Ultrasound assessment of selected peripheral nerve pathologies. Part III: Injuries and postoperative evaluation

Abstract

The previous articles of the series devoted to ultrasound diagnostics of peripheral nerves concerned the most common nerve pathologies, i.e. entrapment neuropathies. The aim of the last part of the series is to present ultrasound possibilities in the postoperative control of the peripheral nerves as well as in the diagnostics of the second most common neuropathies of peripheral nerves, i.e. posttraumatic lesions. Early diagnostics of posttraumatic changes is of fundamental importance for the course of treatment and its long-term effects. It aids surgeons in making treatment decisions (whether surgical or conservative). When surgical treatment is necessary, the surgeon, based on US findings, is able to plan a given type of operative method. In certain cases, may even abandon the corrective or reconstructive surgery of the nerve trunk (when there are extensive defects of the nerve trunks) and instead, proceed with muscle transfers. Medical literature proposes a range of divisions of the kinds of peripheral nerve injuries depending on, among others, the mechanism or degree of damage. However, the most important issue in the surgeon-diagnostician communication is a detailed description of stumps of the nerve trunks, their distance and location. In the postoperative period, ultrasound is used for monitoring the operative or conservative treatment effects including the determination of the causes of a persistent or recurrent neuropathy. It facilitates decision-making concerning a repeated surgical procedure or assuming a wait-and-see attitude. It is a difficult task for a diagnostician and it requires experience, close cooperation with a clinician and knowledge concerning surgical techniques. Apart from a static assessment, a dynamic assessment of possible adhesions constitutes a crucial element of postoperative examination. This feature distinguishes ultrasound scanning from other methods used in the diagnostics of peripheral neuropathies.

Introduction

Until recently, the diagnostics of peripheral nerve pathologies was based on a clinical examination, neurophysiological examinations (electromyography, EMG) as well as on magnetic resonance imaging (MRI). At present, however, more and more clinicians refer their patients to ultrasound examination (US) for the purposes of both diagnostics and control of the surgical or conservative treatment effects. US is also becoming a common instrument for anesthesiologists and is used for monitoring local anesthesia. Entrapment neuropathies, which were discussed in parts II and III of the series, constitute the most common indication for peripheral nerve US. However, ultrasound examination is also used in the diagnostics of injuries and postoperative complications.

Nerve injuries

The diagnosis of nerve injury, especially at an early posttraumatic stage, is of fundamental importance. A surgeon decides about the treatment which may be either conservative or surgical. He or she expects that a diagnostician will not only determine the extent of damage (complete or partial), but also, in the case of complete damage, assess the condition of the nerve stumps and their reciprocal distance.

US examination is of particular significance in the case of extensive injuries of the extremities (after traffic accidents or accidents involving agricultural machinery) with resulting massive damage of soft tissues often including its defects (fig. 1 A) that complicate the clinical examination. Fractures resulting from such injuries are usually treated by metal implant fixation (fig. 1 B) which renders further EMG or MRI examinations impossible. Therefore in such cases, ultrasound becomes a method of choice both in initial assessment and in monitoring the course of treatment.

Fig. 1.

A. Massive tissue damage after a traffic accident. B. Radial nerve trunk (arrows) in a close proximity to the bone plate (a thread for a screw is visible)

There are several classifications of injuries, but the division made by Seddon in 1943⁽¹⁾ and by Sunderland in 1951⁽²⁾, are the most universal.

Seddon classification distinguishes three degrees of nerve damage:

1. Neuropraxia – consists in the interruption of nervous impulse conduction without the loss of axonal continuity (fig. 2). It constitutes the most benign form of nerve injury and apparently results from biochemical changes at the level of the synapses. Nevertheless, it is also reported that the cause may be the damage of the myelin sheath⁽³⁾. The process of regeneration lasts several weeks.

2. Axonotmesis – consists in the injury of the axon and the myelin sheath with the preservation of the continuity of the endoneurium, perineurium and epineurium. Histologically, retrograde changes of Wallerian degeneration occur with possible restoration which proceeds spontaneously and slowly, at the pace of 1 mm per day. After 2–3 weeks, fibrillation changes are observed on EMG in the muscles supported by the damaged nerves.

3. Neurotmesis – constitutes the most serious nerve damage consisting in a total severance of the axons and their myelin sheaths as well as of the endoneurium and perineurium. The epineurium, however, may maintain its continuity, which, intraoperatively, may mimic the preserved continuity of the entire nerve trunk. Only upon surgical inspection, can the submyelinic damage be confirmed.

