Introduction
Brachial plexus injuries are exam favourites because they force you to combine anatomy, mechanism of trauma, and pattern recognition. Instead of memorizing lists, you can localize most lesions in seconds if you understand how traction, compression, and fractures affect specific parts of the plexus. This article presents fast, high-yield clinical pearls with direct links back to core anatomy for rapid revision.
Quick Brachial Plexus Refresh
The brachial plexus is formed by the anterior rami of C5–T1. It is organised into roots, trunks, divisions, cords, and terminal branches. Roots and trunks lie in the neck, divisions are posterior to the clavicle, and cords sit in the axilla.
To orient yourself quickly, review:
- Roots and trunks → C5–T1 roots and upper, middle, lower trunks
- Divisions and cords → anterior and posterior divisions forming lateral, medial, and posterior cords
All the classic trauma patterns simply stretch or compress one of these levels.
Pearl 1: Erb Palsy Means Downward Traction at C5–C6
Erb palsy is the prototype upper trunk (C5–C6) lesion. It follows excessive separation between head and shoulder, stretching the upper elements of the plexus near the C5–C6 roots and upper trunk.
Key Exam Findings
- Arm hangs in the classic "waiter’s tip" posture.
- Loss of shoulder abduction (supraspinatus, deltoid) and external rotation (infraspinatus, teres minor).
- Weak elbow flexion (biceps brachii, brachialis supplied by musculocutaneous nerve).
- Forearm often held in pronation.
Clinical pearl: If shoulder abduction and external rotation are lost but wrist and finger extension are preserved, localise to an upper trunk lesion rather than a distal nerve.
Pearl 2: Klumpke Palsy Starts in the Hand, Not the Shoulder
Klumpke palsy is a lower trunk (C8–T1) lesion caused by excessive upward traction on the limb (e.g. hanging onto an object during a fall, breech extraction). It affects the inferior aspects of the plexus where the lower trunk is formed.
Key Exam Findings
- Severe weakness of intrinsic hand muscles with prominent clawing of the fingers.
- Weak finger flexion (long flexors partly supplied by C8–T1).
- Sensory loss over the medial forearm and medial hand.
- Possible Horner syndrome if adjacent sympathetic fibres are involved.
Clinical pearl: If the hand looks abnormal (wasting, clawing) but shoulder and elbow power are largely preserved, think lower trunk rather than an isolated peripheral nerve.
Pearl 3: Axillary Nerve Injury Has a Single Signature Zone
The axillary nerve winds around the surgical neck of the humerus and is highly vulnerable in shoulder dislocation and surgical neck fractures. Core anatomy and clinical features are reviewed under axillary nerve clinical correlation.
Key Exam Findings
- Loss or marked weakness of shoulder abduction between 15–90° (deltoid, teres minor).
- Flattened shoulder contour with loss of normal deltoid bulk.
- Sensory loss over the "regimental badge" area on the lateral shoulder.
Clinical pearl: After any shoulder dislocation, always test active abduction and sensation over the lateral shoulder. Failure to do this is a common OSCE trap.
Pearl 4: Radial Nerve Lesion Pattern Depends Entirely on Level
The radial nerve passes from axilla to arm, through the spiral groove, and into the forearm before dividing into superficial and deep branches. Each segment has a characteristic lesion pattern. Examination strategies and typical injuries are summarised in clinical correlation for large nerves of the arm.
High Lesion (Axilla)
- Often due to crutch misuse or prolonged compression.
- Wrist drop with loss of finger extension.
- Sensory loss over the dorsum of the hand and posterior forearm.
Midshaft Lesion (Spiral Groove)
- Classically follows a midshaft humeral fracture.
- Wrist drop persists, but triceps may be relatively preserved (branches arise proximally).
Posterior Interosseous Nerve Lesion
- Finger extension weakness or loss.
- No sensory deficit, because superficial radial nerve carries cutaneous fibres.
Clinical pearl: Wrist drop with sensory loss suggests a more proximal lesion. Wrist drop without sensory disturbance suggests a distal lesion, often posterior interosseous nerve.
Pearl 5: Median Nerve — Opposition Loss Is the Fastest Test
The median nerve passes from arm through the cubital fossa, forearm, and carpal tunnel to supply thenar muscles and lateral digits. Clinical patterns of injury at elbow and wrist are outlined under large nerves clinical correlation.
High Lesion (Around Elbow)
- Often due to supracondylar fracture of humerus.
- Loss of forearm pronation.
