis served as reference to quantify the degree of stenosis. The absolute degree of stenosis was featured by complete disappearance of cerebrospinal fluid (CSF) around and in-between the roots on the axial-transverse T2-weighted images. The whole surface of the sac displayed low signal intensity in this condition (Fig. 4). In turn, a relative degree of stenosis was considered when incomplete disappearance of bright CSF was present. Fig. 4: Absolute (D and E) and relative stenosis (B and C). AP diameter and DSCA measurements. Two quantitative parameters were recorded using the measurement options of the Carestream software: the paradigmatic antero-posterior diameter of the sac (AP diameter in mm) [28]; and the dural sac cross-sectional area (DSCA in mm²) [16]. The AP diameter was measured from black to black [17]. The DSCA was measured by manual planimetric contouring of the dural sac using the freehand drawing option with mouse [23]. The SPSS Release 15.0 software for statistical analyses (SPSS Inc., Chicago, IL, U.S.A.) was used for data processing. Relationships between nerve root pattern (straight vs serpentine vs loop), degree of stenosis (absolute vs relative), and quantitative parameters (AP diameter and DSCA) were analyzed using non parametric Kruskal-Wallis test.

5 Relationships between root pattern and degree of stenosis were assessed using Chi-Square test. Statistical significance was thresholded at p <0.05. Results Out of 143 records, a total of 102 patients fulfilling criteria for eligibility were included into the analysis. There were 45 males and 57 females with a mean age of 69 years (range, years). 41 patients (40.2 %) had a pattern of straight nerve roots (Fig. 3a), 37 (36.3 %) had a looping pattern (Fig 3b) and 24 (23.5 %) had a serpentine one. Loops were observed above and below the site of maximal stenosis. Absolute stenosis was observed in 81 cases (79.4 %) and relative stenosis in 21 cases (20.6 %). Mean AP diameter was 6.2 +/- 1 mm and mean DCSA was /- 16 mm2 in cases of relative stenosis and 4.7 +/-1 mm2 and /- 13 mm², in cases of absolute stenosis. This difference was significant (p<0.001 for AP diameter and p<0.004 for DSCA). Table 1 shows the main parameters according to the group of root pattern (loops, serpentine or straight). There was no difference in patients age. The presence of loops or serpentine pattern was associated with more severe stenosis: not only significantly smaller AP diameter and smaller DCSA but almost systematically absolute stenosis (36/37 and 23/24, p<0.001), while with straight roots absolute stenosis was only present in half of the cases (22/41, p <0.001). Table 1 Total Straight group Serpentine Loop group group Number of cases Mean age, +/- SD (years) / / /- 14 Mean AP diameter, +/- SD (mm) / / /- 1 Mean DCSA, +/- SD (mm 2 ) / / /- 12 Relative Stenosis frequency 21/102 19/41 1/24 1/37 Absolute Stenosis frequency 81/102 22/41 23/24 36/37 Discussion This work is certainly not the first description of root trajectory abnormalities neither with myelography [24] nor with MRI [22]. However we thought it was important to draw or re-draw attention on this feature for multiple reasons. First of all, trying to understand or teach the pathogenesis of the LSS symptoms is much easier keeping in mind this feature. Roots are so compressed that they do not glide freely anymore in the canal. It is a straightforward explanation, very easy to understand, and complementary to the rather complex vascular theory of the LSS. Consequently, drawings like that of Hacker et al [12] in Figure 2 should be found in standard textbooks. Secondly, it is not a rare abnormality. Finally, we hope that correct anatomic description will help future investigations regarding the clinical significance of the loop or serpentine roots aspect.

