Laboratory Studies
Routine studies for ulnar nerve entrapment are ordered to rule out anemia, diabetes mellitus, and hypothyroidism and include the following:
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Complete blood cell (CBC) count
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Urinalysis
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Fasting blood glucose
Depending on the specific clinical situation, the following tests may be considered as well:
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Hemoglobin A1C [111]
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Antinuclear antibody
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Erythrocyte sedimentation rate
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Renal function tests
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Paraproteinemia workup (serum protein electrophoresis with immunofixation)
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Angiotensin-converting enzyme
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Lyme serology
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Thyroid function tests
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Vitamins B-12, B-1, and B-6
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Folate level
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Methylmalonic acid
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Tissue transglutaminase antibody
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Gliadin antibody
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HIV serology
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Hepatitis serologies
Radiography
Radiographs of the neck should be obtained if cervical disk disease is suspected to rule out cervical ribs. Likewise, chest radiographs should be obtained if a Pancoast tumor or tuberculosis is suspected.
Radiographs of both the elbow and wrist radiographs are mandatory in ulnar nerve compression because double-crush syndrome may be present. In addition, entrapment of the ulnar nerve may occur at more than one level.
Radiographs of the elbow reveal abnormal anatomy, such as a valgus deformity, bone spurs or bone fragments, a shallow olecranon groove, osteochondromas, and destructive lesions (eg, tumors, infections, or abnormal calcifications). If there is a history of trauma or arthritis, a cubital tunnel projection radiograph should be obtained to exclude medial trochlear lip osteophytes. If a supracondylar process on the medial aspect of the humerus is suspected, an elbow radiograph should be obtained 5 cm proximal to the medial epicondyle.
Radiographs of the wrist reveal fractures of the hook of the hamate, dislocations of the wrist bones, and soft-tissue masses and calcifications to a lesser extent.
Ultrasonography
High-resolution ultrasonographic examination of peripheral nerves may be used to support the clinical and electrophysiologic diagnosis of compressive neuropathy. It may also help identify specific compressive etiologies of nerve injury (eg, tumors or cysts) and visualize structural nerve changes. Advantages of ultrasonography include the following:
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Unlike computed tomography (CT) or magnetic resonance imaging (MRI), ultrasonography provides real-time evaluation of nerve displacement or compression during movements of adjacent joints
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Ultrasonography is noninvasive, cheap, portable, and well-tolerated
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Ultrasonography is readily available (though technicians with specific experience in peripheral nerve ultrasonography may not be)
The ultrasonographic finding that seems to be most helpful in this setting is a change in the diameter of a nerve at the site of compression. Just proximal to the site of compression, nerve swelling can often be seen.
A small study suggested that using a ratio of the cross-sectional nerve area at maximal enlargement and an uninvolved site could improve diagnostic accuracy. [117] Using this ratio did not improve diagnostic accuracy over what could be achieved simply by looking for the point of maximal swelling. However, the ratio did help distinguish compressive neuropathies from other systemic processes associated with diffuse nerve enlargement (eg, diabetes and polyneuropathy).
Subsequently, another study examined nerve vascularity in 137 patients afflicted with ulnar neuropathy at the elbow and determined that increased intraneural vascularization visualized by ultrasonography was indicative of axonal damage. [118]
Chiou et al used high-resolution ultrasonography and found that the mean value of the ulnar nerve area at the medial epicondyle level in symptomatic patients was significantly more extensive than that of the control group and that of the unaffected, contralateral side. [119] They concluded that if the ulnar nerve area was greater than 0.075 cm2 at the level of the medial epicondyle, the patient probably had cubital tunnel syndrome.
An area in which ultrasonography may be particularly useful in evaluating traumatic peripheral nerve injuries. In one study, 20 fresh cadaver arms were disarticulated, and the median, ulnar, or radial nerves were randomly transected in zero, one, or two locations per arm. [120] Sham incisions were performed throughout the extremity. The peripheral nerves were then systematically scanned by ultrasonographers blinded to the transection sites.
The investigators found that high-resolution ultrasonography could identify transected nerves with 89% sensitivity and 95% specificity. [120] The diagnostic accuracy improved throughout the study: With the first 10 arms, the ultrasonographer correctly identified the transection in 77% of cases, whereas with the final 10 arms, the accuracy was 100%.