- Weak wrist flexion with ulnar deviation.
- "Hand of benediction" when attempting to make a fist (unable to flex index and middle fingers fully).
Low Lesion (Carpal Tunnel or Wrist Laceration)
- Thenar muscle wasting and flattening.
- Loss of thumb opposition (opponens pollicis).
- Sensory loss over lateral 3½ digits on the palmar surface.
Clinical pearl: In any suspected lesion, test thumb opposition first; it is a rapid, high-yield screen for median nerve dysfunction.
Pearl 6: Ulnar Nerve — The More Distal the Lesion, the Worse the Claw
The ulnar nerve is vulnerable at the medial epicondyle and at the wrist (ulnar tunnel / Guyon canal). Injury patterns and key clinical correlations are summarised in the same large nerve clinical section.
High Lesion (Around Elbow)
- Less obvious clawing because long flexors (FDP to ring and little fingers) are weakened.
- Weak finger abduction and adduction due to interossei paralysis.
- Sensory loss over medial hand and medial 1½ digits.
Low Lesion (Wrist / Guyon Canal)
- More pronounced clawing of ring and little fingers (FDP intact but intrinsics lost).
- Marked wasting of interossei and hypothenar muscles.
- Similar sensory loss distribution.
Clinical pearl: Dramatic clawing indicates a distal lesion, while a high lesion gives less pronounced clawing but more global weakness in the ulnar territory.
Pearl 7: Supraclavicular vs Infraclavicular Injuries Follow the Clavicle
The clavicle separates lesions affecting roots and trunks from those involving cords and terminal branches. The relevant anatomical layout is shown under components of the brachial plexus.
Supraclavicular Injuries
- Typically due to high-energy traction.
- Involve roots and trunks.
- Can produce widespread muscle weakness and multi-dermatomal sensory loss.
- May be associated with diaphragmatic weakness if C5 contribution to phrenic nerve is affected.
Infraclavicular Injuries
- Often due to shoulder dislocation or penetrating trauma in the axilla.
- Involve cords or terminal branches.
- Deficits tend to follow specific named nerves (axillary, radial, median, ulnar).
Clinical pearl: If weakness and sensory changes are patchy but span several named nerves, think plexus-level involvement instead of a single peripheral nerve.
Pearl 8: Diffuse Pain Patterns Point to Plexus, Not Single Nerves
Peripheral nerve lesions usually cause pain and sensory changes limited to a single cutaneous territory. In contrast, brachial plexus injuries often produce more diffuse, poorly localised pain that radiates across multiple regions of the limb.
Clinical pearl: Pain or paresthesia that crosses classic median, ulnar, and radial territories should raise suspicion of a plexus lesion even before full motor testing is complete.
Pearl 9: Mechanism First, Nerve Second
In exams and real practice, the quickest way to localise is to start with the mechanism of injury, then match it to the most likely level. A few high-yield associations:
- Neck forced away from shoulder → upper trunk lesion (Erb palsy) at C5–C6.
- Arm suddenly pulled upward → lower trunk lesion (Klumpke palsy) at C8–T1.
- Shoulder dislocation or surgical neck fracture → axillary nerve injury.
- Midshaft humeral fracture → radial nerve in spiral groove, risk of wrist drop.
- Supracondylar humeral fracture → median nerve and brachial artery in the cubital fossa.
- Medial epicondyle fracture → high ulnar nerve lesion.
Clinical pearl: In OSCE stations, describe the mechanism in anatomical language. Examiners reward candidates who tie external trauma vectors to internal plexus topography.
Pearl 10: Motor Mapping Is More Reliable Than Sensory Mapping
Sensory territories are helpful, but often less precise in acute trauma and in anxious patients. Motor testing, especially of key groups (deltoid, wrist extensors, interossei, thenar muscles), offers a faster and more reliable map of lesion level.
Clinical pearl: Start with a brief motor screen: shoulder abduction, elbow flexion/extension, wrist extension, finger abduction, thumb opposition. Once you identify the main motor deficit, use sensory testing and reflexes to refine localization.
Summary
Brachial plexus injuries are far easier to handle when you anchor them to three elements: the plexus layout, the direction of traumatic force, and a small set of key motor findings. By focusing on upper vs lower trunk patterns, axillary and radial nerve injury signs, and the characteristic behaviour of median and ulnar lesions at different levels, you can localise most injuries in seconds and present a clear, anatomy-based explanation in both exams and clinical practice.