Anatomy

The median nerve arises from the union of fibers from the lateral and medial fascicles of the plexus brachialis, most commonly from roots C6–T1. It is the main nerve of the anterior muscle group of the forearm and a significant nerve of the hand. Cross-sectional anatomy is key to understanding the sonographic appearance, as ultrasound displays structures in individual slices. Below we will review five key levels of cross-sectional imaging that we will follow along the nerve.

Overview of key cross sections
Overview of key cross sections

Axilla

In the axilla, the n. medianus is formed by the union of the radix lateralis from the fasciculus lateralis and the radix medialis from the fasciculus medialis of the plexus brachialis. On ultrasound examination, it is usually well recognizable as the most anteriorly located nerve relative to the axillary artery.

Axilla
Figure 1: Cross-section of the axilla. D - m. deltoideus, PM - m. pectoralis major, LHBB - caput longum m. biceps brachii, SHBB - caput breve m. biceps brachii, CB - m. coracobrachialis, mc - n. musculocutaneus, M - n. medianus, U - n. ulnaris, R - n. radialis, Tmaj - m. teres major, TLa - caput laterale m. triceps brachii, TLo - caput longum m. triceps brachii, H - humerus

Figure 1: Cross-section of the axilla. D - m. deltoideus, PM - m. pectoralis major, LHBB - caput longum m. biceps brachii, SHBB - caput breve m. biceps brachii, CB - m. coracobrachialis, mc - n. musculocutaneus, M - n. medianus, U - n. ulnaris, R - n. radialis, Tmaj - m. teres major, TLa - caput laterale m. triceps brachii, TLo - caput longum m. triceps brachii, H - humerus

Arm

In the arm, it runs in the neurovascular bundle together with the a. brachialis. Proximally it is more often lateral to the artery, distally it crosses to its medial side. It lies between the m. biceps brachii and m. triceps brachii. For ultrasound, it is crucial: the n. medianus is a typical satellite of the a. brachialis in the brachium.

Arm
Figure 2: Cross-section of the arm. B – m. biceps brachii, Br – m. brachialis, CB – m. coracobrachialis, H – humerus, TLa – caput laterale m. triceps brachii, TM – caput mediale m. triceps brachii, TLo – caput longum m. triceps brachii, R – n. radialis, M – n. medianus, U – n. ulnaris

Figure 2: Cross-section of the arm. B – m. biceps brachii, Br – m. brachialis, CB – m. coracobrachialis, H – humerus, TLa – caput laterale m. triceps brachii, TM – caput mediale m. triceps brachii, TLo – caput longum m. triceps brachii, R – n. radialis, M – n. medianus, U – n. ulnaris

Elbow

In the cubital fossa, the n. medianus lies medial to the a. brachialis and beneath the lacertus fibrosus. In this region, the orientational scheme BAM applies well: biceps tendon – brachial artery – median nerve. Distally, it continues between the humeral and ulnar heads of the m. pronator teres, which is one of the common sites of possible compression.

Elbow
Figure 3: Cross-section of the elbow. Br – m. brachioradialis, ECRL – m. extensor carpi radialis longus, ECRB – m. extensor carpi radialis brevis, B – tendon of m. biceps brachii, S – r. superficialis n. radialis, P – r. profundus n. radialis (n. interosseus posterior), M – n. medianus, PT – m. pronator teres, U – n. ulnaris, FCU – m. flexor carpi ulnaris, A – m. anconeus

Figure 3: Cross-section of the elbow. Br – m. brachioradialis, ECRL – m. extensor carpi radialis longus, ECRB – m. extensor carpi radialis brevis, B – tendon of m. biceps brachii, S – r. superficialis n. radialis, P – r. profundus n. radialis (n. interosseus posterior), M – n. medianus, PT – m. pronator teres, U – n. ulnaris, FCU – m. flexor carpi ulnaris, A – m. anconeus

Forearm

In the forearm, it first runs under the arch of the m. flexor digitorum superficialis and then between the superficial and deep flexor layers. In the middle part of the forearm, it is mainly related to the FDS, FDP, and FPL. Distally, it becomes more superficial and lies close to the tendon of the m. flexor carpi radialis.

Important branches:

  • n. interosseus anterior – arises in the proximal forearm and runs deeper along the interosseous membrane

  • ramus cutaneus palmaris – arises proximal to the carpal tunnel and travels superficially to the palm

Forearm
Figure 4: Cross-section of the forearm. Br – m. brachioradialis, ECRL – m. extensor carpi radialis longus, ECRB – m. extensor carpi radialis brevis, EDC – m. extensor digitorum communis, EDM – m. extensor digiti minimi, ECU – m. extensor carpi ulnaris, AbPL – m. abductor pollicis longus, EPB – m. extensor pollicis brevis, EPL – m. extensor pollicis longus, FCR – m. flexor carpi radialis, PL – m. palmaris longus, FDS – m. flexor digitorum superficialis, FPL – m. flexor pollicis longus, FDP – m. flexor digitorum profundus, FCU – m. flexor carpi ulnaris, M – n. medianus, U (nerve) – n. ulnaris, Rs – r. superficialis n. radialis, Rp – r. profundus n. radialis (n. interosseus posterior), AIN – n. interosseus anterior, R – radius, U (bone) – ulna

Figure 4: Cross-section of the forearm. Br – m. brachioradialis, ECRL – m. extensor carpi radialis longus, ECRB – m. extensor carpi radialis brevis, EDC – m. extensor digitorum communis, EDM – m. extensor digiti minimi, ECU – m. extensor carpi ulnaris, AbPL – m. abductor pollicis longus, EPB – m. extensor pollicis brevis, EPL – m. extensor pollicis longus, FCR – m. flexor carpi radialis, PL – m. palmaris longus, FDS – m. flexor digitorum superficialis, FPL – m. flexor pollicis longus, FDP – m. flexor digitorum profundus, FCU – m. flexor carpi ulnaris, M – n. medianus, U (nerve) – n. ulnaris, Rs – r. superficialis n. radialis, Rp – r. profundus n. radialis (n. interosseus posterior), AIN – n. interosseus anterior, R – radius, U (bone) – ulna

Wrist

In the wrist region, the n. medianus enters the carpal tunnel beneath the flexor retinaculum. This is its most important entrapment location.

Wrist
Figure 5: Cross-section of the wrist. FCR – m. flexor carpi radialis, M – n. medianus, U – n. ulnaris, P – os pisiforme, H – os hamatum, C – os capitatum, S – os scaphoideum

Figure 5: Cross-section of the wrist. FCR – m. flexor carpi radialis, M – n. medianus, U – n. ulnaris, P – os pisiforme, H – os hamatum, C – os capitatum, S – os scaphoideum

Practical anatomical landmarks for US

  • in axilla: anterior to a. axillaris

  • in arm: adjacent to a. brachialis

  • in cubital fossa: BAM, beneath lacertus fibrosus

  • distal to elbow: between heads of m. pronator teres

  • in forearm: under the arch of FDS

  • in wrist: in carpal tunnel beneath retinaculum

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