AUTHOR AND EDITOR INFORMATION

Section 1 of 12  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• Acknowledgments
• Multimedia
• References

INTRODUCTION

Section 2 of 12  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• Acknowledgments
• Multimedia
• References

Background

The ulnar nerve is an extension of the medial cord of the brachial plexus. This is a mixed nerve that supplies innervation to muscles in the forearm and hand and provides sensation over the medial half of the fourth and the entire fifth digit of the hand, the ulnar part of the palm, and the ulnar portion of the posterior aspect of the hand (dorsal ulnar cutaneous distribution). The most common site of entrapment is at or near the elbow region, especially in either the region of the cubital tunnel or the ulnar groove. The second most likely location of entrapment is at or near the wrist, especially in the area of the anatomic structure called Guyon canal. However, entrapment can occur in the forearm between these 2 regions, below the wrist within the hand, or above the elbow.

As diagnostic and surgical methods have evolved over the past 100 plus years, our ability to recognize and describe the sites of entrapment have improved. However, the terminology has become confusing because not all clinicians use the terms in the same way.

Let us first look at ulnar entrapments in the elbow region, the most common location. The 2 most commonly used (and misused) terms for such entrapments are tardy ulnar palsy and cubital tunnel syndrome.

Panas first described what we now often call tardy ulnar palsy in 1878. He presented 3 cases in which either prior trauma or osteoarthritis gradually caused damage to the ulnar nerve. The basic idea behind using the word tardy was that the problem appeared late after an injury or a long course of osteoarthritis (possibly together with an old injury) as opposed to a more immediate or early palsy in which the ulnar nerve showed dysfunction directly after trauma, such as what might occur in an injury that caused either total or partial transection. Subsequent to Panas' paper, other case reports appeared. John Murphy published the first case in American literature in 1914. The initial cases of tardy ulnar palsy were usually associated with trauma (eg, fractures in the region of the elbow), and the typical site of nerve entrapment was the ulnar groove, ie, the location between the medial epicondyle of the humerus and the olecranon. So in addition to a time-based definition (ie, tardy=appears some years after trauma), an anatomical aspect of the term came to pass (ie, tardy=usually in or very near the ulnar groove).

Later, physicians began to recognize ulnar entrapments in the humeroulnar arcade (HUA). This is the region of the aponeurosis of the 2 heads of the flexor carpi ulnaris (FCU) muscle. The aponeurosis is a fibrous or membranous sheet that connects muscles to bones or other structures that the muscles move. The aponeurosis can be thought of as a flattened tendon. The first description of an ulnar nerve entrapped in this region, together with its surgical decompression, was given by Buzzard and Sargent in 1922. The next published description was by Osborne in 1957. In 1958, Feindel and Stratford reported 3 more such cases and coined the term cubital tunnel syndrome to describe the effects of the ulnar nerve entrapment at the HUA. Numerous other reports then followed.

Our current state of knowledge is still incomplete, but now we can identify approximately 5 (or 4 or 6) sites in the elbow region at which the ulnar nerve is most likely to be compressed. The word approximately is used deliberately, because some of the sites are so close together that certain authorities lump and split them differently to get a different number. This article principally follows the classification of Posner³¹, with some comments about the classification of other authors. The sites, according to Posner, are as follows:

1. Above the elbow in the region of the intermuscular septum
   
   
   • Chapman⁴² divides this into 2 regions—the arcade of Struthers and the medial intermuscular septum. Via the standard anatomic definition, the arcade of Struthers is a thin fibrous band that usually extends from the medial head of triceps to the medial intermuscular septum. It is often said to be about 6-10 cm proximal to the medial epicondyle.
     
