eMedicine Specialties > Neurology > Electromyography and Nerve Conduction Studies

Ulnar Neuropathy: Differential Diagnoses & Workup

Author: Christina J Azevedo MD, Staff Physician, Department of Neurology, Dartmouth-Hitchcock Medical Center
Coauthor(s): Stephen A Berman, MD, PhD, Professor, Department of Internal Medicine, Section of Neurology, Dartmouth Medical School; Chief, Neurology Service, White River Junction Veterans Medical Center
Contributor Information and Disclosures

Updated: Dec 11, 2009

Differential Diagnoses

Alcohol (Ethanol) Related Neuropathy
Amyotrophic Lateral Sclerosis
Cervical Spondylosis: Diagnosis and Management
Traumatic Peripheral Nerve Lesions

Other Problems to Be Considered

Brachial plexopathy
Cervical radiculopathy
Peripheral polyneuropathy
Cervical disk syndromes

Workup

Laboratory Studies

• Consider the following peripheral neuropathy workup depending on the specific clinical situation:
  • Complete blood count
  • Fasting blood glucose
  • Hemoglobin A_1C
  • Antinuclear antibody
  • Erythrocyte sedimentation rate
  • Renal function tests
  • Paraproteinemia workup
  • Angiotensin-converting enzyme level
  • Lyme serology
  • Thyroid function tests
  • HIV serology
  • Hepatitis serology and workup

Imaging Studies

Ultrasonography

Ultrasonographic examination of peripheral nerves may be used to support the clinical and electrophysiologic diagnosis in a compressive neuropathy. It may also be helpful in identifying specific compressive etiologies of nerve injury (tumors, cysts, etc) and in visualizing structural nerve changes. Advantages of ultrasonography include the following:

• Unlike CT or MRI, ultrasonography provides real time evaluation of nerve displacement/compression during movements of adjacent joints.
• Ultrasonography is noninvasive, cheap, portable, and well-tolerated by patients.
• Ultrasonography is readily available (however, technicians who are experienced in peripheral nerve ultrasonography may not be readily available).
• The peripheral nerve can be followed for much of its course in an extremity.^{38,40,42,43,44}

The ultrasonography finding that seems to be the most useful is a change in the diameter of a nerve at the site of compression. Just proximal to the site of compression, swelling of the nerve can often be seen. One small study hypothesized that using a ratio of the cross-sectional nerve area at the site of maximal enlargement and at an uninvolved site could improve diagnostic accuracy. Using this ratio did not add any diagnostic accuracy to simply looking for the point of maximal swelling; the ratio did, however, help distinguish compressive neuropathies from other systemic processes associated with diffuse nerve enlargement (eg, diabetes, polyneuropathy).⁴¹

One specific area in which ultrasonography may be useful is the evaluation of traumatic peripheral nerve injuries. In one interesting study, 20 fresh cadaver arms were disarticulated, and the median, ulnar, or radial nerves were randomly transected in 0-2 locations per arm. Sham incisions were performed throughout the extremity. The peripheral nerves were then systematically scanned by ultrasonographers who were blinded to the sites of transection. High-resolution ultrasonography was able to identify transected nerves with 89% sensitivity and 95% specificity. The diagnostic accuracy improved throughout the study; with the first 10 arms, the ultrasonographer correctly identified the transection in 77% of cases. With the final 10 arms, the accuracy was 100%.

This suggests that the experience of the ultrasonographer plays a vital role in the use of ultrasonography in peripheral nerve injury. The study suggests that ultrasonography may be useful both in prognostication of nerve injury when an experienced ultrasonographer can assess for a partial versus complete nerve injury, and/or in localizing a nerve transection for possible surgical repair.³⁹

Magnetic resonance imaging

MRI is being increasingly used in the evaluation of peripheral neuropathies, including ulnar neuropathy. In most patients, history, physical examination and electrophysiologic testing is sufficient to make the diagnosis of ulnar neuropathy. However, there may be a subgroup of patients with inconclusive findings on the standard evaluation in whom MRI may be beneficial.

