We welcome the reported description of indigo carmine in intra-detrusor botulinum toxin (BTX) injections  as a modification of the injection technique to be used for training or research purposes. While the dye helps to identify the sites where BTX has been applied and avoid a possible overlap that could affect outcome, recent ultrastructural and immunohistochemical studies raise the question of where the BTX is actually acting. Although the original hypothesis for its mechanism of action was of a paralysis of the detrusor, as shown by urodynamic studies , there appear to be no structural differences in the detrusor before and after BTX . By contrast, preliminary reports show that ‘intra-detrusally’ delivered BTX affects the bladder's afferent pathways, inducing significant and enduring decreases of the expression of sensory receptors in possibly intact suburothelial nerves . This raises the possibility of action at a suburothelial level through diffusion of the toxin solution from the injection sites. Further investigation is required to exclude an effect of BTX on the detrusor and confirm the presumed changes at an urothelial/suburothelial level, for which indigo carmine enhanced injections could be a valuable tool. If BTX acts at this level, further modification of the injection technique would include a direct delivery of the toxin solution to the suburothelium, and a first report has shown encouraging results .
The above-mentioned ultrastructural and immunohistochemical studies were conducted after injections in a half-full bladder [3,4]. A modification of the injection technique in a fully distended bladder, as suggested by Schulte-Baukloh and Knispel , would increase the risk of activating sensory receptors in the urothelium and suburothelial nerves. This in turn could induce detrusor contraction in an already overactive bladder and, also, increase bladder sensation and make the procedure difficult to tolerate, when being performed with local anaesthesia .
Another issue raised by Schulte-Baukloh and Knispel  is the backflow of toxin solution from injection sites in the bladder lumen when the procedure is used in a ‘half-full’ bladder, but not in a ‘full’ bladder. However, standardisation of a ‘full bladder’ volume might be difficult even with previous urodynamics, especially in patients whose underlying neurological pathologies are not stable, e.g., those with multiple sclerosis, as opposed to a ‘half-full’ bladder volume, which has been standardised to 100 mL . Furthermore, the diameter of the injection needle used to inject BTX via a rigid cystoscope has been reported to be 6 F , equivalent to 2 mm, whereas the needle used in the minimally invasive technique described by Harper et al.. was 27 G, equivalent to only 0.36 mm. Thus, the use of such a fine injection needle should minimize the risk of toxin backflow even in a ‘half-full’ bladder.