To the Editor:
We congratulate Ivanusic et al1 on their rigorous attempt to elucidate the mechanism of action of the erector spinae plane (ESP) block. There is no disputing the anatomical findings in this cadaveric study; however, they remain at odds with the clinical effects reported to date. In our original article,2 we too initially thought to attribute the analgesic effect in our sentinel patient to lateral spread and blockade of
lateral cutaneous branches. However, once the destructive rib lesions were discovered, it became obvious that this could not be the only explanation. We note that this disconnect between cadaveric investigation and clinical effect has been reported for other fascial plane blocks. Mayes et al3 failed to find consistent spread to intercostal nerves with injection performed superficial to the ribs and concluded that this could not provide effective rib fracture analgesia. Yet, there is good clinical evidence to the contrary.4,5 By the same token, the data of Ivanusic et al1 suggest ESP should not work for bony trauma, yet it does.6 We recently adopted a different investigational tack by performing magnetic resonance imaging to compare the ESP and retrolaminar block in cadavers.7 In this smaller study, we ob served radiological spread to the intercostal spaces, neuroforaminal areas, and epidural space with the ESP block. Nevertheless, we agree with Ivanusic et al1 that the physically detectable spread in cadaveric studies may underestimate the true extent of injectate spread. This is apparent even in live human subjects. Marhofer et al8 elegantly showed that sensory dermatomal block was much greater than visible magnetic resonance im aging spread following single-injection thoracic paravertebral blockade.

One theory that may go some way to ward reconciling these conflicting results involves the concept of differential blockade. In the ESP and other fascial plane blocks where local anesthetic is deposited distant from target nerves, it may be that the volume or mass of local anesthetic that reaches the nerves is sometimes so small as to escape detection by conventional means or to produce discernible sensory and motor block but is nevertheless sufficient to produce analgesia. Very low concentrations of local anesthetic applied to nerves will preferentially block unmyelinated Cfibers over the larger A-delta and A-gamma fibers.9 This was illustrated by the discrepancy between analgesia, the extent of cold sensation, and pinprick sensation (and large interindividual variation) reported by Kunigo et al10 in their recent study of serratus plane block. We and others have similarly observed clear evidence of analgesia without
clinically detectable sensory loss following ESP block. The data of Ivanusicetal do not necessarily contradict this theory; injectate did reach the ventral rami and the dorsal root
ganglia, just not to a physically detectable extent in the majority of their specimens.1 We may have reached our limits in attempting to understand the mysteries of the ESP and other fascial plane blocks with cadaveric studies. What is clearly needed now are high-quality controlled clinical trials to demonstrate their clinical efficacy, which is ultimately what is most important for patient care.

Ki Jinn Chin, FRCPC
Department of Anesthesia
University of Toronto
Toronto, Ontario, Canada

Sanjib Das Adhikary, MD
Department of Anesthesia
Penn State Hershey Medical Center
Hershey, PA

Mauricio Forero, MD
Department of Anesthesia
McMaster University
Hamilton, Ontario, Canada

This study was attributed to Toronto Western Hospital, University of Toronto, Toronto, Ontario, Canada
The authors declare no conflict of interest.

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