Relationship between vertebral bodies, spinal cord and spinal radicles
Introduction
Recently, while taking a bedside clinic on neurology, I along with my students realised that we knew so little about the vertical relationship between vertebral bodies, spinal cord and radicles which are coming out of the cord. We got confused for the lowest part of spinal cord (conus) and cauda medullaris.
I thought of writing my know how on the subject and try to see that if doubt solves.
Assumption of the model
We know that in adult the spinal cord ends at lower end of LV1 (1st lumbar vertebral body). We also know that for the neonate (new born), this level is at LV4.
So we can assume that at the start spinal cord ended at SV4, meaning both spinal cord and its protective cover (vertebral bodies) were at the same level. With the growing fetus, vertebral body grows (lengthens) caudally while spinal cord (being composed of neurons with very limited growth potential) does not grow to the same length leading to the confusion about the levels of vertebral body, spinal cord and radicles.
Naming the radicles, vertebral bodies
1st nerve root arises out between occiput and 1st vertebral body, it is known as 1st cervical nerve root
So for the cervical nerve roots, a particular numbered nerve root comes out above that particular numbered vertebral body. Example, 3rd cervical radicle comes out of the vertebral body above CV3 (between CV2 and CV3).
Nerve root coming out between CV7 and TV1 is known as 8th cervical nerve root (not 1st thoracic nerve root as expected). And 1st thoracic nerve root arises out between TV1 and TV2. So, for thoracic vertebra downwards a particular numbered nerve root comes out below that particular numbered vertebral body.
Also, the final nerve root (4th sacral nerve root) arises between SV4 and coccyx.
Situation in fetus
Following figure depicts the situation in fetus where vertebral bodies are being developed around the spinal cord. Length of vertebral bodies is same as that of the spinal cord.
In the above figure, it can be seen that the nerve roots are arising from the respective spinal levels and are coming out of the vertebral foramina. So, spinal levels correspond to the respective vertebral bodies level.
Situation in adult
As described earlier in assumption section, with growth of fetus, vertebral bodies grow more than the spinal cord leading to the discrepancy in the respective levels.
The following figure depicts the situation in adult.
Implications
The above sketch shows following implications:
Spinal cord ends at lower end of LV1, meaning that we cannot have myelopathy below LV1 level. The area is known as cauda equina region. Any lesion at levels below LV1 will lead to confusing picutre of patchy LMN/neuropathic dificit depending on vertical level (LV1 - LV5, SV1 - SV5) and horizontal level (medial vs lateral). There will not be any evidence of myelopathy.
a. Medially situated lesions. As sacral nerve roots exit out of the vertebral canal the last, they are situated most medially. Medially situated lesions will lead to involvement of sacral roots at all levels (from LV2 to SV4).
b. Laterally situated lesions. Depending on the vertical site of the lesion, nerve roots will get involved. A lesion at LV4 will lead to involvement of L4 onwards nerve roots (as nerve roots till L3 have already exited out of the spinal canal).
The relationship between vertebral body number and spinal level can be found at this site. Excerpt for the same is given below.
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 is only spinal roots, called the cauda equina.Lesion at a particular vertebral body will lead to involvement of respective (same numbered) exiting nerve root, involvement of spinal segment (level depends on the level of vertebral body involvement) and UMN involvement below the spinal segment involvement. Example, a lesion at TV1 will lead to involvement of C8/T1 nerve roots (which are exiting out from intervertebral foramina), T4 spinal segment involvement (LMN lesion) and UMN involvement below T4 level.
You are requested to comment and put up your suggestions. I will be grateful.
Suman