6 Abnormalities in the roots trajectories were observed quite frequently. Systematic review of our pre-operative MRI revealed a frequency of 60%. Much more than what was expected. However, this observation is concordant with some data present in previously published studies. Suzuki et al [29] reviewed 1256 myelographies and detected 130 contrast block in standing position (diagnostic of significant anatomic lumbar spine stenosis). Redundant nerve roots were observed in 42% of these patients with block. An older study by Tsuji et al [31] report 29% of loops and 39% of serpentine roots on myelograms of 56 patients with degenerative spinal stenosis. Ono et al [22] described 28% of nerve roots loop shaped depicted on pre-operative MRI of 146 patients presenting an L4-L5 spondylolystesis and local stenosis. Minh et al [20] report a group of 68 patients with one level stenosis with a rate of 33.8% of redundant nerve root see on sagittal MRI images only. Roots trajectory abnormalities in the LSS are presented here as a radiological sign of compression severity. We compared it to three classic signs: measurements of severity: APdiameter, DSCA and absolute character of the stenosis. We deliberately focused on the dural sac (rather than, i.e. bony dimensions) since we think that space available for the roots is the key issue. Antero-posterior diameter of the canal is a very common anatomic criteria of LSS. A study of Eisenstein [7] demonstrates that only AP diameter is required to diagnose dural canal stenosis. For Malghem et al [17], this measure must be done from two important point of reference, black to black, on T2 MRI sequences (either axial or sagittal, Fig. 4 B, D and F). In others terms, it is the distance from the anterior to posterior wall of the radicular and liquid column of the dural sac. In the same way, Perron et al. in a paper about congenital stenosis, referring to classic literature [7,19,26] defined the absolute character of the stenosis when its AP diameter is smaller than 10 mm [23]! Nevertheless, in case of severe hypertrophy of facet joints, the AP diameter measure becomes less adequate. Conversly, some patients may be asymptomatic with absolute stenosis (less than 10mm AP-diameter) since their canal is wider than deep [8,10,13]. Some authors [9] conclude that AP spinal canal diameter is not predictive of clinical symptoms associated with LSS. For McCall [19], measurement of the DCSA is more important than AP diameter since the shape of the canal may vary, leaving more or less space for the roots. The critical level of DCSA is demonstrated as 75 mm 2 by the classic cadaveric experiment of Schönström [25]. Basing our indications on clinical considerations we subjectively selected quite severe stenosis since our mean AP-diameter was 5mm and mean DSCA was 52mm 2. In this paper, we used the term absolute stenosis, not as an a priori criteria based on a 10mm AP-diameter [23], but rather based on the fact that there is -or not- some extra space available for the roots. Hence, when some CSF is still seen around the roots, the stenosis is not considered as absolute. Absolute stenosis can be considered as the MRI analog of the complete block of contrast seen in myelography (Fig. 4D and E). This study is the first or report a relationship between root trajectory abnormality and anatomic severity of the stenosis using MRI. This was not reported on previous papers [20, 22, 29], maybe because of difficulties encountered in practical measurements of the DSCA or AP-diameter. As well, in many cases, the measurement is not easy and using T1 as well as T2 weighted axial images is useful. None of them investigated the absolute character of the stenosis. Roots trajectory abnormalities were found almost exclusively in association with absolute stenosis (59 / 61 cases). Further, we observed loops and serpentine aspect above as well as below the level of spine stenosis (Fig. 1 and 3). This could not be observed with myelography for technical reasons and has not been described yet in MRI. As Verbiest said, the orthostatic factor in the origin of the symptoms may be explained by an in increase of pressure on the dural sac during the upright position [32]. Other authors

7 [21,34,35] have confirmed lower detection at of loops and serpentines when the spine is in flexion or in a neutral position. Consequently, MRI may underestimate the compression. Some patients with LSS report night pain and this symptom may be explained by our data (loops in dorsal decubitus). Fig. 5: Nerve root trajectory abnormality below the L3-L4 stenosis (A). The loop pattern is defined by the horizontal trajectory depicted on the axial T2 weighed images (B and C). New definitions of abnormal root trajectory are proposed in this paper. Terms need to be explained and detailed. We propose to use the term loop when roots have a horizontal trajectory. This can be seen on axial T2 weighted images (Fig. 5b and c) as any linear nerve root trajectory in the central part of the canal. More easily, one can see it on sagittal sequence, where it appears rounded, making or making a U turn and has an antero-posterior horizontal trajectory. When the root has a left-right horizontal trajectory, it will be seen as a black point in the middle of the CSF (Fig. 1 left, black arrow above). Looking for trajectory abnormalities was found easier when adapting the contrast and brightness to the canal content and when passing few sagittal contiguous images back and forth (in a dynamic way). This last detail helps to detect abnormal root trajectory. Further investigations on the clinical significance of the root trajectory abnormalities on MRI are certainly needed since conflicting observations have been reported in different situations [20, 22]. We hope that precise anatomical criteria for the definition of these abnormalities in MRI will help for further investigations.

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