Evaluation of the ulnar nerve with shearwave elastography: a potential sonographic method for the diagnosis of ulnar neuropathy

Evaluation of the ulnar nerve with shearwave elastography: a potential sonographic method for the diagnosis of ulnar neuropathy

REVIEWED BY Craig Winnett | ASA SIG: Musculoskeletal

REFERENCE | Authors: Kim S & Lee GY

WHY THE STUDY WAS PERFORMED

Cubital tunnel syndrome causes neuropathy of the ulnar nerve at the elbow. It is the 2nd most common neuropathy of the arm after carpal tunnel syndrome. Cubital tunnel syndrome occurs at the cubital tunnel of the medial elbow, where compression and/or irritation of the ulnar nerve results. Symptoms of cubital tunnel syndrome include focal pain over the ulnar nerve at the elbow, and numbness and tingling in the 4th and 5th fingers, especially when the elbow is bent. Clinical diagnosis can be aided with nerve conduction and electromyography studies. However, these studies can be painful, cause injury to the nerve and can elicit false negative (normal) results in the early stages of symptoms.

Due to its high spatial resolution of nerves, dynamic ultrasound imaging can assess the appearance of the ulnar nerve and assess for signs of irritation or compression of the nerve. Ultrasound is also useful to assess for structural causes of compression and/or irritation to the ulnar nerve at the cubital tunnel, including bony protrusions, accessory muscles and subluxation of the ulnar nerve. A pathological ulnar nerve may demonstrate the following changes seen on ultrasound. These include focal hypoechoic change, thickening of the nerve proximal to the site of compression, alteration of the normal fascicle pattern, perineural oedema and increased intraneural vascularity on colour Doppler. Cross-sectional area (CSA) measurement of the nerve on ultrasound has been shown in previous studies to be useful with a cut-off normal value of 9 mm2. An increase in the area size above 10 mm2 has been linked with positive ulnar neuropathy (with a sensitivity of approximately 60%).

Shear-wave elastography(SWE)is a new developing technique and can be used to quantitatively measure the stiffness of soft tissue, including muscles, tendons and masses. The use of SWE to assess the stiffness of nerves is a new area of research. Recent research into the applications of SWE has shown a potential to help with the diagnosis of neuropathies. It has been hypothesised that a swollen neuropathic nerve would have a stiffer, higher SWE measurement compared with a normal nerve. This study aimed to investigate whether SWE could identify increased stiffness of affected ulnar nerves at the cubital tunnel. This study also investigated CSA measurements of the ulnar nerve in relation to the ulnar nerve with neuropathy compared to unaffected ulnar nerves.

HOW THE STUDY WAS PERFORMED

The study reviewed 57 patients who had current elbow pain symptoms. The patients underwent an ultrasound scan with B-mode and SWE to examine the ulnar nerves. The study was performed by 2 radiologists using a Samsung RS85 ultrasound machine and a 2-9MHZ linear transducer. The radiologist did not know the patient’s clinical history or the results of the electrodiagnostic studies. The CSA of the ulnar nerves was measured proximal to the cubital tunnel, at the cubital tunnel and distal to the cubital tunnel. The cut-off for normal ulnar nerve CSA of 9 mm2 was used at all points. Ulnar nerves were considered swollen when their CSA was measured above this. SWE was measured using a 5 mm2 area box over the nerve at the same sites where the CSA was measured. Ten of the patients had known ulnar neuropathy confirmed with electromyography. The remaining 47 patients had either symptoms of medial or lateral epicondylitis.

WHAT THE STUDY FOUND

Patients with ulnar neuropathy were shown to have significant swelling of the nerve as indicated by an increased cross-sectional area (CSA) of 13 mm2 at the cubital tunnel, compared to 8.7 mm2 for the remaining patients. All patients had a normal CSA measurement of the ulnar nerve (less than 9 mm2) at the proximal cubital tunnel and distal cubital tunnel sites. This study showed that the patients with a pathological ulnar nerve had a normal ulnar nerve CSA measurement proximal and distal to the cubital tunnel.

This study went on to demonstrate that patients with ulnar neuropathy had significantly higher measured SWE values at the site of inflammation/irritation of the ulnar nerve at the cubital tunnel. The ulnar neuropathy group had an SWE measurement of 66 kPa, compared to 21–33 kPa for the two other patient cohorts. The study also demonstrated that the ulnar nerve SWE measurements proximal to the cubital tunnel and distal to the cubital tunnel were deemed normal for both the ulnar neuropathy patients and the remaining patients.

SWE seems to be a new, reliable, and simple quantitative diagnostic technique to aid in the precise diagnosis of ulnar neuropathy at the cubital tunnel.

RELEVANCE TO CLINICAL PRACTICE

Clinically, CSA measurements of an ulnar nerve at the cubital tunnel are technically non-reliable and subjective due to the tricky anatomy to get a true trans-cross-section of the ulnar nerve consistently without over-measuring the nerve in an oblique view. SWE presents as a possibly more reliable adjunct to the diagnosis of ulnar neuropathy in conjunction with B-mode, colour Doppler and dynamic ultrasound assessment.

SWE is showing promising results to be a reliable and simple quantitative diagnostic technique to aid in the diagnosis of neuropathies including the ulnar nerve at the cubital tunnel. A limitation of this study was the small cohort of patients for the data collection. Further study is also needed to investigate if stiffness SWE values do or don’t increase in early neuropathic and chronic neuropathic states of the ulnar nerve.

While more research is needed in this field, including establishing normal and abnormal ranges for SWE nerve measurements, the early studies are suggesting that SWE can add quantitative data to aid confidence and increase the sensitivity of ultrasound in the diagnosis of neuropathies, cubital tunnel syndrome, and potentially other sites of nerve entrapment.