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The Peripheral Nerves: Update on Ultrasound and Magnetic Resonance Imaging I

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The Peripheral Nerves: Update on Ultrasound and Magnetic Resonance Imaging I The peripheral nerves: update on ultrasound and magnetic resonance imaging I. Möller1, M. Miguel2, D.A. Bong1, F. Zaottini3, C. Martinoli4 1Instituto Poal de Reumatologia, ABSTRACT of the nerve along its trajectory along University of Barcelona, Spain, The motor and sensory branches of with immediate one-to-one compari- and EULAR Working Group Anatomy the somatic peripheral nervous system son with the contralateral structures (6, for the Image; (PNS) can be visualised by different im- 7). In addition, US-guidance has led to 2Department of Pathology and Experimental Therapeutics, Human aging systems. This article focuses on the development of a variety of inter- Anatomy and Embryology Unit, imaging of peripheral nerves by mag- ventional procedures. The use of US is University of Barcelona, Spain; netic resonance imaging (MRI) and becoming widespread in providing ac- 3Department of Health Sciences, high-resolution ultrasound (US). The curate and safe regional anesthesia as DISSAL, University of Genoa; anatomic basis of the peripheral nerve well as focal and regional pain manage- 4 Department of Health Science, image, common pathologies and clini- ment. It has also becoming an increas- University of Genoa, Ospedale cal value of US and MRI imaging of pe- ingly important component of muscu- Policlinico San Martino, Genoa, Italy. ripheral nerves are reviewed. loskeletal specialties such as physical Ingrid Möller, MD medicine and rehabilitation and sports Maribel Miguel, MD David A. Bong, MD Introduction medicine. Federico Zaottini, MD Nerve pathology may be a cause of Carlo Martinoli, MD chronic pain and disability. The initial Anatomical considerations Please address correspondence to: diagnostic evaluation of the periph- The PNS includes spinal nerves that Dr David A. Bong, eral nervous system (PNS) involves a originate from the combination of dor- Instituto Poal de Reumatologia, thorough patient history and physical sal and ventral roots of the spinal cord Calle Castanyer 15, examination. Neurophysiologic testing and the cranial nerves originating from 08022 Barcelona, Spain. is often required owing to the difficul- the forebrain and brain stem. The dorsal E-mail: [email protected] ties in precisely assessing the degree of roots of spinal nerves encompass sen- Received on September 11, 2018; nerve damage and to differentiate be- sory axons, whereas the ventral roots accepted on September 13, 2018. tween demyelination and axonal degen- contain motor axons. Soon after cross- Clin Exp Rheumatol 2018; 36 (Suppl. 114): eration. These tests measure electrical ing the intervertebral foramina, spinal S145-158. activity of nerves according to different nerves split into a delicate posterior © Copyright CLINICAL AND parameters, such as latency, amplitude (dorsal) ramus innervating muscles and EXPERIMENTAL RHEUMATOLOGY 2018. and conduction velocity (1). The accu- skin of the back and a large anterior Key words: peripheral nervous racy of nerve conduction studies may, (ventral) ramus innervating limb mus- system, anatomy, image, magnetic however, be influenced by a variety of cles and the skin of the anterior aspect resonance, high-resolution ultrasound factors, including age, gender, height of the body. Both rami are hybrids as and body mass index (2, 3). they contain sensory and motor fibres. The relatively recent application of im- Cranial nerves may be sensory or mo- aging techniques, such as ultrasound tor. Nerve fibres (axons) are surrounded (US) and magnetic resonance (MR) by a myelin sheath or non-myelinated, imaging, have allowed detailed visuali- such as in the case of the smaller sen- sation of the peripheral nervous system sory nerves. Groups of axons are bun- providing information concerning mor- dled together and embedded in a deli- phological alteration of the nerve and cate connective tissue, the endoneurium surrounding tissues (4, 5). Imaging also (Fig. 1). Each bundle of endonerium- may be used to assess the status of in- embedded axons is then surrounded by nervated musculature that aid in under- a distinct membrane, the perineurium, lying diagnosis, localisation of disease to form a fascicle. and recognition of the degree of nerve In larger nerves, the perineurium acts as damage. In addition, it can be very use- a “blood-nerve barrier” and is in con- ful in monitoring of response to therapy tinuity with the pia-arachnoid of the and prognosis for recovery. US has meninges being formed by layers of some advantages over MR, including flat cells in a matrix of collagen fibres its availability and that it permits higher that are arranged concentrically around Competing interests: none declared. resolution