[Sabio]

The herniation of the nucleus pulposus is most commonly in the posterolateral direction as it is the weakest part of the surrounding annulus fibrosus.

So each inter vertebral disk (nucleus pulposus) will press on the corresponding spinal nerve causing specific anatomic (dermatomic distribution of the affected areas)

The question is what disc number corresponds to what spinal nerve number?

All the spinal nerves are named by the vertebra above it, so for example inter vertebral disc T11-T12 will press on the T11 nerve as it exits in the inter vertebral foramen right below T11 vertebra

There's an exception to this rule..

Because we have more cervical spinal roots (eight) than we have vertebra (seven), then the first seven spinal roots exit above the corresponding numbered vertebra.

So a C4-C5 disk herniation will press on C5 nerve.

Click on this thumbnail to view

Copyright notice: this image was modified from this image posted in Wikimedia Commons

It's also important not confuse the roots (ventral and dorsal contained within the meninges, see image) with the rami (ventral and dorsal, the dorsal ramus is usually not drawn in such an image because it sprouts immediately and loops back to the posterior body supplying the muscles of the back and zygahypophyseal joints)

 

[lee-usmle]

I always remember only one disc and everything above it and below it will come automatically in my mind

I always remember between C7 and T1 vertebra we have spinal nerve C8

 

[dr.muhamad]

then how can we get S1 compressed???

if you say between L5-S1 is gonna be L5??

 

[CHAKRAM]

further queries

wats the case in case of lubmar disc herniation

more simple formula -
relation between spinal segment and cord segment

cervical vertebrae add 1 to vertebral level
upper thoracic vertebrae add 2 to vertebral level
lower thoracic vertebrae add 3 to vertebral level
T 10 VERTEBRAE all dorsal segments over
T 12 vertebrae all lumbar segments over
At L1 all sacral segments over
below L1 Cauda equina

further for pictorially understanding zygophyseal joint, consult http://en.wikipedia.org/wiki/File:Gray90.png

 

[rsin21]

can sum1 plz explain how this nerve compression works...i have hard time grasping the concept...

 

[Swanked]

It's just the larger of the two numbers.

For the USMLE purposes, it's likely going to be best to think the larger number in any given scenario. The below information is a description of why this is the case, a why there is good reason for confusion.

Most all disk herniations occur between L4-L5, L5-S1, C5-C6, and C6-C7 (huge number) - in each of these cases, it's the larger number that is compressed. For example, herniation between L4 and L5, L5 nerve roots are compressed. Between L5 and S1, S1 nerve roots are compressed. Between C5 and C6, C6 nerve roots are compressed, etc. etc.

This list below should help a bit conceptually, but remember, it's unlikely you will be tested on the USMLE for thoracic herniation, because (1) it's hugely uncommon, it's almost always cervical or lumbar herniation (about 95% of the time), and (2) it defies the basic rules important for identifying the compressed nerve (e.g. absent achilles tendon reflex --> likely S1/S2 damage, if herniation was the cause, likely L5-S1 disk herniation).

C1 nerve
C1 vertebrae (herniation here at C1 - C2 disk --> compresses C2 nerve)
C2 nerve
C2 vertebrae (herniation at C2 - C3 disk compresses C3 nerve)
C3 nerve
C3 vertebrae (C3 -C4 disk herniation compresses C4 nerve)
...
C7 nerve
C7 vertebrae (C7 - C8 disk herniation compresses C8 nerve)
C8 nerve

Disk herniation at cervical levels caused compression of the nerve in the adjacent plane - i.e. just lateral to it, This is because of the structure of the cervical spinal cord is drastically different than the lumbar vertebrae. Thoracic vertebrae appears to be similar to some of the cervical levels - however thoracic levels are rarely involved in disk herniations, and it's probably not a test point.

T1 vertebrae (T1 - T2 disk herniation, could compress T1 - probably)
T1 nerve
T2 vertebrae (T2-T3 disk herniation, could compress T2 - maybe)
T2 nerve
T3 vertebrae (T3-T4 disk herniation, T3 compression, or not)
T3 nerve
......

At some point - perhaps at the thoracic level, and most certainly at the lumbar vertebrae, the structure changes to that the nerve exiting below the site of herniation is more commonly compressed:

T12 vertebrae
T12 nerve
L1 vertebrae (L1-L2 disk herniation, likely to compress L2 nerve)
L1 nerve
L2 vertebrae (L2-L3 disk herniation, likely to compress L3 nerve)
L2 nerve
....
L5 vetebrae
L5 nerve (L5-S1 disk herniation, likely to compress S1)
Fused sacral vertebrae
S1 nerve

So choosing the larger number can be applied yet again, but for different reasons. In > 95% of any case you come across, it's just the greater number (between L4/L5 vertebrae --> L5 nerve is compressed).


