Natural history of extruded lumbar intervertebral disc herniation Tatsuhiko Henmi *, Koichi Sairyo **, Shunji Nakano **, Yoshiji Kanematsu *, Tomomasa Kajikawa *, Shinsuke Katoh **, Vijay K Goel† *Department of Orthopedic Surgery, Health Insurance Naruto Hospital, Tokushima, Japan;**Department of Orthopedic Surgery, The University of Tokushima School of Medicine, Tokushima, Japan;and &dagger;Department of Biomedical Engineering, The University of Toledo, Ohio, USA Abstract:We studied the natural history of extruded lumbar intervertebral discs using MRI. Forty-nine patients with lumbar disc herniation were included in this study. Ages ranged from19to57. On the T2-weighted sagittal MR image, the signal intensity in the herniated mass was measured and the ratio to that in the original nucleus (i.e, nucleus pulposus from which they extruded) was calculated (signal intensity ratio; SIR). The relationship with SIR and duration of illness was evaluated. In ten patients who were re-examined by MRI after conservative treatment, the size of the herniation measured by T1-weighted axial MR image was compared before and after treatment. The signal intensity of HNP became higher than that of the original nucleus immediately following herniation and thereafter decreased with time, suggesting that initial hydration of the HNP occurred shortly after herniation followed by dehydration of the HNP. The size of the HNP with a SIR value of1.2and higher on T2-weighted MR images decrease with time, however, the HNP with a SIR below1.2did not show any size reduction. The SIR of1.2and higher is a good indicator predicting spontaneous reduction of the HNP. Dehydration in the HNP may play an important role in the reduction of the lumbar disc herniation. J. Med. Invest. 49:40-43, 2002 Keywords:MRI/ Lumbar Spine/ Nucleus pulposus/ Herniation INTRODUCTION Spontaneous reduction of the herniated nucleus pulposus (HNP) in the lumbar spine has been well documented (1-3, 6-8). The proposed mechanisms of spontaneous reduction of the HNP with time included retraction by posterior longitudinal ligament tension, dehydration, resorption by macrophage phagocytosis,lymphatic drainage and immunologic reaction (1-4, 6-7). However, several studies have shown that dehydration is one of the major contributors to reduction of the HNP. For example, Bozzao et al. (1)and Saal et al. (6), based on qualitative information of MRI signals of the HNP (as judged by the brightness on MRI films), found a spontaneous reduction in the patients whose HNP were bright on T2-weighted images. Since T2-weighted MR images represent water content, these observations suggest that dehydration could be a factor. These authors, however, did not quantify the threshold values of MRI signal intensity that may predict spontaneous reduction of the HNP. Thus, the purpose of the present study is to quantify signal intensity of HNP as compared to the nucleus where it originated from and to correlate the signal intensity with the reduction in size. Such an investigation will enable identification of patients in whom spontaneous reductions of HNP occur. PATIENTS AND METHODS All consecutive patients between January 1992and February 1994 diagnosed with lumbar disc herniation were included in this study, with the following exceptions. Patients suffering from previous symptoms suggesting an earlier HNP were excluded. Patients presenting with non - extruded disc herniations and sequestration on MR images, and/or patients who had other lesions such as spondylolysis, spondylolisthesis, spinal canal stenosis and multiple disc herniations were excluded. Due to these restrictions, forty-nine patients qualified for the present study. There were32men and17women with a mean age of36.0years (range19to57years;standard deviation=12.1years). The HNPs were found at L3-4in one case, L4-5in31cases and L5-S1in17cases. T1-and T2-weighted MR images showing extruded nucleus pulposus were obtained at the time of the first examination. The duration of illness, defined as the time elapsed from occurrence of the leg symptoms to the first MRI examination, for each patient was also recorded. Thirty patients subsequently were elected for surgery and19for conservative treatment. We recommended follow-up MRIs to patients in the conservative treatment group. Ten agreed, and nine objected due to the time and cost involved. For the ten patients who agreed, the MRI was taken again after six to12months following the first visit, Table1. MRI apparatus used in this study had a 0.5 Tesla superconducting magnet (MRT-50, Toshiba, Japan). Spin-echo sequences employed on a condition of500ms/40ms (TR/TE) for T1-weighted images and2000ms/120ms (TR/TE) for T2-weighted images, with6mm of slice thickness. Signal intensity of HNP. On the T2-weighted sagittal MR image, we established regions of interest (ROI) in the center of the HNP and in the center of nucleus from which it extruded (original nucleus), (Figure1). Signal intensity of the ROI was measured, and the ratio of the signal intensity (SIR;Signal Intensity Ratio) in the herniated mass to that of the original nucleus was calculated. In this study, we attempted to clarify the natural history of herniated nucleus pulposus. Therefore, the SIR of the HNP was compared with the original nucleus, not with the normal disc such as the disc at L1/2level. The relationship between the duration of illness and SIR was also examined. Volumetric evaluation. On the T1-weighted axial image, the percent space-occupying ratio (SOR) of the herniation to the spinal canal was calculated by using imaging software, Figure2. The SOR between initial and follow-up MRIs were compared by calculating percent reduction ratio as100-(SOR on follow-up scan/SOR on initial scan×100). RESULTS Signal intensity of HNP. As shown in Figure1, the SIR was measured. The