These findings suggest that the experience of the ultrasonographer has a vital effect on the diagnostic utility of ultrasonography in peripheral nerve injury. Thus, ultrasonography may be useful in determining the prognosis for nerve injury when an experienced ultrasonographer can distinguish between partial and complete injury, in localizing a nerve transection for possible surgical repair, or in both. [120, 121]
A further consideration is the comparison of the localizations derived from sonographic and electrophysiological methods. Simon et al compared the lesion localization obtained by careful short-segment neurophysiological inching studies with that obtained by ultrasonographic methods. [122] For the larger segment across the entire elbow, the overall drop in conduction velocity correlated well with the maximum cross-sectional area (CSA) and maximal degree of the hypoechoic fraction. However, on the short segments in patients whose lesion was apparently at the medial epicondyle as judged by neurophysiological methods, the maximal sonographic abnormalities were detected both slightly proximally and distally to the medial epicondyle. It was concluded, therefore, that the ultrasonographic images may see secondary changes adjacent to the primary site of damage.
Magnetic Resonance Imaging
MRI is increasingly used to evaluate peripheral neuropathies, including ulnar neuropathy. [123] In most patients, history, physical examination, and electrophysiologic (EP) testing are sufficient to diagnose ulnar neuropathy, and MRI is not necessary. However, there may be a subgroup of patients with inconclusive findings on the standard evaluation for whom MRI may benefit. [124]
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 usually is isointense to slightly hyperintense to surrounding muscle. Normal nerves do not enhance after the administration of gadolinium.
Possible changes that could be seen in neuropathies include increased signal intensity within the nerve on T2-weighted sequences. On MRI, increased signal intensity is a better indicator of ulnar nerve entrapment than enlargement of the nerve.
Neurogenic muscle edema can be seen as early as 24-48 hours after denervation, and short T1 inversion recovery (STIR) sequences are susceptible to that. It contrasts with EP 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 also be seen with MRI. [125]
Several small studies have attempted to address the use of MRI in the diagnostic evaluation of ulnar neuropathy. For example, Vucic et al identified 52 patients who met clinical criteria for ulnar neuropathy based on either sensory symptoms or motor weakness in the ulnar nerve distribution; all underwent EP testing. [126] In 63%, the EP studies were diagnostic of ulnar neuropathy at a specific location, commonly at the elbow. In 37%, the studies were non-localizing according to the American Association of Electrodiagnostic Medicine criteria.
All 52 patients also underwent MRI scanning, which revealed abnormalities in 90%. [126] Of those patients who had diagnostic EP studies, 94% had an abnormal MRI; of those who had nondiagnostic EP studies, 84% had an abnormal MRI. The authors concluded that MRI was “more sensitive” than neurophysiologic testing and that the sensitivity of MRI did not change, regardless of the EP results.
Andreisek et al. assessed 51 patients with clinically evident neuropathies in the radial, median, or ulnar nerve, which were referred to their center for MRI scans of an upper extremity. [127] This study aimed to assess the impact of the MRI results on clinical decision-making and patient management.
Andreisek et al found only a weak-to-moderate correlation between MRI results and clinical findings—not surprisingly, given that clinical findings imply physiologic dysfunction of the nerves. In contrast, MRI findings can evaluate nerve morphology alone. [127] The most effective use of MRI in this study seemed to be in cases where the cause of the symptoms was unclear; in this situation, MRI reportedly identified the symptom etiology in 93% of cases. In addition, it resulted in a moderate-to-major impact on treatment in 84% of the patients in this subgroup.
These seemingly positive results notwithstanding, some caveats remain. First, the criteria for diagnosing neuropathy on MRI scans are not well defined. Second, the clinical significance of certain MRI findings has been questioned. Husarik et al performed MRI elbow scans in 60 asymptomatic patients and found that 60% had increased ulnar-nerve signal intensity without concomitant changes in their medial or radial nerve. This study suggests that increased signal intensity should not be the sole criterion in evaluation for possible ulnar neuropathy. [128]
Britz et al examined the use of MRI with a STIR sequence to diagnose cubital tunnel syndrome. [129] They studied 31 elbows with documented ulnar nerve entrapment and found increased signal intensity in the ulnar nerve in 97% of cases and enlargement of the ulnar nerve in 75%.