Recap:

Why does C6 get compressed with C5-C6 disk herniation? Because it compresses the nerve on the same plane (C1 nerve exits above C1 vertebrae).
Why does L5 get compressed with a L4-L5 disk herniation? Because it compresses the nerve exiting below (nerve exits between L5-S1 vertebrae).

In the long run, it's going to be (likely) best to think the larger number of the two given will be compressed. I have not read of a set of rules saying between T1-T2 vertebrae, T1 nerve will most likely be compressed (however that seems to be the case), or between T11-T12 vertebrae, T11 nerve will be compressed (again, I have not read about this). Considering these thoracic herniations only make up a few percentile (some 4% or so) of the total cases of all herniations, it's just not something to focus on. For the USMLE, just pick the larger of the two.

 

[shine4ever]

For the USMLE purposes, it's likely going to be best to think the larger number in any given scenario. The below information is a description of why this is the case, a why there is good reason for confusion.

Most all disk herniations occur between L4-L5, L5-S1, C5-C6, and C6-C7 (huge number) - in each of these cases, it's the larger number that is compressed. For example, herniation between L4 and L5, L5 nerve roots are compressed. Between L5 and S1, S1 nerve roots are compressed. Between C5 and C6, C6 nerve roots are compressed, etc. etc.

This list below should help a bit conceptually, but remember, it's unlikely you will be tested on the USMLE for thoracic herniation, because (1) it's hugely uncommon, it's almost always cervical or lumbar herniation (about 95% of the time), and (2) it defies the basic rules important for identifying the compressed nerve (e.g. absent achilles tendon reflex --> likely S1/S2 damage, if herniation was the cause, likely L5-S1 disk herniation).

C1 nerve
C1 vertebrae (herniation here at C1 - C2 disk --> compresses C2 nerve)
C2 nerve
C2 vertebrae (herniation at C2 - C3 disk compresses C3 nerve)
C3 nerve
C3 vertebrae (C3 -C4 disk herniation compresses C4 nerve)
...
C7 nerve
C7 vertebrae (C7 - C8 disk herniation compresses C8 nerve)
C8 nerve

Disk herniation at cervical levels caused compression of the nerve in the adjacent plane - i.e. just lateral to it, This is because of the structure of the cervical spinal cord is drastically different than the lumbar vertebrae. Thoracic vertebrae appears to be similar to some of the cervical levels - however thoracic levels are rarely involved in disk herniations, and it's probably not a test point.

T1 vertebrae (T1 - T2 disk herniation, could compress T1 - probably)
T1 nerve
T2 vertebrae (T2-T3 disk herniation, could compress T2 - maybe)
T2 nerve
T3 vertebrae (T3-T4 disk herniation, T3 compression, or not)
T3 nerve
......

At some point - perhaps at the thoracic level, and most certainly at the lumbar vertebrae, the structure changes to that the nerve exiting below the site of herniation is more commonly compressed:

T12 vertebrae
T12 nerve
L1 vertebrae (L1-L2 disk herniation, likely to compress L2 nerve)
L1 nerve
L2 vertebrae (L2-L3 disk herniation, likely to compress L3 nerve)
L2 nerve
....
L5 vetebrae
L5 nerve (L5-S1 disk herniation, likely to compress S1)
Fused sacral vertebrae
S1 nerve

So choosing the larger number can be applied yet again, but for different reasons. In > 95% of any case you come across, it's just the greater number (between L4/L5 vertebrae --> L5 nerve is compressed).


Recap:

Why does C6 get compressed with C5-C6 disk herniation? Because it compresses the nerve on the same plane (C1 nerve exits above C1 vertebrae).
Why does L5 get compressed with a L4-L5 disk herniation? Because it compresses the nerve exiting below (nerve exits between L5-S1 vertebrae).

In the long run, it's going to be (likely) best to think the larger number of the two given will be compressed. I have not read of a set of rules saying between T1-T2 vertebrae, T1 nerve will most likely be compressed (however that seems to be the case), or between T11-T12 vertebrae, T11 nerve will be compressed (again, I have not read about this). Considering these thoracic herniations only make up a few percentile (some 4% or so) of the total cases of all herniations, it's just not something to focus on. For the USMLE, just pick the larger of the two.

Thank you for pointing out whats important for the step and what's not. Saved me some time.

 

[Lhoyt001]

Perfect almost, but one question.... it's at the point of the "transition"

"C7 vertebrae (C7 - C8 disk herniation compresses C8 nerve)
C8 nerve"----- from your message.

There actually is NO C7-C8 disc herniation, because there is no C8 vertebrae. So it would actually be a C7-T1 herniation. Correct me if I am wrong, but-this being the case- I feel this would be one exception to the rule. IF USMLE asks which nerve at a C7-T1 herniation is affected, the T1 (the bigger number) would NOT be correct. I believe that C8 would be the answer. This fits the same answer as what you posted, however, the vignette would have read "C7-T1 herniation" so using the rule of thumb of choosing the higher number (T1>C8) would be inappropriate in this one instance. Would you agree?

