Ultrasound to guide nerve transfer healing and muscle re-innervation

INTRODUCTION

There are many ways to assess muscle re-innervation following brachial plexus nerve transfers, which include electromyography (EMG), nerve conduction studies (NCS), and clinical evaluation using the Medical Research Council (MRC) grading.^[1-5] With EMG, the re-innervating muscle exhibits motor unit potentials (MUPs) with longer duration and larger amplitude, often accompanied by increased polyphasia. The percentage of polyphasic MUPs serves as an indicator of the degree of ongoing spontaneous re-innervation. Over time, re-innervated MUPs gradually lose their polyphasic morphology while retaining their larger amplitude and, to a lesser extent, their longer duration. The chronic morphology of MUPs indicates successful muscle reinnervation.^[1,3,5] In the NCS, the re-innervating muscles have an increase in compound motor action potential. It is challenging to compare these findings with clinical recovery, and it depends on the technical expertise involved.

Clinical examination findings of re-innervation of the muscle will take several weeks to several months. The nerve regeneration occurs at 1 mm/day, and it takes a minimum of 3–6 months to appreciate elbow flexion.^[6] Re-innervation of the biceps becomes efficient and functional when a sufficient number of axons reach their target, along with appropriate motor afferent re-innervation, ensuring adequate muscle strength and elbow flexion. Furthermore, muscle tenderness is an indicator of muscle re-innervation, which can be appreciated by pressing the biceps muscle. This deep palpation tenderness is a good prognostic sign predicting expectant muscle contractions. It takes approximately 3 months to have biceps tenderness, 4 months to notice visible contraction, and 12 months to perceive functional movement.

Assessing nerve pathology can be effectively achieved using ultrasound. In a transverse section, a normal nerve displays small hypoechoic areas separated by hyperechoic septa, creating a “honeycomb-like” appearance.^[2] These hypoechoic areas represent nerve fascicles, while the echogenic septa correspond to the interfascicular perineurium. Longitudinal sections reveal the fascicular architecture, resembling a “bundle of straws.”^[2] Leveraging these characteristics, we used ultrasound to evaluate nerve healing at the distal nerve transfer sites and to observe the biceps muscle’s reaction to electrical stimulation of the donor nerve, thereby confirming muscle re-innervation.

This technical note aims to introduce ultrasound as a simple, non-invasive, and reliable tool for evaluating muscle re-innervation and nerve transfer site healing in distal nerve transfers for brachial plexus injuries. To illustrate its application, we present a case study involving an ulnar nerve fascicle transfer to the biceps muscle in a patient with a C5,6 brachial plexus injury.

TECHNIQUE

We utilize high-frequency linear-array probes (6–15 MHz) for ultrasound imaging, as they offer superior spatial resolution and optimal images with a standard gel interface. Occasionally, low-frequency probes (transducers) are employed for greater tissue penetration depth; however, they are less effective for visualizing superficial structures.^[7] The ulnar nerve is visualized and the ulnar fascicle transfer to the biceps branch of the musculocutaneous nerve is visualized. Dynamic imaging is performed during the elbow’s active and passive flexion and extension, primarily to evaluate the biceps muscle. In addition, color Doppler imaging is used to assess vascularity around the nerve healing site, providing valuable information on the status of nerve repair.

ILLUSTRATIVE CASE

A 25-year-old male sustained a C5, C6 brachial plexus injury and developed shoulder and elbow weakness. The surgical intervention included brachial plexus exploration in the neck, spinal accessory nerve transfer to the suprascapular nerve, ulnar nerve fascicle transfer to the biceps branch of the musculocutaneous nerve, and long head of triceps branch transfer to the axillary nerve. The sutures were removed at two weeks. Post-operative electrical stimulation was initiated at six weeks. In addition, passive therapy and shoulder and elbow rehabilitation were started. At three months, ultrasound imaging of the arm confirmed the well-coapted nerve transfer site and demonstrated biceps muscle contractions on cutaneous electrical stimulation of the ulnar nerve. This method could be considered in all nerve transfer surgeries to assess the healing and muscle re-innervation at an earlier rehabilitation stage.

Procedure and findings

At 3 months, an ultrasound of the arm can reveal the nerve transfer site and its healing status. It will show the normal fascicular architecture over the ulnar nerve and its donor nerve fascicle harvested in the proximal arm. By following the donor fascicle, the suture site and the recipient biceps branch of the musculocutaneous nerve can be identified [Figure 1]. Additionally, ultrasound demonstrated biceps muscle contractions in response to cutaneous electrical stimulation of the ulnar nerve [Figure 2] [Video 1]. This non-invasive technique effectively monitored the progress of nerve healing and muscle re-innervation, confirming successful nerve transfer and early muscle recovery.

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Figure 1:

The intraoperative picture shows the ulnar fascicle transfer to the biceps branch of the musculocutaneous nerve.

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Figure 2:

The ultrasound image of the same patient shows the normal fascicular pattern of the ulnar nerve, ulnar nerve fascicle (yellow), suture site (S), and the recipient nerve biceps branch of the musculocutaneous nerve.

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CONCLUSION

Ultrasound is a valuable tool for diagnosing and monitoring early re-innervation following distal nerve transfers in brachial plexus injuries. It provides a non-invasive, reliable, and patient-friendly method for assessing nerve transfer site healing and muscle contraction in response to donor nerve stimulation. By incorporating ultrasound into the postoperative management protocol, clinicians can enhance the monitoring and outcome of nerve transfer surgeries, ensuring better recovery and function for patients with brachial plexus injuries. We acknowledge that ultrasound is primarily a diagnostic modality. However, providing early evidence of re-innervation and confirming the success of the nerve transfer can guide rehabilitation efforts and potentially optimize functional recovery.

Further research is needed to validate the use of ultrasound in this context and to establish standardized protocols for its application using a prospective study comparing ultrasound with EMG, NCS, and clinical examination in a larger patient cohort.