Excerpt from Ulnar Neuropathy

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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.

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