Fig. 2.

Radial nerve neurapraxia (arrows) in a motorcyclist resulting from the compression of the nerve at the level of the spiral groove by a protective shoulder pad: section area of the compressed nerve – 38 mm², and of the contralateral, unaffected one – 18 mm²; K.R. – humerus

In Sunderland classification, five degrees of nerve injury are distinguished. When compared with Seddon classification, the first degree corresponds to neurotmesis and the second to axonotmesis. The third degree is a disruption of the axons and endoneurium with the preservation of the perineurium and epineurium. In the fourth degree, only the epineurium is preserved and in the fifth one, the entire nerve trunk is severed with two stumps visible in a certain distance from each other⁽³⁾.

In neurotmesis, surgical reconstruction of the nerve continuity is necessary. Preferably, this should be done in the first week following the injury. A precise diagnosis, which is possible by means of ultrasound, is of vital importance since EMG examination in such an early stage is not reliable.

When taking into account the mechanisms of injuries, three types of nerve injuries may be distinguished:

1. stretching;

2. crushing/bruising [in Polish literature called choking (Polish: zdławienie)];

3. severance.

Injuries in the tension mechanism may accompany joint dislocations (e.g. in the dislocation of the knee joint the fibular nerve may become damaged)^{(4, 5)}. In this mechanism, the brachial plexus may undergo damage during a motorcycle accident (fig. 3).

Fig. 3.

Brachial plexus at the level of the scalene muscles – visible difference in thickness between normal and affected sides (asterisks – trunks of the plexus)

The ultrasound presentation of this type of injury encompasses a wide range of morphological changes. During the examination, one should assess the continuity of the epineurium, the continuity and homogeneity of the bundle structure of the nerve and its vascularization. Edema at the preserved epineurium presents itself as a spindle-like thickening of the trunk outlines. The measurement of the sectional area at the apex of this thickening is the most reliable and repeatable parameter used in control examinations. The assessment of the inner structure of the nerve should not pose any problems when high-frequency transducers are applied. In a well-produced image, one can even determine the number of damaged fascicles. At the level of damage, a hematoma is visible in various phases of its development depending on the time which lapsed since the injury.

In the case of partial damage, “a neuroma-in-continuity” may arise in the regeneration process. Such a neuroma is a lesion of a spindle-like shape which models a part of intact fascicles (fig. 4 A, B). It is necessary to assess its length and diameter. When the continuity of the nerve is disrupted (fig. 5 A, B), it is essential to measure both the distance between the stumps and the distance between the intact, “healthy” nervous tissue. In this mechanism, the lesions involve a long fragment of the nerve trunk with the disruption of the continuity of particular fascicles or fibers at many levels (fig. 6 A–C).

Fig. 4.

A. Intraoperative image of “a neuroma-in-continuity” of the median nerve at the level of the forearm lifted by a surgical instrument. B. Ultrasound presentation of “a neuroma-in-continuity” (asterisk) on the superficial branch of the radial nerve (arrows) after compressive injury

Fig. 5.

A. Complete severance of the digital nerve; nerve stumps are lifted on a surgical instrument. B. Ultrasound image with visible stumps of the digital nerve (asterisks); nerve trunk (arrows)

Fig. 6.

Patient after multiple organ injury manifesting symptoms of radial nerve injury. US scan shows two-level damage – injury of the radial nerve trunk at the level of the humeral shaft and damage of the branch of the posterior interosseous nerve (PIN) at the level of the supinator muscle of the forearm (S). A. Longitudinal section of the PIN (asterisks) at the level of the supinator muscle – clear thickening of the PIN is visible. B. Transverse section at the level of the humeral shaft – edema and elimination of the radial nerve structure (arrows). K.R. – humerus. C. Typical presentation of a falling hand

US findings enable to plan the method of surgical treatment (suturing the ends of the nerves or, if the resection of a long fragment of the damaged nerve is necessary, using a graft or neurotube; in extreme cases, nerve transfer is needed). During a preoperative examination, one may simultaneously asses the nerve which may be used for a potential transplant and check it for possible asymptomatic pathologies such as benign proliferative lesions (fig. 7) or anatomical variants. As a result, a prepared surgeon will be able to perform the procedure faster and reduce unnecessary surgical cuts. Nerves which are usually used for transplants are the following three sensory nerves: sural, medial antebrachial cutaneous and medial brachial cutaneous⁽⁶⁾.

