Assessing the brachial plexus is a fundamental skill for clinicians managing trauma, orthopaedic injuries, and neurological conditions. This intricate network of nerves originates from the cervical and upper thoracic spine, weaving together to provide motor and sensory function to the entire upper limb. A systematic brachial plexus exam allows for the localization of injury, differentiation between root, trunk, or peripheral nerve pathology, and the monitoring of recovery or progression of disease.
Anatomy and Function of the Brachial Plexus
The brachial plexus is formed by the ventral rami of C5 through T1, with contributions often described as roots, trunks, divisions, cords, and branches. The roots merge to form the superior, middle, and inferior trunks, which subsequently divide into anterior and posterior divisions. These divisions regroup into the posterior, lateral, and medial cords, named by their relationship to the second part of the axillary artery. Each terminal branch, including the musculocutaneous, median, ulnar, radial, and axillary nerves, carries specific motor and sensory fibers that must be tested individually to ensure a comprehensive evaluation.
Indications for Performing the Examination
Clinical indications for a brachial plexus exam are vast and include trauma such as shoulder dystocia during childbirth, falls from height, or high-speed motor vehicle collisions where traction injuries are common. Patients presenting with neck pain radiating into the arm, numbness, tingling, or weakness in specific nerve distributions warrant this assessment. Additionally, it is crucial for monitoring conditions like thoracic outlet syndrome, cervical radiculopathy, or peripheral nerve compressions, and for pre-operative baseline documentation in cases requiring shoulder or arm surgery.
Step-by-Step Assessment Technique
A thorough examination follows a logical sequence, moving from inspection to palpation, motor testing, sensory evaluation, and reflex assessment. The clinician should begin by observing the posture of the limb, looking for atrophy, fasciculations, or obvious deformities. Palpation of the supraclavicular fossa helps identify masses or tenderness over the trunks, while assessing the range of motion of the neck can reveal secondary mechanical causes of nerve tension or compression.
Motor Function Testing
Motor assessment is the cornerstone of the exam, targeting specific muscles innervated by the major nerve branches. The examiner should test shoulder abduction (deltoid, axillary nerve), external rotation (infraspinatus, suprascapular nerve), elbow flexion (biceps, musculocutaneous nerve), wrist extension (extensor carpi brevis and longus, radial nerve), finger flexion and extension (median and ulnar nerves), and intrinsic hand function, particularly the adduction and abduction of the fingers (hypothenar and interossei muscles, ulnar nerve).
Sensory and Reflex Evaluation
Sensory testing involves mapping light touch or pinprick across the dermatomes of the upper limb to identify specific nerve root or peripheral nerve involvement. The radial, median, and ulnar nerves have distinct sensory territories that should be compared side-to-side. While the brachial plexus primarily supplies the upper limb, reflex testing such as the biceps (C5-C6) and triceps (C7) reflexes provides valuable insight into the integrity of the corresponding nerve roots and their synapses within the cord.
Interpreting Findings and Localization
Pattern recognition is essential when interpreting the results of the brachial plexus exam. An injury to the upper trunk, often termed Erb's palsy, typically presents with the classic "waiter's tip" posture: the arm hangs adducted and internally rotated, with the wrist flexed. Conversely, a lower trunk injury, or Klumpke's palsy, affects the intrinsic hand muscles and can cause a "claw hand" deformity. Isolated nerve injuries, such as a radial nerve palsy causing wrist drop, help differentiate between plexus-level damage and peripheral nerve compromise.
