Ultrasound imaging, anatomy and histology of nerves and fasciae: They never walk alone

Abstract

Dear Editor, Ultrasound (US) imaging is an important diagnostic complement to clinical and electrophysiological assessments as regards peripheral nerve pathologies, eg compressive/traumatic injuries, polyneuropathies and tumors.1 In this sense, it has been established as the firstchoice method for imaging the peripheral nerves in the extremities.2 Technically, depending on the topographic anatomy, either curvilinear or linear transducers are used during scanning, and eventually the cross-sectional area of the nerve and/or the loss of fascicular echotexture are the (semi)quantified parameters in many of the studies.2 While the advantages US imaging over magnetic resonance imaging (MRI) in the assessment of peripheral nerve pathologies are its low-cost, better soft tissue resolution, ability to scan the entire nerve dynamically/comparatively, the main disadvantage is perhaps its operator dependency, requiring a long learning curve.3 Of note, an US examination should not be confined to only the pertinent nerve itself but should comprise evaluation of all the anatomical structures around the nerve as well. Bignotti et al2 demonstrated that in 22.4% of the patients with peripheral nerve disorders, US evaluation has shown extraneural findings. In addition to the well-known impact of muscles and vessels in various dynamic nerve entrapment syndromes, Stecco et al4 also highlighted the possible role of neighbouring fascial layers in the causation of the entrapments. Likewise, being aware of the fact that nerves and fasciae naturally run together, fascial planes have recently been targeted for nerve blocks in anesthesiology.5 Furthermore, the relationship between the cutaneous nerves and the subcutaneous layers have also been reported.6 To this end, in this study, the aim of the authors is to clarify the relationship among fasciae and nerves and the need why fasciae have to be included in the sonographic nerve imaging. First and foremost, the nerves are coated and immersed in fascial layers, ie they use the spaces between the two adjacent fascial layers to be aligned throughout the body. This way, the fasciae protect the nerve as well as (with the loose connective tissue in between) allow its gliding during movements. Second, each nerve is enveloped by connective tissue layers that are organised as a telescopic structure.7 Anatomically, axon groups constitute the nerve fascicles, ie the functional units of a peripheral nerve. Each individual axon is surrounded by the endoneurium, while the perineurium surrounds each group of axons forming a nerve fascicle. Nerve fascicles can be grouped together by connective and adipose tissues that contain blood vessels. Finally, the peripheral nerve is covered by a thin layer of connective tissue, the epineurium, which limits the spread of injected fluids but allows its physiological diffusion. The paraneurium (perineural sheath), which is composed of connective and adipose tissues, covers the epineurium and connects the nerve to the neighbouring structures (Figure 1)—also allowing enough gliding and preventing nerve impingement. Any modification in this