Nerves of the Upper Limb & The Brachial Plexus


Summary

  1. Key anatomy of the brachial plexus
    • Nervous supply of the upper limb (shoulder, arm, and hand)
  2. Clinical relevance of key lesions

Supplied structures

  • The brachial plexus supplies all of the shoulder and upper limb except for:
    1. The trapezius (supplied by CNXI)
    2. Some skin on the shoulder (superficial cervical plexus)
    3. Posteromedial aspect of upper arm (the intercostobrachial nerve – a branch of the 2nd intercostal nerve)

Gross structure

      • There are 5 key anatomical parts to the brachial plexus described from proximal to distal: (Randy Travis Drinks Cold Beer)
        • 5 Roots  –
        • 3 Trunks
        • 6 Divisions
        • 3 Cords
        • 5 terminal Branches
        • Each of the five sections can usually be located using a landmark as described below (although there may be some variation)

      • Roots – These start in the spinal cord
        • Arise from C5-T1
        • Landmark – usually pass inferolaterally between the anterior and middle scalenes
      • Trunks – develop as the roots merge
        • 3 trunks to consider:
          • Superior/Upper (C5/C6)
          • Middle (C7)
          • Inferior/lower (C8/T1)
        • Landmark – usually found between scalenes and 1st rib
      • Divisions – develop as the trunks split again (and will form into cords)
        • 3 anterior divisions of each trunk
        • 3 posterior divisions of each trunk
        • Landmarkdivide around the 1st rib and pass under the clavicle
      • Cords
        • 3 cords – termed according to relation to the axillary artery
          • Posterior (made up of all 3 posterior divisions thus contains all C5-T1)
          • Lateral (anterior divisions of Superior and Middle cords – contains C5-C7)
          • Medial (anterior division of Inferior cord – contains C8-T1)
        • Landmarkrun alongside the axillary artery lateral to the clavicle
      • Branches – Mainly at the terminal cords (Remember AR MM U) but some occur earlier
        • Earlier branches (Landon Donovan  was 1st in Super Soccer )
          • Long thoracic nerve (C5-C7)- Innervates serratus anterior
          • Dorsal scapular nerve (C5)- innervates rhomboids and levator scapulae
          • 1st intercostal nerve (T1)
          • Nerve to Subclavius (C5-C6/upper trunk)
          • Suprascapular nerve (C5-C6/upper trunk) Innervates infraspinatus + supraspinatus

Cord Branches

Posterior cord

  • The posterior cord has 4 branches (STAR)
  1. Subscapular nervehas 2 main branches 
    • Upper branch – Supplies subscapularis
    • Lower branch – Supply subscapularis and teres major
  2. Thoracodorsal nerve – aka nerve to latissimus dorsi
  3. Axillary nerve
    • Course: passes posterior to the surgical neck of humerus
      • Therefore vulnerable to fractures found here
    • Supply:
      • Motordeltoid and teres minor and lateral shoulder
      • Sensory – skin over the lateral deltoid (via superior lateral cutaneous nerve of the arm)
        • Injury causes ‘regimental badge sign’ and failure to abduct
  4. Radial Nerve
    • Course:
      • Initially runs in the radial groove in posterior humerus
      • Then comes out anterior to the lateral epicondyle (where it bifurcates – see below)
    • Branches
      • Gives off 3 cutaneous branches in the upper arm
        • Posterior brachial cutaneous
        • Inferior lateral brachial cutaneous
        • Posterior antebrachial cutaneous
      • Splits into 2 branches at the lateral epicondyle
        • Superficial branch of radial nerve – cutaneous supply to skin of thumb and posterior hand
        • Deep branch of radial nerve – supplies extensor muscles of forearm
    • Supply:
      • Motor – triceps, brachioradialis, and wrist extensors
      • Sensory – posterior arm, forearm, and hand
    • Injury – 3 main mechanisms of injury
      • Fracture of body of humerus can cause the radial groove to displace and damage the nerve
      • Vulnerable to mid-shaft fractures
      • Axilla compression (e.g. crutches)
    • Injury results in wrist drop (failure to extend hand + wrist + elbow )

Lateral cord

  • The lateral cord has 3 branches
  1. Lateral pectoral nerve– supplies the common pectoral nerve (together with medial pectoral nerve – see below)
  2. Musculocutaneous nerve
    • Course: anterior part of upper arm
      • Passes through corachobrachialis near the axilla to come anterior to it
      • Continues into forearm as the lateral cutaneous nerve of the forearm
    • Supply:
      • Supplies corachobrachialis, brachialis and biceps brachii via branches – i.e. the elbow flexors
    • Injury
      • Also injured in surgical neck of humerus fractures
      • Injury results in failure of elbow flexion
  3. Median nerve (Part of this also comes from the medial cord)
    • Courseanterior to elbow (in antecubital fossa) and between the pronator teres muscle

    • Supply
      • Motor supply – lateral wrist flexors and LOAF muscles (Lumbricals 2/3, Opponens pollicis, abductor pollicus,
      • Sensory supply – lateral 3.5 fingers
    • Injury:
      • 3 main mechanisms
        • Carpal tunnel syndrome
        • Supracondylar humeral fracture
        • Lunate dislocation
      • Distal injury (at wrist) results in:
        • Loss of lateral hand sensation
        • Loss of thumb apposition + abduction + Thenar atrophy + results in:
        • Ape hand deformity – the thumb cannot be abducted or opposed
        • The unopposed adductor pollicis exacerbates this by adducting/externally rotating the thumb
          • This is the only muscle of the thumb innervated by ulnar n. so remains intact

