Ultrasonographic evaluation of the median nerve: normal and variant anatomy and appearance

The median nerve arises from both medial and lateral cords of the brachial plexus, but the location of this merge can vary. It has been found that both cords can merge at the axillary fossa (60%), below the axillary fossa at the level of the pectoralis major muscle (39%), or at the level of medial part of the humeral shaft (1%)⁽¹⁾. The relation of the median nerve with the brachial artery in the arm has also been described. The median nerve enters the upper arm laterally to the brachial artery. In most cases (92%), at the level of insertion of the coracobrachialis muscle (anteriomedial surface of humerus distal to lesser tubercle) the median nerve crosses the brachial artery in front from the lateral to medial sides. In some cases, the median nerve can cross the brachial artery posteriorly⁽⁶⁾.

Variations at the elbow mainly impact the relationship between the median nerve and the pronator teres muscle. In a vast majority of cases (95%), the nerve passes between both heads of the pronator teres muscle. Rarely, the median nerve passes between the pronator teres muscle and the flexor digitorum profundus muscle, or goes through the humeral head of the muscle, or the brachialis muscle (Fig. 2). When Struthers’ ligament (an anatomical variation found in 1–3% of the population) is present, the median nerve passes behind it, and this may be a potential compression site⁽¹⁾. Regardless of the site where the median nerve is compressed, this condition is called pronator teres syndrome, since the compression itself often occurs between the heads of the pronator teres muscle. In the majority of people, both (humeral and ulnar) heads are present (86%), and the median nerve can course between the two heads of the pronator teres muscle (72%), posteriorly to the two heads (3%) or through the ulnar head of the pronator teres muscle (11%). In some cases (14%), the ulnar head is absent, in which case the median nerve courses through the humeral head (3%) or posteriorly to the humeral head of the pronator teres muscle (11%)⁽⁷⁾.

Fig. 2.

Median nerve variations at the level of the carpal tunnel might be the most widely described site mainly due to the high prevalence of carpal tunnel syndrome and surgical decompression therapy in this region. The Poisel classification regards the branching types of the recurrent branch of the median nerve (a.k.a. thenar motor branch):

Type I or extraligamentous (approximately 45% of all cases) where the recurrent branch divides distal to the flexor retinaculum in a retrograde manner.

Type II or subligamentous (about 30%) where the recurrent branch divides at the level of the carpal tunnel, runs under the flexor retinaculum, and reaches the thenar muscle.

Type III or transligamentous (about 25%) where the recurrent branch divides in the carpal tunnel and crosses the flexor retinaculum.

Some authors have also described a preligamentous form in which the recurrent branch divides proximal to the carpal tunnel and runs in front of the flexor retinaculum⁽⁸⁾. Furthermore, in a vast majority of cases the recurrent branch of the median nerve divides from the radial side of the nerve, but in about 2% of all cases it arises from the ulnar side⁽⁵⁾.

Three years later, in 1977, Lanz expanded the Poisel classification (Fig. 3):

Fig. 3.

Fig. 4.

Some authors propose to modify the Lanz classification by including trifurcation of the median nerve (Fig. 5) in the 3^rd group because even though bifurcation of the median nerve has shown not to be associated with carpal tunnel syndrome, theoretically an increase in cross-sectional volume of the nerve within the carpal tunnel might contribute to compression. Furthermore, there is a risk of injury to the median nerve if trifurcation goes undiagnosed⁽¹⁰⁾.

Fig. 5.

If clinically necessary, both arms should be assessed preoperatively because no one can surely assume the anatomical pathway of the median nerves between the arms, as only 72% of patients had an identical nerve course on both sides^(1,5).

In addition to various branching sites of the median nerve, many authors in anatomical research papers have observed communicating branches between the median nerve and the musculocutaneous nerve, emerging directly from the lateral cord of the brachial plexus in the axilla⁽¹¹⁾. The overall prevalence of the median nerve and musculocutaneous nerve branches is estimated to be about 30%, and anastomoses are bilateral in up to 14.1% of cases^(1,8). Choi et al. described three patterns of anastomosis between the median and musculocutaneous nerves⁽⁸⁾ (Fig. 6):

Fig. 6.

Another extensively studied anastomosis is between the median nerve and ulnar nerve a.k.a. the Martin–Gruber communicating branch (Fig. 7). Its incidence varies greatly depending on the study, from 7% to 40% (with a mean prevalence of 19.5%)^(1,12).

Fig. 7.

In most of these cases, the axons course in direction from the median nerve (or interosseous anterior nerve) proximally to the ulnar nerve distally. Usually, the Martin-Gruber anastomosis emerges 1 to 9 cm below the transepicondylar line⁽¹⁾ (Fig. 7).

