Where The Heck Is My Pinched Nerve Coming From? Dermatomes, Sciatica And Lower Back Pain
How do doctors determine where my pinched nerve is?
Which tests best determine the offending nerve?
What can I do to help with this trapped nerve?
I get dozens of emails everyday from people with detailed MRI reports with no information about their history or examination.
A typical report might look like this:
“L4 – L5 shows broad based posterior protrusion causing indentation of thecal sac, filar roots, severe narrowing of neural foramina and mild compression of exiting, traversing nevre roots (L > R)
L5 – S1 disc shows diffuse annular bulge causing mild narrowing of neural foramina and mild compression of exiting nerve roots.”
They invariable want to know what they can do. Should they get surgery or for most people, they want to know what exercise they can do to “cure” the problem.
I can’t give advice to people with just an MRI. It’s not a diagnosis and the MRI is often wrong.
The dilemma is that while MRI is considered a gold standard in some circles research has found the MRIs are not correlated to the pain.
An MRI is very sensitive but not very specific. In other words you can have all kinds of things going on in the MRI that is not “normal” but still have no pain. All those things wrong on MRI doesn’t point out your pain.
For example you can have three disc herniations on MRI but one or none of them can be causing your pain. This is quite common.
“It is very rare for an MRI. to come back with the words ‘normal study,“ said Dr. Christopher DiGiovanni, a professor of orthopedics and a sports medicine specialist at Brown University. “I can’t tell you the last time I’ve seen it.” From the New York Times
Where Is My Pinched Nerve?
Dermatomes are the key. Dermatomes are an area of your skin where you feel the pain, numbness or tingling. Your nerves from your spine called the spinal nerve each supply a different area of your skin.
Your little toe and all the other toes and your achilles area is usually supplied by one nerve called the S1 nerve from your lower back. While your big toe and the top of your foot and side of your lower leg is supplied by another nerve called the L5 also from your lower back.
All the nerves have their own separate areas. These areas on the skin are called dermatomes.
Dermatomes can vary between different people but for most people it’s not too different. It’s like your nose can be in different areas on different people. Some higher, some lower but I haven’t seen anybody with a nose on their forehead yet.
The dermatomes are so reliable, they are better than the neurological examination which we do to help make your diagnosis.
Whenever someone comes in with sciatica it’s important have your reflexes, muscles strength and skin sensitivity checked which is the what the typical neurological examination consists of.[1-2]
Research found that the dermatomes or the patch of skin affected by the spinal nerve is what is found in the text books 88% of the time for the L4 nerve, 82% for the L5 nerve and 83% of the time. 
While the percentage of accuracy is pretty good you have to remember that 12-18 people have their dermatomes in the “wrong” or unexpected place.
It doesn’t mean your chiropractor will get your diagnosis wrong though. This is when experience and correlation with other results come into play.
A lot of different diagnosis can affect the L4 dermatome. Disc herniations, Osteoarthritis or wear and tear degeneration also called degenerative disc disease when the disc of the spine are involved. Your can get lateral stenosis which usually due to bony spurs from the joints of the spine.
Between the L34 vertebrae is the L34 disc. The L4 nerve comes between these two vertebrae.
When your L4 nerve is pinched 88% of you will have the pain affecting the big toe and the inside part of the shin just above the ankle going up along the front of the shin and eventually to the side of the thigh .
Weakness of your quads is another sign if the part of the nerve that controls your muscles gets pinched. In these cases squatting may be difficult.
Between the L45 vertebrae is the L45 disc. The L5 nerve comes out between these two vertebrae.
Discs can herniate in any direction. They can herniate forward, backwards and sideways. Disc are generally oval in shape when seen from aboe, so they can herniate at any point of a 360 degree circle. Luckily the disc herniations have a general pattern which is to herniate backwards and slightly away from the center.
This type of backwards and off center herniation is called a parmedian (paracentral) or lateral recess herniation.
Your disc will often herniated this way as that is the weak point for most people as their is a strong ligament directly behind or posterior to the disc called the PLL or (posterior longitudinal ligament).
While your anatomy plays a big part in which way your disc will herniated, what you do also makes a difference. Most people bend forward to shave, put on make up and pick things up. You sit all day in a chair while you work or bend forward to pick things up all day.
Either way you are putting pressure on the front of your disc. When you put pressure on the front of the disc like a jelly doughnut the jelly will come out the back. Only that ligament the PLL is right in the center so the jelly comes out a little to the right or the left of the center.
Paramedian comes from “Para” which means “adjacent to” and median which means which means at the midline. So the disc is herniated adjacent to the midline.
Lateral recess comes from “lateral” which means side and “recess” which means depression or cleft. The disc coming out slightly to the side in the space between the spinal cord and piece of bone of the vertebrae. This space is the “recess” or cleft.
Either way paramedian and lateral recess mean the same thing a disc herniation that is pushing out slightly to the right or left of center.
When your L45 disc herniates in a paramedian, paracentrally or in the lateral recess. For most of you or 82% of you will feel the effects of the pinched nerve starting with your big toe going up the along the top of the foot the outer portion of the shin and up the side of the thigh to the buttock and to the L5 vertebrae area
If the muscle portion of the L5 nerve is damaged then you make have trouble with foot drop. Where your toes keep hitting the floor and you can lift them up. Your walking would be strange as you will then lift that bad foot up higher to compensate.
In this case you would trip and fall more.
Your L5S1 disc often herniates at the paramedian, paracentral or the lateral recess. Remember that paramedian, paracentral and lateral recess all mean the same thing a disc that herniates next to the center line.
For 83% of you when the disc herniates there the L5 nerve will become pinched when the disc herniates in the paramedian, or lateral recess.
You will feel burning or numbness or pain at the baby toe going towards the heel and up the side or back of the leg up the thigh to the buttock.
If the muscle control part of the nerve is pinched then you may have trouble doing ten or more heel raises.
The achilles reflex may also be absent.
By providing you with this information I hope to empower you by being able to understand generally what is going on with your pinched nerve whether it is from disc herniations, stenosis, or degnerative changes from osteoarthritis.
Tell us what you think in the comments below and like us on Facebook. I will answer all questions in the comments section here at this downtown Toronto Chiropractic clinic.
1. Quraishi NA,Taherzadeh O,McGregor AH, Hughes SP, Anand P, Correlation of nerve root pain with dermatomal sensory threshold and back pain with spinal movement in single level lumbar spondylosis. J Bone Joint Surg Br. 2004 Jan;86(1):74-80.
2. Hancock MJ, Koes B, Ostelo R, Peul W. Diagnostic accuracy of the clinical examination in identifying the level of herniation in patients with sciatica. Spine 2011; 36:E712-E719.
3. Nitta H, et al. “Study on dermatomes by means of selective lumbar spinal nerve root block.” Spine 1993;18:1782-1786.
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