Findings in this prospective study of patients undergoing decompression for lumbar canal stenosis support the usefulness of early postoperative MR imaging.
Epidural hematomas are frequent in early postoperative MRI after decompression surgery in lumbar spine in asymptomatic patients with varying prevalence [6
]. In our series postoperative epidural hematoma with compression of dural sac was seen also in patients with asymptomatic postoperative course. In our study 42.5% of operated patients with uneventful course had an epidural hematoma. However, the hematoma and dural sac compression were significantly larger in patients with postoperative symptoms requiring revision. We found a statistically significant difference between the postoperative dural sac areas in asymptomatic patients compared to patients who needed revision surgery.
In asymptomatic patients with postoperative hematoma, the preoperative and postoperative cross-sectional areas of the dural sac were similar. This could indicate that resected parts were replaced by epidural hematoma and no major dural tube expansion occurred in the early postoperative period in these patients.
Until now it is not known, what are the differences in MRI between the patients requiring revision surgery for symptomatic epidural compression by hematoma in the early postoperative period and those with uneventful course, since mass effect in early postoperative MRI is a common finding and can be regarded as “normal” [6
]. In the literature the correlation between clinical symptoms and radiological findings of early postoperative MRI is found to be low [5
Most studies concerning early postoperative MRI included a variety of pathologies, e.g. disc herniations and patients with instrumentation which may have additional signal changes [8
]. Since discectomy and instrumentation represent additional factors which obscure the early postoperative signals we analyzed only patients undergoing decompression for lumbar canal stenosis without discectomy or instrumentation. In the patients with spinal instrumentation postoperative measurements could be difficult because of obscuring signals.
Ross et al. [6
] in a prospective study of 15 patients found extensive soft-tissue changes in MRI obtained immediately postoperatively. They concluded that these changes severely limit the usefulness of MRI in the evaluation of early postoperative symptoms. They also stated that identification of hemorrhage is possible due to the distinctive signal on T1-weighted images. After decompression the regions of the missing laminas, ligamentum flavum and spinous process were replaced by a variable amount of posterior soft-tissue edema exhibiting heterogenous intermediate signals at T1-weighted pulse sequences and isointense to increased signals at T2-weighted sequences.
Awwad et al. [5
] reviewed the findings of immediate postoperative MRI of 10 patients who had decompression surgery without postoperative adverse symptoms. In 9 out of 10 patients severe thecal sac compression was present, greater than that on preoperative MRI. The authors concluded that severe spinal canal compression can be a normal finding in postlaminectomy spine and that the MRI appearance in such instances is not significant in the absence of compressive clinical symptoms. They stated that these findings on immediately postoperative MRI may lead to incorrect conclusion that there is a surgical complication needing evacuation.
Kotilainen et al. [7
] performed MRI on the first postoperative day after percutaneous nucleotomy or microdiscectomy in 44 patients. In 86% of the patients an extradural hematoma was found. All patients who underwent decompression for lumbar disc herniation and with large hematoma in early postoperative MR imaging had a complete resolution of sciatica. In patients with medium hematoma and small hematoma sciatica resolved in a lower rate, 88 and 74%, respectively. Further, patients with no hematoma in early postoperative MR imaging showed a reduction in sciatica only in 67%. The author concluded that the presence of hematoma was not associated with poor short-term prognosis.
Sokolovsky et al. [8
] in a prospective study determined in 57 patients the incidence, volume and extent of postoperative epidural hematoma resulting in dural sac compression with MRI 2 to 5 days after surgery. This group included various pathologies with and without fusion. As much as 58% of patients developed an epidural hematoma. The postoperative dural sac area ranged from 44 to 194% of preoperative area. On average the postoperative dural sac area was 32% smaller than preoperative at the maximum site of compression due to the mass effect of subfascial hematoma. In our series, we found a statistically significant increase of the dural sac area postoperatively with a median of 178% in all asymptomatic patients.
In a second study Sokolovsky et al. [9
] compared the asymptomatic patient group with two retrospective groups with severe peri-incisional pain (12 patients) and postoperative cauda equine syndrome (5 patients). In this study absolute measurements of dural sac area did not differ significantly between groups. They found that the critical ratio was the only measurement to differ significantly among the 3 groups. Mean critical ratio was 0.8 for asymptomatic patients, 0.5 for patients with pain and 0.2 for patients with cauda equine symptoms.
Schönstrom et al. [13
] in an in vitro study calculated the CSA at which a further constriction caused a pressure increase among the nerve roots and called this the critical size. The average corresponding critical size was 76.9 mm2
at the level L2, 71.5 mm2
at level L3 and 64.8 mm2
at L4. Measuring the CSA with CT or MR has been shown to be a reliable method to diagnose lumbar stenosis [13
Until now there are no reports determining an absolute critical value of the cross-sectional area of dural sac in early postoperative MRI which could indicate clinical significance. We found in 75% of the patients in revision group a maximal postoperative dural sac area of 58.5 mm2
or less (ratio 0.62), whereas in the asymptomatic group only 25% of patients had an area of 96.5 mm2
or less (ratio 1.21). This indicates that in the investigated patients a critical value in between 58.5 and 96.5 mm2
exists which may produce symptoms. Probably this value is similar to that proposed in preoperative images by different authors to differentiate moderate from severe stenosis, approximately 75 mm2
]. In fact, considering only levels with hematoma in our asymptomatic group 75% patients had a dural sac area at least of 75 mm2
(Q1, Table ).
Bolender et al. [15
] found central lumbar stenosis if the cross-sectional area of the dural sac was 100 mm2
or less, early stenosis or likely stenosis if the area was 100–130 mm2
. Normal canal dimensions were given with a mean of 180 ± 50mm2
Other authors also defined the size of spinal canal of more than 130 mm2
as normal, between 130 and 100 mm2
as borderline or early stenosis. Values below 100 mm2
are generally accepted as stenosis and values below 75 mm2
] or 70 mm2
] as absolute stenosis.
According to our results a dural sac area of 75 mm2 in the early postoperative period probably represents a threshold which could help to differentiate patients at risk for development of new symptoms from those with uneventful outcome in the early postoperative period.
In our study the median size of hematomas differed significantly between the two groups. The median of hematoma in the revision group was approximately twice as large as in the asymptomatic group.
In the asymptomatic group 25% of patients had a hematoma area greater than 216.5 mm2 and 75% of patients in the revision group had a hematoma area which was larger than 259.2 mm2. This means that in the investigated patients there could be a critical hematoma area between 216.5 and 259.2 mm2 which can lead to clinically significant dural sac compression.
Further, in our study there was evidence of a statistically significant difference in postoperative bony areas between the groups. The revision group had greater surgical bony decompression compared to the asymptomatic group. This probably indicates that more extensive bony resection may lead to more exposure of epidural veins and soft tissue which can provoke larger hematomas.
We conclude that MRI is useful for adequate evaluation of postoperative epidural hematoma and dural sac compression in early postoperative period after lumbar spinal decompression for degenerative stenosis. Early postoperative epidural hematoma was seen in 42.5% in patients without any symptoms. The size of hematoma correlates with the development of symptoms. The size of hematoma and the degree of dural sac compression were significantly larger in patients with symptoms. The median area of postoperative hematoma at the operated level was 176 mm2 in asymptomatic patients and 365 mm2 in symptomatic patients. The median cross-sectional area of the dural sac at the operated level was 128.5 and 0 mm2 in asymptomatic and symptomatic patients. Dural sac area of less than 75 mm2 in early postoperative MRI indicated clinical significance.