What is the structure and function of the spinal cord at the C4 level?

Structure of the Spinal Cord at C4 Level

Anatomical Organization

The C4 spinal cord level contains neural pathways that control diaphragmatic breathing, upper extremity function (particularly shoulder and upper arm movements), and serves as a critical transition zone between cervical and upper thoracic innervation patterns. 1

Key Structural Components

• Gray Matter Organization: The C4 level contains anterior horn motor neurons that innervate the diaphragm via the phrenic nerve (C3-C5 roots, with C4 being the primary contributor), as well as motor neurons for shoulder elevation/depression and upper arm musculature 2, 3

• White Matter Tracts: At C4, the lateral corticospinal tracts carry descending motor signals to all four extremities, while the dorsal columns transmit ascending sensory information including proprioception and fine touch 1

• Nerve Root Contributions: The C4 nerve root emerges at this level and contributes to the brachial plexus formation (though the main brachial plexus derives from C5-T1, with occasional C4 contribution) 1

Functional Significance

Motor Control

• Respiratory Function: C4 level injury critically affects diaphragmatic function, as the phrenic nerve originates primarily from C4, making this level the boundary between ventilator-dependent and independent breathing 4, 2

• Upper Extremity Control: Injury at C4 typically results in complete tetraplegia with loss of hand function, bilateral weakness affecting biceps, triceps, wrist extensors, and finger flexors 1, 5

• Shoulder Function: The C4 level contributes to shoulder elevation/depression (shoulder shrug) movements, which may be preserved or partially preserved depending on injury completeness 3

Sensory Pathways

• Dermatomal Distribution: The C4 dermatome covers the lower neck and upper shoulder region, with sensory changes below this level indicating cord involvement 6

• Sensory Level Determination: Cord edema or compression at C4/C5 typically manifests with sensory changes below the level of injury, often with patchy distribution or a defined sensory level 1, 5

Clinical Implications of C4 Pathology

Expected Neurological Deficits

• Complete C4 Injury: Results in tetraplegia with loss of all extremity function, abdominal respiration only, bladder-bowel disturbance, and anesthesia below C5 6

• Incomplete C4 Injury: May present with bilateral hand weakness, gait disturbance, hyperreflexia, and variable sensory changes, with potential for recovery following the central cord syndrome pattern 1, 5, 6

Respiratory Considerations

• Tracheotomy Requirements: Approximately 47% of patients with C3/4 spinal cord injury require tracheotomy due to severe respiratory disturbance from phrenic nerve involvement 4

• Ventilatory Support: Functional electrical stimulation of abdominal muscles can augment tidal volume by an average of 220 ml in patients with C4-C7 injuries, though this represents a compensatory mechanism for impaired diaphragmatic function 2

Prognosis and Recovery Patterns

• Mortality Risk: Patients with complete C3/4 injuries (Frankel A) have significant mortality risk, with approximately 33% mortality in long-term follow-up studies 4

• Recovery Potential: Among C3/4 injury patients, approximately 53% show improvement in Frankel classification over time, with recovery often following a central cord syndrome pattern (lower extremities recovering before upper extremities, and proximal before distal) 4, 6

Diagnostic Correlation

Imaging-Clinical Correlation

• MRI Findings: Cord edema at C4/C5 should correlate with clinical myelopathy including bilateral hand dysfunction, not isolated findings like single finger drop 5

• Signal Changes: T2-weighted imaging showing cord signal changes from C4-C6 indicates significant cord injury and predicts bilateral upper extremity involvement with potential lower extremity signs 1

Common Pitfalls

• Anatomical Mismatch: Isolated finger drop with MRI showing C4/C5 cord edema suggests alternative diagnoses such as C6/C7 radiculopathy or peripheral nerve pathology rather than true C4 myelopathy 5

• Comprehensive Examination Required: Testing must include all upper extremity myotomes systematically, reflexes, gait assessment, and sensory level determination to accurately localize pathology to the C4 level 5