The lower trunk brachial plexus represents a critical neuroanatomical junction where the complex network of the brachial plexus converges. Formed by the union of the C8 and T1 nerve roots, this specific trunk serves as the primary conduit for motor and sensory information destined for the intrinsic muscles of the hand and the medial aspect of the upper limb. Understanding its precise anatomy, surgical vulnerability, and the pathologies affecting it is essential for clinicians managing upper extremity dysfunction, as lesions here can produce profound and functionally limiting deficits.
Anatomical Structure and Organization
Anatomically, the brachial plexus is traditionally divided into roots, trunks, divisions, cords, and branches. The lower trunk is the most caudal of the three primary trunks, originating at the level of the cervico-thoracic inlet. It is specifically composed of the anterior rami of the C8 and T1 spinal nerves, which merge to form a single trunkous structure. This trunk then promptly divides into an anterior and posterior division, each of which will subsequently contribute to the medial and lateral cords, respectively, ultimately giving rise to the terminal branches that innervate the hand.
Motor Functions and Clinical Significance
The motor fibers traversing the lower trunk are indispensable for the fine motor control required in hand function. These fibers primarily supply the intrinsic muscles of the hand, including the interossei, the thenar muscles (excluding the lateral two lumbricals), and the medial two lumbricals. Injury to the lower trunk results in a characteristic motor deficit known as Klumpke's palsy, which manifests as an inability to flex the metacarpophalangeal joints and extend the interphalangeal joints of the fingers, leading to a "claw hand" deformity. Furthermore, weakness in finger flexion and adduction significantly impacts grip strength and dexterity.
Sensory Distribution and Patient Presentation
Sensory information from the lower trunk is conveyed to the medial aspect of the arm, forearm, and hand. The specific cutaneous territory includes the ulnar side of the forearm and the palmar and dorsal surfaces of the medial one and a half fingers (the little finger and the ulnar half of the ring finger). Patients with a lower trunk lesion will often report numbness, tingling, or paresthesia in this specific distribution. The combination of medial sensory loss and intrinsic muscle weakness creates a distinct clinical picture that guides diagnostic investigation toward the brachial plexus rather than more proximal or distal pathologies.
Etiology of Lower Trunk Injuries
Trauma is the most common cause of lower trunk brachial plexus injuries, typically occurring through traction or compression mechanisms. A classic scenario involves a sudden upward traction force on the shoulder, such as when a child is lifted by the arm, leading to obstetric brachial plexus palsy during delivery or falls in adults. Additionally, direct penetrating trauma, severe blunt trauma to the shoulder girdle, or pathological processes such as Pancoast tumors (apical lung cancers) can compress or infiltrate the lower trunk. Inflammatory conditions, though less common, may also target this region, leading to neuropraxia or more severe axonal degeneration.
Diagnostic Approaches and Assessment
Accurate diagnosis relies on a thorough clinical examination complemented by electrodiagnostic studies. Clinicians assess specific muscle groups innervated by the lower trunk, such as the abductor digiti minimi, first dorsal interosseous, and flexor digitorum profundus to the ring and little fingers. Nerve conduction studies and electromyography are invaluable tools for localizing the lesion to the lower trunk and differentiating between pre-ganglionic and post-ganglionic injuries. Magnetic resonance imaging (MRI) with neurovascular-specific sequences may be utilized to visualize structural lesions, tumors, or post-traumatic neuromas in the supraclavicular fossa.