Diffusion-tensor imaging (DTI) is an advanced quantitative MRI technology. [130] This has previously been used mainly to image central myelin tracts, but it is also applicable to peripheral myelin. As with central myelin, quantitative measures of MRI parameters, including fractional anisotropy (FA), can be obtained. For example, qualitative FA maps of nerve tracts allow observers to detect peripheral neuropathy with a sensitivity of 80% and a specificity of 83%, which are high enough to make them quite useful. A prospective study of 39 patients showed a positive correlation between the clinical findings, electrodiagnostic tests, and DTI parameters in carpal tunnel syndrome patients. [131] . Neuropathic segments typically show lower FA values compared to healthy nerve segments. [132, 133]
The role of MRI in the evaluation of the ulnar and other peripheral neuropathies continues to evolve. At this point, it is reasonable to conclude that MRI may be a helpful adjunct in select cases, either when a specific compressive lesion (eg, a mass) is suspected or when a patient with the clinical syndrome of ulnar neuropathy has a nondiagnostic EP test. Further research is required to develop standardized criteria for diagnosing ulnar neuropathy on MRI, to improve the diagnostic accuracy. [127, 128, 129, 134]
Electromyography and Nerve Conduction Studies
Electromyography (EMG) and nerve conduction studies are indicated to confirm the area of entrapment, document the extent of the pathology, and detect or rule out the possibility of a double-crush syndrome. [135, 136, 137, 138] In recent entrapments of the ulnar nerve, motor and sensory conduction velocities are more valuable, whereas, in chronic neuropathies, conduction velocities and EMG are helpful because EMG is capable of showing axonal degeneration.
EMG is not essential when the diagnosis of cubital tunnel syndrome is obvious on clinical examination; a false test result can be misleading and hinder rather than aid diagnosis. However, it is important to perform EMG when the diagnosis of cubital tunnel syndrome is unclear or when it is necessary to determine the efficacy of conservative treatment. [139]
Basic sensory and motor nerve parameters measured in nerve conduction studies include latency, amplitude, and conduction velocity. Electrodes (metallic reusable or pregelled disposable tape) are placed over the main belly of the active muscle (eg, the abductor digiti quinti or the first dorsal interosseous muscle) [109] and the tendon of the fifth or first digit. The ulnar nerve is stimulated at the wrist, above and below the elbow; this helps localize the site of involvement.
Short-segment stimulation (also known as the inching technique), in which the nerve is stimulated over 1- to 2-cm intervals, can increase the sensitivity of the procedure and may improve localization by helping the examiner judge whether a blockage is infracondylar (ie, near the cubital tunnel) or higher (ie, near the ulnar or epicondylar groove, the location associated with tardy ulnar palsy). (See the image below.)
Inching technique used to isolate conduction block in left ulnar nerve. Note significant amplitude drop at 305 mm, which correlates with position 2 cm above medial epicondyle. This is example of supracondylar block. Image courtesy of A S Lorenzo, MD.
Findings are considered positive for cubital tunnel syndrome when the motor conduction velocity across the elbow is less than 50 m/s or when the difference between the motor conduction velocity across the elbow and that below the elbow exceeds 10 m/s.
If the point of maximum conduction delay and drop in amplitude of the compound muscle action potential (CMAP) is at or just proximal to the medial epicondyle, compression of the ulnar nerve is probably at the level of the epicondylar groove. If the point of maximum conduction delay and drop in CMAP amplitude is 2 cm distal to the medial epicondyle, compression is probably in the cubital tunnel. Unfortunately, false-positive results are obtained in 15% of cases.
It should be kept in mind that in any given case, it is impossible to know the exact course the ulnar nerve follows. Considerable anatomic variation exists from person to person, and even controlling the elbow angle does not determine precisely where the nerve is running beneath the skin. Thus, the examiner does not know exactly where the nerve is being stimulated. In addition, the takeoff point of the impulse may not be precisely under the stimulator.
A good percentage of experienced electromyographers believe that in many, if not most cases, the best that can be done is to establish whether a blockage exists at the elbow. Often, even that cannot be accomplished with certainty. The relevant anatomic issues have been discussed more fully by Campbell. [41]
Reservations aside, the reader is invited to try the inching technique and make an individual assessment of its potential utility in his or her own situation. This might include the following steps:
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After performing the inching technique, report to the prospective surgeon where you think the blockage might be with respect to anatomic landmarks
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Make it clear that this is a tentative assessment (ie, done to the best of your ability but not to be taken as definite)
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Ask the surgeon to tell you where the blockage actually seemed to be after surgery was performed
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Keep track of your findings compared with surgical findings, and draw your own conclusions about how accurate the inching method is in your own hands.
