Multimodality imaging review of ulnar nerve pathologies

Abstract

The ulnar nerve is the second most commonly entrapped nerve after the median nerve. Although clinical evaluation and electrodiagnostic studies remain widely used for the evaluation of ulnar neuropathy, advancements in imaging have led to increased utilization of these newer / better imaging techniques in the overall management of ulnar neuropathy. Specifically, high-resolution ultrasonography of peripheral nerves as well as MRI has become quite useful in evaluating the ulnar nerve in order to better guide treatment. The caliber and fascicular pattern identified in the normal ulnar nerves are important distinguishing features from ulnar nerve pathology. The cubital tunnel within the elbow and Guyon’s canal within the wrist are important sites to evaluate with respect to ulnar nerve compression. Both acute and chronic conditions resulting in deformity, trauma as well as inflammatory conditions may predispose certain patients to ulnar neuropathy. Granulomatous diseases as well as both neurogenic and non-neurogenic tumors can also potentially result in ulnar neuropathy. Tumors around the ulnar nerve can also lead to mass effect on the nerve, particularly in tight spaces like the aforementioned canals. Although high-resolution ultrasonography is a useful modality initially, particularly as it can be helpful for dynamic evaluation, MRI remains most reliable due to its higher resolution. Newer imaging techniques like sonoelastography and microneurography, as well as nerve-specific contrast agents, are currently being investigated for their usefulness and are not routinely being used currently.

Introduction

Neuropathic symptoms related to the ulnar nerve commonly involve compression of the nerve at various sites in the upper extremity, and the ulnar nerve is the second most commonly entrapped nerve after the median nerve.^1,2 These conditions can be of acute onset following trauma or delayed in onset due to other causes such as infections/inflammatory conditions, tumors, benign or malignant, and sometimes normal variant anatomy which predisposes some patients and/or external pressure on the ulnar nerve from pathologies arising from tissues within the immediate vicinity along the course of the ulnar nerve.^3–6 Although detailed neurological evaluation and electrophysiological studies are often used for evaluation of the nerve, imaging is a vital component of any workup in order to provide a greater understanding of the underlying etiology and pathology in order to guide treatment. ⁷ The advent of advancements in newer and/or better imaging techniques, such as high-resolution ultrasonography and magnetic resonance imaging (MRI), the peripheral nerves in all areas of the body are increasingly being studied and evaluated. Importantly, the role that imaging plays in this type of pathology is expected to continue to increase over time.^1,8 Although ulnar nerve imaging is less frequently encountered in clinic practice, having sound knowledge of ulnar nerve pathologies and imaging features is a helpful adjunct to all physicians who may potentially treat a patient presenting with this type of pathology.

Anatomy

The ulnar nerve is one of the major terminal branches of the brachial plexus as it originates from the medial aspect of the spinal cord and then arises as the anterior division of the inferior trunk of the brachial plexus. The ventral rami of the C8 and T1 nerve roots, as well as occasionally the C7 nerve roots, contribute to the ulnar nerve. The ulnar nerve begins within the axilla in close relation to the axillary artery and vein. The arm itself is then divided into anterior and posterior muscular compartments by the intermuscular septum which exists as a fascia extending from the lesser tuberosity of the humerus to the medial epicondyle portion of the humerus. The ulnar nerve then continues its course within the anterior compartment of the arm posteromedial to the brachial artery.^9,10 The ulnar nerve extends through the medial intermuscular septum as it passes through the arcade of Struthers which is a V-shaped opening 8–10 cm proximal to the medial epicondyle ultimately reaching the posterior compartment where it courses between the intermuscular septum and the medial head of the triceps muscle.^3,10,11 The arcade of Struthers is a band of deep brachial fascia measuring approximately 3.75 cm^3,10 and attaches to the medial head of the triceps to the medial intermuscular septum. Specifically, the arcade of Struthers is a potential site of ulnar nerve compression. The intermuscular septum otherwise thickens and broadens to attach to the medial epicondyle from which the ulnar nerve courses posteriorly. Potentially, the ulnar nerve can be compressed by the septum in patients who have underwent an anterior transposition procedure or have an unstable ulnar nerve that tends to sublux over the medial epicondyle. For this reason, routine excision of the septum is performed in the anterior transposition procedure. Another potential site of compression is by the medial head of the triceps muscle and an anomalous anconeus epitrochlearis muscle which is usually located just proximal to the medial epicondyle where the ulnar nerve appears superficial and is often palpable. The ulnar nerve then passes posterior to the medial humeral epicondyle within the superficial condylar groove referred to as the cubital tunnel (Table 1). ¹⁰ The roof of this tunnel is formed by the cubital tunnel retinaculum (Osborne’s ligament) proximally and the flexor carpi ulnaris (FCU) aponeurosis (Osborne’s fascia) distally (Figure 1). Osborne’s ligament prevents the anterior subluxation of the ulnar nerve during flexion. Any space occupying lesion within the tunnel can potentially compress the ulnar nerve. In the forearm, the ulnar nerve passes between the two heads of the FCU and the ulnar nerve can potentially be compressed between these two anatomic bodies. ¹⁰ The ulnar nerve then courses between the FCU and the flexor digitorum profundus muscle within the forearm.^9,10 In the distal third of the forearm, the dorsal cutaneous branch arises from the ulnar nerve piercing the fascia of the FCU muscle to enter the dorsal aspect of the forearm while the main trunk continues to course along the FCU becoming superficial to the flexor retinaculum at the level of the wrist. The ulnar nerve turns lateral to the pisiform bone and, along with the ulnar artery, enters a fibro-osseous tunnel known as Guyon’s canal (Table 2). The ulnar artery is lateral to the ulnar nerve within the canal (Figure 1(g)). ¹⁰ The most common sites of ulnar nerve neuropathy are the cubital tunnel and Guyon’s canal.^9,10,12,13

Table 1.

