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Spinal Cord Atrophy
Spinal Cord Atrophy
Wise Young, Ph.D. M.D.
W. M. Keck Center for Collaborative Neuroscience
Rutgers University, Piscataway, NJ 08854-8082
Last Updated: January 9, 2002
People sometimes hear about atrophy in their spinal cord. Magnetic resonance images of their spinal cords may show a narrower spinal cord and cysts in the cord. Some are afraid that the atrophy means that the spinal cord has been damaged above or below the injury site and may have implications for recovery or regeneration. This article considers the causes spinal cord atrophy, what should happen, and what should not occur after injury.
Trauma damages spinal cord neurons and axons at the injury site. Axons are parts of neurons and cannot survive long when disconnected from the cell body. Neurons that are more than two segments away from the injury site should not die. Axons that are still connected to the neurons will die back only a short distance. Some shrinkage or atrophy of the injury site is normal. Since ascending and descending axons constitute 50-70% of the volume of the spinal cord, loss of disconnected portions of axons may result in some atrophy of the spinal cord above and below the injury site.
The spinal cord has ascending and descending tracts. Spinal tracts run in three sets of columns: posterior (dorsal in animals), lateral, and anterior (ventral in animals). These columns are sometimes called white matter because they are myelinated and appear whitish in color. For the sake of consistency with animal literature, I will use the terms dorsal and ventral rather than posterior and anterior. Lateral is the same in both human and animals. The spinal cord is an extension of the brainstem and is part of the central nervous system.
Spinal cord neurons are situated in the central part of the spinal cord. Because they are not myelinated, areas with neurons appear grayish and are called gray matter. Sensory neurons are located just outside the spinal cord on the spinal roots, in little swellings on the spinal roots called dorsal root ganglia. Motor neurons that directly innervate muscles of the arms and legs are respectively located in the cervical and lumbar segments. The cervical and lumbar parts of the spinal cord are larger than the thoracic cord which contain only 10-20% gray matter.
Ascending sensory tracts are mostly situated in the dorsal and lateral columns. Dorsal root ganglion neurons send axons that enter the spinal cord and ascend in the dorsal columns to the brainstem. The spinothalamic tract carries pain and temperature sensations and ascend in the lateral column to the thalamus. Several smaller sensory tracts in the lateral and ventral (or anterior) columns carry signals to the brainstem and cerebellum. In addition, sympathetic tracts are mostly situated in the lateral columns.
Descending tracts carry motor signals from the brain to the spinal cord. The corticospinal tract runs from the motor cortex in the brain to the spinal cord and is located in the lateral column (note that in rats, the corticospinal tract is situated in the dorsal columns). The rubrospinal tract runs from the red nucleus in the midbrain to the spinal cord and is also situated in the lateral columns. The reticulospinal and vestibulospinal tracts run from the brainstem to the spinal cord and are situated in the ventral columns.
Trauma kills cells and damages axons at the injury site. Therefore, there is cell loss and atrophy at the injury site. The atrophy may not be apparent for days or weeks after injury because the acutely injured spinal cord swells. On magnetic resonance images, the injury site may appear brighter, i.e. shows higher intensity signal, because of the increased water content of the spinal cord. The increased signal intensity at the injury site should diminish with time as the edema subsides.
Axons that have been disconnected from their neurons by the injury will die. The isolated axons surprisingly may survive for days or even weeks. Eventually, however, they degenerate. With sufficient axonal loss, oligodendroglial cells (the cells that myelinate axons in the central nervous system) also die. Degenerating white matter tracts undergo a process called Wallerian degeneration. Macrophages and other inflammatory cells cluster in the degenerating tracts and play an important role in clearing cellular debris.
Neurons more than two segment above or below the injury site should survive. If there is gray matter loss at some distance from the injury site, this would suggest that there has been injury at several levels of the cord. While axons that have been detached from the cell bodies die, axons that are still attached to the cell generally die back only one segment or less. Gray matter loss that is more than two segments from the trauma site would suggest compression or trauma to multiple segments.
Some people may have had injury due to ischemia (loss of blood flow). Ischemia typically damages gray matter more than white matter. Also, ischemia damages small axons more than large myelinated axons. In the thoracic spinal cord, gray matter constitute 20% or less of the spinal cord. Thus, such people may show little or no atrophy of the affected spinal cord segments. However, ischemia of the cervical or lumbar cord may cause loss of gray matter.
