Ultrasound Scanning Protocol in the Assessment of Ulnar Neuropathy at the Elbow

Summary

This is a standardized protocol for evaluating the ulnar nerve at the elbow using ultrasound.

Abstract

Ulnar neuropathy at the elbow is commonly encountered in clinical practice and is the second most common entrapment neuropathy. Left untreated, ulnar neuropathy at the elbow can result in significant disability due to loss of dexterity and grip strength secondary to the weakness of intrinsic hand muscles. Precisely localizing a lesion in ulnar neuropathy can be challenging with electrodiagnostic testing alone. Ultrasound is a relatively quick and useful adjunctive diagnostic modality in overcoming this limitation, as an increase in the cross-sectional area (CSA) of the nerve is a common and validated finding in ulnar neuropathies at the elbow. Sonographic assessment of the nerve's echotexture and vascularity can provide additional diagnostic clues. Ultrasound also offers the unique benefit of detecting ulnar nerve subluxation or dislocation out of the retroepicondylar groove during dynamic assessment, although the clinical significance of this is controversial. Finally, ultrasound can also identify structural abnormalities leading to nerve compressions, such as the presence of bony abnormalities, scar tissue, and space-occupying lesions. These findings may influence management strategies and surgical planning. This protocol aims to illustrate the technique of static and dynamic sonographic imaging of the ulnar nerve around the elbow as a complement to electrodiagnostic testing in the assessment of ulnar neuropathy at the elbow.

Introduction

Ulnar neuropathy at the elbow (UNE) is the second most common entrapment neuropathy¹. Electrodiagnostic testing is an important diagnostic modality but has low sensitivity and specificity in diagnosing UNE in the setting of mild pathology and pure axonal injury². There are several different sites of entrapment that may occur at or near the elbow. The most common locations of entrapment around the elbow are at the retroepicondylar groove and under the humeroulnar aponeurotic arcade (in the true cubital tunnel). However, more proximal locations, such as the Arcade of Struthers and medial intermuscular septum, are also possible³. In the last several years, ultrasound has emerged as a useful tool in identifying the location of ulnar nerve lesions in the setting of abnormal but non-localizing electrodiagnostic findings⁴. In fact, an expert consensus published in Clinical Neurophysiology in 2021 recommended using both ultrasound and electrodiagnostics in evaluating UNE⁵. This protocol is therefore intended to be used as an adjunctive diagnostic modality to electrodiagnostic studies rather than a replacement.

We find it easiest to perform ultrasound and electrodiagnostics together because the results of both modalities provide more information than either modality alone. Furthermore, ultrasound can be performed in a matter of minutes by a proficient sonographer. It is therefore feasible for patients to complete both types of testing in the same encounter. Given this, electrodiagnosticians may find the most benefit in this technique. However, it may also be useful in other settings, such as outpatient sports medicine clinics, during the comprehensive musculoskeletal assessment of elbow complaints. This protocol outlines the steps of the sonographic evaluation of the ulnar nerve with the patient positioned at 90°of elbow flexion. Various techniques are described in the literature about scanning and patient position. Some sonographers choose to examine the patient with the elbow in full extension¹. The advantage of the technique described in this protocol is related to changes in the cross-sectional area (CSA) of the ulnar nerve at the elbow in a flexed versus extended position. A systematic review and meta-analysis of sonographic normal values of the ulnar nerve demonstrated that most studies obtained their measurements with the elbow in 90° of flexion⁶.

The goal of this method is to (1) provide an accurate and specific location of ulnar nerve lesions at or near the elbow, if possible; (2) identify structural variations or abnormalities such as space-occupying masses, accessory muscles, scar tissue, or osseous changes that may be contributing to ulnar nerve compression; and (3) identify nerve hypermobility in the form of subluxation or dislocation out of the retroepicondylar groove during dynamic assessment. Please note that while this protocol focuses on the location of the elbow, we include instructions for scanning the entire length of the nerve per expert consensus guidelines⁵. Reporting these findings may guide management and aid in surgical planning. Overall, ultrasound is noninvasive, well-tolerated, and less expensive than other imaging modalities, such as magnetic resonance imaging.