Boundaries of the cubital tunnel. ⁹

Boundary	Structures
Roof	Cubital tunnel retinaculum (Osborne’s ligament) proximally and FCU aponeurosis (Osborne’s fascia) distally
Floor	Elbow joint capsule and medial or ulnar collateral ligament
Medial	Medial epicondyle of the humerus
Lateral	Olecranon process of the ulna and tendinous arch formed by humeral and ulnar heads of FCU

Figure 1.

(a) Axial PD and (b) T1 sagittal of elbow demonstrate the ulnar nerve (black arrow) behind the medial epicondyle (ME) in the cubital tunnel, along with the posterior recurrent ulnar artery (white arrowhead). The roof of the cubital tunnel is formed by cubital tunnel retinaculum (white arrow) proximally and FCU aponeurosis distally. Posterior bundle of medial collateral ligament (black arrowhead) forms the floor of the tunnel. (c) Axial T1 and (d) coronal T1 of the wrist at Guyon’s canal demonstrated the ulnar artery (white arrow) lies radial to the ulnar nerve (black arrow). Hook of the hamate forms important landmark (HH). The roof is formed by volar carpal ligament (black arrowhead), and the floor consists of the transverse carpal ligament (white arrowhead) and hypothenar muscles. (e) Transverse ultrasound view of the cubital tunnel demonstrates the boundaries of the tunnel ME and olecranon process. The ulnar nerve is located deep to the cubital tunnel ligament (white arrows). Normal ulnar nerve (black arrow) demonstrating multiple nerve fascicles as hypoechoic areas surrounded by echogenic interfascicle epineurium which gives it a honeycomb appearance. (f) Sagittal view ultrasound demonstrates long hypoechoic nerve (black arrow) fascicle with linear echogenic interfascicle epineurium giving it a fine striated pattern. (g) Transverse ultrasound view of Guyon’s canal demonstrates the pisiform bone medially. The ulnar nerve (black arrow) is located medial to the ulnar artery (white arrowhead). Transverse carpal ligament (white arrow) forms the floor of Guyon’s canal. CT, carpal tunnel; C, capitate bone; HH, hook of hamate; TP, trapezium; PT, pronator teres; OLC, olecranon; P, pisiform bone.

Table 2.

Boundaries of Guyon’s canal or ulnar tunnel.^8,9,11

Boundary	Structures
Roof	Volar carpal ligament, palmaris brevis, and hypothenar connective tissue
Floor	Transverse carpal ligament, pisohamate and pisometacarpal ligaments, and fibers of opponens digiti minimi
Medial	Pisiform bone, abductor digiti minimi, and distal tendon of FCU
Lateral	Hook of hamate and transverse carpal ligament.

Clinical presentation and evaluation

Ulnar neuropathy presents with muscular weakness and/or sensory deficits and can result in handgrip weakness and atrophy of the intrinsic hand muscles when severe. ¹⁴ The pattern of sensory and motor loss can help in localization of the site of the actual nerve injury. ¹⁵ Electrodiagnostic studies like electromyography, nerve conduction velocity, and quantitative neurosensory testing are often used to confirm and assess the severity of the neuropathy. ¹⁶ These studies however are not only discomforting to the patients but may also not provide the precise information with respect to the location of the injury, the extent of the injury, or even the underlying etiology of ulnar nerve pathology. It is important to note that these tests are indeterminate in nearly one-third of cases. ¹⁷ Imaging, however, certainly as an adjunct, can facilitate localization, potentially identify the underlying disease process, and guide treatment.^8,18 The pathophysiology of nerve injuries, clinical presentation, and evolution often correlate with electrodiagnostic studies and imaging findings (Table 3).

Table 3.

Classification of nerve injury with clinical, electrodiagnostic, and imaging correlation.^1,19,20

Seddon	Sunderland	Etiology	Pathophysiology	Recovery	Surgical intervention required	Nerve conduction study	Electromyography	Ultrasound findings	MRI findings
Neuropraxia	Grade I	Local ischemia, traction, mild crush or compression	Axons intact	Full, few hours to weeks	None	Focal partial or complete conduction block	Poor	Hypoechoic nerve	T2 hyperintensity
Axonotmesis	Grade II	Nerve crush	Connective tissue intact, damage to axons	Full, weeks to months	None	Partial/complete conduction block proximally and distally	Abnormal activity	Hypoechoic and thickened nerve	T2 hyperintensity and thickening of nerve. T2 high signal in affected muscles
Axonotmesis	Grade III		Discontinuity of axon, myelin, and endoneurium	Incomplete, usually slow, months	None or neurolysis			Hypoechoic and thickened nerve. Involved muscles show altered echogenicity	T2 hyperintensity and thickening of nerve. T2 high signal in affected muscles.
Axonotmesis	Grade IV		Discontinuity of axon, myelin, endoneurium, and perineurium.	Incomplete and variable. Months to years.	Nerve repair, graft or transfer			Fusiform hypoechoic lesion continuous with nerve. Loss of fascicular pattern. Altered muscle echogenicity	Focal enlargement of nerve with increased signal and loss of fascicular pattern. Increase signal in involved muscles
Neurotmesis	Grade V	Nerve laceration and transection	Rupture of nerve	Incomplete, months to years	Nerve repair, graft, or transfer	Complete conduction block proximally and distally	Abnormal activity	Denervation changes in muscles. Neuroma at proximal end	End neuroma at proximal end with denervation changes in muscle
	MacKinnon grade VI	Stab and gunshot wounds, closed traction damages	Mixed injury	Incomplete, months to years	Neurolysis, nerve repair, graft or transfer		Abnormal activity	Hypoechoic enlarged nerve with scarring discontinuity or neuroma formation	Heterogenous nerve. Muscle denervation changes

