**5. Tissue plane dynamics and some misunderstandings**

In 2006 we published a paper in Anesthesiology which was provocatively entitled 'The Sheath of the Brachial Plexus: Fact or Fiction?' [7]. It may have been better entitled 'The Sheath of the Brachial Plexus: Actual Anatomy or Concept?', as negative reaction to the title may have distracted from what we believed to be the importance of the paper. This was a discussion about tissue planes and their significance to the practice of regional anaesthesia and to our knowledge the first time this had been directly addressed in the literature.

The notion of the brachial plexus 'sheath' has been attributed to various authors [124–126]. Two of these authors were in fact referencing the brachial fascia which is the deep investing fascia of the arm [125, 126], and one also referenced the intermuscular septum of the arm [126]. This is confusing as neither fascia is in intimate relationship with the neurovascular bundle.

Winnie [11] subsequently suggested that the 'sheath' was merely the final part of a tubular prolongation of the prevertebral fascia and promoted the concept of a continuous fascia-enclosed space extending from the cervical transverse processes to several centimetres into the arm. He likened brachial plexus anaesthesia to epidural anaesthesia where, once the space had been entered only a single injection was needed, an analogy intended to stop practitioners performing multiple injections which thereby increased the chances of neural injury. Various publications subsequently presented findings which significantly modified his concept [14, 16, 18, 20, 23, 28, 68].

The discussion section in our paper addressed tissue planes and their dynamics. The tough tissues of the prevertebral fascia give way rapidly to much thinner, softer and translucent connective tissues which encircle and entwine the brachial plexus and blood vessels [8]. As a collective, these thin and translucent connective tissues form the tissue planes of the brachial plexus. Indeed, in the current age of ultrasound-guidance the presence of these tissue planes is well accepted [9].

Within the tissue planes there may be minimal room for expansion at any one point and therefore flow has to occur along the tissue planes according to resistances encountered along the way [7]. The layers of connective tissue are not homogeneous, do not necessarily interconnect, and can hinder or prevent diffusion. Injection at one point does not guarantee spread elsewhere [8]. At the level of the trunks and divisions of the brachial plexus, the neural elements reorganise significantly and their associated tissue planes interconnect. This is readily observable during surgical dissection [8]. This arrangement allows for a more even spread of solution, a feature which has indeed been observed clinically [9, 127–130]. The belief that supraclavicular blocks are more effective because the neural elements are closer together [131] is better explained by the interconnection of tissue planes at this level. By contrast, at axillary level where the nerves do not interconnect and the tissue planes containing each nerve are largely separate [14, 16, 18], efficacy is lower unless each nerve was blocked separately. Side effect profiles can also be explained by spread of injectate via tissue planes, e.g., phrenic paresis with subomohyoid suprascapular nerve block [132].

The sheath concept also does not take into account the impact that surrounding rigid anatomy has on flow dynamics. Our work on the 'axillary tunnel' [133] calculated the dimensions of the tunnel and explained the significant impact of the varying dimensions. The injected dye did not spread as in a cylindrical tube but followed the contours of the rigid anatomy. The volume of the axillary tunnel at any one point was less than 10 ml, and so flow inevitably occurred along the tunnel. The tunnel had two constrictions and flow of injectate from the needle tip could be anterograde, bidirectional or retrograde depending on where the point of injection occurred in respect of these constrictions. The more lateral constriction was clearly the obstruction to flow at this lower level [133] rather than the head of the humerus as previously described [134]. Historic dye studies of the brachial plexus [11, 134] fitted with our predictions of flow patterns based on the contours of the rigid anatomy.

The CT dye studies from the axillary tunnel work also revealed the reason we were able to avoid the phrenic nerve during anaesthesia and analgesia for shoulder surgery [135]. We were manipulating the tissue planes of the brachial plexus by injecting into the tissue planes posterior to the artery, with retrograde spread restricted to these same tissue planes, well away from the anteriorly situated phrenic nerve. This phenomenon has subsequently been demonstrated by another group [136].

Given that brachial plexus regional anaesthesia had been based for many years on the concept of the sheath, it was unsurprising that a cadaveric study was subsequently published demonstrating apparent macroscopic evidence of the brachial plexus sheath [137]. We had significant issues with this evidence, including: 1. a significant connective tissue structure was demonstrated covering the emerging nerve roots in the root of the neck, but this was the prevertebral fascia. There were difficulties with calling this the brachial plexus sheath, partly because it already had an anatomic name. 2. the brachial plexus, as revealed prior to disappearing from view under the clavicle, was covered by a thin layer of opaque connective tissue. It was agreed by both sets of authors that this was what had been identified as the enveloping tubular structure called the sheath. This opaque connective tissue enveloping

#### *The Tissue Plane DOI: http://dx.doi.org/10.5772/intechopen.99533*

the plexus in the cadaver was remarkable for its difference to the equivalent tissue in a patient undergoing surgical dissection of the plexus [8], and we believe that this most likely reflected post-mortem changes in connective tissues [5, 6]. Interestingly, not all investigators using cadaveric specimens have encountered an opaque layer of connective tissue surrounding similar major nerves or plexuses. Indeed, they have echoed our words of 'thin, transparent and fragile' when describing such connective tissues [138].

We concluded that tissue planes, in conjunction with the influence of surrounding rigid anatomy, provided a better explanation for outcomes in brachial plexus regional anaesthesia than the concept of a sheath.
