(Radiology. 2001;220:208-212.)
© RSNA, 2001
Medullary Cone Movement in Subjects with a Normal Spinal Cord and in Patients with a Tethered Spinal Cord1
Theodoor D. Witkamp, MD,
William P. Vandertop, MD, PhD,
Frederik J. A. Beek, MD, PhD,
Nicolette C. Notermans, MD, PhD,
Robert H. J. M. Gooskens, MD, PhD and
Paul F. G. M. van Waes, MD, PhD
1 From the Departments of Radiology (T.D.W., F.J.A.B., P.F.G.M.v.W.), Neuromuscular Diseases (N.C.N.), and Child Neurology (R.H.J.M.G.), University Hospital Utrecht, HPnr: E.01.132, Heidelberglaan 100, NL-3584 CX Utrecht, the Netherlands; and the Department of Neurosurgery, Free University Amsterdam, the Netherlands (W.P.V.). Received December 9, 1999; revision requested January 24, 2000; revision received December 13; accepted January 15, 2001. Address correspondence to T.D.W. (e-mail: j.m.m.vanamstel@azu.nl).
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ABSTRACT
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PURPOSE: To compare movement of the normal medullary cone when the patient has changed from a supine to prone position with that in patients with known or suspected tethered spinal cord syndrome.
MATERIALS AND METHODS: Fifty-six individuals divided into three groups were examined with lumbar spine magnetic resonance (MR) imaging performed with the patient in the prone and supine positions. Group 1 consisted of 15 healthy volunteers and six patients with a herniated disk; group 2, 25 patients clinically suspected of having a tethered cord; and group 3, 10 patients who previously had undergone tethered cord surgery.
RESULTS: All group 1 subjects showed distinct and statistically significant medullary cone movement (range, 21%–41%); no patient in group 3 showed movement (Wilcoxon rank sum test, P < .001). In group 2, the 20 patients in whom a definite diagnosis of tethered cord syndrome was made on the basis of initial supine MR image findings showed no movement, whereas two of five patients with normal supine MR images had abnormal and decreased cone movement at prone imaging.
CONCLUSION: Prone MR imaging has no additional value when the supine MR image has clearly shown the cause of tethering or in patients who have undergone tethered cord surgery, but it can provide additional information in patients clinically suspected of having a tethered cord and in whom supine MR imaging depicted no abnormalities.
Index terms: Spinal cord, MR, 36.121411, 36.121412 • Spinal cord, abnormalities, 36.1481, 36.1482
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INTRODUCTION
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The tethered spinal cord usually is defined as a low-lying medullary cone. In routine practice, the clinical diagnosis of a tethered spinal cord is rejected or confirmed on the basis of magnetic resonance (MR) imaging findings by using the normal shape and location of the cord and cone as discriminative factors. The causes of tethering are various and include a tight terminal filum, intradural lipoma, and retethering following surgery for a myelomeningocele (1–5). Surgical untethering usually stops the clinical deterioration and sometimes even reverses symptoms (6–8). Therefore, an accurate neuroradiologic diagnosis is needed to help the clinician confirm the clinical diagnosis.
There have been published reports of patients with tethered cord syndrome who at myelography, computed tomography, or MR imaging showed a normal cone shape and location, as well as those with an abnormally low cone location without obvious tethering causes (9,10). In such cases, additional information is needed to make a definite diagnosis. This can be obtained by adding dynamic information to the static information obtained at standard MR imaging. The spinal cord normally exhibits anteroposterior and craniocaudal movement at MR imaging, whereas the tethered spinal cord exhibits decreased or no movement. These findings have been reported by McCullough et al (10), who studied the movement of the cervical spinal cord with MR imaging; by Naidich et al (11), Tamaki et al (12), and Schumacher et al (13), who studied the spinal cord with ultrasonography (US); and by Scatliff et al (14), who studied the movement of the tethered and normal spinal cord with myelography. Dynamic information obtained in this manner is either invasive (ie, myelography), obtained in another anatomic location (ie, cervical spine) then in the primary region of interest, or obtained with another imaging method (ie, US).
Although the position of the normal spinal cord changes under the influence of gravity when the body moves from the supine to prone position, reduced or absent movement is expected when the spinal cord is tethered. We investigated whether this hypothesis could be proved— noninvasively and without changing the region of interest or the imaging modality—by examining healthy volunteers and patients clinically suspected of having tethered spinal cord syndrome.