Imaging of the ulnar nerve

Ultrasonography and MRI are useful modalities for evaluating the ulnar nerve due to the high soft tissue resolution. Radiographs and computerized tomography (CT) are more reserved for the evaluation of bony and soft tissue pathologies that can predispose to ulnar nerve pathology.^8,21,22

Ultrasound is often the initial imaging modality used because it is non-invasive and cost-effective. On longitudinal view, the nerves appear as a uniform tubular bundle of multiple parallel bands of hypoechoic nerve fascicles with a hyperechoic interfascicular epineurium and reticular or honeycomb appearance on cross-sectional view (Figure 1(e) and (f)). ²³ Sonographic features that suggest neuropathy include fusiform hypoechoic swelling (e.g., a maximum cross-sectional area of 8–10 mm² at the level of the medial epicondyle) as well as the loss of the normal fascicular pattern or findings related to external compression of the nerve with surrounding soft tissue edema. ¹⁴ High-frequency linear transducers, spatial compounding, and artifact reduction software have led to high resolution ultrasonography with spatial resolution superior to MRI for small caliber nerves. Unfortunately, high-frequency linear transducers have limited depth penetration and are not useful for the evaluation of patients with large body habitus or deeper structures like the brachial plexus. ²³ Intraneural vascularization during the interrogation of the ulnar nerve with Doppler was more likely to be seen in patients with ulnar neuropathy which reached statistical significance in some studies. However, the sensitivity of this finding in the detection of ulnar neuropathy is poor, particularly in mild cases in comparison with more severe cases where pathology is more likely to be visualized. Consequently, this finding can be used as a severity indicator.^24,25 Another advantage of ultrasound is its ability to provide useful real-time images assessing the dynamic evaluation of the ulnar nerve during flexion and extension maneuvers of the elbow. ²⁶

MRI is superior to ultrasound in the evaluation of complex and deeper structures like the brachial plexus in addition to the characterization of lesions secondary to improved soft tissue differentiation and simultaneous evaluation of denervated muscles. Magnetic resonance neurography (MRN) is an advanced imaging technique specifically designed for the optimal evaluation of peripheral nerves using a high strength magnet, phase array coils, better gradient performance, wider bandwidths, and spin echo technique to produce high spatial resolution, high contrast resolution, and thinner slice images.^14,23 Parallel acquisition techniques enable improved temporal resolution and faster acquisition times. Higher echo times (>60–70 ms) can avoid magic angle artifacts and improve conspicuity of nerve from surrounding fat and edema. ¹⁷ Multiplanar imaging using a combination of T1, T2 fat saturation (T2FS), short tau inversion recovery (STIR), proton density (PD), and post-contrast T1 fat-saturation sequences is common. Axial T1 and PD are useful for the assessment of anatomy while fluid sensitive sequences like STIR and T2FS are useful for the detection of edema. ¹⁴ Frequency selective or adiabatic inversion recovery fat suppression sequence provides better fat suppression with improved signal-to-noise ratio than conventional STIR sequence. ¹⁴ Poor shimming, motion artifacts, field inhomogeneity, and susceptibility from nearby metal can decrease the image quality. STIR sequences are useful in later two scenarios. ¹⁷ On T1-weighted (T1W) sequences, normal nerves appear as fascicles that are isointense to muscle in a hyperintense background and peripheral ring of fat containing interfascicular and outer epineurium. They have uniform contour and taper gradually. This fascicular pattern is seen in T1W and T2 weighted (T2W) images. On STIR or T2FS, nerve appears iso to minimally hyperintense. Increased nerve caliber (>8 mm² cross-sectional area at medial epicondyle), increased T2 signal, subluxation, flattening, disruption of fascicles, and contrast enhancement are abnormal. Contrast enhancement of nerve usually indicates breach in blood–nerve barrier. Acutely denervated muscle is hyperintense on fluid-sensitive sequences due to edema and fluid shifts in compartments while chronic denervation of muscle leads to atrophy and fatty infiltration with hyperintense signal on T1.^14,23 Magic angle artifact is a potential pitfall. Mild increased T2 signal intensity of the nerve is often seen in small and fixed areas (55° to the magnetic field) of normal nerve. Additionally, heterogeneous fat suppression can also mimic pathology. Mild hyperintensity is also commonly observed in asymptomatic patients in nerves predisposed to subclinical traction/friction because of their superficial location or around the joints, such as the ulnar nerve in the Guyon canal or C8 and T1 nerve roots at the thoracic outlet. Presence of additional signs of neuropathy like loss of fascicular pattern, proximal nerve enlargement with flattening at entrapment site, and denervation muscle changes improves accuracy. ²⁷ A few recent studies have evaluated the utility of contrast-enhanced 3D MR neurography and showed improved signal-to-noise ratio due to better background signal suppression and increased conspicuity of smaller branches.^28,29 Unfortunately, this did not lead to an overall improvement in lesion detection or accuracy.^30,31