On MRI, normal nerves appear as smooth, round, or ovoid structures that are isointense to surrounding muscles on T1-weighted sequences. There is often a rim of hyperintense signal on T1. On T2-weighted images, the nerve is normally isointense to slightly hyperintense with respect to surrounding muscle. Normal nerves do not enhance after administration of gadolinium.

Possible changes that could be seen in neuropathies include increased signal intensity within the nerve on T2-weighted sequences. Neurogenic muscle edema can be seen as early as 24-48 hours after denervation, and STIR sequences are particularly sensitive for that. This is to be contrasted with electrophysiologic testing, in which changes after denervation are not seen for 1-3 weeks. After months of denervation, fatty muscle atrophy is seen. Changes in the surrounding structures that may be related to the neuropathy in question, such as osteoarthritis, synovitis, or tumors, can be seen with MRI as well.⁷⁷

Several small studies exist that attempt to address the use of MRI in the diagnostic evaluation of ulnar neuropathy. In one study by Vucic et al, 52 patients were identified who met clinical criteria for ulnar neuropathy, based on either sensory symptoms or motor weakness in the distribution of the ulnar nerve. All of these patients underwent electrophysiologic testing. In 63%, the electrophysiologic studies were diagnostic of an ulnar neuropathy at a specific location, commonly at the elbow. In 37%, the EP studies were nonlocalizing based on the criteria of the American Association of Electrodiagnostic Medicine.

All patients subsequently underwent MRI scanning as well, which revealed abnormalities in 90% of patients. In the subgroup of patients who had diagnostic EP studies, 94% had an abnormal MRI; in those who had nondiagnostic EP studies, 84% had an abnormal MRI. The authors conclusion was that MRI was "more sensitive" than neurophysiologic testing, and that the sensitivity of MRI does not change, regardless of the EP results.⁷⁶

Another study by Andreisek et al looked at 51 patients with clinically evident neuropathies in either the radial, median, or ulnar nerves who were referred to their center for MRI scans of an upper extremity. This study was designed to assess the impact of the MRI results on clinical decision making and patient management. In summary, this study found only a weak/moderate correlation between MRI results and clinical findings, which the authors felt was not surprising given that clinical findings imply physiologic dysfunction of the nerves, whereas MRI findings can evaluate nerve morphology alone. The authors reported that the greatest use of MRI in this study seemed to be in the patients in whom the etiology of their symptoms was unclear; in these cases, the MRI scan was said to have found the etiology of the symptoms in 93% of cases. This resulted in a moderate to major impact on treatment in 84% of patients in this subgroup.⁷⁹

Despite the seemingly positive results of these 2 studies, some caveats should be applied. Firstly, imaging criteria for diagnosing neuropathy on MRI scans are not well-defined. Furthermore, the clinical significance of certain MRI findings has been called into question. A study by Husarik et al took 60 asymptomatic patients and performed MRI scans of their elbows. In these healthy volunteers, 60% had increased signal intensity of their ulnar nerves without accompanying changes in the medial or radial nerves. This study suggests that an increase in signal intensity should not be used as the only criterion when evaluating for possible ulnar neuropathy.⁸⁰

The role of MRI in the evaluation of ulnar and other peripheral neuropathies continues to evolve. At this point, it seems safe to conclude that MRI may be a useful adjunct in select cases, either when a specific compressive lesion such as a mass is suspected, or when a patient with the clinical syndrome of ulnar neuropathy has nondiagnostic electrophysiologic tests. To improve diagnostic accuracy, further research is required to develop standardized criteria to make the diagnosis of ulnar neuropathy on MRI.

Other Tests

• Nerve conduction studies
• This test measures basic sensory and motor nerve parameters such as latency, amplitude, and conduction velocity.
• Electrodes (metallic reusable or pregelled disposable tape) are placed over the main belly of the muscle (active) such as the abductor digitorum quinti (ADQ) or first dorsal interosseous⁴⁶ (FDI) and the tendon of the fifth or first digit, respectively.
• The ulnar nerve is stimulated at the wrist and above and below the elbow. This helps localize the site of involvement.
• Short segment stimulation (also known as the inching technique) can increase the sensitivity of this method and can possibly improve the localization by helping the examiner judge whether a blockage is infracondylar (ie, near the cubital tunnel) or higher, near the ulnar grove (ie, near the location associated with tardy ulnar palsy) (see Media file 3). In fact, one can try to choose one of the 5 or 6 locations previously mentioned; however, the exact course of the ulnar nerve is impossible to know in any given case. Considerable anatomic variation exists from person to person and even controlling the angle of the elbow does not determine exactly where the nerve is running beneath the skin. Thus, one really does not know exactly where the nerve is being stimulated. The take-off point of the impulse may not be exactly under your stimulator.
  