Even if the inching technique does not yield the exact localization of the lesion, the attempt to use it may be helpful in and of itself as it makes the clinician more conscious of the anatomy.
Martin-Gruber anastomosis
The anatomic variant known as Martin-Gruber anastomosis is seen during routine nerve conduction studies and can pose a diagnostic dilemma if not identified. A unique innervation pattern occurs between the forearm's median and ulnar nerves. [140]
In a Martin-Gruber anastomosis, a crossover of axons from the anterior interosseous nerve (the exclusively motor branch of the median nerve) 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).
The Martin-Gruber anomaly 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, though a gene controlling this occurrence has not been identified.
The fibers involved are from the C8/T1 nerve roots. Three patterns of Martin-Gruber anastomosis are commonly recognized, as follows (see the image below):
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Type I (second most common pattern) - The hypothenar muscles are involved
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Type II (most common pattern) - The crossover fibers innervate the first dorsal interosseous muscle
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Type III (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
Normal median and ulnar patterns are compared with those of 3 commonly recognized types of Martin-Gruber anomaly.
In a patient who does not have a Martin-Gruber anastomosis, stimulating the median nerve at the wrist produces a CMAP amplitude at the thenar eminence (eg, abductor pollicis brevis), 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 signal dispersion.)
In a patient with the anomaly, however, the wrist response is smaller because many axons from the median nerve have already crossed. Contributions from now median-innervated ulnar intrinsic hand muscles falsely increase the elbow response.
The converse is true with ulnar nerve stimulation during recording over the hypothenar eminence (abductor digiti quinti) or the first digital interosseous muscle; median nerve fibers innervate ulnar muscles in hand, and the elbow response is smaller (see the images below). Again, this could be mistaken for a conduction block. Accordingly, a Martin-Gruber anastomosis should be excluded before an ulnar conduction block is diagnosed.
First 3 traces correspond to ulnar compound muscle action potential (CMAP) amplitude during recording at abductor digiti quinti (ADQ) and stimulating at wrist, below elbow, and above elbow, respectively. Fourth trace corresponds to stimulation of median nerve at elbow during recording at ADQ. Although CMAP amplitude is reduced markedly above elbow, this is compensated for by adding response seen after stimulation of median nerve; this represents Martin-Gruber anastomosis.
First 3 traces correspond to stimulation of ulnar nerve during recording at first dorsal interosseous (FDI) muscle at wrist, below elbow, and above elbow, respectively. Fourth trace corresponds to stimulation of median nerve at elbow during recording at FDI muscle; this represents Martin-Gruber anastomosis.
These relations can be visualized even more clearly by considering the characteristic EMG patterns with respect to the corresponding anatomy (see the image below).
In those with Martin-Gruber anomaly who have no other significant neuropathy or nerve compression, stimulation of specific nerves at different sites yields differing results. With the median nerve, stimulation at the elbow yields larger compound muscle action potential (CMAP) at hypothenar muscles, first dorsal interosseous (FDI) muscle, or thenar muscles (or combination thereof) than does stimulation at the wrist. With the ulnar nerve, stimulation at the wrist yields larger CMAP at hypothenar muscles, FDI muscle, or thenar muscles (or combination thereof) than does stimulation at the elbow. In this context, "larger" and "smaller" generally refer to amplitude differences ≥1.0 mV.
The distinctions between the three major types of Martin-Gruber anastomosis, the tests performed to confirm them, and the possible areas of clinical confusion are summarized in the table below.
