Demographics

After above mentioned selection from an original pool of 856 patients, there were a total of 104 patients analyzed. This included 70 patients with EL and 34 without EL. There were 43 men and 61 women in the study. A binomial test of these two groups resulted in a p < 0.05, confirming that the numbers of men and women in both groups were statistically equal. The average age of the patients was 61.8 ± 2.8 years. Those with EL had an average age of 61.7 and those without, 61.8.

Identification of EL

T1 weighted MR images were used for identification of EL as displayed in the sacral area on the sagittal image (Figure 1A). As shown in Figure 1B, T2 weighted images may not allow enough contrast between fluid and fat.Out of 70 EL patients, 46 were classified as mild. Figure 2A exemplifies mild EL: enhanced visibility of the L5 roots and mildly indented thecal sac. Sixteen patients were classified with moderate severity, as exemplified in Figure 2B: enhanced L5 roots and cross sectional area of epidural fat greater than that of the thecal sac. Figure 2C is an image at L5/S1 and is indicative of severe EL: the thecal sac is compressed to form a Y-shape. There were eight patients with the classification of severe EL. Figure 2D is an image taken at the L4 level and illustrates the case where although the thecal sac and epidural fat are equal, the neural elements are compressed significantly thus considered severe.A misdiagnosis that may occur is that arachnoiditis is confused with EL. The sagittal MR image may show severe EL and an irregularly compressed dural sac confusable with arachnoiditis (Figure 4). Using the T1 image, enhanced contrast will help the clinician to distinguish the two.

Figure 4 T1 sagittal image confusable with arachnoiditis. The arrow at S1 points to excessive anterior fat and a compressed thecal sac. Full size image

Linear image measurements of epidural and of subcutaneous fat

The anterior and posterior epidural, and posterior subcutaneous fat measurements were made in both EL and non-EL control groups using a Fuji PACS system in the sagittal (Figure 3A) and axial planes (Figure 3B). The measurements of fat varied slightly between the orientations. Measurements in the sagittal plane were more consistently replicable than in the axial plane. Sagittal epidural fat measurements were combined and reported as total epidural fat. The total epidural fat for patients with EL was significantly greater than the total for non-EL control group as determined by WMW test (7.2 ± 0.4 mm vs. 5.0 ± 0.4 mm, p < 0.01). Qualitative visual analysis of lipomatosis grades revealed a nearly linear correlation with total epidural fat measurements (Figure 5A). The comparison of total epidural fat of males and females showed no difference within their respective groups. However, females had significantly more posterior subcutaneous fat than males within their respective groups, p < 0.01. Posterior subcutaneous fat was also significantly greater in patients with EL than non-EL (37.5 ± 2.6 mm vs. 27.9 ± 2.0 mm, p < 0.01).

Figure 5 Grade of lipomatosis severity increases with epidural fat and BMI. (A) Holistic grading approach of EL scales with quantitative total epidural fat measurements. (B) BMI was statistically different between control and EL group, but not different between grades of EL. Full size image

Increased body mass index and triglycerides are associated with epidural lipomatosis (EL)

The average BMI for all patients in the study was 33.7 ± 0.7 (obese). The average BMI for men was 33.4 ± 1.1 and 33.6 ± 0.9 for women. Both T-test and WMW test had p-values of 0.8 and 0.15, respectively, suggesting no difference in BMI based on gender.The average BMI for patients with no EL was 29.2 ± 0.9 and those with EL was 36.0 ± 0.9. A weighted T-test and WMW test had p < 0.01, suggesting there is a difference in BMI between the two groups. An ANOVA with Tukey post-hoc test on BMI between different grades of lipomatosis revealed significant differences for all grades of EL severity when compared to the control group (p < 0.01), but no difference comparing the different grades of lipomatosis (Figure 5B). BMI was found to be statistically significant for the logistic regression model with a p < 0.01. The probability of developing EL was linear with increasing likelihood as BMI increases, until a BMI of 35 after which point it plateaus (Figure 6A).

Figure 6 Probabilities of developing EL computed by logistic regression. (A) Probability computed with BMI as the variable. (B) Probability computed with number of prior epidural steroid injections as the variable. Full size image

Those with EL had an average triglycerides value of 250 ± 30 and those without EL had an average of 186 ± 21 mg/dL. To account for the fact that triglycerides values were positively skewed in both groups, a WMW test was used. Triglycerides in the EL group were significantly elevated compared to the control group with p < 0.01. Triglycerides do not independently determine the probability of developing EL with a p > 0.01 in a logistic regression model.

Severity of lipomatosis on patient’s symptoms

The chief complaints, causes, and associated complaints were determined from history and physical examination as specified in Table 1. Twenty-eight (40%) EL patients complained of pain on walking relieved in a few minutes with sitting (neurogenic claudication). Forty-two (60%) complained of sciatica. EL was the primary cause of sciatica in 9 of 42 (21%) but may be contributory in other patients. The impact of EL on patient’s symptom impact was then graded as none, minimal, moderate and high. The Spearman chi-square p-value for EL severity vs. impact was <0.0001 showing that there is strong association between EL severity and symptomatic impact (Table 2). The early stages of the development of EL had low or no symptomatic impact.

Table 1 Categories of Chief Complaint Full size table

Table 2 Severity of EL and its relationship to gradation of impact Full size table

Epidural steroid injections, stress, & alcohol consumption

The number of ESI deliveries was significant in an independent logistic regression model with a p < 0.01. Absence of ESI deliveries or one ESI delivery did not increase the patient’s odds of developing EL. After two ESI the odds of developing EL was 66%. After three ESI, the odds were 98%. Four or more ESI increased the odds approaching 100%.

The average number of ESI delivered in the patients with no EL was 1.0 ± 0.0, and there were no patients who received more than one. The average number of ESI delivered was 1.8 ± 1.5 to the EL group. There were two patients in the EL group who did not receive any ESI. Their BMI were 40 and 42.

It was noted in the charts of ten patients in the EL group who self-reported stress and eight who self-reported alcohol consumption. No patients in the control group reported stress or alcohol consumption.