     
   • Considerable anatomic variation exists and, in fact, there is outright controversy about the arcade of Struthers. One controversy is trivial. No evidence exists that Dr. Struthers discovered this structure or even knew about it. His name was attached to it by Kane et al¹⁵ in their 1973 paper. A recent autopsy study by Siqueira of 60 upper limbs found a structure reasonably approximating the definition given above in 8 limbs (13.5%).³⁵ Ulnar nerve entrapment occurred in none of them (but there was no reason to clinically expect that there might have been). Bartels et al could not find this structure in their dissections and they doubt that it exists.³ Wehrli and Oberlin have described a different structure in the same region, the internal brachial ligament rather than the arcade of Struthers, which might be involved in ulnar entrapment in some cases.⁴⁰ Interestingly, Struthers did describe the existence of this structure, but not in relation to ulnar nerve entrapment. Wehrli and Oberlin advocate "cancelling the concept of the arcade of Struthers." In contrast, von Schroeder and Scheker find yet another structure, a fibrous tunnel in roughly the same region.⁴¹ They say that the ulnar nerve goes through this tunnel and could be trapped therein. Rather than cancelling the arcade of Struthers, they propose to call their structure the arcade of Struthers. Settling this controversy is beyond the scope of this article. Suffice it to say that in rare cases, the ulnar nerve is compressed considerably above the ulnar groove and that surgeons may find it entrapped in a fibrous/ligamentous structure that may correspond to one of the terms mentioned above.

2. Medial epicondylar region: Ulnar compression in this region is generally from a valgus deformity of the bone. If a patient is placed in standard anatomical position with palms rotated toward front, thumb away from midline, valgus deformity means the elbow would be deformed away from midline of the body.

3. Epicondylar groove: This is the same as the ulnar groove. It is a bit distal to the medial epicondyle (or at least to the beginning of it).
   • Using slightly different terminology, Campbell lumps the medial epicondylar region and the epicondylar groove together as the area of the retrocondylar groove.
     
     
   • Chapman considers the medial epicondylar region and the epicondylar groove to be the area of the medial epicondyle.
     
     
   • Both the medial epicondylar region and the epicondylar groove are generally considered to be the classical location (or locations) for the tardy ulnar palsy.
     
     
   • In the author's personal experience, electromyographers and orthopedic surgeons more commonly refer to a tardy ulnar palsy at the retrocondylar groove, thus using the Campbell terminology.
     
     
4. The region of the cubital tunnel: The main source of compression is a thickening of the Osborne ligament.
   
   
   • Campbell's classification is basically the same for this region, except he no longer uses the term cubital tunnel. He refers to this as the region of the HUA, apparently because he believes so many clinicians now use the term cubital tunnel too loosely to refer to virtually anywhere in the elbow.
     
     
   • Chapman divides this region into 2 parts—the cubital tunnel and Osborne fascia. This is a good example of the difficulty with the terminology. Different terms are used for locations that are virtually the same. For all practical purposes, certainly for anything one can distinguish on EMG, Osborne ligament=Osborne fascia=the HUA.
     
     
   • The cubital tunnel is the space bounded by the following:
     
     
     • The medial epicondyle (medial border)
       
       
     • The olecranon (lateral border)
       
       
     • The elbow capsule at the posterior aspect of ulnar collateral ligament (floor)
       
       
     • The humeroulnar arcade (Osborne fascia or ligament) (roof)
5. The region at which the ulnar nerve exits from the FCU at which the usual cause of compression is the deep flexor-pronator aponeurosis. Campbell and Chapman also list this as the final site at the elbow.

   
   
   This is a schematic diagram of the elbow region. The 5 main sites as given by Posner are labeled 1-5. Other sites and structures are also named. The main regions of interest are circled with pastel colored arrows. Sites 2 and 3 are close together and distinguishing them by EMG and nerve conduction studies is not possible. The term ulnar groove or retrocondylar groove are used to describe this location.
   

After the ulnar nerve passes distal to the elbow, it makes several important divisions. The first branches to come off are those that go to the FCU. Further distally, the branches to the flexor digitorum profundus muscles of digits 4 and 5 arise.

As the ulnar nerve courses down the forearm toward the wrist, the dorsal ulnar cutaneous nerve leaves the main branch. A little further down, the palmar cutaneous branch takes off. Thus, neither of these 2 branches go through Guyon canal. The remainder of the ulnar nerve does enter Guyon canal at the proximal portion of the wrist. This is bounded proximally and distally by the pisiform bone and the hook of hamate bone. It is covered by the volar carpal ligament and the palmaris brevis muscle. Although the nerve could be injured or entrapped at any point along its course, the 4 most common locations in relation to Guyon canal are shown in the following image.



This diagram shows the ulnar nerve distal to the elbow region. The dorsal ulnar cutaneous nerve (lavender) branches off the main trunk (blue). Although the course is not followed in detail after that, the lavender region on the sensory dermatome diagram shows where this sensory nerve innervates the 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 a branch to the palmaris brevis muscle. The deep terminal branch has no corresponding skin area because it is solely motor innervating the muscles shown, as well as some others not explicitly depicted. Of course the nerve could be pinched or injured anywhere, but the sites listed with Roman numerals I-IV are the relatively common sites.