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MATERIALS AND METHODS
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Subjects
A total of 56 individuals, divided into three groups, were examined. Institutional review board approval was given, and informed consent was obtained from all volunteers and patients after the nature of the additional procedure had been explained.
Group 1 consisted of 15 healthy volunteers (eight male, seven female) and six consecutive patients (five male, one female) with a herniated disk or nonneurogenic micturition disturbances. Their ages ranged from 3 to 40 years (mean age, 15 years).
Group 2 consisted of 25 consecutive patients (11 male, 14 female) aged 3–46 years (mean age, 14.7 years) in whom a tethered spinal cord was clinically suspected by a neurologist and a neurosurgeon.
Group 3 consisted of 10 consecutive patients (seven male, three female) aged 3–26 years (mean age, 12.7 years), all of whom had undergone surgery for a tethered cord and later presented with progressive neurologic deficits indicative of possible retethering. Six of these patients had a myelomeningocele, and four had a lipomyelomeningocele.
MR Imaging Evaluation
In group 1, lumbar spine MR imaging was performed in the 15 healthy volunteers placed in the supine and prone positions. T1-weighted spin-echo MR images (500/20 [repetition time msec/echo time msec], 3–5-mm section thickness, 350-mm field of view) in the sagittal and transverse planes were obtained. The six patients were then examined with the protocol used for groups 2 and 3.
In groups 2 and 3, the lumbar spine was examined by using a standard screening protocol for MR imaging of the lumbar spine. This consisted of obtaining sagittal T1-weighted (500/20, 3–5-mm section thickness, 350-mm field of view) and T2*-weighted gradient-echo (615/27, 25° flip angle) images and was completed by obtaining transverse T1- and T2*-weighted images through a region of interest, depending on the abnormality found on the sagittal images. The patients were then placed in the prone position, and the same surface coil as that used for supine imaging (40 x 10 cm) was placed on the back of each patient. The acquisition of T1-weighted images in the sagittal and transverse planes of the lumbar spine was repeated in this position. The same parameters as those used for standard supine imaging were used. For comparison purposes, the field of views for prone and supine sagittal imaging were identical.
To make an initial diagnosis, the standard screening MR images were evaluated separately, without knowledge of the clinical data, by two neuroradiologists (T.D.W., F.J.A.B.). The position of the medullary cone was described in relation to the spinal vertebrae. A normal image was defined as that showing the medullary cone situated above or at the level of the L1-2 intervertebral disk, with a normal medullary cone shape and without a visible terminal filum or a filum smaller than 2 mm in diameter (15). Tethering was considered to be present when (a) the medullary cone was lower than the L1-2 intervertebral disk and/or had an abnormal shape or (b) the terminal filum was thicker than 2 mm, regardless of the level of the cone (16,17). On both the supine and prone images, the following measurements were obtained: (a) the distance between the medullary cone and the ventral dura with the patient in the supine position (As), (b) the distance between the medullary cone and the ventral dura with the patient in the prone position (Ap), and (c) the width (W) of the dural sac at this level (Fig 1). The average of the measurements obtained by the two neuroradiologists was used for data analysis.
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Figure 1a. Drawings show the measurements made on (a) supine and (b) prone MR images. The vertical black structure at the top of the drawing is the spinal cord and medullary cone. Ap = distance between the medullary cone and the ventral dura at MR imaging with the patient in the prone position, As = distance between the medullary cone and the ventral dura at MR imaging with the patient in the supine position, W = width of dural sac at the level of the tip of the medullary cone. Percentage of movement = (As - Ap)/(W x 100).
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Figure 1b. Drawings show the measurements made on (a) supine and (b) prone MR images. The vertical black structure at the top of the drawing is the spinal cord and medullary cone. Ap = distance between the medullary cone and the ventral dura at MR imaging with the patient in the prone position, As = distance between the medullary cone and the ventral dura at MR imaging with the patient in the supine position, W = width of dural sac at the level of the tip of the medullary cone. Percentage of movement = (As - Ap)/(W x 100).
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Data Analysis
To evaluate the movement of the medullary cone, a percentage of movement was calculated by using the formula (As - Ap)/(W x 100). To compare groups for all parameters, the Wilcoxon rank sum test was performed by using the Statistical Package for the Social Sciences software (SPSS, Chicago, Ill).