  A good percentage of experienced electromyographers believe that usually the best that can be done is to say whether or not a blockage exists at the elbow. Often even that cannot be done for sure. For a discussion of the anatomic variation, see Campbell, 1991.²⁴ The reader is invited to try the inching technique and report to the prospective surgeon where he or she thinks the blockage might be with respect to anatomical landmarks. Report this as tentative information, ie, the best you can do but not to be taken as definite, and ask the surgeon to tell you where the blockage actual seemed to be if surgery is performed. Keep track and draw your own conclusions about how accurate this method is in your own hands. Even attempting to use the inching technique may help you by making you more conscious of the anatomy, even if it does not give you the exact localization.
• Needle electrode examination
• Evaluation of motor unit morphology and recruitment patterns
• Ascertains ongoing loss of muscle fibers via detection of abnormal spontaneous activity (eg, fibrillation potentials and fasciculations)
• Checks the integrity of the muscle membrane to expand differential diagnosis (eg, myotonia, paramyotonia, periodic paralysis) as manifested by increased insertional activity such as complex repetitive discharges, myokymia, and (para)myotonic discharges
• Histologic studies
• Nerve enlargement in cases of entrapment typically occur proximal to the point of compression.
• Nerve compression leads to a cascade of edema, demyelination, inflammation, axonal loss, fibrosis, and remyelination with subsequent thickening of the perineurium and endothelium.

Martin-Gruber anastomosis

This anatomic variant is seen during routine nerve conduction studies and can pose a diagnostic dilemma if not identified as such. Martin originally described it in 1763. Gruber's paper appeared over a century later in 1870. It is an 47

• In Martin-Gruber, a crossover of axons from the anterior interosseous nerve (exclusively motor branch of the median) to the ulnar nerve in the forearm usually occurs. In such cases, no sensory fibers are involved in the crossover. However, in a small minority of cases, the crossover can occur from the main median trunk (in which case some sensory nerve fibers may cross over as well).
• Martin-Gruber occurs in 10-30% of individuals and 60-70% of those affected show the anomaly bilaterally. In some families, an autosomal dominant inheritance is possible, although a gene controlling this occurrence has not been identified.
• The fibers involved are from the C8/T1 nerve roots. Three patterns of Martin-Gruber are commonly recognized. In type II, the most common pattern, the crossover fibers innervate the first dorsal interosseus (FDI). In type I, the next most common pattern, the hypothenar muscles are involved. In type III, the least common pattern, the thenar muscles, typically the adductor pollicis and the flexor pollicis brevis rather than the abductor pollicis brevis, are involved. Sometimes other muscles, including forearm muscles such as the flexor digitorum superficialis and the flexor digitorum profundus, are involved as well. These 3 common types are delineated in the following image.
  
  

  

  The normal median and ulnar pattern are compared with that of the 3 commonly recognized types of the Martin-Gruber anomaly.
  

  [ CLOSE WINDOW ]

  

  The normal median and ulnar pattern are compared with that of the 3 commonly recognized types of the Martin-Gruber anomaly.
  

• In the patient without Martin-Gruber anomaly, stimulating the median nerve at the wrist produces a compound muscle action potential (CMAP) amplitude at the thenar eminence (eg, abductor pollicis brevis [APB]) that is essentially the same size as the thenar CMAP produced by elbow stimulation (the CMAP produced by wrist stimulation could be a bit larger because stimulating further away from the ultimate target muscle gives a little more temporal dispersion of the signal). With the anomaly, however, the wrist response is smaller because many axons from the median nerve have crossed already. Contributions from now median-innervated ulnar intrinsic hand muscles falsely increase the elbow response.
• The converse applies with ulnar nerve stimulation, when recording over the hypothenar eminence (ADQ) or FDI, as median nerve fibers are innervating ulnar muscles in the hand, and the elbow response is smaller (see Media files 5-6). This could be mistaken for a conduction block. Thus, a Martin-Gruber anastomosis should be excluded prior to diagnosing ulnar conduction block.
• To see these relationships even more clearly, the image below shows the same anatomical diagrams as the image above, plus the corresponding EMG patterns.
  