Pathophysiology

The nerve, axon, and myelin can be affected. Within the axon, fascicles to individual muscles may be involved selectively. Axonal involvement leads to motor unit loss and amplitude/area reduction. Conduction block implies impaired transmission through a segment of nerve. In the absence of changes indicating axonal damage, conduction block implies myelin damage to the involved segment. Significant slowing of conduction and/or significant spreading out of the temporal profile of the recorded response (ie, abnormal temporal dispersion) with preserved axonal integrity suggests demyelination.

Frequency

United States

In the general population, abnormalities in the ulnar nerve at the elbow in asymptomatic subjects are common (about 40%).

The elbow is the second most common site of nerve entrapment in the upper extremity, the first being the wrist (ie, carpal tunnel syndrome).

Mortality/Morbidity

Delayed recognition of ulnar neuropathy at the elbow or wrist or unsuccessful surgical intervention can lead to loss of function due to prolonged axonal degeneration.

Sex

No gross anatomical differences in the course of the nerve are noted between the sexes. However, the following have been noted. 

• Men develop perioperative ulnar neuropathies at the elbow more frequently than women.


• Women have more fat content in the medial elbow overlying the tubercle of the ulnar coronoid process (2-19 times more).


• The tubercle of the coronoid process is 1.5 times larger in men.

CLINICAL

Section 3 of 12  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• Acknowledgments
• Multimedia
• References

History

Both the onset and progress of the symptoms can be variable. Although the answer is frequently negative, tone should ask specifically about trauma and pressure to the arm and wrist, especially the elbow, the medial side of the wrist, and other sites close to the course of the ulnar nerve.

• Many patients complain of sensory changes in the fourth and fifth digits. Rarely, a patient actually notices that the unusual sensations are mainly in the medial side of the ring finger (fourth digit) rather than the lateral side, corresponding to the textbook sensory distribution. Sometimes the third digit is also involved, especially on the ulnar (ie, medial) side. The sensory changes can be a feeling of numbness or a tingling or burning. Pain rarely occurs in the hand. Complaints of pain tend to be more common in the arm, up to and including the elbow area. Indeed, the elbow is probably the most common site of pain in an ulnar neuropathy. Occasionally, patients specifically say “I have pain in my elbow,” “I have pain in my funny bone,” or even “I have pain in this little groove in my elbow,” but usually they are not quite so explicit unless prompted. Patients rarely notice specific muscle atrophy.
• Weakness may also be a presenting complaint, but the complaint may be expressed in subtle ways.
• • One traditional sign of ulnar neuropathy, Wartenberg sign, is actually a complaint of weakness. The patient complains that the little finger gets caught on the edge of the pants pocket when he put his hand into the pocket (usually it's a male who has such pockets). At first, that complaint seems surprising because most physicians remember that finger abduction is governed by the ulnar nerve. So the physician might think that with an ulnar neuropathy, the patient would have less tendency to have the little finger abducted and thus caught on the edge of the pocket. But adduction is also ulnar. In particular, the patient cannot pull the fifth digit tightly against the fourth because of weakness of the ulnar innervated third palmar interosseus muscle.
    
    In addition, the muscle that extends the fifth digit at the metacarpal phalangeal joint is radially innervate and it inserts on the ulnar side of the joint. Normally this muscle is opposed by ulnar innervated muscles that flex the joints. But with an ulnar neuropathy, the muscle is relatively unopposed so it pulls the finger up and to the ulnar side. This is the perfect position to catch onto the edge of the pocket.
  • The patient also may express the complaint of weakness by saying “my grip is weak.” Many of the grip muscles are ulnar. Also, when someone tries to grip powerfully, the hand usually deviates in the ulnar direction under the influence of the flexor carpi ulnaris. If this ulnar deviation is impaired, the grip mechanism does not work optimally even for the muscles that are unimpaired.
  • Sometimes a patient notices that his pincer grip (pinching with the thumb and index finger) is weak. Two of the key muscles involved in this movement are the adductor pollicis (which adducts the thumb) and first dorsal interosseus, which adducts the index finger. Not only may the pincer grip be weak in an ulnar neuropathy, the median innervated flexor pollicis longus partially compensates for the weakened adductor pollicis and the thumb flexes at the distal joint. Usually a patient does not notice the thumb flexion, but when demonstrated by the examiner, this flexion is considered to be Froment sign.