Table. Types of Martin-Gruber Anastomosis (Open Table in a new window)
Type |
Anatomy |
Most Characteristic Finding |
Confirmation |
Additional Verification |
Potential Clinical Confusion |
I |
Crossover fibers innervate hypothenar muscles |
Ulnar stimulation at wrist* produces larger hypothenar CMAP than stimulation at elbow |
Stimulation of median nerve at elbow† produces response at hypothenar muscles |
Hypothenar CMAP from ulnar stimulation at wrist is equal to hypothenar CMAP from ulnar stimulation at elbow plus hypothenar CMAP from median stimulation at elbow |
Smaller response from proximal stimulation could be mistaken for conduction block |
II |
Crossover fibers innervate FDI muscle |
Ulnar stimulation at wrist produces larger FDI CMAP than stimulation at elbow |
Stimulation of median nerve at elbow produces response at FDI |
FDI CMAP from ulnar stimulation at wrist is equal to FDI CMAP from ulnar stimulation at elbow plus FDI CMAP from median stimulation at elbow |
Usually none, because FDI muscle is not usually recording site; if it is used, conduction block could be suspected, as in type I |
III |
Crossover fibers innervate thenar muscles (typically ADP and FPB) |
Elbow stimulation of median nerve produces greater thenar response than wrist stimulation |
Ulnar stimulation produces thenar CMAP with initial positive deflection; it is higher with wrist stimulation than with elbow stimulation |
For thenar CMAP amplitudes, median elbow stimulation amp is equal to median wrist stimulation amplitude plus ulnar wrist stimulation amplitude minus ulnar elbow stimulation amplitude |
Can complicate median nerve studies, especially when carpal tunnel syndrome is involved |
ADP—adductor pollicis; CMAP—compound motor (or muscle) action potential; FDI—first dorsal interosseous; FPB—flexor pollicis brevis. *Ulnar stimulation at the wrist yields larger CMAP at hypothenar muscles, FDI, or thenar muscles (or sometimes a combination of these) than stimulation at the elbow. †Median stimulation at the elbow yields larger CMAP at hypothenar muscles, FDI, or thenar muscles (or sometimes a combination of these) than stimulation at the wrist. Note: "Larger" and "smaller" generally mean amplitude difference ≥1.0 mV. |
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Two potentially critical 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. The explanation is that the median nerve axons travel 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%. However, 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, representing contributions from ulnar-derived muscles in the thenar eminence. [141, 142, 143, 144]
Riche-Cannieu anastomosis
Another anomaly that can complicate diagnostic studies is the Riche-Cannieu anastomosis (see the image below).
Riche-Cannieu anastomosis is communication between the recurrent branch of the median nerve and the deep branch of the ulnar nerve in the hand. Although it is present in 77% of hands, it yields highly variable degrees of detectable physiologic difference. In many hands, it contributes little and does not affect diagnostic findings at all. The most common effect is probably to give ulnar innervation to some muscles usually innervated by the median nerve, median innervation to muscles usually innervated by the ulnar nerve, or both. The most extreme version is the so-called all-ulnar hand (very rare). Two examples of confusion this might cause are as follows: (1) a median lesion could cause denervation in typically ulnar muscle, such as adductor digiti minimi (adductor digiti quinti) or first dorsal interosseous muscle, and (2) an ulnar lesion could cause denervation in typically median muscle, such as flexor pollicis brevis or abductor pollicis brevis.
Histologic Studies
Nerve enlargement in cases of entrapment typically occurs 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.
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A schematic diagram of the elbow region with 5 main sites (as given by Posner) labeled 1-5; other sites and structures are also named. The main regions of interest are colored circles. Sites 2 and 3 are close together and cannot be distinguished by means of electromyography and nerve conduction studies. This location is referred to as ulnar (or epicondylar) groove.
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The diagram shows the ulnar nerve distal to the elbow region. The dorsal ulnar cutaneous nerve (lavender) branches off the main trunk (purple). Although the course is not followed in detail after that, the lavender region on the sensory dermatome diagram shows where this sensory nerve innervates skin. Similarly, the palmar cutaneous sensory nerve (yellow) branches off to innervate the skin area depicted in yellow. The superficial terminal branch is mostly sensory (see green-colored skin on palmar surface), though it also gives off a branch to the palmaris brevis. The deep terminal branch has no corresponding skin area, because it is solely motor-innervating the muscles shown, as well as others not explicitly depicted. A nerve could be pinched or injured anywhere, but sites labeled I-IV are more commonly involved.
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Inching technique used to isolate conduction block in left ulnar nerve. Note significant amplitude drop at 305 mm, which correlates with position 2 cm above medial epicondyle. This is example of supracondylar block. Image courtesy of A S Lorenzo, MD.
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Normal median and ulnar patterns are compared with those of 3 commonly recognized types of Martin-Gruber anomaly.