If I am correct, I believe your post is perfect minus that one "exception" I mentioned

For the USMLE purposes, it's likely going to be best to think the larger number in any given scenario. The below information is a description of why this is the case, a why there is good reason for confusion.

Most all disk herniations occur between L4-L5, L5-S1, C5-C6, and C6-C7 (huge number) - in each of these cases, it's the larger number that is compressed. For example, herniation between L4 and L5, L5 nerve roots are compressed. Between L5 and S1, S1 nerve roots are compressed. Between C5 and C6, C6 nerve roots are compressed, etc. etc.

This list below should help a bit conceptually, but remember, it's unlikely you will be tested on the USMLE for thoracic herniation, because (1) it's hugely uncommon, it's almost always cervical or lumbar herniation (about 95% of the time), and (2) it defies the basic rules important for identifying the compressed nerve (e.g. absent achilles tendon reflex --> likely S1/S2 damage, if herniation was the cause, likely L5-S1 disk herniation).

C1 nerve
C1 vertebrae (herniation here at C1 - C2 disk --> compresses C2 nerve)
C2 nerve
C2 vertebrae (herniation at C2 - C3 disk compresses C3 nerve)
C3 nerve
C3 vertebrae (C3 -C4 disk herniation compresses C4 nerve)
...
C7 nerve
C7 vertebrae (C7 - C8 disk herniation compresses C8 nerve)
C8 nerve

Disk herniation at cervical levels caused compression of the nerve in the adjacent plane - i.e. just lateral to it, This is because of the structure of the cervical spinal cord is drastically different than the lumbar vertebrae. Thoracic vertebrae appears to be similar to some of the cervical levels - however thoracic levels are rarely involved in disk herniations, and it's probably not a test point.

T1 vertebrae (T1 - T2 disk herniation, could compress T1 - probably)
T1 nerve
T2 vertebrae (T2-T3 disk herniation, could compress T2 - maybe)
T2 nerve
T3 vertebrae (T3-T4 disk herniation, T3 compression, or not)
T3 nerve
......

At some point - perhaps at the thoracic level, and most certainly at the lumbar vertebrae, the structure changes to that the nerve exiting below the site of herniation is more commonly compressed:

T12 vertebrae
T12 nerve
L1 vertebrae (L1-L2 disk herniation, likely to compress L2 nerve)
L1 nerve
L2 vertebrae (L2-L3 disk herniation, likely to compress L3 nerve)
L2 nerve
....
L5 vetebrae
L5 nerve (L5-S1 disk herniation, likely to compress S1)
Fused sacral vertebrae
S1 nerve

So choosing the larger number can be applied yet again, but for different reasons. In > 95% of any case you come across, it's just the greater number (between L4/L5 vertebrae --> L5 nerve is compressed).


Recap:

Why does C6 get compressed with C5-C6 disk herniation? Because it compresses the nerve on the same plane (C1 nerve exits above C1 vertebrae).
Why does L5 get compressed with a L4-L5 disk herniation? Because it compresses the nerve exiting below (nerve exits between L5-S1 vertebrae).

In the long run, it's going to be (likely) best to think the larger number of the two given will be compressed. I have not read of a set of rules saying between T1-T2 vertebrae, T1 nerve will most likely be compressed (however that seems to be the case), or between T11-T12 vertebrae, T11 nerve will be compressed (again, I have not read about this). Considering these thoracic herniations only make up a few percentile (some 4% or so) of the total cases of all herniations, it's just not something to focus on. For the USMLE, just pick the larger of the two.

 

[htan01]

"C7 vertebrae (C7 - C8 disk herniation compresses C8 nerve)
C8 nerve"----- from your message.

There actually is NO C7-C8 disc herniation, because there is no C8 vertebrae. So it would actually be a C7-T1 herniation. Correct me if I am wrong, but-this being the case- I feel this would be one exception to the rule. IF USMLE asks which nerve at a C7-T1 herniation is affected, the T1 (the bigger number) would NOT be correct. I believe that C8 would be the answer. This fits the same answer as what you posted, however, the vignette would have read "C7-T1 herniation" so using the rule of thumb of choosing the higher number (T1>C8) would be inappropriate in this one instance. Would you agree?

If I am correct, I believe your post is perfect minus that one "exception" I mentioned

very well done

 

[kiratd]

Why does L5 get compressed with a L4-L5 disk herniation? Because it compresses the nerve exiting below (nerve exits between L5-S1 vertebrae).
Why does that happen?
I guess, its because of cauda equina. The root at the same level, like L4 in L4/L5 herniation, is more lateral and is exiting, but L5 is at the spot where disc herniates (postero-laterally). Is that correct?