Physical

On physical examination, numerous findings offer clues to the existence of ulnar compression. 

• In addition to assessing sensation and testing individual muscle strength, inspection of the hand may reveal a clawed posture (called main en griffe in French).
  
  
  • Several factors contribute to the clawed appearance. Wasting of the intrinsic muscles of the hand make it look bonier. The fourth and fifth digits extend at the metacarpal phalangeal joint because the extensors at that joint are radially innervated, whereas the flexors are innervated by the ulnar. Also, the fifth digit deviates slightly in the medial direction because, as explained for Wartenberg sign, the muscle that extends the fifth digit at the metacarpal phalangeal joint is radially innervated and it inserts on the ulnar side of the joint. The fourth and fifth interphalangeal joints flex because for them the extensor muscles are also ulnar and the natural tension of the muscles and tendons in the absence of strong muscle activity in either direction leads to flexion. The first 3 digits are extended at both the metacarpalphalangeal joints and the interphalangeal joints because of the unopposed radial nerve innervation. All these factors make the hand look somewhat like a claw.
  • A different interpretation of the posture is that it looks like the hand gesture that a Catholic priest makes in the process of conferring a blessing, and thus it is sometimes called the “benediction sign” or the “benediction hand.”
• Froment sign is an observable sign that correlates with the complaint of weakness of the ability to pinch normal and strongly between the first and second digits.
  
  
  • This sign is sometimes elicited by asking the patient to grasp a piece of paper between the thumb and index finger. Ordinarily, the grasp is tight and the patient makes heavy use of the adductor pollicis to adduct the thumb and the first dorsal interosseus to move the index finger.
  • In addition to overt weakness of the pinch, the examiner also notes that the thumb flexes at the interphalangeal joint because the flexor pollicis longus activates in an attempt to compensate for the weakness. Thus, in addition to the weakness, the examiner sees the flexion of the tip of the thumb.
• Ulnar neuropathy at the elbow
  
  
  • Positive Tinel sign at the elbow: The examiner taps with a reflex hammer over the ulnar nerve in the ulnar groove and a little further distal over the cubital tunnel. The test is positive if the patient experiences definite paresthesias in the ulnar portion of the hand, especially the last 2 digits. This test is not considered highly sensitive, but it is considered to be quite specific if performed properly (eg, not hit too hard). If the examiner hits hard enough, many normal individuals experience paresthesias in the fourth and fifth digits. Assuming the complaint is unilateral, the opposite side is a good control for this. Sometimes palpating the nerve in the ulnar groove may produce a similar result.
  • Atrophy and muscle weakness: The most important ulnar hand muscles to test are the first dorsal interosseous and the abductor digiti minimi (abductor digiti quinti). In the forearm, the flexor digitorum profundus of the fourth and fifth digits (which flexes the distal phalanges of those fingers) and the flexor carpi ulnaris (flexion at the wrist in the ulnar direction) are valuable to examine. Of these latter 2 muscles, it is not uncommon for the flexor carpi ulnaris to be spared in ulnar lesions near the elbow, especially the lower (more distal) lesion near the elbow. Sparing occurs because the branch to the flexor carpi ulnaris splits off from the main trunk prior to (ie, above or proximal to) the compression.
    
    The ulnar muscles should not be examined in isolation from other muscles. In particular, several key muscles with C8/T1, lower trunk, medial cord innervation should be examined, especially the abductor pollicis brevis (a thenar muscle typically involved with carpal tunnel syndrome, the major compressive median nerve neuropathy) and the median innervated long thumb and index finger flexors.
    
    If both the ulnar intrinsics hand muscles and the ulnar forearm muscles are involved, then an ulnar nerve lesion should be suspected in the region of the elbow (or, very rarely, above the elbow region). If the ulnar forearm muscles are spared, considering the possibility of a lesion at the wrist is reasonable, but extra caution is warranted in this case. Sometimes the forearm muscles are spared with a lesion near the elbow, especially if the lesion is in the lower elbow region in or around the cubital tunnel. Even for higher elbow lesions, there can be considerable selectivity in which muscles are affected because the ulnar nerve is organized into a number of separate fascicles. Sometimes some fascicles are severely affected by whatever is pinching the nerve and other fascicles are unaffected. If other C8/T1, lower trunk, medial cord muscles are affected, a C8/T1 radiculopathy or a brachial plexus lesion may be the cause.

