Pathophysiology

The spinal column is composed of 33 bony vertebrae with interspersed cartilaginous intervertebral disks. The typical vertebra consists of an anterior vertebral body and a posterior vertebral arch. These elements form the borders of the vertebral foramen, which contains and protects the spinal cord. The superior and inferior articular processes arising from most vertebrae allow the spine to be a strong yet flexible structure. The pedicles and laminae form the sides of the vertebral arch and are notched to allow for the passage of nerves and blood vessels.

The spinal cord is a cylindrical structure arising at the base of the brain and passing through the skull at the foramen magnum. It is surrounded and protected by three layers of meninges as well as cerebrospinal fluid. Thirty-one pairs of spinal nerves exit the spinal column via the intervertebral foramen. The spinal nerves are formed by the junction of the anterior and posterior nerve roots as they exit from the spinal cord.

The spinal cord consists of both white and gray matter. In general, the white matter is the outer covering of the cord. It contains the nerve fibers running up and down the spinal cord in tracts. The gray matter is made up of nerve cells and is formed in the shape of an H when viewed on cross section ( Hg.31,-1).

FIG. 31-1. Section of the spinal cord, showing the white and gray matter, spinal roots, and spinal nerve (Adapted with permission from Snell RS: Clinical Anatomy for Medical Students. Philadelphia, Lippincott-Raven, 1973, p. 828.), as well as the efferent portion of the autonomic nervous system. (Used with permission from Snell RS: Clinical Neuroanatomy for Medical Students. 4th ed. Philadelphia, Lippincott-Raven, 1997, p. 463.)

The autonomic nervous system, which maintains the internal balance of the body's many systems, has two main divisions: the sympathetic and parasympathetic. The sympathetic nervous system activates the "fight or flight" response, increasing the heart rate and blood pressure and constricting arterioles of the skin and intestines in order to redistribute blood flow, preferentially to the brain, heart, and skeletal muscle. The parasympathetic nervous system has the opposite effect, slowing the heart rate, decreasing blood pressure, and increasing the peristaltic activity of the gastrointestinal tract.

The anatomy of the autonomic nervous system is quite complex (Fig 31-1). The outflow portion of the sympathetic system starts with neuron cell bodies located in the lateral gray horns of the first thoracic to the second lumbar segments. In some cases they may extend to the third lumbar segment. These cells are controlled by the hypothalamus via descending tracts of the reticular formation. The axons from the sympathetic nerve cells in the lateral gray horns leave the spinal cord in the anterior nerve roots and connect to the ganglia of the paraspinal sympathetic trunk. The sympathetic trunk is located along each side of the spinal column and extends along the entire length of the vertebral column. Axons arising from neurons in the sympathetic ganglia then travel throughout the body. The sympathetic fibers that innervate the heart arise primarily from the second to fourth thoracic segments.

The anatomy of the parasympathetic system is very different. The majority of the parasympathetic system is carried along the cranial nerves, although there is a portion that involves the second to fourth sacral segments of the spinal cord. The parasympathetic axons synapse with the cranial nerves in peripheral ganglia close to or within the target organ. The parasympathetic axons from the sacral segments form the pelvic splanchnic nerves. The portion of the parasympathetic system that innervates the heart originates in the dorsal nucleus of the vagus nerve and travels to the heart via the vagus nerves.

In evaluating patients with spinal cord injuries, the concepts of primary and secondary cord injury are important. When the spinal cord is initially injured, the pathologic picture may be relatively benign, showing some scattered hemorrhages and edema.7 Several weeks later, the appearance is much worse, with large cavities surrounded by gliosis and fibrosis.7

These primary or initial changes are caused by the traumatic event, which can cause compression, laceration, or stretching of the cord. 8 Over several days to weeks, the initial injury evolves to what is termed secondary cord injury. Spinal cord ischemia has been suggested as the principal etiology of secondary changes, although other mechanisms may exist.38 Ischemia of the spinal cord can be caused by a variety of events. The blood supply to the spinal cord is, in general, not very substantial, and can be easily disrupted by either local trauma to the small anterior and posterior spinal arteries or injury and thrombosis to a large regional vessel, such as the great radicular artery of Adamkiewicz. General systemic hypotension and shock, if severe enough, can cause a low-flow state such that blood flow to the cord is compromised, even with an intact arterial supply.

The clinical relevance of secondary injury is that a patient's presentation can change in the period following the traumatic event. An incomplete lesion can evolve to a complete injury, or the level of injury can become higher because of the cord changes that occur during secondary cord injury.

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