  

  

  In people without the Martin-Gruber anomaly who do not otherwise have significant neuropathy or nerve compressions, here is what happens when the relevant nerves are stimulated. Median stimulation: Stimulation at the elbow yields a larger compound muscle action potential (CMAP) at the hypothenar muscles, the first dorsal interosseus (FDI), or the thenar muscles (or a combination of these) than does stimulation at the wrist. Ulnar stimulation: Stimulation at the wrist yields a larger CMAP at the hypothenar muscles, the FDI, or the thenar muscles (or a combination of these) than does stimulation at the elbow. Larger and smaller generally means a difference of 1.0 millivolt in amplitude or more.
  

  [ CLOSE WINDOW ]

  

  In people without the Martin-Gruber anomaly who do not otherwise have significant neuropathy or nerve compressions, here is what happens when the relevant nerves are stimulated. Median stimulation: Stimulation at the elbow yields a larger compound muscle action potential (CMAP) at the hypothenar muscles, the first dorsal interosseus (FDI), or the thenar muscles (or a combination of these) than does stimulation at the wrist. Ulnar stimulation: Stimulation at the wrist yields a larger CMAP at the hypothenar muscles, the FDI, or the thenar muscles (or a combination of these) than does stimulation at the elbow. Larger and smaller generally means a difference of 1.0 millivolt in amplitude or more.
  

  
  
• The table below explains in words why the patterns look the way they do. If you really want to understand this, you may need to print these out and sketch them yourself a few times. This table describes the 3 major types of the Martin-Gruber anastomosis and shows the pattern of CMAP response at the thenar eminence, the FDI, and the hypothenar muscle in people who have the Martin-Gruber anomaly but do not otherwise have significant neuropathy or nerve compressions.
  
  Table 1. Martin-Gruber Anastomosis

Open table in new window

CMAP = compound motor (or muscle) action potential;

FDI = first dorsal interosseus

ADP = adductor pollicis

FPB = flexor pollicis brevis

Note: Larger and smaller generally means a difference of 1.0 millivolt in amplitude or more.

Median stimulation:

Stimulation at the elbow yields a larger CMAP at the hypothenar muscles, the FDI, or the thenar muscles (or sometimes in a combination of these) than does stimulation at the wrist.

Ulnar stimulation:

Stimulation at the wrist yields a larger CMAP at the hypothenar muscles, the FDI, or the thenar muscles (or sometimes in a combination of these) than does stimulation at the elbow.

• Two potentially important diagnostic implications are associated with this Martin-Gruber anomaly.
• First, in cases of carpal tunnel syndrome (ie, median mononeuropathy at the wrist), the larger median CMAP amplitude at the elbow has an initial positive (ie, downward) deflection, which is not seen at the wrist. This is explained by the fact that the median nerve axons are traveling slower through the carpal tunnel so that the median-innervated ulnar hand muscles conduct first, leading to a volume-conducted response that is manifested by a positive deflection. If carpal tunnel syndrome is suspected clinically, the chance of a false-negative result on nerve conduction testing is still about 8-10%. Given that the anomaly exists 15-31% of the time, a chance still exists of diagnosing carpal tunnel syndrome electrically.
• Second, in suspected cases of ulnar neuropathy at the elbow or forearm, a reduced-to-absent response would be expected proximally with sparing of the wrist responses, provided that no diffuse severe axon loss has occurred. To disprove a true ulnar neuropathy, stimulation of the median nerve at the elbow would lead to a wrist response that, when added to the response achieved by stimulating the ulnar nerve at the elbow, would equal a difference of less than 20-25% between elbow and wrist, which is acceptable as normal temporal dispersion. Stimulation of the median nerve at the wrist should lead to a small response, as this would represent contributions from ulnar-derived muscles in the thenar eminence.