Diffusion-tensor imaging (DTI) is another newer MRI technique based on diffusion-weighted imaging techniques. Peripheral nerves are highly organized bundle of neurons. There is propensity of water to diffuse along the longitudinal axis of nerve rather than perpendicular to it. This is termed as anisotropy. It helps to differentiate nerves from the surrounding isotropic tissues in which water molecules usually do not have any preferential direction of motion. The degree of diffusion anisotropy can be measured by DTI. ²³ Fractional anisotropy represents asymmetry of diffusion within a voxel. ³² Patients with neuropathy due to the disruption of organized nerve bundles will have decrease in fractional anisotropy. ²³ DTI may be more sensitive than T2W images for the detection of subclinical ulnar neuropathy but is not yet widely used for clinical purposes. ³³ The diffusion asymmetry in different directions in each voxel is calculated with mathematical 3D constructions to generate a 3D course of nerve. This technique is referred to as tractography. ²³

Sonoelastography assesses the elastic property of tissue and measures stiffness of the peripheral nerve and surrounding tissue by measuring the tissue strain in response to an applied stress. In shear wave elastography, the stiffness of the tissue is estimated as a function of the shear wave velocity after the application of vibration stress. ²³ Sonoelastography in the assessment of nerve tissue has not been validated for clinical use, but studies have demonstrated very high sensitivity and specificity and may find its role in the evaluation of neuropathy in future.^16,23

Microneurography is a technique of obtaining higher spatial resolution images of nerve fascicular structure using high magnetic field strength of 7T or 9T and specific coils. These high strength magnets are mostly limited for research purposes but in future may be useful for visualizing microstructure of nerve. ²³

Nerve-specific contrast agents are not approved for use yet but may have role for imaging of the nerves in future. One of the nerve-specific contrast agent is Gadofluorine M that selectively binds to regions of demyelinated nerve. Superparamagnetic contrast agents like iron oxide containing agent may localize to areas of inflammation resulting in hypointense signal on T2-weighted sequences. ²³

Ulnar nerve pathologies

Mechanical injury to the ulnar nerve

The ulnar nerve can be compressed anywhere along its course but is most common at the cubital tunnel in the elbow and Guyon’s canal at the wrist.

Trauma

Acute injury-related palsy is seen in approximately one percent of patients with elbow trauma. ³ Direct impact to the nerve in the wrist or the elbow can cause ulnar nerve edema, compression by displaced bone fragment, or adjacent soft tissue inflammation. ¹⁴ Fracture of the medial epicondyle/olecranon process of ulna or fracture dislocation at elbow can impinge upon the ulnar nerve in the cubital tunnel. ³ The ulnar nerve has a superficial course near the wrist where it passes under the Guyon’s canal and prone to injuries. Penetrating injuries can lead to discontinuity of the ulnar nerve. ¹

Tardy ulnar nerve palsy

Tardy ulnar nerve palsy is characterized by delayed onset ulnar neuropathy due to nerve compression resulting from a myriad of etiologies. ¹ Chronic nerve entrapment can lead to characteristic findings like nerve swelling proximally and attenuated caliber (notch sign) distally at the site of compression. On ultrasound, nerve is hypoechoic with blurring of the normal fascicular pattern. On MRI fluid-sensitive sequences, the nerve at the site of compression is hyperintense in signal with proximal fascicular enlargement Degenerating nerve and denervated muscle may also appear hyperintense distal to the site of compression. ²³

Overuse and chronic trauma

Chronic abnormal position or repetitive motion can cause compression of the ulnar nerve as it courses through tight spaces at the elbow and the wrist predisposing them to ulnar neuropathy (Figure 2(a)). Certain professions and sports like baseball pitchers are prone to ulnar neuropathy at the elbow due to repetitive valgus forces. ¹⁴ Recurrent elbow dislocation can also result in ulnar neuropathy. ³⁴

Figure 2.

(a) Ulnar neuropathy due to overuse showing mild enlarged distal ulnar nerve with increased T2 signal on fat saturated axial MR image at the wrist (white arrow), normal signal of median nerve for comparison (black arrow). (b) Plain radiograph of the elbow lateral view demonstrates severe erosive changes of the articular surfaces of the elbow with joint effusion (black arrow) and soft tissue fullness due to synovitis with elbow subluxation causing tardy ulnar nerve palsy.

Deformity, degeneration, and arthritis

Acquired deformity from old fractures, osteophytes from osteoarthritis, and hardware around the elbow can result in tardy nerve palsy. One such example is cubitus varus deformity following old supracondylar fracture of the humerus. Joint effusion and synovial thickening seen in arthritis can potentially displace the ulnar nerve out of the cubital tunnel (Figure 2(b)).^1,14 Metallic K wires and percutaneous screws used in fractures of the olecranon process can impinge upon the nerve causing chronic nerve irritation. ¹ Atrophy of the hypothenar and interossei muscles suggests long-standing ulnar neuropathy.^1,14

Postsurgical scar tissue

Surgery in the vicinity of the ulnar nerve can lead to scar formation which can compress the ulnar nerve leading to ulnar neuropathy. On MRI, scar tissue is generally hypointense on both T1W and T2W images. It can demonstrate variable enhancement pattern depending on time and presence of granulation tissue. ¹⁴