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RESULTS
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Group 1
All images obtained in group 1 (n = 21) were normal. In all individuals, distinct movement of the medullary cone was evident on both the sagittal and transverse images (Fig 2). The distance between the medullary cone and the ventral dura at MR imaging ranged from 10 to 18 mm (mean, 12 mm) with the patient in the supine position and from 5 to 10 mm (mean, 7 mm) with the patient in the prone position. Cone movement ranged from 4 to 8 mm (Fig 3). The width of the dural sac at the level of the cone ranged from 5 to 10 mm. Medullary cone movement ranged from 21% to 41% (mean, 33%). This movement was significantly greater than that in either group 2 or group 3 (Wilcoxon rank sum test, P < .001).
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Figure 2a. Spin-echo MR images (500/20) of the lumbar spine obtained in a healthy volunteer in the (a) supine and (b) prone positions. Distinct movement (33%) of the medullary cone (c) is visible.
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Figure 2b. Spin-echo MR images (500/20) of the lumbar spine obtained in a healthy volunteer in the (a) supine and (b) prone positions. Distinct movement (33%) of the medullary cone (c) is visible.
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Figure 3. Scatterplot shows the variation in range of movement among the three subject groups. All patients are included, and one square dot may represent more than one patient. 1 = healthy volunteers, 2 = patients clinically suspected of having a tethered cord, 3 = patients previously operated on for a tethered spinal cord.
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Group 2
In three patients in group 2 (n = 25)—that of patients clinically suspected of having a tethered spinal cord—the supine images were considered to be normal. In one of these three patients, the prone images could not be evaluated owing to extreme movement artifacts. In the two other patients, the prone images showed only slight cone movement—2 mm in both—which resulted in 11% movement and 13% movement (Figs 3, 4). In one of these two patients, tethering was confirmed at surgery; the other patient refused to undergo surgery. One-year clinical follow-up showed no progression of the neurologic problems.
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Figure 4a. Spin-echo (a) supine and (b) prone MR images (500/20) of the lumbar spine obtained in a patient clinically suspected of having a tethered cord. The supine MR image is normal, whereas the prone image shows only slight (11%) medullary cone (c) movement. A tethered spinal cord was found at surgery.
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Figure 4b. Spin-echo (a) supine and (b) prone MR images (500/20) of the lumbar spine obtained in a patient clinically suspected of having a tethered cord. The supine MR image is normal, whereas the prone image shows only slight (11%) medullary cone (c) movement. A tethered spinal cord was found at surgery.
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In 22 patients, the diagnosis of a tethered spinal cord was made on the basis of standard supine MR image findings—specifically, an abnormally low medullary cone position. In two of these 22 patients, the prone images could not be evaluated owing to movement artifacts; in five patients, thickening of the terminal filum was diagnostic (Fig 5). Thirteen patients had transdural lipoma, and two had a split cord malformation (ie, diastematomyelia). In none of the patients with diagnostic prone images (18) was there a measurable change in the medullary cone between the supine and prone positions.
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Figure 5a. Spin-echo (a) supine and (b) prone MR images (500/20) of the lumbar spine obtained in a patient with a tethered cord due to a lipomatous thickened terminal filum. No medullary cone movement is visible. The arrowheads point to the stretched spinal cord and medullary cone.
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Figure 5b. Spin-echo (a) supine and (b) prone MR images (500/20) of the lumbar spine obtained in a patient with a tethered cord due to a lipomatous thickened terminal filum. No medullary cone movement is visible. The arrowheads point to the stretched spinal cord and medullary cone.
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Group 3
Because all patients in group 3 (n = 10) previously had undergone surgery for tethered cord syndrome, the cause of tethering was clearly identified. In none of these patients was there a sign of cord movement at the level of the suspected tethering.
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DISCUSSION
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In this study, we measured the movement of the medullary cone in patients with and in those without tethered cord syndrome. The normal spinal cord is held in position by the dentate ligaments, the nerve roots, and the terminal filum (19). As the dentate ligaments stop at the 11th thoracic vertebra, the cone is able to move more easily than is possible at the higher regions. This information, combined with US (9,11,12), myelographic (14), and dynamic MR imaging (10) data, leads to the hypothesis that the medullary cone should show detectable movement at MR imaging when the patient changes from the supine to prone position. Our study data show that in healthy subjects, the movement of the medullary cone is substantial when the patient changes to the prone position (on the order of 33%). In our study, there were no healthy subjects in whom the position of the medullary cone showed no measurable change.
