Vertebral vs. Cord Segmental Levels
People with spinal cord injury are often told that they have an injury at a given spinal cord level. They are often told that the injury is “complete” or “incomplete.” They sometimes are told that they have a bony fracture or other involvement of one or more spinal vertebral levels. They may also be told that they are classified according to the American Spinal Injury Association (ASIA) Classification, as an ASIA A, B, C, D, or E. What are the meanings of the different spinal cord injury levels, the definition of complete and incomplete injury, and the different classifications of spinal cord injury? In the early 1990’s, there was no single definition of level, completeness of injury, or classification. Doctors frequently had different definitions of spinal cord injury levels and complete and incomplete injuries. In this article, I will try to explain the currently accepted definitions of spinal cord injury levels and classification.
Vertebral vs. Cord Segmental Levels
The spinal cord is situated within the spine. The spine consists of a series of vertebral segments. The spinal cord itself has neurological segmental levels which are defined by the spinal roots that enter and exist the spinal column between each of the vertebral segments. As shown in Figure 1 to the right, adapted from a spinal anatomy web site at Emory University, the spinal cord segmental levels do not necessarily correspond to the bony segments. The vertebral levels are indicated on the left side while the cord segmental levels are listed for the cervical (red), thoracic (green), lumbar (blue), and sacral (yellow) cord.
Vertebral segments – There are eight cervical (neck), 12 thoracic (chest), five lumbar (back), and five sacral (tail) vertebrae. The spinal cord sends roots that exit the spinal canal between vertebral bodies. Spinal cord segmental levels are defined by their roots but are not always situated at the corresponding vertebral levels. For example, the C8 cord segment is situated in the C7 vertebra while the T12 cord is situated in the T8 vertebra. The lumbar cord is situated between T9 and T11 vertebrae. The sacral cord is situated between the T12 to L2 vertebrae, as shown.
Spinal Roots – The spinal roots for C1 exit the spinal column at the atlanto-occiput junction. The spinal roots for C2 exit the spinal column at the atlanto-axis. The C3 roots exit between C2 and C3. The C8 root exits between C7 and T1. The first thoracic root or T1 exits the spinal cord between T1 and T2 vertebral bodies. The T12 root exits the spinal cord between T12 and L1. The L1 root exits the spinal cord between L1 and L2 bodies. The L5 root exits the cord between L5 and S1 bodies.
The Cervical Cord – The first and second cervical segments are special because they hold and pivot the head. The back of the head is called the Occiput. The first cervical vertebra, upon which the head is perched, is sometimes called Atlas, after the Greek mythological figure who held up earth. The second cervical vertebra is called the Axis, upon which Atlas pivots. The interface between the occiput and the atlas is called the atlanto-occiput junction. The interface between the first and second vertebra is called the atlanto-axis junction. The C3-4 cord contains the phrenic nucleus. The cervical cord innervates the deltoids (C4), biceps (C4-5), wrist extensors (C6), triceps (C7), wrist extensors (C8), and hand muscles (C8-T1).
The Thoracic Cord – The thoracic vertebral segments are defined by those that have a rib. These vertebral segments are also very special because they form the back wall of the pulmonary cavity and the ribs. The spinal roots form the intercostal (between the ribs) nerves that run on the bottom side of the ribs and connect to the intercostal muscles and associated dermatomes.
The Lumbosacral Cord – The lumbosacral vertebrae form the remainder of the segments below the vertebrae of the thorax. The lumbosacral spinal cord, however, starts at about T9 and continues only to L2. It contains most of the segments that innervate the hip and legs, as well as the buttocks and anal regions.
The Cauda Equina – In humans, the spinal cord ends at L2 vertebral level. The tip of the spinal cord is called the conus. Below the conus, there is a spray of spinal roots that is frequently called the cauda equina or horse’s tail. Injuries to T12 and L1 vertebra damage the lumbar cord. Injuries to L2 frequently damage the conus. Injuries below L2 usually involve the cauda equina and represent injuries to spinal roots rather than the spinal cord proper.
In summary, spinal vertebral and spinal cord segmental levels are not necessarily the same. In the upper spinal cord, the first two cervical cord segments roughly match the first two cervical vertebral levels. However, the C3 through C8 segments of the spinal cords are situated between C3 through C7 bony vertebral levels. Likewise, in the thoracic spinal cord, the first two thoracic cord segments roughly match first two thoracic vertebral levels. However, T3 through T12 cord segments are situated between T3 to T8. The lumbar cord segments are situated at the T9 through T11 levels while the sacral segments are situated from T12 to L1. The tip of the spinal cord or conus is situated at L2 vertebral level. Below L2, there are only spinal roots, called the cauda equina.