Brachial Plexus and Upper Extremity Nerves

• Root level (C5-T1): Five nerve roots emerge from the intervertebral foramina; C5 and C6 unite to form the upper trunk, C7 forms the middle trunk, and C8 and T1 unite to form the lower trunk. Proximal injuries at this level cause root avulsion or stretch injuries with severe, permanent neurological deficit.

• Trunk level divisions and cords: Each trunk divides into anterior and posterior divisions; the three posterior divisions unite to form the posterior cord, anterior divisions of upper and middle trunks form the lateral cord, and the anterior division of lower trunk becomes the medial cord. Lesions here affect multiple nerve territories in specific patterns.

• Cord to terminal nerve branching: The three cords divide into six terminal nerves: musculocutaneous (from lateral cord), axillary and radial (from posterior cord), and ulnar, median, and medial antebrachial cutaneous (from medial and lateral cords). Terminal nerve injuries cause circumscribed motor and sensory deficits in single nerve distributions.

• Mechanism of injury: Traction injuries stretch and tear nerve fibers; compression injuries from trauma, tumors, or anatomical variants cause ischemia and demyelination; and penetrating injuries directly sever nerve tissue. The degree of injury ranges from neurapraxia (reversible conduction block) to axonotmesis (axonal loss with intact connective tissue) to neurotmesis (complete transection).

• Anatomical vulnerability zones: The plexus is vulnerable at the interscalene groove where roots pass (prone to stretch), behind the clavicle (prone to compression and traction), and in the axilla where terminal nerves are superficial. Root avulsion specifically occurs when excessive traction pulls the nerve root away from the spinal cord, causing irreversible injury.

Classic Upper Trunk (Erb's) Palsy (C5-C6 lesion)

• Presents with loss of shoulder abduction and external rotation (supraspinatus and infraspinatus paralysis), loss of elbow flexion and supination (biceps and brachialis paralysis)
• Results in the characteristic "waiter's tip" position: adducted and internally rotated shoulder with extended elbow
• Common from shoulder dislocation, birth trauma (from traction during difficult delivery), or motorcycle injury
• Sensory loss over lateral shoulder and upper lateral arm

Classic Lower Trunk (Klumpke's) Palsy (C8-T1 lesion)

• Presents with intrinsic hand muscle paralysis (lumbricals and interossei) causing a "claw hand" deformity with hyperextended MCP joints and flexed IP joints
• Loss of finger flexion and wrist flexion from median and ulnar nerve involvement
• May include Horner's syndrome if T1 root is avulsed, due to involvement of sympathetic fibers
• Results from traction injuries pulling the arm upward or penetrating injuries to the lower neck/axilla
• Sensory loss in medial forearm and hand (C8-T1 dermatome)

Middle Trunk Lesion (C7)

• Rare isolated injury; presents with wrist drop and finger extension loss (radial nerve) along with some median nerve findings
• Sensory loss in middle finger and dorsal hand

Posterior Cord Lesion

• Causes radial nerve palsy: wrist drop, loss of finger extension, loss of thumb extension and abduction
• Loss of elbow extension (triceps), loss of sensation in dorsal first web space
• Results from axillary nerve compression (also from posterior cord): loss of shoulder abduction and external rotation

Lateral Cord Lesion

• Musculocutaneous nerve paralysis: loss of elbow flexion and supination, sensory loss along lateral forearm
• Partial median nerve involvement possible

Medial Cord Lesion

• Ulnar nerve deficits: intrinsic hand weakness, sensory loss to medial hand and ulnar forearm
• Median nerve hand branch involvement possible

Important Clinical Pearls

• Bilateral symptoms or progressive worsening suggests plexopathy rather than mononeuropathy and requires imaging
• Horner's syndrome with brachial plexopathy is pathognomonic for preganglionic (root level) injury with T1 involvement
• Axillary nerve is commonly injured due to its vulnerability as it passes through the quadrangular space; isolated axillary palsy causes loss of shoulder abduction and sensory loss over lateral shoulder (badge area)

• Clinical examination: Systematically test each nerve territory (see above presentations) and perform sensory testing in dermatomal/peripheral nerve distributions; compare bilaterally. Identification of sensory level (dermatomal) suggests root involvement, while peripheral nerve pattern suggests distal injury.