• Ulnar neuropathy at or distal to the wrist

  • Weakness of the interossei and hypothenar muscles only with no sensory loss: This would most likely be due to compression of the deep motor branch in the hand after it had separated from the superficial terminal sensory branch but before the branch to the hypothenar muscles had taken off.
  • Interosseus weakness only with no sensory loss: This would most likely be due to compression of the deep motor branch after the branch to the hypothenar muscles has taken off.
  • Weakness of the interossei and hypothenar muscles with sensory involvement in the fifth digit: This would suggest involvement in Guyon canal with compression of both the deep motor branch and the superficial terminal sensory branch. This might be said to be the typical or classical Guyon canal pattern.
  • Pure sensory loss with normal dorsal ulnar cutaneous sensory nerve, normal palmar cutaneous sensory nerve, and normal motor responses: This would imply injury to the superficial terminal sensory branch alone, probably a compression distal to Guyon canal.
  • Interossei weakness and sensory loss with preserved function in the hypothenar and dorsal ulnar cutaneous territories: This would imply a compression of the deep motor branch and the superficial terminal sensory branch distal to the point where the sub-branch to the hypothenar area (eg, the ADM) had split off the deep motor branch.

Sensory examination

• Adding information from the sensory examination to that of the motor examination helps to localize the ulnar lesion. The image below, which has been discussed earlier in the context of the anatomy of the ulnar nerve, shows the ulnar sensory regions on the hand.
  
  

  
  
  
  This diagram shows the ulnar nerve distal to the elbow region. The dorsal ulnar cutaneous nerve (lavender) branches off the main trunk (blue). Although the course is not followed in detail after that, the lavender region on the sensory dermatome diagram shows where this sensory nerve innervates the 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 a branch to the palmaris brevis muscle. The deep terminal branch has no corresponding skin area because it is solely motor innervating the muscles shown, as well as some others not explicitly depicted. Of course the nerve could be pinched or injured anywhere, but the sites listed with Roman numerals I-IV are the relatively common sites.

• Although the area of the palmar cutaneous sensory nerve can extend a bit more proximal than shown, if the sensory involvement extends more than an inch above the wrist crease along the medial aspect of the forearm, the nerve roots (C8/T1) or brachial plexus most likely are involved (but in some cases this could be in addition to an ulnar injury).
• As previously noted, both the palmar cutaneous sensory branch of the ulnar and the dorsal ulnar cutaneous branch come off of the main ulnar branch above (proximal to) the wrist. Thus, a lesion exclusively at the wrist (Guyon canal) would miss these branches and the superficial terminal branch would be the only sensory involvement. However, a physician must be cautious in interpretation. Typically, neuropathic damage, whether generalized or related to nerve compression, affects (or is perceived to affect) the most distal parts of the nerves preferentially. A compression at Guyon canal might be perceived by the patient and might be detectable on examination only in the tips of the fingers. Thus, the compression would appear to be affecting only the superficial terminal branch.

Causes

• Ulnar nerve at or near the elbow
• • Compression during general anesthesia
  • Blunt trauma
  • Deformities (eg, rheumatoid arthritis)
  • Metabolic derangements (eg, diabetes)
  • Transient occlusion of brachial artery during surgery
  • Subdermal contraceptive implant
  • Venipuncture
  • Hemophilia leading to hematomas
  • Malnutrition leading to muscle atrophy and loss of fatty protection across the elbow and other joints

  • Cigarette smoking

• Ulnar neuropathy at or distal to the wrist (ie, at Guyon canal)
• • Ganglionic cysts
  • Tumors
  • Blunt injuries with or without fracture
  • Aberrant artery
  • Idiopathic

DIFFERENTIALS

Section 4 of 12  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• Acknowledgments
• Multimedia
• References

Other Problems to be Considered

Brachial plexopathy
Cervical radiculopathy
Peripheral polyneuropathy
Cervical disk syndromes

WORKUP

Section 5 of 12  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• Acknowledgments
• Multimedia
• References

Lab 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 - To detect cysts in the Guyon canal and to assess ulnar nerve diameter at the elbow

• Magnetic resonance imaging - To detect structural abnormalities along the course of ulnar nerve accounting for compression (eg, fibrous bands)

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 interosseous (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, 1991.⁹ 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 anomalous innervation pattern occurring between the median and ulnar nerves in the forearm.

