Nerves of the Upper Limb & The Brachial Plexus

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Summary

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

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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)

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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
      • Landmark – divide 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)
      • Landmark – run 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

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Cord Branches

Posterior cord

• The posterior cord has 4 branches (STAR)

1. Subscapular nerve – has 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:
     • Motor – deltoid 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 )

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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)
   • Course – anterior to elbow (in antecubital fossa) and between the pronator teres muscle

1. • 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

1. • • 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

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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

1. • Supply:
     • Motor – flexor 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.

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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

• • Features – 4 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 paralysis – loss 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