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

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.

Representative 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 arching course. The ulnar nerve typically appears round, oval, or triangular structure in the retroepicondylar groove and has 1-3 fascicles at this location (Figure 3)^3,7,8.

A normal nerve is relatively uniform in CSA throughout, with some degree of tapering that occurs naturally in the distal portions of the nerve. However, it is considered normal for the ulnar nerve to have a CSA that is about 0.5-1.0 mm² larger at the medial epicondyle than it is just proximal or distal to this⁹. It is important to note that the CSA can change depending on whether the elbow is in flexion or extension¹⁰. The mean CSA of the ulnar nerve at the elbow was between 6 and 7 mm² in one study¹¹. Subjects in this study were positioned with their arms in 90° of abduction and slightly bent. A systematic review and meta-analysis found that performing ultrasound of the ulnar nerve in elbow flexion resulted in a lower CSA. The majority of the included studies performed their scans with the elbow in 90° of flexion; other studies had a range of elbow flexion from 40°-135°. Using a lower resolution probe (<15 MHz) was also correlated to a lower CSA⁶.

There is no consensus on optimal positioning of the elbow while performing an ultrasound of the ulnar nerve; therefore, it is most important that the examiner use a consistent technique. This is especially true in side-to-side comparisons. We prefer the elbow in a flexed position as the majority of studies evaluating CSA of the ulnar nerve at the elbow was performed in this position. However, some examiners find it to be less technically challenging to evaluate the ulnar nerve at the elbow in the extended position², especially when visualizing the nerve in the long axis. This may also be preferable in cases where the patient is unable to tolerate the flexed elbow position.

Common ultrasound findings in ulnar neuropathy at the elbow
Studies have demonstrated that the CSA of the healthy ulnar nerve at various locations rarely exceeds 10 mm², and this is therefore, a generally accepted cut-off value for a normal range of CSA¹². Increased CSA of the ulnar nerve has correlated well to axonal injury on electrodiagnostic studies^13,14.

Dynamic US evaluation of the ulnar nerve allows real-time visualization of ulnar nerve mobility during elbow flexion and extension. In most individuals, the ulnar nerve maintains its location at the retroepicondylar groove during elbow flexion. In some individuals, the ulnar nerve will dislocate to a position either at the medial epicondyle (subluxation) or completely over the medial epicondyle (dislocation) during full elbow flexion (135°) and then relocate back to the groove during extension^7,15. This is readily observed when present (Figure 4). It should be noted that there is a significant controversy over the clinical significance of ulnar nerve hypermobility and the terminology used to describe it. Some use the terms subluxation and dislocation interchangeably, although the degree of mobility differs between the two, as described above. It has been hypothesized that hypermobility can lead to traction and/or friction injuries of the nerve, though it is well reported that this finding is present and asymptomatic in a significant portion of the population¹⁶.

There are several structural, morphologic, and dynamic abnormalities of the ulnar nerve and adjacent bony and soft tissue structures associated with UNE. One example of anatomic variation associated with UNE is the presence of a small muscle known as the anconeus epitrochelaris. This muscle originates at the medial epicondyle and inserts onto the olecranon, effectively acting as the roof of the cubital tunnel. The presence of this muscle may reduce space within the retroepicondylar groove and subsequently increase pressure on the ulnar nerve. Other reported variations in muscle anatomy, such as an accessory fourth head of the triceps brachii or additional slips of triceps brachii muscle tissue, may lead to compression of the ulnar nerve at the distal humerus¹⁷. Bony abnormalities leading to nerve compression include the presence of osteophytes or osseous reorganization the following trauma or surgery. Masses such as arteriovenous malformations, ganglion cysts, and neoplasms may also lead to nerve compression¹.

Intraneural vascularization, assessed by power Doppler, is more frequent in patients with UNE than in healthy controls and correlates with the severity of axon loss in electrodiagnostic studies¹⁸. The loss of a normal inner fascicular pattern contributes to a hypoechoic appearance in injured nerves¹⁹. However, the relationship between hypoechogenicity of the ulnar nerve at the elbow and electrodiagnostic abnormalities has not been well studied. Some studies have attempted to measure echogenicity quantitatively with a hypoechoic fraction, though more prospective studies are required to validate this²⁰.

Figure 1: Patient position for ulnar nerve ultrasound. (A) The patient is supine for ulnar nerve ultrasound with the arm in abduction at the shoulder, flexion at the elbow, and supination at the forearm (B) The lateral end of the transducer is placed 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. (C) Probe position for evaluation below the elbow. (D) Probe position for evaluation above the elbow. Please click here to view a larger version of this figure.

