Unsuspected spondylolysis in patients with lumbar disc herniation on MRI: The usefulness of posterior epidural fat.

BACKGROUND AND PURPOSE: Lumbar disc herniation (LDH) at the level of non-displaced spondylolysis (isthmic spondylolisthesis) is an uncommon association rarely evaluated in the literature. In this study, authors examine whether the continuous posterior epidural fat between the dura mater and spinous process (continuous double-hump sign) at the level of LDH is a valuable tool to identify patients with non-displaced spondylolysis on MRI.

METHODS: Eighteen patients (group 1) presented at our department (2000-2010) with a LDH associated with an undiagnosed non-displaced spondylolysis on MRI. Spondylolysis was confirmed by direct visualization of the defect on CT-scan, dynamic radiography, MRI or at surgery. To validate this method, we made the same evaluation in 20 surgically treated patients (group 2) with a one-level LDH without spondylolysis.

RESULTS: In all patients of group 1 spondylolysis was unsuspected on the MRI report. However, a positive "continuous double-hump sign" was seen in 16 patients. The spondylolysis was recognized on MRI in six cases, on CT-scan in nine cases and on dynamic radiography in one case. Two cases were diagnosed surgically. In group 2, only one patient had a positive "continuous double-hump sign". This new sign had a specificity of 95%, sensitivity of 88.88%, and accuracy of 92.10% for diagnosis of non-displaced spondylolysis.

CONCLUSIONS: Non-displaced spondylolysis may be associated with adjacent LDH. Although uncommon, it is important for neurosurgeons to be aware of this association because of its implication on the therapeutical management. MRI is not always sufficient to recognize a non-displaced spondylolysis with certainty; however "continuous double-hump sign" may be used as a simple valuable diagnosis tool.