Incidence of asymptomatic early postoperative epidural hematoma in minimally invasive lumbar fusions: an exploratory prospective observational study.

BACKGROUND: The incidence of all postoperative spinal epidural hematomas (SEH) following open lumbar decompression and fusions reportedly ranges from 58% to 89%. However, the incidence of asymptomatic SEH and the radiographic parameters predicting symptomatic SEH have not been established. The objective was to determine the incidence of SEH and the radiologic parameters that predict SEH becoming symptomatic following minimally invasive transforaminal lumbar interbody fusions (MiTLIF). METHODS: Between January 2017 and January 2018, we prospectively studied a series of consecutive patients without new neurologic symptoms following MiTLIFs. The enrolled patients underwent gratis magnetic resonance imaging (MRI) of the lumbar spine. An additional consecutive series of patients who developed symptomatic SEH, confirmed on MRI, were retrospectively analyzed as a control. Using Merge, we measured the level of maximal compression (MC): dural sac-to-spinal canal diameter, length of MC, the volume of the thecal sac, and the ratio of preoperative/postoperative dural-sac cross-sectional area [critical ratio (CR)]. Univariate analyses compared patient hematoma risk factors between asymptomatic and symptomatic patients, radiographic parameters between asymptomatic and symptomatic compressions, and between symptomatic and asymptomatic hematomas. P<0.05 was considered significant. RESULTS: Twelve patients developed symptomatic SEH, while 4 demonstrated asymptomatic SEH, and 26 were without hematoma. The incidence of asymptomatic SEH rate was 13.3% [95% confidence interval (CI): 3.8-30.7%] for MiTLIFs. SEH cross-sectional area was significantly larger in symptomatic than asymptomatic SEH (1.88±0.70 vs. 1.04±0.19, P=0.002). Between the cohorts, post-operative thecal sac cross-sectional area was significantly larger in the no hematoma group than asymptomatic or symptomatic groups (1.22±0.6 vs. 0.80±0.22 vs. 0.62±0.28, P=0.008), and CR was greatest (1.24±0.55 vs. 0.70±0.13 vs. 0.57±0.20, P<0.001), respectively. CONCLUSIONS: The incidence of SEH in asymptomatic patients following MiTLIF was lower than previously described for open surgeries. The CR may aid in determining if compression is causing symptoms.