Ulnar nerve subluxation/snapping triceps syndrome

Snapping triceps syndrome is a rare condition in which distal portion of the triceps dislocates over the medial epicondyle during flexion or extension of the elbow. ³⁵ It may coexist with ulnar nerve dislocation. Subluxation of the ulnar nerve at the cubital tunnel or pain from a dislocating triceps segment results in symptoms. It is commonly seen in men who are manual workers or athletes or have varus deformity of the elbow from prior trauma. The first symptoms usually occur in adolescence or early adulthood. ³⁵ The position of the transition between the muscular and the tendinous insertion of the medial head determines the distance between the medial edge of the triceps muscle and the medial epicondyle. A more distal transition results in shorter distance which predisposes to snapping triceps. The ulnar nerve runs between the medial edge of the triceps and the medial epicondyle. Triceps during flexion of elbow gets wider as it is compressed against distal humerus. This predisposes to subluxation anteriorly over the medial epicondyle, first the ulnar nerve at 70–90 degrees of flexion and triceps muscle later with increasing angle of flexion. Fourth head of triceps is rare anatomical variation where additional head arises from proximal humerus and spirals around long head of triceps to course medially. Unlike the three heads of triceps that form a common tendon prior to insertion to olecranon process, fourth head terminates in distal aspect of common triceps tendon. When fourth head of the triceps contracts, it can pull the common tendon and medial head of triceps along with it medially decreasing the distance between medial epicondyle and medial head of triceps predisposing to snapping triceps syndrome. A shallow groove and insufficiency of the cubital tunnel retinaculum are potential anatomical causes of a dislocating ulnar nerve. Snapping triceps on flexion alone may not result in ulnar neuropathy unless associated with dislocation of the ulnar nerve.^35,36 Dynamic evaluation of the triceps insertion and medial epicondyle during flexion and extension is possible with ultrasound making it imaging modality of choice (Figure 3). ³⁷ Failure of conservative therapy for 3–6 months may suggest a need for surgical intervention that requires fixing both the ulnar nerve and the triceps problems. Anterior transposition of the ulnar nerve may need to be considered if ulnar nerve dislocation is present. ³⁵

Figure 3.

(a, b, c, and d) Dynamic greyscale ultrasound of the medial elbow region with image A in neutral position the triceps (T) and ulnar nerve (white arrow) are seen. At complete flexion of the elbow in image D showing dislocation of the ulnar nerve and part of medial most triceps. (e) Axial STIR MRI of the elbow shows thickened ulnar nerve with increased signal suggesting neuritis (white arrow). Note more medial position of the ulnar nerve in cubital tunnel. ME, medial epicondyle.

Accessory anconeus epitrochlearis

This accessory muscle is a normal variant found in approximately 15% (3%–34%) of the population. ³⁸ It is usually unilateral but can be bilateral in nearly one-quarter of the patients. Osborne’s ligament is thought to be the remnant of accessory anconeus epitrochlearis muscle. During elbow flexion, retinaculum becomes taut reducing the size of tunnel by half. The accessory muscle forms the roof of the cubital tunnel further reducing its size and predisposing to compression of the ulnar nerve most commonly at the cubital tunnel groove (Figure 4).^14,39,40 However, some authors argue against increased risk of ulnar neuritis due to the presence of accessory anconeus epitrochlearis as the incidence of accessory muscle in patients with neuritis is not higher than in general population, and some authors even suggest it may actually be protective against the development of ulnar neuropathy.^39,41 Therefore, hypertrophied or edematous accessory muscle probably results in ulnar neuropathy. Patients with ulnar neuropathy undergo complete excision of the accessory muscle which is widely accepted as a definitive treatment. Transposition of the ulnar nerve is controversial and usually depends on surgeon’s choice. ⁴²

Figure 4.

(a) Axial fat saturated STIR images show focal thickening and edema of the ulnar nerve posterior to the elbow (white arrow). (b) More distal T1-weighted image showing overlying accessory anconeus epitrochlearis muscle causing focal compression (black arrow). Ulnar nerve neuritis most likely due to compression by accessory epitrochlearis muscle.

Metabolic

Diabetes mellitus is the most common cause of peripheral neuropathy from microvascular injury and oxidative stress leading to nerve hypoxia or ischemia. ¹⁴ In high-resolution ultrasonography, diabetic neuropathy appears as enlargement of the ulnar nerve with loss of honeycomb appearance and fuzzy margins. ⁷