Other (rarer) variations are possible, including:

Another communicating branch between the median nerve and ulnar nerve at the forearm is the Marinacci anastomosis (with estimated frequency of at least 0,7%), which in contrast to the Martin-Gruber anastomosis, goes in the opposite direction⁽¹⁾.

Riche´–Cannieu communicating branch is an anastomosis between the recurrent motor thenar branch of the median nerve and the deep branch of the ulnar nerve at the thenar eminence⁽¹³⁾. Some authors state the prevalence of the Riche´–Cannieu anastomosis at 55%-70%, though this anastomosis is not widely known and reported^(1,13).

The Berrettini communicating branch is an anastomosis between common digital nerves that arise from the ulnar nerve and median nerve in the palmar surface of the hand⁽¹³⁾.

A summary of the main anatomical interneural anastomoses of the median nerve is illustrated in Fig. 8.

Fig. 8.

The diagnostic information which can be obtained during sonography of peripheral nerves is to large extent influenced by the operator and the availability and technical considerations of state-of-the-art equipment⁽¹⁴⁾. For peripheral nerve sonography, the type of transducer depends on the anatomical region, but superficially lying median nerves should be scanned with a high-frequency, linear array transducer⁽¹⁵⁾. At sites where the surface does not allow adequate contact between the probe and skin, the best choice is a hockey-stick probe, however they are characterised by a relatively small field of view⁽⁴⁾. The visualisation of superficial structures with modern transducers is possible at frequencies of up to 17 MHz⁽¹⁴⁾. When imaging anatomical structures, the depth should be kept to a minimum, the focus values should be directed to the area of interest, and gains adjusted in order to differentiate the nerve fascicles⁽¹⁶⁾.

The nerves are depicted as well-circumscribed, roundish hypoechoic fascicles with a honeycomb-like pattern within the hyperechoic epineurium on transverse imaging⁽¹⁵⁾. The nerves are easier to detect when they are coursing among hypoechoic muscles than those among hyperechoic fat tissue, which is why a nerve of a physically active patient is better visualised than the same nerve examined in a patient with atrophic muscles⁽⁴⁾. Also, the nerves are compressible and alter their shape depending on the volume of the anatomical spaces within which they course, so when assessing nerve compression, it is advised to measure the cross-section area (CSA), and for the median nerve this is done at four sites: proximal border of the pronator quadratus muscle, proximal to the carpal tunnel inlet, at tunnel inlet, and at tunnel outlet (Fig. 1). Maximal CSA of 12 mm² or more at any site is diagnostic of carpal tunnel syndrome⁽²⁾. It is also good to remember that when the nerves cross tight passages, for example, neural foramina and osteofibrous tunnels, changes in echotexture can be detected, with a more homogeneous hypoechoic appearance due to the tighter packing of the fascicles and local reduction in the volume of the epineurium⁽⁴⁾.

It is preferable to examine the median nerve in transverse plane to be able to easily follow the nerve pathway throughout the limb. Proximally, the median nerve is located between the biceps brachii and brachialis muscles in the bicipital fossa, and it lies lateral to the brachial vein and artery^(4,15) (Fig. 9).

Fig. 9.

Important landmarks that a specialist should look out for at the elbow include the brachial artery, pronator teres muscle, and ulnar artery. At the elbow, the median nerve courses through the Lacertus tunnel, where the median nerve lies medial to the brachial artery (which branches into the radial and ulnar arteries), and between the humeral and ulnar heads of the pronator teres muscle, more lateral the distal part of biceps brachii can be visualised (“BAM” view)^(1,4,15).

The landmark ulnar artery is seen deep to the ulnar head of the pronator teres. The median nerve runs along the medial aspect of the ulnar artery in the forearm. In the middle part of the forearm, the median nerve moves distally to the ulnar artery and travels between the flexor digitorum profundus muscle and the flexor digitorum superficialis^(2,4) (Fig. 10).

Fig. 10.

At the wrist, the median nerve is superficially located, just deep to the palmaris longus tendon and flexor retinaculum, lateral to the FDS tendons, and medial to the FCR (Fig. 11).

Fig. 11.

At the wrist level, the anisotropy of the tendons surrounding the median nerve can be noted. In contrast to the nerve, the tendons will be hypoechoic and hyperechoic, and the median nerve will remain hypoechoic and fascicular⁽⁴⁾ (Fig. 11). In the carpal tunnel, the median nerve is superficial and radial to the deep and superficial flexor digitorum, and superficial and ulnar to the tendon of the flexor pollicis longus muscle⁽²⁾ (Fig. 11).