Continued compression or tethering of the cord may cause ischemia over many levels of the cord. Cord compression may be due to indentation of the cord by bone, disc, intraspinal or extra-spinal cyst. Normally, there should be cerebrospinal fluid all around the cord. The dura or membranous sac in which the spinal cord resides should not be contact with the spinal cord. Tethering of the cord implies adhesions of the dura to the cord or surrounding tissues, causing tension of the spinal cord during movement.
Obstruction of cerebrospinal fluid flow may lead to the development of cysts in the spinal cord. Called syringomyelia or syrinx, cysts may extend long distances above and below the injury site. Cysts may occupy as much as 50-90% of the volume of the spinal cord close to the injury site. The cyst usually can be seen on magnetic resonance images (MRI) of the cord, viewed from the side or axially (coronal). Due to the cyst, the outer diameter of the spinal cord may be close to or even bigger than normal.
Finally, loss of axons does not imply death of the neurons that they connect to. Neurons at and close the injury site may die but neurons more than two segments above and below the injury site typically survive. Many investigators have looked for neuronal atrophy in the brain or in the spinal cord below the injury site in both human and animals after spinal cord injury. Although some specialized populations of neurons may be lost, a majority of neurons in the brain and the spinal cord below the injury do not degenerate after spinal cord injury.
With these points in mind, let us consider spinal cord atrophy that may occur with injury to four levels of the spinal cord: C4, T4, L1, and Cauda Equina.
- Injury site. There should be substantial atrophy of the spinal cord at C4, due to death of cells in both gray and white matter. Frequently, in addition to atrophy, there may be a cyst in the spinal cord.
- Above C4. The injury should interrupt ascending axons in the dorsal and lateral columns. Therefore, the dorsal and lateral column above C4 should show atrophy. Some of the sensory axons in the dorsal and lateral column come from C1-3 and they should not be affected by the injury. However, since the injury interrupts many axons coming spinal segments below C4, the dorsal column above C4 may show substantial atrophy. The lateral column should not show much atrophy because it has descending tracts.
- Below C4. The injury interrupts descending axons in the lateral and ventral columns. Therefore, the lateral and ventral columns below the injury site should show some shrinkage. The lateral column contains some ascending axons and thus should not show as much atrophy. However, most of the ventral column is composed of axons that come from the brainstem, the atrophy of the ventral columns should be substantial and extend all the way down the spinal cord.
- Injury site. There should be substantial atrophy or a cyst at the T4 cord due to death of the cells in gray and white matter.
- Above T4. The injury should interrupt ascending axons from the lower spinal cord. These form a majority of the axons in the dorsal column between T4 and T1. However, many sensory axons enter the spinal cord and ascend the dorsal column in the cervical segments. Therefore, the dorsal column atrophy should prominent between T1 and T4 but less so in the cervical segments. There should be some shrinkage of the lateral columns but minimal shrinkage of the ventral columns.
- Below T4. The injury should interrupt descending axons from the brain and brainstem, as well as from the cervical spinal cord. This should be associated with some shrinkage of the lateral column and marked atrophy of the ventral column below T4. The dorsal column should show minimal shrinkage.
- Injury site. Most of the lumbar cord is located at the L1-L2 vertebral segments. This is sometimes called the lumbar enlargement where many of the neurons for the legs live. There may be marked atrophy or a cyst at L1.
- Above L1. There should be marked atrophy of the dorsal column in the thoracic spinal cord extending up to the cervical cord. However, the lateral and ventral columns should be only minimally affected.
- Below L1. Because most of the lumbar cord is located at L1-L2 vertebral segments and gray matter occupy ~50% of the cord at these levels, there should be only minimal shrinkage of the cord below the injury site.
Cauda equina injury.
- Injury site. The cauda equina is located below the L2 vertebral level. Since it is composed of spinal roots from L1 through S5, it is not spinal cord injury per se. However, because it damages the motor and sensory axons coming from and to the spinal cord, it causes paralysis and sensory loss.
- Above L1. Because the injury damages sensory axons that go into the dorsal column, some shrinkage of the dorsal column may occur. However, the atrophy should be minor.
In summary, trauma damages cells and axons in the spinal cord and should cause atrophy or a cyst at the injury site. Spinal axons ascend and descend in white matter columns in the spinal cord. Loss of ascending axons cause dorsal column atrophy above the injury site, particularly in thoracic spinal cord. Ventral columns may shrink below the injury site but since ventral columns are small, these changes are often not apparent. Lateral columns, because they contain ascending and descending axons, may show partial atrophy. These changes do not necessarily mean damage to the spinal cord above or below the injury site. On the other hand, continued indentation of the cord and increased signal intensity, loss of gray matter or syringomyelic cysts extending more than two segment from the injury site are abnormal.
©Wise Young PhD, MD