Vascular

Hypothenar hammer syndrome

Ulnar artery in Guyon’s canal is relatively fixed to the surrounding structure over a length of 2–3 cm and vulnerable to injury. Skin, subcutaneous tissue, palmaris brevis muscle, and superficial aponeurosis overlying the ulnar artery are thin and cannot provide enough protection. They can easily get compressed against the hook of hamate resulting in damage to arterial intima and thrombus formation. Distal embolization can also lead to ischemia of digital arteries. Hypothenar hammer syndrome occurs due to repetitive compression or blunt trauma to the distal ulnar artery or proximal portion of superficial palmar arch. It is often seen in carpenters and butchers who may repeatedly use their hypothenar eminence instead of hammers as well as in athletes playing sports such as basketball and volleyball. The incidence of hypothenar hammer syndrome in patients referred for hand ischemia ranges between 1.1 and 1.6% which would be expected to be even lower for general population.^43,44 These patients develop signs and symptoms of ischemia in involved digits with characteristic sparing of the thumb. The close anatomical proximity of sensory portion of the ulnar nerve and artery in Guyon’s canal may result in sensory symptoms like paresthesia or hyperesthesia. Rarely, pulsatile mass may be felt over the hypothenar eminence in some of the patients who develop ulnar artery aneurysm. Most patients will have positive Allen’s test. Ultrasound and MRI can be useful for the assessment of vessels as well as surrounding soft tissue structures (Figure 5). CT angiography can be performed for the assessment of small arteries of hand and evaluation of underlying bony abnormalities. Conventional catheter angiography is the gold standard for detailed assessment of hand ischemia and often provides helpful insights necessary for surgical planning. Corkscrew appearance or beaded irregular appearance of distal ulnar artery with or without aneurysm or occlusion is a classical finding in angiography. Other pathologies like vasculitis or thromboembolic disease process that predispose to hand ischemia can mimic hypothenar hammer syndrome. Symmetrical involvement and lower extremity preference favor the diagnosis of thromboangiitis obliterans. Hand arm vibration syndrome can be distinguished by the involvement of thumb and hyperemic phase during Raynaud’s phenomenon. Initial treatment approach includes life style modifications like smoking cessation, hyperlipidemia treatment, and vasodilators. Vascular intervention is indicated for acutely symptomatic patients. Surgical options include resection and vein grafting for patients with poor collateral or ligation of the ulnar artery to prevent further embolism to digital arteries. ⁴⁴

Figure 5.

(a) Greyscale ultrasound of the distal forearm 4 cm above the wrist joint showing thrombosed distal ulnar artery (black arrows) and increased thickness of adjacent ulnar nerve (white arrows) yet maintaining its normal fibrillar pattern. (b) Color Doppler ultrasound confirms complete occlusion distally. (c) T1-weighted MR coronal image shows characteristic corkscrew appearance of the ulnar artery (black arrow) seen in hypothenar hammer syndrome with displacement of the ulnar nerve (white arrow).

Epitheloid hemangioendothelioma

It is a rare locally aggressive vascular tumor of endothelial origin often seen in middle-aged population. It has intermediate malignant potential, but occasional distant metastases have been reported.^45,46 These tumors arise most commonly in the lung, liver, and soft tissue with few case reports of involvement of the ulnar artery. ⁴⁵ It appears as poorly circumscribed painful soft tissue mass that may result in peripheral neuropathy.^45,47 Diagnosis is made by pathological evaluation, and imaging cannot differentiate it from other vascular entities like sarcoma. EHE usually appears as an well-defined hypoechoic mass on grayscale ultrasound with low resistance type of arterial flow on Doppler imaging (Figure 6(a)). Infrequently, they may appear iso or hyperechoic. ⁴⁵ CT may be useful in the evaluation of distant metastasis or nodal involvement. These tumors are ¹⁸Fluorine positron emission tomography avid and useful for staging and assessment of residual tumor or disease recurrence. On MRI, it may appear as well defined or irregular heterogeneous T1 iso to hypointense and T2 hyperintense mass with areas of internal septations and flow voids on T2W images (Figure 6(b)). Low signal in T1W and T2 W images suggests hemosiderin deposition. ⁴⁵ These tumors when they arise from neurovascular bundle may mimic neurogenic tumors like Schwannoma which is more common than these vascular tumors.^47,48

Figure 6.

(a) Color Doppler ultrasound just above the wrist showing a well-circumscribed hypoechoic soft tissue mass arising from the wall of ulnar artery (white arrow) with mild compression on vascular lumen and abutting the ulnar nerve (black arrow). (b) Axial T1-weighted image shows the eccentric mass arising from wall of the ulnar artery (black arrow) and thickened ulnar nerve (white arrow) due to tumor infiltration. (c) Axial MRI T2 fat saturated image shows the diffusely infiltrative tumor involving the ulnar artery (black arrow). Surgery was performed, and histopathological diagnosis was epithelioid hemangioendothelioma of the ulnar artery.

Neoplasms

Ganglion cyst

Ganglion cysts are benign cystic masses that most commonly occur at the wrist. ⁴⁹ They usually arise from joint capsules and are connected to the joint by a stalk. They may also originate from tendon sheaths and rarely arise from a nerve sheath. ⁵⁰ The etiology is unknown, but thought to be related to degeneration of soft tissue structures surrounding a joint. They are most commonly seen in the volar side of the wrist. ⁴⁹ Impingement of the ulnar nerve in Guyon’s canal is rare; however, several cases of intraneural and extraneural ganglion cysts have been implicated.^50–53 Multiple cases of ganglion cysts at the elbow causing ulnar neuropathy at the cubital tunnel have also been reported.^54–57

On ultrasound, simple ganglion cysts appear as anechoic areas with a thin wall, no Doppler flow, and variable acoustic enhancement.^50,58,59 Complex cysts may include features such as internal septations, thick cyst wall, color Doppler flow, and a partial solid component. ⁵⁸ On MRI, uncomplicated cysts are observed to be typically T1 hypointense and T2 homogeneously hyperintense rounded or lobular masses with a thin rim (Figure 7).^51,53,57 Post-contrast sequences demonstrate enhancement of the outer rim and internal septations. ⁵⁹ Ganglion cysts that are complicated by inflammation, infection, or bleeding may appear more heterogeneous T1 isointense to hyperintense due to a high protein content in the fluid or hemorrhage, thickening of the wall and internal septations, surrounding edema in the soft tissues, and a partial solid component.^59,60 However, the absence of central enhancement differentiates these cysts from solid lesions. ⁵⁹

Figure 7.