Protocol

The protocol follows the guidelines of the Wake Forest School of Medicine Research and Ethics Committee, and informed written consent was obtained from patients before deidentifying and including the ultrasound images in this document.

1. Patient positioning, knobology, and basic tools of assessment (Figure 1)

1. Patient positioning
   1. Ask the patient to lie supine with the affected arm abducted and externally rotated at the shoulder and elbow flexed at 70°-90°(Figure 1A). For patients with shoulder pathology and limited range of motion, place a pillow under the patient's arm to limit the degree of external rotation at the shoulder.
2. Ultrasound set up
   1. Turn on the ultrasound machine and enter patient information.
   2. Press the button labeled Exam (Figure 2) on the keyboard of the ultrasound machine.
   3. Use the cursor to navigate the screen, click on Upper Extremity to choose the appropriate preset, and click on 12 Mhz Linear Transducer to choose the recommended probe.
3. Image optimization
   1. Apply a generous amount of ultrasound gel to the transducer and place the probe on the patient's arm.
   2. Attempt to visualize the ulnar nerve in the area of interest (see the next section for regional assessments).
   3. Set the initial depth between 2-3 cm using the depth knob (Figure 2). Adjust accordingly to optimize the visualization of the ulnar nerve.
   4. Use the focus knob (Figure 2) to ensure that the ulnar nerve is visualized within the focal zone at the top of the screen.
   5. Use the gain knob (Figure 2) to increase or decrease the brightness of the signal to optimize the visualization of the ulnar nerve.
4. Measuring the cross-sectional area and storing images
   1. Press the Freeze button (Figure 2) at the location of a suspected lesion as signified by an abrupt change in CSA or echotexture.
   2. Double-click the Measure button (Figure 2) to choose the cross-sectional area tool.
      NOTE: The cursor will appear as a small circle with a dot in the center after the double-click.
   3. Drag the cursor around the circumference of the nerve within the hyperechoic rim of the nerve and hit Enter when complete. Ensure that the measured CSA appears in the lower left-hand corner of the screen.
   4. Press the P1 button (Figure 2) to store the image.
5. Assessing vascularity
   1. Press the Color button (Figure 2) to use the Doppler tool and move the box over the nerve.
   2. Document the evidence of blood flow within the nerve by freezing the screen and saving the image.
   3. Press the Color button again to remove the Doppler tool.
6. Long-axis visualization of the nerve
   1. With the ulnar nerve centered on the screen in the area of interest, slowly turn the transducer 90°.
   2. Look for focal constrictions or fusiform enlargement in this view. Freeze the screen and store images in areas of interest.