Figure 2: Knobology. The Exam button is within the green box. Gain control is located within the blue box. Color Doppler is located within the red box. Focal zone (Focus) control is within the yellow box. Depth control is within the purple box. The P1 button for photos and videos is located within the black box. The Freeze button is within the orange box. Please click here to view a larger version of this figure.

Figure 3: Normal appearance of the ulnar nerve below, at, and above the elbow on ultrasound. (A) The ulnar nerve CSA at the wrist; the trace tool is used to outline it. The appearance of fascicles within the nerve can be observed despite being slightly hypoechoic compared to the below or above elbow locations. (B) The ulnar nerve in a cross-sectional view at the level of the proximal forearm (below the elbow). (C) The ulnar nerve in a cross-sectional view at the level of the distal arm (above the elbow). Please click here to view a larger version of this figure.

Figure 4: Appearance of the ulnar nerve when dislocated laterally over the medial epicondyle. Entrapment of the ulnar nerve results in an enlarged cross-sectional area with associated hypoechogenicity, as illustrated in this cross-sectional view. The manual trace tool is used to outline the cross-sectional area in yellow. This CSA is taken at the level of the medial epicondyle and is 13 mm2, considered a focal enlargement at the elbow consistent with ulnar neuropathy. Note the position of the nerve at the lateral rather than medial side of the medial epicondyle, depicting a complete dislocation of the nerve out of the retroepicondylar groove. Please click here to view a larger version of this figure.

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 epicondyle⁵. Especially in cases of mild pathology, ultrasound is able to localize ulnar neuropathies with abnormal but non-localizing electrodiagnostic results²¹. Of course, the clinical context is imperative to consider when interpreting electrodiagnostic and sonographic findings and considering treatment strategies. The time course and severity of symptoms, along with the history of prior management, should guide decisions around surgical versus conservative management.

The use of Doppler flow has less robust evidence as a diagnostic modality for ulnar neuropathy, but there is evidence to support that the presence of intraneural vascularization correlates with more severe disease¹⁸ and is associated with axonal damage²². With regard to nerve echotexture, it is generally accepted that injured peripheral nerves often appear more hypoechoic¹⁹. As previously mentioned, however, the normal ulnar nerve can appear slightly hypoechoic at the medial epicondyle⁷. Given this, there is less agreement about the clinical significance of hypoechogenicity in the evaluation of UNE⁵. However, a completely anechoic appearance of the nerve or an abrupt change in echotexture in the course of the nerve have not been reported as normal findings in ultrasound of the ulnar nerve at the elbow. We have chosen to include an evaluation of echogenicity in the protocol because it adds minimal time to the exam and may provide supportive evidence of a lesion. Additionally, echotexture evaluation in ulnar nerve ultrasound is still recommended by some of the best-known experts in the field^1,2.

The evaluation of ulnar innervated muscles can also be performed in the sonographic assessment of ulnar neuropathy. Muscles that are denervated often appear hypoechoic and atrophic, and comparisons can be made between muscles of the ipsilateral upper extremity with different peripheral nerve innervation¹. The pattern of muscle abnormalities may further support the diagnosis of ulnar neuropathy or another diagnosis on the differential.

There are several limitations to this protocol. Performing ultrasound around bony prominences can be technically challenging and complicate direction visualization and measurement of the nerve. Using the ultrasound gel liberally can assist with this. It is also recommended to adjust the degree of elbow flexion accordingly to obtain the best angle for visualization. As mentioned, the degree of elbow flexion does impact the CSA of the nerve and should be considered when interpreting the results. The variability in the degree of elbow flexion across studies attempting to establish normal values is indeed a limitation. The value in sonography is also very much operator-dependent, and many examiners will not have had the opportunity to learn neuromuscular ultrasound in training. Additionally, while ultrasound is undoubtedly less expensive to the healthcare system than other imaging modalities such as MRI, the upfront cost to an institution for a high-quality ultrasound may be an obstacle in the setting of low but improving reimbursement for these studies.

In conclusion, ultrasound is a well-validated, noninvasive diagnostic tool that should be used in conjunction with electrodiagnostic in the assessment of UNE. More research should be conducted to evaluate the clinical significance of nerve hypoechogenicity and intraneural vascularization in UNE. Expert consensus on the optimal degree of elbow flexion would improve the standardization of protocols and assist in the interpretation of results. Guidance on the ultrasound assessment of postsurgical UNE will also be informative, and the current literature on this topic is sparse. Finally, more robust studies on the impact of ultrasound findings on surgical outcomes will assist both providers and patients in decision making regarding procedural interventions.

Materials

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