(a) Axial T2-weighted MRI image showing a well-defined hyperintense structure (white arrow) in the wrist causing extrinsic impression upon the ulnar nerve at Guyon’s canal was found to be a ganglion cyst. (b) Axial T1-weighted image showing hyperintense signal due to proteinaceous contents (black arrow). Extrinsic impression upon the ulnar nerve (white arrow) can be seen.

Neurogenic tumors

Schwannomas are the most common peripheral nerve sheath tumors (PNSTs). ⁶¹ Neurofibromas are another common type of PNST. Both mostly occur sporadically but can be associated with neurofibromatosis. They are slow-growing, benign masses that may cause symptoms of nerve compression due to mass effect. Neurofibromas can undergo malignant transformation, particularly when associated with neurofibromatosis. ⁶²

Schwannomas and neurofibromas can look similar on imaging. Ultrasound demonstrates homogenous, well-defined, hypoechoic lesions with posterior acoustic enhancement (Figure 8(a)). They are differentiated from lymph nodes by the absence of an echogenic central hilum. ⁶³ On MRI, benign PNSTs are characterized by an oval or fusiform T2 hyperintense mass with avid contrast enhancement (Figure 8(b) and (c)).^1,62,63 There is commonly a “target sign” on T2 with central low intensity signal and hyperintense peripheral signal (Figure 8(d) and (e)).^61,64 This is due to cystic degeneration and is usually seen with larger masses. The split-fat sign shows a thin peripheral rim of hyperintense fat surrounding the mass, best seen on T1. ¹ It is not always possible to differentiate schwannomas and neurofibromas with MRI. ⁶¹ One indication is the presence of a capsule. Schwannomas are usually encapsulated, while neurofibromas are not. It may be possible to identify a capsule on MRI as low-intensity rim best seen on T2. ⁶¹ Schwannomas do not infiltrate the nerve fascicle and are located eccentrically in relation to the involved nerve, whereas neurofibromas invade the nerve fascicle and the nerve passes through the center of the lesion.^62,63 So during surgical resection, schwannomas can generally be resected without damaging the underlying nerve, while neurofibromas often cannot be separated from the nerve fibers without causing damage. ⁶²

Figure 8.

(a) Greyscale ultrasound of the left wrist shows a solid hypoechoic mass (white arrowhead) in close relation to the ulnar nerve (white arrow). (b & c) Axial T1 MRI of the left arm (a) demonstrates a well-defined isointense mass (white arrow) arising from the ulnar nerve was proven to be schwannoma. The lesion was T2 bright (image not shown) and showed central enhancement in post-contrast images (c), a feature helpful in distinguishing neurogenic tumors from a ganglion cyst. Sagittal (d) and Axial (e) T2-weighted lumbar spine images of a patient with neurofibromatosis type I show circumscribed lesion anterior to L5 vertebra with peripheral high signal and central low signal giving classical target sign appearance (white arrow), presumed to be neurofibroma. (f) Axial T2-weighted image of cervical spine showing fusiform dilated lower cervical nerve root (white arrow) affecting the ulnar nerve and plexiform neurofibromas (black arrow) of neck soft tissue. (g, h, and i) Axial fat-saturated MRI of the wrist (g) shows a well-defined T2 hyperintense mass (white arrow) which is hypointense on T-1 weighted image (h). Neurogenic origin is characteristic on MRI and was thought to be schwannoma preoperatively. Histopathology (i) showed multifocal cellular areas with peripheral palisading consistent with Verocay bodies and admixed within the lesion are areas of more fibrotic stroma with a “shredded carrot” appearance and scattered entrapped mast cells, consistent with a neurofibroma. (j) Axial T1-weighted image showing a large well-defined mass in the mid and distal arm arising from the ulnar nerve (white arrow) which was rapidly growing in size with associated pain in a patient with known NF-1. (k) Coronal post-contrast T-1 weighted images show non-uniform intense enhancement (white arrow) proven to be MNST. Note the splaying of nerve (black arrow). (l) Axial T2-weighted image of MNST tumor shows a large hyperintense mass with heterogeneous central low signal giving target-like appearance (white arrow), a feature more frequently seen in benign neurogenic tumors than MNST.

Hybrid PNSTs are a distinct type of benign tumors that have features of multiple conventional nerve sheath tumors. The most common type is hybrid schwannoma/perineuromas. Hybrid neurofibroma/schwannomas are associated with neurofibromatosis or schwannamatosis and have a risk of malignant transformation. ⁶⁵ Hybrid PNSTs usually arise from peripheral nerves and present as painless masses in the dermis or adipose tissue (Figure 8(g)–(i)). There is a paucity of descriptions of the imaging features of these tumors in the literature. ⁶⁶

Malignant PNSTs can arise de novo or from malignant transformation of neurofibromas, or more rarely from schwannoma. About 50% of malignant PNSTs are associated with neurofibromatosis type 1. These tumors are usually high-grade sarcomas that are often rapidly enlarging. Imaging often cannot differentiate malignant PNST from benign tumors. Some findings that favor a malignant lesion include large size (>5 cm), heterogeneity, irregular borders, bone destruction, peritumoral edema, and absence of a target sign.^62,63 Heterogenic appearance after contrast enhancement due to areas of necrosis may be seen with malignant tumors; however, benign lesions may have similar appearance due to degeneration (Figure 8(j) and (k)).^62,63 Although target sign is rare in malignant PNST compared to benign tumors, presence of a target sign should not be used to rule out malignant PNST (Figure 8(l)). ⁶⁴

