Adult Brachial Plexus Injury

The brachial plexus is the network of nerves that carries motor and sensory signals from the lower cervical and first thoracic spinal nerves out to the whole upper limb. An adult traumatic brachial plexus injury is usually a high-energy traction injury - the classic patient is a young man thrown from a motorcycle, whose head and shoulder are forced apart so violently that the nerves are stretched, ruptured, or pulled out of the spinal cord altogether.

The result can be devastating: a partly or completely paralysed, numb, and often painful arm in a young, working person. For the exam, this topic rewards a clear, structured approach rather than memorised lists. Three threads run through everything: work out exactly where and how badly the plexus is injured (especially preganglionic versus postganglionic), act within the time window before denervated muscle is lost, and rebuild the limb in order of usefulness - elbow flexion first, then shoulder, then (rarely) hand.

It is worth separating this from brachial plexus birth palsy, a different obstetric traction problem in neonates with its own classification (Narakas) and a high rate of spontaneous recovery. The adult injury is higher-energy, recovers far less spontaneously, and the surgical decisions are driven hard by timing and by the preganglionic/postganglionic distinction.

You cannot reason about a plexus injury without the anatomy, but you only need it at the level the examiner tests: which section is injured, and what that predicts.

The five sections run proximal to distal and are easy to remember as "Read Those Damn Cadaver Books" - Roots, Trunks, Divisions, Cords, Branches.

• Roots (C5, C6, C7, C8, T1): the spinal nerves emerging from the neck. The dorsal root ganglion (DRG) sits on the sensory root - its position is the anatomical basis of the whole preganglionic/postganglionic split.
• Trunks: C5+C6 form the upper trunk, C7 alone the middle trunk, C8+T1 the lower trunk - these lie in the supraclavicular region.
• Divisions: each trunk splits into an anterior and posterior division behind the clavicle.
• Cords (lateral, posterior, medial): named for their relation to the axillary artery, lying in the infraclavicular region.
• Branches: the terminal nerves - musculocutaneous, axillary, radial, median, ulnar.

Branches that come off very proximally are diagnostic flags. The dorsal scapular nerve (rhomboids) and long thoracic nerve (serratus anterior) arise from the roots themselves; the suprascapular nerve comes off the upper trunk. If these muscles are paralysed, the lesion is very proximal - pushing you toward a preganglionic root-level injury.

Pathophysiology: how the injury happens

Adult plexus injury is overwhelmingly a traction (stretch) injury. When the head and shoulder are forced apart - the classic motorcyclist thrown over the handlebars - the angle between neck and shoulder widens violently and the nerves are stretched beyond their limit.

• Where the nerve fails depends on the force and the anatomy. The upper roots (C5-C6) are tethered and tend to rupture distal to the DRG (postganglionic), whereas the lower roots (C8-T1) are less supported and are more prone to being avulsed from the cord (preganglionic). This is why a Horner's sign (T1) signals a severe, proximal, lower-root injury.
• Once an axon is disconnected from its cell body it undergoes Wallerian degeneration, and the denervated muscle it supplied begins an irreversible clock toward fibrosis. Regenerating axons advance only slowly (on the order of about a millimetre a day), which is precisely why reinnervation distance and timing dominate the surgical strategy - the closer and the sooner you can deliver new axons to a muscle, the better.
• Other mechanisms exist: a sharp/open laceration divides the plexus cleanly (favourable for early direct repair), and an infraclavicular injury is often produced by a shoulder dislocation or proximal humerus fracture dragging on the cords and branches.

Adult plexus injuries are classified in several overlapping ways. The exam wants you to combine them: where along the plexus, how severe the nerve lesion, and which functional pattern.

A plexus assessment is a viva favourite because it is so structured. Frame it as history, then a systematic limb examination, then the special signs that localise the level.

Investigations exist to answer one question: for each root, is the lesion preganglionic or postganglionic, and is there anything to repair?

Management is best structured as who to operate on, when, and what to do - then the non-surgical care that runs alongside.

Adult brachial plexus injury is a high-yield exam topic precisely because it integrates so much: applied neuroanatomy (the five sections and the dorsal root ganglion), examination skills (the special signs that localise a lesion), investigation logic (why a preserved SNAP in a numb limb means an avulsion), the physiology of denervation and the time window, and a clear reconstructive ladder. It is also a topic where the examiner wants to see judgement - recognising the limb-threatening vascular injury first, knowing when to wait and when not to, refusing to promise a normal arm, and referring early to a specialist centre. Master the preganglionic/postganglionic split, the timing principle, and the elbow-shoulder-hand order of priorities, and you can reason through almost any scenario they set you.

According to PubMed, the following studies support the key teaching points above.

High-yield, single-best-answer style points that examiners turn into questions.

• Specialist, multidisciplinary care is the consistent global message: traumatic adult plexus injuries do best when managed early in dedicated centres combining orthopaedic/hand, plastic, and peripheral-nerve neurosurgery, with physiotherapy and pain services. Delayed referral is documented as common and as a driver of poorer results.
• Timing principles are broadly agreed: explore sharp/open and vascular injuries early; reconstruct closed traction injuries inside roughly a 3 to 6 month window before denervated muscle is lost; shift to reconstructive (tendon transfer, fusion, free muscle transfer) options for late presentations.
• Nerve transfers have shifted global practice over recent decades: distal nerve transfers (such as the Oberlin transfer for elbow flexion and spinal-accessory-to-suprascapular transfer for the shoulder) are now widely used, shortening reinnervation distance and improving outcomes, while nerve grafting remains the answer for graftable postganglionic ruptures.
• Outcome variation reflects injury severity and access: upper-plexus injuries reliably regain elbow and shoulder function, whereas hand recovery after lower-plexus or pan-plexus injury remains poor everywhere - so realistic goal-setting (a useful helper arm) is a universal part of consent and care.
• Mechanism and demographics differ by region: motorcycle and road-traffic trauma dominate in many low- and middle-income settings, shaping a younger, predominantly male, higher-severity caseload - a global epidemiological pattern rather than any single country's experience.