• Trevor William Watkins, Simon Dupre, and John Richard Coucher.
• Department of Medical Imaging, Princess Alexandra Hospital, Brisbane, Queensland, Australia.
• J Med Imaging Radiat Oncol. 2015 Oct 1; 59 (5): 571-7.

IntroductionTargeted spinal steroid injections are effective in reducing back pain in selected patient populations and carry a small risk of significant adverse neurological outcomes. Recent recommendations are for the use of non-particulate steroid agents for all spinal injections to reduce the risk of neurovascular embolic adverse events. Many injections have used a combination of local anaesthetic agent with the steroid. At our institutions, we have recently observed interactions between ropivacaine and dexamethasone combinations ascribed to the incompatibility of the former with alkaline solutions, resulting in rapid crystallisation. This study has further investigated the combinations of commonly used local anaesthetic and steroid combinations to determine if such precipitation effects are more widespread.MethodsThe commonly used local anaesthetics (lignocaine, bupivacaine, ropivacaine) and the non-particulate steroid dexamethasone sodium phosphate combinations were evaluated macroscopically, microscopically, and pH values measured. Where crystallisation was observed the rate of precipitation and crystal size was measured. Contamination of ropivacaine with sodium bicarbonate solution was also evaluated. Particulate size of the particulate steroid agent betamethasone acetate was evaluated as a comparison.ResultsAll mixtures of ropivacaine and the non-particulate dexamethasone sodium phosphate assessed demonstrated a pH-dependent crystallisation of the solution. No precipitation was demonstrated with the combinations of dexamethasone and lignocaine or bupivacaine. Contamination of ropivacaine with residual sodium bicarbonate in a drawing up needle following air clearing had a precipitation effect.ConclusionWe describe the effect of crystallisation with the combination of ropivacaine and the non-particulate steroid, dexamethasone sodium phosphate, a mixture that has been used in the literature for targeted pain injections. As this may be considered a non-particulate steroid/anaesthetic injectate, this would potentially carry increased risk if inadvertent intravascular injection occurred during a targeted spinal injection, as has been described with particulate steroid agents. This is due to the elevated pH of dexamethasone and the incompatibility of ropivacaine with alkaline solutions.© 2015 The Royal Australian and New Zealand College of Radiologists.

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