Does a diagnostic nerve block predict the outcome of botulinum toxin treatment? A narrative review

Botulinum toxin type A is a first line choice in the treatment of spastic muscle overactivity. However, targeting the muscles involved in the deformity with the appropriate dose as well as choosing the goal to achieve and predicting the expected results can be challenging. Diagnostic nerve block with anaesthetics rapidly and temporarily suppresses overactivity of the selected muscle allowing clinicians to identify the involved muscles and the potential improvement of botulinum toxin injections. This narrative review summarizes the predictive value of the diagnostic nerve block before botulinum toxin injections. In the case of a stiff knee gait, rectus femoris blockade seems to predict knee flexion and gait speed improvement, which is subsequently obtained after rectus femoris botulinum toxin injections, but underestimates improvements in balance. In the case of spastic equinovarus foot, tibial nerve block provides a greater reduction in spasticity. Diagnostic nerve block assessment prior to botulinum toxin type A injections leads to an increase in the number of injected muscles, in the dose per muscle and in the overall cumulative dose. Finally, diagnostic nerve block may help to increase the goal achievement rate. Further well conducted studies are necessary.