-
First 3 traces correspond to ulnar compound muscle action potential (CMAP) amplitude during recording at abductor digiti quinti (ADQ) and stimulating at wrist, below elbow, and above elbow, respectively. Fourth trace corresponds to stimulation of median nerve at elbow during recording at ADQ. Although CMAP amplitude is reduced markedly above elbow, this is compensated for by adding response seen after stimulation of median nerve; this represents Martin-Gruber anastomosis.
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First 3 traces correspond to stimulation of ulnar nerve during recording at first dorsal interosseous (FDI) muscle at wrist, below elbow, and above elbow, respectively. Fourth trace corresponds to stimulation of median nerve at elbow during recording at FDI muscle; this represents Martin-Gruber anastomosis.
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In those with Martin-Gruber anomaly who have no other significant neuropathy or nerve compression, stimulation of specific nerves at different sites yields differing results. With the median nerve, stimulation at the elbow yields larger compound muscle action potential (CMAP) at hypothenar muscles, first dorsal interosseous (FDI) muscle, or thenar muscles (or combination thereof) than does stimulation at the wrist. With the ulnar nerve, stimulation at the wrist yields larger CMAP at hypothenar muscles, FDI muscle, or thenar muscles (or combination thereof) than does stimulation at the elbow. In this context, "larger" and "smaller" generally refer to amplitude differences ≥1.0 mV.
-
Riche-Cannieu anastomosis is communication between the recurrent branch of the median nerve and the deep branch of the ulnar nerve in the hand. Although it is present in 77% of hands, it yields highly variable degrees of detectable physiologic difference. In many hands, it contributes little and does not affect diagnostic findings at all. The most common effect is probably to give ulnar innervation to some muscles usually innervated by the median nerve, median innervation to muscles usually innervated by the ulnar nerve, or both. The most extreme version is the so-called all-ulnar hand (very rare). Two examples of confusion this might cause are as follows: (1) a median lesion could cause denervation in typically ulnar muscle, such as adductor digiti minimi (adductor digiti quinti) or first dorsal interosseous muscle, and (2) an ulnar lesion could cause denervation in typically median muscle, such as flexor pollicis brevis or abductor pollicis brevis.
Tables
Type |
Anatomy |
Most Characteristic Finding |
Confirmation |
Additional Verification |
Potential Clinical Confusion |
I |
Crossover fibers innervate hypothenar muscles |
Ulnar stimulation at wrist* produces larger hypothenar CMAP than stimulation at elbow |
Stimulation of median nerve at elbow† produces response at hypothenar muscles |
Hypothenar CMAP from ulnar stimulation at wrist is equal to hypothenar CMAP from ulnar stimulation at elbow plus hypothenar CMAP from median stimulation at elbow |
Smaller response from proximal stimulation could be mistaken for conduction block |
II |
Crossover fibers innervate FDI muscle |
Ulnar stimulation at wrist produces larger FDI CMAP than stimulation at elbow |
Stimulation of median nerve at elbow produces response at FDI |
FDI CMAP from ulnar stimulation at wrist is equal to FDI CMAP from ulnar stimulation at elbow plus FDI CMAP from median stimulation at elbow |
Usually none, because FDI muscle is not usually recording site; if it is used, conduction block could be suspected, as in type I |
III |
Crossover fibers innervate thenar muscles (typically ADP and FPB) |
Elbow stimulation of median nerve produces greater thenar response than wrist stimulation |
Ulnar stimulation produces thenar CMAP with initial positive deflection; it is higher with wrist stimulation than with elbow stimulation |
For thenar CMAP amplitudes, median elbow stimulation amp is equal to median wrist stimulation amplitude plus ulnar wrist stimulation amplitude minus ulnar elbow stimulation amplitude |
Can complicate median nerve studies, especially when carpal tunnel syndrome is involved |
ADP—adductor pollicis; CMAP—compound motor (or muscle) action potential; FDI—first dorsal interosseous; FPB—flexor pollicis brevis. *Ulnar stimulation at the wrist yields larger CMAP at hypothenar muscles, FDI, or thenar muscles (or sometimes a combination of these) than stimulation at the elbow. †Median stimulation at the elbow yields larger CMAP at hypothenar muscles, FDI, or thenar muscles (or sometimes a combination of these) than stimulation at the wrist. Note: "Larger" and "smaller" generally mean amplitude difference ≥1.0 mV. |
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