dynamic real-time imaging single nerve fascicles (8). The perineu- Clinical and Experimental Rheumatology 2018 S-145 Peripheral nerves: updare on US and MRI / I. Möller et al. the fascicles is referred to as the inter- fascicular epineurium. The amount of interfascicular epineurium is related directly to the number of fascicles and is more abundant in areas requiring greater mobility, such as where nerves cross joints. On the external side, the outer epineurium forms a continuum with surrounding loose areolar connec- tive tissue, the so-called mesoneurium. Nerves are vascularised throughout their entire trajectory and also contain a lymphatic network. The mesoneu- rium contains a complex system of perineural vessels, the so-called “vasa nervorum”, and guarantees some nerve mobility while protecting the microvas- cular architecture. Perineural vessels give off penetrating branches that enter the nerve bundle and run parallel to the fascicles, embedded in the interfascicu- Fig. 1. Microscopic section of peripheral nerve demonstrating the endoneurium (1), perineurium (2) lar epineurium. Along the perivascular and epineurium (3). plexuses, small nerve endings, the “ner- va-nervorum”, provide sympathetic in- nervation to the nerve bundles. Familiarity with nerve trajectory and anatomic passageways is critical for a successful evaluation with high-res- olution US and MR imaging. Nerve localisation, recognition of anatomi- cal variants and detailed knowledge of the areas in which entrapment is most likely to occur enhance the accuracy of imaging evaluation. Knowledge of spe- cific anatomic structures (“landmarks”) or reference points is essential in local- ising and assessing peripheral nerves: 1. nerves pass through narrow anatom- ic passageways in the limbs, the so called osteofibrous tunnels, that typ- ically are delimited by a bony floor and retinacular roof (e.g. the carpal tunnel, where the median nerve may be entrapped) (Fig. 2); 2. nerves often are accompanied by Fig. 2. Carpal tunnel. Axial section through the carpal tunnel at the wrist. Note the median nerve (MN) satellite vessels or guardian muscles in the tunnel which is formed by the overlying flexor retinaculum (FR) which connects the pisiform (P) to the scaphoid tubercle (ST) of the carpus. The flexor tendons (FT) of the thumb and fingers are for long segments of their course also in the tunnel. (e.g. the anterior interosseous nerve accompanied by the anterior inter- rium helps to regulate and, also, protect a plexus, as observed frequently in the osseous artery as it travels down the axons from tensile and compressive median nerve at the antecubital fossa through the forearm; the ulnar nerve stresses. (9). Nerve fascicles are then bound covered by the flexor carpi ulnaris Fascicles may travel directly and inde- together by a tough outer epineurium muscle) (Fig. 3); pendently within the nerve as individ- made up of loose connective tissue to 3. nerves may cross fascial sheets that ual “cords” or “cables” as the median form a typical peripheral nerve (10). overlie muscles or separate compart- nerve does in the distal arm or may in- The space between the outer epineu- ments at specific locations (e.g. the terconnect with other fascicles forming rium and the perinerium surrounding sensory branch of the radial nerve S-146 Clinical and Experimental Rheumatology 2018 Peripheral nerves: updare on US and MRI / I. Möller et al. Fig. 3. Anterior interosseous nerve. Deep topographical dissection of the anterior(volar) forearm showing the anterior interosseous nerve (yellow markers) traveling jointly with the anterior interosseous artery (red markers) on the surface of the interosseous membrane from the antecubital fossa (right) to the wrist (left). piercing the fascia of the distal fore- pear more hyperechoic relative to the arm after leaving the undersurface surrounding perineural fat. This feature of the brachioradialis; the superfi- is, however, inconstant and rarely rec- cial peroneal nerve as it penetrates ognised outside tunnels. Unlike abso- the lower leg fascia 10 cm above the lutely uniform and fibrillar structure of lateral malleolus) (Fig. 4). tendons, the fascicular echotexture of An in-depth knowledge of soft-tissue nerves does not display the property anatomy may help in nerve identifica- of anisotropy. The term “anisotropy” tion and often clarifies the aetiology essentially refers to a characteristic of and pathophysiology of some nerve a certain structure or substance being diseases. directionally dependent. Specifically in US, this represents a dramatic change Ultrasound and MR imaging in ecogenicity (from white “hyperecho- of peripheral nerves ic” to dark grey-black “hypoechoic-an- Ultrasound imaging echoic” when toggling the probe from Peripheral nerves and perineural struc- side-to-side or up-down thus changing tures can be examined using US and the
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