Fig. 7.

Asymptomatic neuroma of the sural nerve (asterisk) detected incidentally, on a longitudinal section. M.B. – gastrocnemius muscle; M.P. – soleus muscle

The injuries resulting from crushing/compression usually concern the nerves located directly on the bone⁽⁷⁾. The image of the damaged nerve may vary depending on the force and repeatability of the injury. Slight, but repeatable injuries lead to a spindle-shaped, reactive thickening of the nerve trunk, a thickening/fibrosis of the epineurium and hyperemia of the perineurium. This may be observed for instance in cyclists who develop neuropathic changes of the superficial branch of the ulnar nerve at the level of the hook of the hamate bone (as a result of microinjuries against the handlebar). A similar mechanism of the injury concerns the deep fibular nerve at the level of the tarsus and is found in soccer players⁽⁸⁾. A single high-energy injury rarely leads to a disruption of the nerve continuity. On US examination, focal of complete blurring of the bundle structure as well as nerve edema and hyperemia are observed. Sometimes, the epineurium itself is also thickened (swollen).

Nerve damage may be partial or complete. In the first days following the injury, especially following a non-transmural injury, the accurate assessment may be prevented by edema and the presence of isoechoic hematoma. In such a situation the examination should be repeated after several days so as to establish an accurate diagnosis not later than within the first week following the injury. This will increase the chance to restore proper functions of the nerve by means of an early surgical intervention⁽³⁾.

In the case of a partial injury to the nerve, one may observe a fragmentary, non-transmural disruption of the epineurium and a part of nervous fascicles (fig. 8 A, B). In an early posttraumatic phase, edema, developing hematoma and nerve hyperemia are expected to appear. In time, a regenerate will be formed of nervous and connective tissues. When the outcome of the healing process of a partial injury is unfavorable, “a neuroma-in-continuity” may appear.

Fig. 8.

A. Intraoperative image of the median nerve at the level of the distal epiphysis of the forearm. B. US image of the injury to the ulnar nerve at the level of the forearm; neuromas on the stumps (asterisks) and scar tissue which joins them (arrow)

In the case of a complete (transmural) injury of the nerve trunk, the ends of the stumps may adhere to each other and thus, mimic a preserved continuity. It is indicated then, to apply pressure with the transducer which will cause their separation and induce a paradoxical reciprocal movement. In time, at the ends of the stumps, neuromas may form. It may happen that the stumps and their neuromas become pulled into the scars of the adjacent muscle or fascia structures (fig. 9). In such a situation, the role of US examination is to precisely determine their location, which will considerably facilitate planning surgical cuts. One may mark the location of the stumps on the patient's skin directly before his or her consultation with the surgeon or perform photographic documentation.

Fig. 9.

US image of the ulnar nerve (asterisks) moved beyond the groove to the apex of the medial epicondyle; protruding joint recess filled with synovial fluid (triangles). After the fracture of the distal epiphysis of the humerus with the dislocation of bone fragments

When discussing the injuries of peripheral nerves, particular attention needs to be drawn to the radial nerve. The most common causes of its pathologies are humeral bone fractures and the consequences of their management. The nerve may undergo damage at the moment of bone breaking when it becomes surrounded by fragments of the fractured bone. Neurapraxia may appear during a surgical procedure when the nerve is retracted by the surgeon assistant by means of, so called, marking bands. A long procedure or too strong fraction may lead to ischemia of the nerve. Similar risk is posed by a prolonged pressure applied by the pressure cuff during surgical procedures or a specific position of the arm during sleep (“Saturday night palsy”). Moreover, when the fracture is fixed by means of a bone plate, a nerve may become entrapped between the plate and bone or between the screw and plate. Another hazardous phase of the healing process is the bony buildup at the site of the fracture and the deformation of the bone with the appearance of a bone collar, known as callus. Sometimes, it pulls the radial nerve in and surrounds it with bony build-up^{(3, 6)}. In the case of a nonunion of the fractured bone, a hypertrophic pseudarthrosis with fibrous tissue cuffs may appear (fig. 10 A, B).

Fig. 10.

