Benefits include excellent resolution, dynamic field of view
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High-resolution ultrasound has emerged as a useful tool to guide the management of patients with peripheral nerve entrapments, tumors, trauma and other surgically amenable pathology. The procedure is a valid and reliable method of evaluating peripheral nerves,1 offering excellent resolution and a flexible, dynamic field of view.
Neuromuscular ultrasound offers particular utility when skillfully combined with clinical examination findings and electrodiagnostic techniques. When paired with electromyography (EMG), the traditional gold standard for evaluating peripheral nerve disease, ultrasound meaningfully impacts the clinical approach in up to 43 percent of cases, typically by identifying potentially surgically amenable intraneural and adjacent pathology as well as variant anatomy.2 In patients with suspected traumatic nerve lesions, ultrasound modifies the treatment plan in 58 percent of cases, primarily by providing early evidence of nerve discontinuity.3 Ultrasound also can identify symptomatic peripheral nerve lesions not apparent by EMG.
Ultrasound and MRI are both increasingly used for visualizing peripheral nerves. A recent study comparing the two modalities suggested that ultrasound is more sensitive than MRI, has equivalent specificity and is better at identifying multifocal lesions.4 For these reasons, ultrasound is typically the initial imaging modality for peripheral nerve assessment, except when nerves lie very deep within the body or beneath bone.
Neuromuscular ultrasound is used as part of the comprehensive evaluation offered by Cleveland Clinic’s Peripheral Nerve and Plexus Surgery Program, a specialized multidisciplinary clinic designed to diagnose and treat brachial and lumbosacral plexus disorders as well as focal neuropathies of the upper and lower extremities, including peripheral nerve tumors, trauma and entrapment.
This clinic is directed out of Cleveland Clinic’s Neuromuscular Center, which is actively involved in neuromuscular ultrasound guideline development through participation in the American Association of Neuromuscular and Electrodiagnostic Medicine (AANEM) Ultrasound Task Force.5
Neuromuscular Center staff have likewise trained numerous medical professionals in neuromuscular ultrasound technique through lectures and hands-on instruction at Cleveland Clinic’s main campus, at the Wake Forest Baptist Medical Center Program for Medical Ultrasound, and at annual meetings of the American Academy of Neurology, the American Society for Neuroimaging and the International Society of Peripheral Neurophysiological Imaging.
Here are some leading indications for which specialists in the Neuromuscular Center and elsewhere are applying neuromuscular ultrasound as its use continues to evolve.
Carpal tunnel syndrome (CTS) is the most common peripheral nerve entrapment. Increased cross-sectional area of the median nerve at the level of the pisiform bone (a marker of the proximal carpal tunnel) is considered the most reliable and clinically useful parameter, and it is accurate for the diagnosis of CTS.5
In addition to diagnosing CTS, ultrasound can identify structural causes of CTS and important anatomic variations that impact the surgical approach. Persistent median artery (PMA) within the carpal tunnel (estimated incidence of 10 to 26 percent) also can be demonstrated. When not identified preoperatively, PMA can complicate an endoscopic carpal tunnel release ‒ or an open release if a tourniquet is used. Ultrasound imaging may thus guide CTS surgical planning and improve patient outcomes.
There is increasing evidence that ultrasound can localize ulnar nerve entrapment at the elbow when EMG is equivocal and that it can identify relevant pathology and anatomic variants.6 At the same time, interest is growing in the use of ultrasound to guide surgical intervention for patients with ulnar nerve entrapment and to diagnose patients with deterioration after ulnar nerve transposition.7
Patients with foot drop may be diagnosed with a common peroneal neuropathy at the fibular head based on EMG findings. Although either long-standing compression or acute trauma affecting the nerve may be the cause, an intraneural ganglion cyst is identified in up to 18 percent of these patients (see Case Profile 1, below).
When identified in a timely manner, intraneural ganglion cysts are surgically amenable, with good postoperative outcomes. The presence of pain at the knee or neuropathic pain in the peroneal nerve distribution, a mass lesion, and fluctuating symptoms increase the pretest probability of finding an intraneural ganglion cyst, particularly in patients with no history of weight loss, immobility or leg crossing.8
Peripheral nerve tumors also are readily identified by ultrasound. Tumor types include lesions derived from adjacent non-neural sheath tissues, such as desmoid tumor and nodular fasciitis (see Case Profile 2, below), as well as benign peripheral nerve sheath tumors (e.g., schwannomas, neurofibromas, perineurioma and granular cell tumor) and malignant peripheral nerve sheath tumors.
The primary role of ultrasound in nerve tumor management is localization for biopsy/surgical planning. It also has been suggested that serial evaluation of asymptomatic lesions ‒ monitoring for change in size or morphology ‒ may prove useful in patients with known neurocutaneous disorders, such as neurofibromatosis.
Identification of complete nerve transection can guide the decision to pursue earlier surgical intervention in patients with nerve trauma. EMG cannot differentiate complete nerve transection until reinnervation begins beyond six weeks. Ultrasound is both sensitive and specific for early identification of transection, and is used in acute presurgical planning to localize the injury site and proximal/distal nerve stumps. For management of remote nerve trauma, ultrasound can identify stump neuromas and reveal excessive perineural scar tissue.
Patients who remain symptomatic after peripheral nerve exploration and surgical intervention present a special clinical challenge. Although delayed recovery is expected, early identification of graft discontinuity, nerve encasement by scar tissue or neuroma formation prompts surgical revision and potentially improves patient outcomes.
A 49-year-old man presented with a painful right foot drop. He had no history of weight loss, immobility or leg crossing. EMG showed severe axon loss affecting the tibialis anterior and peroneal longus muscles. Ultrasound revealed an oblong tender, hypoechoic, power-Doppler-negative lesion within the peroneal nerve with significant posterior acoustic enhancement (longitudinal and transverse views below), consistent with an intraneural ganglion cyst. Surgical decompression of the peroneal nerve with external and internal neurolysis and removal of the intraneural ganglion cyst were performed (intraoperative image below), with resolution of pain and improvement of ankle and toe dorsiflexion to near full power.
A 31-year-old woman presented with dorsal left hand and forearm numbness that progressed to wrist and finger drop over several months. Neurological exam revealed sensory loss in the distribution of the left radial nerve and weakness of left wrist/finger extension. EMG showed a left radial neuropathy. Ultrasound (longitudinal view above) revealed a hypoechoic, noncompressible, power-Doppler negative soft tissue mass (arrows) within the antecubital fossa completely surrounding and compressing the radial nerve. The epineurium was intact within the mass. The mass was resected surgically with decompression and internal/external neurolysis of the radial nerve (intraoperative image above). Pathology revealed nodular fasciitis, a benign mesenchymal tumor arising from fascia. The patient’s pain resolved, and weakness was significantly improved at her four-month follow-up appointment.
Dr. Shook is a staff neurologist in the Neuromuscular Center in Cleveland Clinic’s Neurological Institute.
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