2. Regional assessment of the ulnar nerve throughout its course

1. Assessment of the distal ulnar nerve segment (wrist to elbow)
   1. Identify the ulnar nerve at the wrist where it lies immediately adjacent to the ulnar artery.
   2. From this location, follow the ulnar nerve proximally through the forearm, where it runs between the flexor carpi ulnaris and flexor digitorum profundus, and continue to the level of the medial epicondyle. Adjust the depth accordingly to keep the ulnar nerve in view.
   3. Identify and document a description of any visually apparent changes in nerve echotexture or CSA in the wrist and forearm segment. Measure CSA, assess vascularity, and store images in any area of interest.
      NOTE: See the referenced articles for a discussion regarding the normal appearance of the ulnar nerve, including echotexture^1,7.
   4. Identify and record a description of any identifiable structural abnormalities such as masses, foreign bodies, or scar tissue (see discussion for details). Store images of interest.
   5. Visualize the nerve in the long axis (see step 1.6) at any location of increased CSA, change in echotexture, or structural abnormality. Freeze the screen and store relevant images.
   6. Document the specific location of any abnormalities by measuring the distance from a bony prominence such as the ulnar styloid (in the distal segment) or medial epicondyle (at the elbow or proximal segment) to the area of interest with a tape measurer.
2. Assessment of the ulnar nerve at the elbow
   1. Place the lateral end of the transducer at the medial epicondyle and the medial end of the transducer at the olecranon; the nerve runs directly between these landmarks at the retroepicondylar groove (Figure 1B).
      NOTE: The presence of these bony prominences requires a significant amount of gel to maintain the contact between the transducer and the patient skin.
   2. Adjust the gain, focus, and depth to obtain the optimal image of the nerve at this location.
   3. Scan the entire segment of the ulnar nerve in the elbow, moving the transducer several centimeters in both a proximal (Figure 1B) and distal (Figure 1C) direction from the medial epicondyle while keeping the nerve centered.
      NOTE: The true cubital tunnel is the location immediately distal to the medial epicondyle as the ulnar nerve enters the forearm between the ulna, heads of the flexor carpi ulnaris, and humeroulnar aponeurotic arcade.
   4. Visually identify the maximal point of the enlargement of the nerve in this region and measure CSA. Ensure the transducer is directly perpendicular to the nerve when identifying the area of maximal enlargement, as tracing the nerve from an oblique angle can artificially increase the CSA.
   5. Repeat steps 1.3-1.6 in this region. When visualizing the ulnar nerve in the long axis at the elbow, reposition the elbow into an extension to capture the entire nerve segment of interest.
3. Dynamic assessment of the ulnar nerve at the elbow
   1. Place the transducer in the position between the medial epicondyle and olecranon as described previously. Keep the transducer fixed at the medial epicondyle to keep the ulnar nerve in view.
   2. Flex the elbow (135°) while maintaining this transducer position and assess ulnar nerve mobility.
      NOTE: The olecranon often moves out of view during deep elbow flexion, and the distal aspect of the medial head of the triceps may come into view.
   3. Record a video of the dynamic assessment but hitting the P1 button, which will then be recorded for 3 s from the time the button is pressed.
   4. Document any evidence of subluxation (migration of the ulnar nerve over part of the medial epicondyle) or dislocation (complete migration of the ulnar nerve over the entire medial epicondyle).
4. Assessment of the proximal ulnar nerve (elbow to axilla)
   1. From the medial epicondyle, scan the ulnar nerve proximally through the Arcade of Struthers and medial intermuscular septum of the arm up to the level of the axilla, where it lies adjacent to the axillary artery.
      NOTE: The ulnar nerve remains superficial in the proximal arm and is typically visualized just medial to the brachial artery in this segment.
   2. Repeat steps 1.3-1.6 in this region.

Results

Normal appearance of the ulnar nerve at the elbow
Nerves are classically described as having a "honeycomb" appearance on ultrasound in the short axis. This appearance is due to the fact that each individual fascicle is hypoechoic (dark) appearingand the surrounding perineurium is hyperechoic (light) appearing. The result is a circular structure with speckled appearance. Note that the normal ulnar nerve often appears slightly hypoechoic at the medial epicondyle due to anisotropy from its arc...

Discussion

Ulnar neuropathy is commonly encountered as entrapment neuropathy in clinical practice. Correct diagnosis and localization aid surgical planning and treatment outcome¹⁰. It is recognized by expert consensus that ultrasound and electrodiagnostics together are more informative than either modality on its own. This expert consensus also maintains that ultrasound assessment should include measurement of CSA in regions where a lesion is suspected and an evaluation of nerve mobility at the medial epico...

Materials

Name	Company	Catalog Number	Comments
Transducer	GE	H48062AB	L4-12T
Ultrasound	GE	H8041EG	LOGIQ e
Ultrasound gel	Aquasonic	E8365BA	250 mL bottles