Longitudinal (A) and transverse (B) sections of the radial nerve trunk (little arrows) adjacent to the pseudarthrosis of the humeral shaft (large arrow)

Postoperative nerve evaluation

Ultrasound evaluation of nerves after surgical procedures requires extensive experience. The image of the nerve is frequently considerably changed which results from the healing physiology of soft tissues with scar formation. Prior to the examination, a diagnostician should have access to a complete documentation concerning the conducted surgery.

After surgery in entrapment syndromes, it is essential to assess the continuity of the epineurium, the continuity the bundle structure as well as the radical character of the procedure. One should pay attention to possible fibrous bands remaining after the procedure, which may compress the nerve e.g. the transverse carpal ligament that is not completely cut (fig. 11 A, B). Hyperemia and edema of the nerve may persist for many weeks. In the early postoperative period, all fluid collections in direct surrounding of the nerve and inflammatory granulation will indicate a pathology.

Fig. 11.

A. Intraoperative image of the fibers of the transverse carpal ligament left after a procedure, which constitute a cause of persistent nerve compression. B. Postoperative ultrasound image in a patient with persistent symptoms of carpal tunnel syndrome: the fibers of the transverse carpal ligament left inside after a surgery (asterisks), the median nerve (arrows), persisting “notch” sign (thick arrow)

In the case of surgery of entrapment syndromes, which were not treated for many years, the return of correct bundle echostructure of the nerve is not always observed despite clinical improvement (fig. 12).

Fig. 12.

Persistent degenerative changes in the median nerve (arrow) despite the transverse carpal ligament reconstruction. Patient with long-lasting symptoms of the median nerve compression neuropathy. After the procedure, night pains stopped, but hypoesthesia of the pulps of fingers I–III persisted

If the ends of the nerve were sutured, one-level scar is visible. When, however, a graft or neurotube were used, scars are present at two levels. A slight spindle-like thickening of the nerve trunk outlines at the site of the sutures is a correct postoperative presentation. When high-frequency transducers are used, slight, hyperechoic points are clearly visible at the level of the junction. They correspond to microsurgical sutures. A neuroma causes considerable widening of the nerve outlines, which may be irregular, and results in disturbances of its internal echostructure. Not always does it involve the entire section of the nerve; sometimes it affects only single fascicles. On Doppler examination, it shows enhanced vascularization.

A permanent and crucial element of postoperative assessment is a dynamic examination during which a slide of the nerve in relation to adjacent tissues is observed in a longitudinal section. So called “tightening” of the tissues or, in extreme cases, “wrinkling” of the nerve attest to the presence of adhesions⁽⁵⁾ (fig. 13 A–C).

Fig. 13.

Median nerve adhesions (between arrows) with adjacent tissues cause the “wrinkling” effect in the dynamic examination; extension of the wrist (A) and hand flexion (B)

In a postoperative assessment of the ulnar nerve, one should remember about the technique of nerve transposition to the anterior surface of the condyle of the humerus. During US examination, the nerve will be clearly visible on the border of the subcutaneous tissue and fascia on the anterior outline of the epicondyle. Particular attention should be paid to the potential bending site of the proximal fragment of the transposed nerve at the level of the arcuate ligament in relation to the fragment located between the heads of the flexor carpi ulnaris muscle. Considerable degree of such a bending may constitute a cause for the appearance of a secondary neuropathic syndrome⁽³⁾ (fig. 14).

Fig. 14.

After the transposition of the ulnar nerve with the symptoms of secondary neuropathy. Bending (arrow) of the nerve (asterisks) at the site where it enters between the heads of the flexor carpi ulnaris muscle

Conclusion

US scan, together with neurophysiological examinations and MRI, belongs to additional examinations performed in the diagnostics of peripheral nerve pathologies. When high quality apparatus and high-frequency transducers are used, ultrasound allows for an assessment of the entire nerve trunk (as opposed to MRI, where only a selected fragment may be evaluated). Additionally, during US examination, one may correlate the US image with the most painful site. Furthermore, dynamic examination and comparison with the contralateral site may be conducted. When assessing nerves, one should take into account not only the direct symptoms, but also indirect (imaging of the muscles innervated by the assessed nerves), quantitative and qualitative ones. Being familiar with the aforementioned assessment criteria together with the knowledge of anatomy and surgical techniques enable an experienced diagnostician to differentiate between the reasons of neuropathies as well as to precisely determine their location and extent. This, in turn, allows for planning the treatment and monitoring its course.

Conflict of interest

Authors do not report any financial or personal links with other persons or organizations, which might affect negatively the content of this publication and/or claim authorship rights to this publication.