signal intensity of the disc and the original disc in Figure1 looks very similar, and its SIR is calculated as1.1. The SIR more than1.0means the signal intensity of the HNP is higher than that of the original disc, suggesting the HNP is hydrated following the herniation. In case the SIR shows below1.0, the signal of the HNP is lower than that of the original disc, indicating the HNP is suggested to dehydrate. The duration of illness ranged from1day to200days and exhibited a logarithmic decrease in the SIR values, Figure3. The relationship between the duration of illness and the SIR was found to be statistically significant (p<0.05). The result showed that the signal intensity of HNP became higher than that of the original nucleus shortely after herniation occurred and thereafter tended to decrease with time, suggesting that the hydration in disc herniations occurred immediately following event and dehydration in the HNP may progress over time. Volumetric evaluation. Ten patients who underwent a follow-up MRI after their first visit were regrouped into three categories, based on the SIR values of the initial T2-weighted MR images. Five patients had low-SIR (0.8or less); four had high-SIR (1.2and higher) and one had in-between the two (iso-SIR:between0.8and1.2). The size of the HNP of the four patients in the high-SIR group decreased (Reduction ratio:73.8to100%) at the time of follow-up examination. The other two groups did not show any significant changes in the reduction ratio (range-6.0to16.7%). However, in these patients the follow-up between the two MRI sessions was longer than the patients in high-SIR group, Table1. The reduction ratio in high-SIR group was significantly greater than that in iso-and low-SIR groups, (unpaired t-test, p<0.01). DISCUSSION In certain patients spontaneous reduction of lumbar HNP has occurred following conservative treatment (1-3, 6-8), e.g. 48%cases with a reduction of more than70% (1), and14of21patients (2). The present study has also demonstrated this to be true. The spontaneous reduction of the herniated mass observed in the present study showed a significant correlation with SIR at the first presentation. HNPs in high-SIR group (SIR>1.2) showed spontaneous reduction in size with time. The HNPs in high-SIR group showed increased water content compared with the nucleus pulposus of origin. On the other hand, the HNPs presenting with low-SIR lost water following the initial herniation. It seems that disc herniations with high SIR contained a certain amount of water to be dehydrated, while low-SIR masses had already lost water and therefore unable to decrease their volumes further by dehydration. Actually, the correlation between SIR and duration of illness suggested HNP will be hydrated following herniation and thereafter dehydrated with time. Dehydration in the HNP, thus, may play an important role in size reduction of lumbar disc herniation. Bozzao et al (1) and Saal et al. (6) reported cases with HNPs showing spontaneous reduction. In their patients, HNP showed high signal intensity on initial T2-weighted MR images and suggested dehydration to be a factor. The authors did not quantify the threshold values of MRI signal intensity that may suggest spontaneous reduction of the HNP. Our results provide a quantitative support to their observations. Thus, SIRs of1.2and higher can provide us the information to predict which herniation is likely to decrease with time spontaneously. Morphologic changes of HNP were found to correspond to clinical outcomes (2, 3), and an excellent outcome without surgery was more likely in patients in whom the degree of herniation reduction was larger. In this study, we clarified that HNP with high-SIR is likely to reduce in size with time and in the other two groups it seems unlikely. Therefore, if patients presenting with HNP are found to be in the high-SIR group, the symptoms may decrease spontaneously without surgery due to the size reduction. Thus, by measuring the SIR, surgeons can predict the clinical outcomes of patients presenting with HNP. Actually, in the present study clinical symptoms such as low back pain and leg pain of all patients in high-SIR group decreased at the final follow-up in corresponding to the reduction in size. The predictive value of our findings could also influence on the health care cost and will be the focus of further studies. Based on the results in this study, we proposed1.2in the SIR to be predictive value of the natural reduction in size of HNP. However, we just reviewed only ten patients. Thus, the value might not be completely correct. Further investigations should be required to elucidate the exact value of SIR, which can predict the natural reduction of HNP. ACKNOWLEDGMENT The authors wish to thank Prof. L.J.Grobler, Department of Orthopaedic Surgery, The University of Iowa, for his help during the editing process. REFERENCES 1.Bozzao A, Gallucci M, Masciocchi C, Aprile I, Barile A, Passariello R:Lumbar disc herniation. MR imaging assessment of natural history in patients treated without surgery. Radiology185:135-141, 1992. 2.Delauche-Cavaillier MC, Budet C, Laredo JD, Debie B, Wybier M, Dorfmann H, Ballner I: Lumbar disc herniation. 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AJNR 6:331-335, 1985. 8.Yukawa Y, Kato F, Matsubara Y, Kajino G, Nakamura S, Nitta H:Serial magnetic resonance imaging follow-up study of lumbar disc herniation conservatively treated for average30months. Relation between reduction of herniation and degeneration of disc. J Spinal Disord 9:251-256, 1996. Received for publication September 25, 2001;accepted November 22, 2001. Address correspondence and reprint requests to Tatsuhiko Henmi, M.D., Department of Orthopedic Surgery, Health Insurance Naruto Hospital, Muya-cho, Naruto, Tokushima772-8503, Japan and Fax:+81-88-683-1860.