• Electrodiagnostic studies (EMG/NCS): Essential for confirming nerve injury, localizing the lesion level (root vs. trunk vs. cord vs. terminal nerve), and assessing severity. Nerve conduction studies show reduced amplitudes (axonal loss) or slowed velocity (demyelination); EMG reveals acute denervation (fibrillations and positive sharp waves after 2-3 weeks) and chronic changes (large motor units); temporal patterns help distinguish neurapraxia (recovery within weeks) from axonotmesis (weeks to months) and neurotmesis (incomplete or no recovery).

• Imaging studies: MRI is first-line for suspected brachial plexopathy, showing nerve enlargement from inflammation, T1 root avulsion (characteristic finding of pseudomeningocele), tumor invasion, or structural compression. CT assesses bony anatomy in trauma. Chest X-ray may show Horner's syndrome findings (apical lung tumor, Pancoast tumor is classic cause of Klumpke's via lower trunk compression).

• Important diagnostic considerations:
• Differentiate plexopathy from radiculopathy (plexus involves multiple roots in non-dermatomal pattern; radiculopathy follows single root distribution and may have neck pain)
• Rule out central cord syndrome (spinal injury) which presents with upper extremity weakness disproportionate to lower extremity involvement
• Assess for progressive symptoms or systemic signs (fever, weight loss, lymphadenopathy) suggesting malignancy or infection rather than traumatic injury

Conservative Management (First-line for most cases)

• Immediate immobilization and ice for acute traumatic injuries to reduce edema
• Early physical therapy and occupational therapy to maintain passive range of motion and prevent contractures while awaiting nerve regeneration
• Sling immobilization for upper trunk injuries; hand splinting for lower trunk injuries to prevent deformity
• Serial examinations every 4-6 weeks for first 3 months to assess for spontaneous recovery (favorable prognostic sign)
• Most neurapraxia injuries resolve spontaneously within 6-12 weeks as conduction block resolves

Surgical Intervention (indicated if no recovery by 3-6 months or evidence of complete transection)

• Timing: Surgery generally deferred until 3-4 months post-injury to allow time for spontaneous recovery of neurapraxia; earlier intervention (2-3 months) if EMG shows complete denervation and no early signs of reinnervation
• Nerve grafting: For complete nerve transection gaps; autograft (gold standard, using sural nerve as donor) preferred over allograft
• Nerve transfer: Transfer uninjured proximal nerve branches to distal nerve segments distal to injury; useful for root avulsion injuries (C5 root avulsed → transfer accessory nerve to suprascapular nerve)
• Tendon transfer: For chronic

Buzzword-to-lesion pairs examiners reuse

• Radial nerve at the spiral groove: mid-shaft humerus fracture → wrist drop with triceps spared (branches leave proximal to the groove). If triceps extension is also lost, the lesion is in the axilla (crutch palsy, Saturday night palsy) — this triceps distinction is the single most common distractor.
• Axillary nerve: surgical neck fracture or anterior shoulder dislocation → deltoid weakness plus numbness over the "regimental badge" area. Do not attribute lost abduction here to the suprascapular nerve.
• Median nerve at the supracondylar humerus: pediatric supracondylar fracture; attempted fist gives the hand of benediction ("pope's blessing"). An isolated inability to make the OK sign is the anterior interosseous nerve — pure motor, no sensory loss.
• Carpal tunnel syndrome: thenar atrophy with sparing of central palm sensation, because the palmar cutaneous branch arises proximal to the flexor retinaculum and passes over, not through, the tunnel.
• Ulnar nerve: medial epicondyle fracture or cubital tunnel entrapment. Ulnar paradox — a proximal lesion causes less clawing, since flexor digitorum profundus to digits 4–5 is also denervated.
• Long thoracic nerve (C5–C7): axillary node dissection or mastectomy → winged scapula from serratus anterior palsy.

Associations and next steps

• Horner syndrome plus lower-plexus findings signals a preganglionic T1 root lesion; in an adult smoker with shoulder/arm pain, think Pancoast tumor — chest imaging is the next step. In trauma, MRI showing a pseudomeningocele confirms avulsion; ACR Appropriateness Criteria support MRI as the primary modality for plexopathy.
• Neonatal Erb palsy after shoulder dystocia: observation with gentle range-of-motion therapy is first-line; most recover spontaneously. Do not order early surgery or immediate EMG.
• Timing of electrodiagnostics: fibrillations require roughly 2–3 weeks to appear (AANEM guidance), so an EMG obtained on day 1 cannot distinguish neurapraxia from axonotmesis.
• Cervical rib / thoracic outlet syndrome compresses the lower trunk, producing intrinsic hand wasting — not an upper-trunk "waiter's tip" picture.