The tethered spinal cord is a developmental abnormality that has been extensively discussed in the literature. MR is regarded as the imaging method of choice to confirm the clinical suspicion of a tethered spinal cord (1). Thanks to detailed anatomic information, it is now possible to identify not only the medullary cone but also the possible causes of tethering (20). In our study patients in whom the cause of tethering was clearly visible on the supine MR images, the medullary cone showed no movement on the prone MR images.
In two cases, the supine MR image was normal. The prone MR image, however, depicted markedly diminished movement. One of these patients was operated on, and tethering caused by a terminal filum was found. In retrospect, this terminal filum was not apparent on the transverse T1-weighted MR image. Only at the level of the first and second sacral vertebrae was a small amount of fat visible in the terminal filum. In this case, the prone image clearly proved to be of additional value. The other patient refused to undergo surgery, so the diagnosis could not be confirmed.
Warder and Oakes (9) found similar cases—that is, a tethered cord with the cone in a normal position—in 13 of 73 cases over a 12-year period. These findings lead to the conclusion that there can be an indication, although limited, for obtaining MR images with the patient in both the prone and supine positions.
Adding a prone position sequence to MR imaging of the normal spine prolongs the examination by approximately 10–15 minutes. In these predominantly young patients, this could lead to increased restlessness. In addition, the quality of prone MR images is always lower compared with that of supine MR images, mainly owing to movement artifacts caused by respiration. These can be reduced by supporting the shoulder and pelvic region, which gives the abdomen more space to move and results in reduced anteroposterior movement of the spine. Placing presaturation slabs ventral to the spine and/or changing the preparation direction from anteroposterior to caudocranial also can reduce artifacts.
Many authors (9,13,17,21) have discussed the problems of diagnosing retethering of the spinal cord. Postoperative scar tissue causes the cord to retether to the dura and/or subcutaneous tissues. We found no signs of cerebrospinal fluid dorsal to the spinal cord on the supine MR images. Thus, in all cases, retethering was diagnosed and later confirmed by the finding of no apparent movement at prone MR imaging.
According to Naidich et al (11), reduced movement of the cervical spinal cord could be an indication of retethering. They used the dynamic information from consecutive US studies to diagnose retethering in children. In our opinion, when tethering is clear on conventional MR images, dynamic information such as that provided on prone images does not provide any substantial additional information.
Vernet et al (18) stated that prone MR imaging has no additional value in the evaluation of spinal cord tethering. They examined postoperatively only those patients with a proved tethered cord, so their results are in accordance with the findings of our study. However, when the results of supine MR imaging alone were compared, our study results show that prone MR images may be helpful in selected cases.
In conclusion, we contend that prone MR images have no additional value in patients in whom the supine MR images have already shown the cause of tethering or in those who have undergone surgery for a tethered cord and are clinically suspected of having retethering. Prone MR images can provide additional information in patients in whom a tethered cord is clinically suspected but the supine MR images initially showed no abnormalities.
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ACKNOWLEDGMENTS
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The authors thank Marijke Eurelings, MD, for valuable assistance with the statistical analyses and the photographic department for their help.
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FOOTNOTES
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Author contributions: Guarantors of integrity of entire study, T.D.W., P.F.G.M.v.W., W.P.V.; study concepts and design, T.D.W., F.J.A.B.; literature research, T.D.W., W.P.V.; clinical studies, T.D.W., W.P.V., R.H.J.M.G.; data acquisition, T.D.W., F.J.A.B., P.F.G.M.v.W.; data analysis/interpretation, T.D.W., W.P.V., N.C.N.; statistical analysis, W.P.V.; manuscript preparation, T.D.W., W.P.V.; manuscript definition of intellectual content, T.D.W., W.P.V., F.J.A.B., N.C.N.; manuscript editing, W.P.V.; manuscript revision/review, all authors; manuscript final version approval, T.D.W., W.P.V., N.C.N.
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ABSTRACT
INTRODUCTION