      • Proximal injury (e.g. at the elbow) also causes other symptoms
        • Causes loss of lumbrical function (which  doesn’t happen with distal lesions)
        • Also causes loss of Flexor Digitus Profundus (to radial 2 fingers)
          • Patients cannot flex the index/middle fingers (at the MCPJ due to loss of lumbricals or at the PIPJ/DIPJ due to loss of (half of) FDP)
          • FDP remains in the ulnar 2 digits so they can flex
        • All of FDS is also lost
        • Extension is intact because the interossei (ulnar) and extensor digitorum (radial) are not affected
        • Therefore, a claw is seen on flexion
          • This leads to the hand of benediction
          • (NB this is better termed ‘pointing finger‘ because the middle finger can actually flex as it is connected to the adjacent ring finger FDP = Quadriga phenomenon)
            • Differentiate from the ulnar nerve injury as it occurs on flexion

Medial cord 

  • The medial cord has 2 terminal and 3 non-terminal branches
  • Non-terminal branches – these branch off the medial cord before it ends
    • Medial cutaneous nerve of the (upper) arm
    • Medial cutaneous nerve of the forearm
    • Medial pectoral nerve (joins lateral pectoral nerve => pectoral nerve)
  • Median nerve (see above)
  • Ulnar nerve
    • Course: runs medially and posterior to medial epicondyle
      • In the forearm, it runs between flexor carpi ulnaris tendon + ulnar artery
      • Passes over flexor retinaculum in the ulnar canal
    • 2 branches for the hand
      • Superficial branch
      • Deep branch
    • Supply:
      • Motorflexor carpi ulnaris, ulnar half of FDP, hypothenar eminence and PAD/DAB
      • Sensory – medial forearm and medial/ulnar 1.5 fingers (i.e. little + half of ring finger)
    • Injury:
      • Mechanisms:
        • Medial epicondyle fracture
        • Hook of Hamate fracture also causes failure of intrinsic hand muscles
      • Distal Injury results in 2 key muscle failures with 3 effects
        1. Failure of PAD/DAB muscles causes failure of abduction/adduction and interosseus wasting (in all fingers)
        2. Failure of the ulnar 2 lumbricals (little/ring)  causes weakness of MCPJ flexion
          • This also causes the ulnar two MCPJs to remain in extension due to unopposed long finger extensor action (extensor digitorum communis/extensor digiti minimi)
        3. Failure of both PAD/DAB and lumbricals together also causes  weakness of IPJ extension (where these muscles also act)
          • This causes IPJ flexion due to unopposed FDP activity
      • Ulnar claw hand develops as a complication of both unopposed IPJ flexion + MCPJ extension
        • Similar appearance to medial claw hand above but occurs on extension rather than flexion
      • Ulnar paradox – ‘a more proximal lesion (e.g. elbow) causes a less deformed ulnar claw’
        • This is because FDP function is also lost to the two fingers
          • Ulnar 2 IPJs (little/ring) are paralysed in full extension (as both FDP + Lumbrical loss means both DIPJ and MCPJ flexion is lost)
            • PIPJ is still normal because FDS is only innervated by median nerve.

Trunk lesions

  • 2 main trunk lesions to consider – split into lower and upper brachial plexus injuries
Erb Duchenne Palsy (Upper Trunk – C5-C6)
  • This is a lesion of the Upper trunk (C5/6) and is the most common (50% of all birth-related neuropraxias)
  • Mechanism
    • Occurs in traumatic birth as neck is flexed away from the shoulder
    • Especially affects posterior cord – axillary, suprascapular, and musculocutaneous nerves
      • Results in atrophy of deltoid, biceps and brachialis, and infraspinatus/teres minor
      • May also involve roots – loss of long thoracic + dorsal scapular nerves

    • Features4 main features
      1. Axillary nerve + musculocutaenous nerve loss leads to failure of the deltoid + corachobracialis muscles
        • This causes failure to abduct shoulder, resulting in an adducted shoulder
      2. MSc nerve loss causes loss of the anterior compartment resulting in:
        • Failure to flex elbow – extended elbow
        • Failure to supinate – Pronated forearm (musculocutaneous nerve – biceps brachii causes supination)
      3. Suprascapular + axillary nerve loss causes failure of  supraspinatus + infraspinatus + teres minor
        • Failure to externally rotate leads to a medially rotated arm
      4. + Winged scapula if long thoracic nerve is lost
  • This all culminates in the waiter’s tip sign
  • Either may be associated with Horner’s syndrome
    • Due to involvement of the stellate ganglion or sympathetic chain
  • Narakas classification can be used to classify Erb’s palsy based on the severity of paralysis

Klumpke’s Palsy (Lower Trunk – C8-T1)
    • This is much rarer than Erb’s palsy
    • Commonly affected in Pancoast tumour or ‘grabbing onto a branch’
    • Median  and ulnar nerves are most affected
      • Ulnar nerve is more severely affected
    • Features:
      • Complete hand paralysisloss of all Lumbricals
      • Loss of forearm flexors + extensors
      • Because intrinsic muscles are more affected, the patient may develop a ‘claw hand’ unopposed finger flexion + arm/wrist extension
      • Can also cause Horner’s Syndrome

Leave a Reply

Your email address will not be published. Required fields are marked *