This anatomy study demonstrates that PICB injections produce consistent lateral spread along the corresponding intercostal neurovascular bundle and less predictable medial spread to the adjacent paravertebral/epidural spaces. Additionally, contiguous endothoracic fascia staining was found in the majority of the dissection. While proximal backtracking of local anesthetics to paravertebral space could not consistently provide explanation of the multi-segmental coverage of PICB; the consistent multi-level somatic block in the clinical findings suggests an additional route for PICB injectate distribution, possibly along the endothoracic fascial plane.
Truncal regional anesthesia techniques such as TPVB and the classic intercostal blocks have been utilized for anesthesia and/or analgesia for patients undergoing breast surgery2,4-5. Recent evidence also suggests that regional anesthesia techniques could potentially reduce the incidence of chronic postsurgical pain and even influence cancer recurrence1,13-14. However, TPVB is considered advanced regional anesthetic technique8 and technically challenging due to difficulties with needle visualization9 and identification of important collateral structures such as pleura, lung10. The classic intercostal nerve block is performed without imaging by landmark technique along the mid-axillary line and is considered an intermediate-difficulty technique8. Usually, it provides only single-dermatome analgesia per injection, therefore necessitating multiple injections to achieve analgesia for breast surgery.5 This can be time-consuming, as well as associated with more patient discomfort and even increased procedural risks.
The proximal portion of the ICS (between the tip of the transverse process medially and the costal angle laterally) contains the intercostal nerves and communicates with the paravertebral space medially. Paraskeuopoulos et al. have demonstrated that as little as 1ml methylene blue injected into the ICS 5 cm lateral to the spinous processes can spread to the paravertebral space11. Therefore, a larger volume PICB may result in spread into the paravertebral space and even the epidural space, providing multilevel analgesia with 1-2 level injections15 offering alternative to TPVB.
As the breast is mainly innervated by T2-T5 spinal nerves3 and the axilla (intercostobrachial nerve, T2) is a common site of persistent pain after axillary node dissection16; we utilize a combined 2nd/4th PICB technique for analgesia after breast surgery. Since pilot single-level cadaver injections demonstrated only 1-3 level spread per injection, the subsequent injections were performed with combined two-level injections, reflected in our current clinical practice. Hypothesizing that the ICSs are smaller cranially, we arbitrarily chose 15 and 25 ml for 2nd and 4th PICB, respectively. Real-time fluoroscopy demonstrated contrast consistently spreading beyond the ICS after the first 5 ml, concordant with the anatomy findings by Moorthy et al17 that intercostal injectate of 5 ml is confined to one ICS, whereas 10 ml spread outside the injected ICS via the potential space between the pleura and the internal intercostal muscle.
The PICBs produced consistent distribution within the injected intercostal space (100% at 2nd and 4th intercostal space) but demonstrated great variability in paravertebral spread (0-7 segments), similar to the variability of paravertebral spread in TPVB described in previous studies 18-19. In our results, the discrepancy between paravertebral spread by anatomy dissection (60% in T3 and 27% in T5) and area of hypoesthesia in clinical finding (100% in T3 and 92% in T5 dermatome) leaves many questions. First, the sensory block area in clinical practice on one hand, and the methylene blue and contrast media distribution in cadavers, on the other hand, may not be truly comparable due to different injectate viscosities and solubilities, different injection rates and pressures, and different tissue density in vivo and postmortem. Second, the ability to assess separately T3 or T5 dermatome sensation, especially when T2 and T4 dermatomes are anesthetized, may be difficult. Finally, while we originally hypothesized that the PICB causes multi-level analgesia through medial communication with the paravertebral space, it is plausible to consider additional non-paravertebral route(s) of distribution.
Our dissections revealed methylene blue spread inside the respective intercostal spaces and along the investing tissues around the injection sites in 80% of the specimens. The endothoracic fascia is interposed between the parietal pleura and the superior costotransverse ligament and extends laterally as an intervening fascia between pleura and internal intercostal membrane. The absence of dye on the visceral pleura and the underlying lung surface (Figure 4) suggests that the injectate spreads above the parietal pleura and the investing layer is the endothoracic fascia. Since the confirmatory sign of a successful ultrasound-guided PICB injection is the anterior displacement of the pleura, the injectate spreads most likely in the IIM-EFPP plane. Moorthy et al17 demonstrated that a 10 ml of intercostal injection can cause multilevel spread (average area of spread of 51.1+/19 cm2) through the potential space between the pleura and the internal intercostal muscle, which supports this hypothesis. The three dissections which revealed no endothoracic or adjacent ICS spread, but extensive paraspinal muscle staining might be explained with inadvertently shallow needle placement causing injectate spread into muscle instead of endothoracic fascia plane. Predictable 2nd and 4th intercostal distribution combined with paravertebral and endothoracic fascia plane spread may present a plausible complex model for reliable dermatomal coverage of PICB in the clinical finding.
Potential advantages of the PICB over TPVB (both with paramedian sagittal US scanning), include superior US-visualization due to shorter skin-to-target distance and more perpendicular US beam-to-pleura orientation, which allows for better pleura visualization. Additionally, the wider sonographic window between the ribs in PICB, than that between the transverse processes in TPVB, allows for the needle trajectory to be less steep and improved needle visualization (unpublished data). Furthermore, the predictably longer distance from the block needle to the spinal canal (lateral to the transverse process) may convey improved safety, especially in patients who are at increased risk of bleeding complications.
Our clinical findings suggest that high-volume two-level PICBs consistently produce sensory block in dermatomes relevant to adequate analgesia after breast surgery, and could logically decrease pain and opioid consumption after mastectomy and lumpectomy. However, our study was not designed and powered to examine differences in pain scores and only demonstrated a trend towards lower pain scores in the PICB group. As the shortcomings of our clinical study stem from its retrospective design with no anesthetic/analgesic standardization, well-controlled prospective trials are needed to further evaluate the analgesic, anesthetic and recovery profiles of PICB.