Background: 
 ESI is a common practice for pain treatment since 1953. In 2014, the FDA issued a warning about ESI. Studies have focused on the effect of the particles size and their ability to generate harmful aggregates. Previous studies have established that sizes over 100 μm are potentially able to occlude blood vessels. 
-Objectives and methods. 
Studying the effect of the time elapsed between mixing the steroid preparation and injection on the number and size of aggregates with sizes above 100 μm. Steroids evaluated were those commonly used in Spain for ESI: betamethasone, triamcinolone, and dexamethasone. 
Samples were examined with a Leica TCS-SP2 microscope at the first, the fifth and the 30th minute. Aggregates observed were manually grouped in the following size range: 0-20, 20-50, 50-100, 100-300, 300-500 and >500 μm. 
Statistical analysis was carried doing a global comparison of the groups using the Kruskal-Wallis test and posthoc comparisons using the Wilcoxon test, adjusting p-values by the Holm method for multiple comparisons. 
-Results:
Aggregates present in triamcinolone and betamethasone samples were statistically larger than in dexamethasone samples. Triamcinolone suspensions produced significantly larger aggregates than betamethasone five minutes after mixing. Triamcinolone preparations produced greater particle aggregates (>500 μm), which were not present in dexamethasone and betamethasone preparations.
-Conclusions:
The size of the particles injected depends on the type of steroid and the time allowed between mixing and injecting.  Waiting time’s longer than 5 minutes between mixing and injecting can predispose the formation of potentially harmful aggregates in triamcinolone and betamethasone samples. The presence of greater particle aggregates (> 500 μm) may occlude some important vessels and arteries with serious adverse results. Vigorous shaking of the injectable could prevent such events.

MICROSCOPIC STUDY OF INJECTABLE STEROIDS USED IN EPIDURAL INJECTIONS: EFFECTS OF DIFFERENT DILUENTS USED IN STEROID´S AGGREGATION ABILITY