Originally Posted by Swanked
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 vertebrae (herniation here at C1 - C2 disk --> compresses C2 nerve)
C2 vertebrae (herniation at C2 - C3 disk compresses C3 nerve)
C3 vertebrae (C3 -C4 disk herniation compresses C4 nerve)
C7 vertebrae (C7 - C8 disk herniation compresses 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)
T2 vertebrae (T2-T3 disk herniation, could compress T2 - maybe)
T3 vertebrae (T3-T4 disk herniation, T3 compression, or not)
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:
L1 vertebrae (L1-L2 disk herniation, likely to compress L2 nerve)
L2 vertebrae (L2-L3 disk herniation, likely to compress L3 nerve)
L5 nerve (L5-S1 disk herniation, likely to compress S1)
Fused sacral vertebrae
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).
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.