Neurolymphomatosis

Neurolymphomatosis is a rare disease characterized by infiltration of the nervous system by lymphoma.^67,68 Prevalence has been reported from 0.2% of all cases of non-Hodgkin’s lymphoma (NHL) to 3% of new intermediate or high-grade cases. It is most frequently associated with NHL (90%) with diffuse large B-cell lymphoma constituting 75% of etiology. Patients generally present with a painful neuropathy of the affected peripheral nerves, and diagnosis is challenging by labs and clinical evaluation alone. Imaging is important in prompt diagnosis and guiding biopsies. ⁶⁸ Nerve biopsy is still considered the gold standard for the diagnosis of neurolymphomatosis. ^{67 18}Fluorodeoxyglucose positron emission tomography generally presents as a linear or fusiform avid mass coursing along a neuronal path (Figure 9). MRI may show diffuse enlargement or multifocal nodularity of the peripheral nerves or plexus with hyperintense signal in T2W images and enhancement in post-contrast images. MRI may also show findings of muscle denervation in the majority of cases. ⁶⁸ Brachial and lumbar plexuses and trigeminal nerves have been reported to have higher predilection for neurolymphomatosis. ⁶⁷

Figure 9.

(a) Contrast-enhanced CT of thoracic inlet with oblique reformats and (b) coronal FDG PET/CT in a patient with diffuse large B cell lymphoma relapse as neurolymphomatosis, showing bilateral thickened bilateral brachial plexuses with increased tracer activity (yellow oval markings).

Infection

Leprosy

Leprosy is a chronic infection by Mycobacterium leprae affecting skin and peripheral nerves due to its preference for cooler areas of the body. The ulnar nerve is commonly affected as it is subcutaneous and at a cooler temperature. The severity of the clinical presentations varies based on the immune response. Tuberculoid leprosy is associated with a robust cell-mediated immunity, while lepromatous leprosy is associated with a weaker immune response. Neuropathy is typically an early sign of disease and typically presents with loss of sensation. During later stages, this may progress to neuropathic pain and motor deficits.

Leprosy infection causes inflammatory damage to the nerve trunks and cutaneous nerves resulting in thickening of the nerves ranging from 40% to 75%. Mononeuritis is the most common presentation of leprosy. ⁶⁹ Ultrasound and MRI can both be used to identify nerve involvement. Ulnar nerve thickening is the main sonographic feature. ⁷⁰ Additional sonographic findings include focal hypoechoic and hyperechoic areas, loss of fascicular pattern, and increased endoneural vascularity on Doppler (Figure 10(a)–(c)). Abscess formation is seen as anechoic area adjacent to or within the nerve on ultrasound.^70,71 This is significant because while acute neuritis can usually be managed with steroids, an abscess often requires surgical decompression. ⁷¹ MRI is particularly useful to rule out nerve abscess. Findings of neuritis include hyperintense signal on T2 and STIR, and enhancement with contrast. ⁷² Abscess appears as well-defined peripheral rim-enhancing lesion. ⁷³

Figure 10.

(a) Greyscale ultrasound image at the level of mid-forearm shows marked thickening of the ulnar nerve with hypoechoic leprosy granulomas (white arrows). (b) Increased vascularity on color Doppler with adjacent fluid collection/abscess (white arrow). (c) Cord-like enlarged nerve during surgical micro-dissection (white arrow). (d) Coronal post-contrast fat-saturated image at the elbow showing diffuse marrow enhancement in the distal humerus and proximal ulna (white arrow) abutting the ulnar nerve in a patient with extrapulmonary tuberculosis.

Tuberculosis

Nervous system involvement is common in extrapulmonary tuberculosis. Tuberculomas are granulomatous foci that typically affect the central nervous system but uncommonly involve peripheral nerves, particularly the ulnar nerve. Due to the rare presentation, these are often misdiagnosed. ⁷⁴ MRI can show a well-defined fusiform mass that is hyperintense on T1 and hypointense on T2 (Figure 10(d)). ⁷⁴

Radiation neuritis

Radiation neuritis is an uncommon but serious complication. Specifically, a total dose of >60 Gy and doses >2 Gy per fraction predispose to radiation-induced neuropathy.^75,76 Side effects of radiotherapy can be subdivided into early and late effects. Early effects are due to electrophysiological and biochemical changes combined with altered vascular permeability which occur within days to a year after irradiation of the nerve. ⁷⁵ Late effects occur due to direct axonal injury, demyelination, extensive fibrosis, and ischemia with compensatory neovascularization within the irradiated nerve. Clinically, it is characterized by gradually progressive paresthesia, paresis, hyporeflexia, pain, and edema. Imaging is helpful in the differentiation of radiation-induced plexopathy from neoplastic brachial plexopathy (Figure 11). ⁷⁶

Figure 11.

Ulnar neuritis in a patient with post-radiation treatment for left breast cancer showing edema of lower nerve roots (C8–T1) (white arrow) on coronal (a) and axial (b) weighted fat-saturated images, with clinical symptoms of ulnar neuropathy due to acute radiation neuritis.

Conclusion

Imaging is gaining an important role in the evaluation and treatment of ulnar nerve pathologies. With these advancements, its utilization is only expected to increase in future as these advancements become more refined and as radiologists and other clinicians encountering these pathologies become more comfortable with these specific techniques and the unique imaging that they provide. It is important, therefore, to understand the imaging findings of the normal anatomy of the ulnar nerve and potential ulnar nerve pathologies in order to provide better care of these patients.

ORCID iD

Ranjit Kumar Chaudhary https://orcid.org/0000-0003-2840-0180