In the realm of spinal cord injuries, C4 represents a critical juncture. Injuries above this level cause paralysis of the diaphragm muscle, making breathing possible only with the assistance of a mechanical ventilator. Injuries at or below C4 preserve the ability to breathe independently. Movement of the head and neck is preserved, while paralysis of the body and legs will occur in patterns reflecting the extent of injury.
Categories of Injury
The spinal cord is organized so that information flows in distinct bundles based on type and destination. A complete injury results in no function below that level. An incomplete injury will cause varying deficits. For example, anterior cord damage leaves motor function intact while central cord damage leads to paralysis of the arms and spares the legs. A condition known as Brown-Sequard syndrome results in weakness or paralysis on one side of the body, called hemiparalysis, and a loss of sensation on the other side. According to the National Spinal Cord Injury Association, since 1988, 45 percent of all injuries have been complete, 55 percent incomplete.
Muscles That Control Vital Functions
The diaphragm, a sheet of muscle at the bottom of the ribcage, contracts to cause a vacuum effect that sucks air into the lungs. The diaphragm might be weakened, though not paralyzed by a C4 injury. Taking deep breaths or coughing is assisted by the scalenus muscles of the neck. Impairment of these accessory breathing muscles, which receive their innervation from C4, can lead to the inability to properly clear the airways, making the individual susceptible to respiratory infections. In fact, pneumonia and pulmonary embolism are two of the leading causes of death for people with spinal cord injuries.
Nerve control of the heart muscle and the muscles that line the blood vessels can be affected in a C4 injury through a process known as autonomic hyperreflexia. In this life threatening situation, sensory messages from the body that convey commonplace messages such the impulse to urinate can lead to an incorrect nervous system response such a spike in heart rate or blood pressure, putting the individual at risk of heart attack or stroke.
Arm, Leg, and Torso Muscles
Muscles of the upper limb that are partly controlled by C4 include the trapezius, deltoid, rhomboids, biceps and levator scapulae. These muscles are associated with movement of the shoulder and arm. Injury here impairs, either fully or partially, activities such as raising the shoulders and flexing the arms.
Statistically, a little more than half of all spinal cord injuries result in quadriplegia. Torso and leg muscles will be greatly affected by injury at the C4 level because all the motor control to these areas is contained here, as individual nerves to these structures branch off the spinal cord at lower levels.