• 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.
  
  
• 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.
  
  
• 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

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. 
    

Riche-Cannieu anastomosis

Another anomaly that can complicate diagnostic studies is the Riche-Cannieu anastomosis. 

TREATMENT

Section 6 of 12  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• Acknowledgments
• Multimedia
• References

Medical Care

• Vasculitic and metabolic evaluation with treatment of underlying condition
• Trials of various classes of pain medications in cases of significant pain or sensory symptoms
• • Nonsteroidal anti-inflammatory drugs (NSAIDs, many classes)
  • Tricyclic antidepressants
  • Anticonvulsants

• Occupational therapy and work hardening programs with design of splints or cushions

Surgical Care

• Surgical exploration in patients with symptoms of intractable and/or if progressive weakness/atrophy, with resection of masses/cysts or sectioning of fibrous bands


• Specific surgical techniques such as medial epicondylectomy, simple release of the flexor carpi ulnaris aponeurosis, and anterior transposition of the nerve


• Correction or stabilization of obvious trauma sites


• Much more detail on the surgical approaches to these problems may be found in Ulnar Nerve Entrapment and Cubital Tunnel Syndrome.

Consultations

• Depending upon etiology, symptoms, and signs, referral to a neurosurgeon, hand surgeon, pain specialist, internist, physiatrist, rheumatologist, occupational therapist, and/or alternative medicine specialist may be appropriate.

MEDICATION

Section 7 of 12  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• Acknowledgments
• Multimedia
• References

The goals of pharmacotherapy are to reduce morbidity and prevent complications.

Drug Category: Tricyclic antidepressants (TCA)

TCAs are effective in painful paresthesias. While the dosages are similar, the drugs in this category vary in their sedative properties. Amitriptyline can be used if the patient suffers from insomnia, while nortriptyline and desipramine are better choices when sedation becomes a problem.

Drug Category: Anti-arrhythmic/local anesthetics

Mexiletine, which has been used in various forms as an antiarrhythmic and local anesthetic, tends to blunt some of the stinging and burning of neuropathic pain in some patients.

Drug Category: Alpha-adrenergic agonists

These agents stimulate alpha2-adrenoreceptors in brain stem and activates an inhibitory neuron.

Drug Category: Narcotics

Although traditionally narcotics have been avoided in peripheral neuropathies, they are useful in many cases.

Drug Category: Anticonvulsants

Many anticonvulsants are used to alleviate painful dysesthesias, which frequently accompany peripheral neuropathies. Although they have many different mechanisms of action, their use to alleviate neuropathic pain probably depends on the fact that they generally tend to reduce neuronal excitability.

FOLLOW-UP

Section 8 of 12  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• Acknowledgments
• Multimedia
• References

Prognosis

• A motor amplitude of 10% of normal or a greatly reduced recruitment of motor units indicates a low likelihood of significant or full recovery.
• Typically, nerves regenerate at a rate of 1 mm/d.
• In some cases, regeneration is accompanied by pain and paresthesias, which are thought to be secondary to random ectopic impulse generation of affected nerves.
• A diameter greater than 3.5 mm on the initial sonogram of the ulnar nerve at the elbow is associated with persistent symptoms or signs regardless of treatment (ie, conservative treatment or surgical treatment).
• The outcome does not correlate with clinical features at baseline or with the duration of symptoms prior to treatment.
• The presence of motor conduction velocity slowing or pure conduction block across the elbow signifies a favorable outcome; these are considered independent prognostic factors.
• A favorable surgical outcome is more likely for sensory function than for motor function; however, overall, a favorable outcome occurs in 85-95% of cases.
• Unfavorable or poor surgical outcome is characterized by the following:
• • Age older than 50 years
  • Coexisting diabetes or other causes of peripheral polyneuropathy

  • Atrophy and ongoing denervation of ulnar-derived muscles

  • Absent ulnar sensory responses

  • Postoperative position of the ulnar nerve in relation to the medial epicondyle

MISCELLANEOUS

Section 9 of 12  

• Authors and Editors
• Introduction
• <