Now that you've learned the Anatomy of the sciatica nerve we can get into what causes sciatica. As I said in the introduction, I am using sciatica to describe a bunch of terms that are commonly debated. Some professionals believe "true" sciatica comes only from the nerve root, but I would argue that sciatica is any change to function, sensation, or pain caused by compromise of the sciatic nerve. Feel free to debate this in my forum.
Sciatica is most often spinal/discogenic in origin. This is why many clinicians take the nerve root compression stance for sciatica. Up to 90% of all cases of sciatica will be cause by on of the following intraspinal conditions.
Bulging or Herniated Disc
This will be the "typical" cause of sciatic for someone with low back pain and radiating pain down the leg. The exact symptoms that you get depend on what level the disc is bulging or herniated. For example, compression at the L5 nerve root may cause low back pain and radiating leg pain numbness and/or pain on the top of the foot and in the webbed space by the big toe. It my also cause weakness in the big toe and a foot drop - inability to lift foot up. This is because the L5 nerve root innervates the extensor muscles of the lower leg and provides sensation to the top of the foot.
This is by far the most common cause of sciatica. Interestingly, recent research has shown that a bulging disc typically causes loss in function and sensation, but not always pain, whereas a herniated disc will cause pain. This is because the nucleus pulposus (the jelly like substance inside the disc) will leak out when the disc is herniated. This jelly has a lot of inflammatory proteins and will act to inflame/irritate the nerve.
Degenerative Disc Disease
In degenerative disc disease there is narrowing of the disc between vertebrae. Degenerative Disc Disease can cause sciatic in a few ways:
- This narrowing of the disc can cause the vertebrae above and below the disc to shift and put pressure on the sciatic nerve.
- Bone spurs from form in and around the disc space - these spurs can sometimes compress nerve roots.
- The nucleus pulposus can leak out of the disc and come into contact with sciatic nerve roots causing inflammation/irritation.
Spondylolisthesis is a condition in which a vertebrae has slipped out of position. This can cause compression at of the nerve. Spondylolithesis can be caused at birth, degenerately (such as above with degenerative disc disorder) or by extreme force.
Stenosis means narrowing. Lumbar stenosis can be either central - narrowing of the spinal canal in the lumbar vertebrae or foraminal - narrowing of pathway where the nerves exit. This condition causes compression on the sciatic nerve and is usually positional, meaning that being in an awkward position can cause the sciatic pain and moving positions can relieve pain.
Sacroiliitis is inflammation in the sacroiliac (SI) joint. Sacroiliitis can affect the sciatic nerve in a couple of ways:
1. Inflammatory mediators released from the SI joint can inflame/irritate the nerve.
2. Oedema (swelling) from the inflammation can compress the nerve.
Sacroiliitis itself can be caused by a range of different things. Pregnancy and heavy lifting being two common ways. In pregnancy a hormone call relaxin is released in order to allow ligaments in the pelvis and ilium to move more freely. This can cause instability around the sacrum and lead to sacroiliitis.
If you remember from the anatomy, the sciatic nerve runs behind the piriformis muscle, or at times through it. If the piriformis muscle goes into spasm then it can cause compression on the sciatic nerve. Piriformis syndrome is more common in women and in runners.
Occasionally, a tumour may cause sciatica - it all depends on the location. For example, a tumour in the femur or pelvis may cause compression of the sciatic nerve.
Obesity may be seen as a cause because it is a risk factor for some of the above mentioned pathologies, such as degenerative disc disease, or disc herniation.
There are other causes of Sciatica, but these are the ones that I would say are most common. It's important to note that sciatica does not always present itself in the same way. As was mentioned above, the exact symptoms you get depend on where the nerve is compromised. It's also interesting to note that not all sciatica will come with low back pain. For example, some people with sacroiliitis will have absolutely no back pain, but yet have pain that radiates from the buttocks down the leg.
Signs and Symptoms
Sciatic can have a combination of the following signs and symptoms. The symptoms you get will depend on the cause and where the sciatic nerve is compromised.
- Low back pain with pain, numbness, or loss of function in legs
- Sharp pain in one part of the leg
- numbness or decreased sensation in all or part of the leg
- Tingling and/or burning sensation in all or part of the leg
- Pain that is worse when lying down or in certain positions
- occasionally an electric shock type feeling in the legs, possibly triggered by movement.
- Weakness in lower leg, foot, and/or toes.
Legrand, E. et al. 2007, Sciatica from disk herniation: Medical treatment or surgery?, Joint Bone Spine, 74, pp. 530-535
Kulcu, D. G. and Naderi, S. (2008), Differential diagnosis of intraspinal and extraspinal non-discogenic sciatica, Journal of Clinical Neuroscience, 15, pp. 1246-1252
Kumar, M. et al. (2011) Epidemiology, Pathophysiology and Symptomatic Treatment of Sciatica: A Review, International Journal of Pharmaceutical & Biological Archives, 2(4), pp. 1050-1061
Stafford, M. A., Peng, P., and Hill, D. A. (2007) Sciatica: a review of history, epidemiology, pathogenesis, and the role of epidural steroid injection in management, British Journal of Anaesthesia, 99(4), pp. 461-73
Wong, M., Vijayanathan, S., and Kirkham, B. (2005), Sacroillitis presenting as sciatica, Rheumatology, 44(10), pp. 1323-1324
Next up, Treatment options for Sciatica.