The sciatic nerve is the largest nerve in the human body and is formed by the union of 5 nerve roots from the lower spine. It passes deep in the buttock and down the back of the thigh all the way to the heel and sole of the foot. The sciatic nerve serves a vital role in connecting the spinal cord with the skin and muscles of the thigh, leg, and foot.1

Sciatica is a type of lumbar radiculopathy. Sciatic nerve pain originates from one or more spinal nerve roots in the lumbar (lower) and/or sacral spine. Watch: Sciatica Overview Video

Any type of pain and/or neurological symptom that originates from the sciatic nerve is referred to as sciatica. The symptoms of sciatica are typically felt along the path of the nerve.

See What You Need to Know About Sciatica

How the Sciatic Nerve is Affected

The sciatic nerve may be irritated, compressed, or inflamed by a number of problems in the lower back, causing sciatica. Sciatica is a type of lumbar radiculopathy, which means that the pain originates from the lumbar and/or sacral nerve roots.

See Lumbar Radiculopathy

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Several common factors may affect the sciatic nerve:

Mechanical compression
Direct physical forces may be applied to the nerve due to the following common conditions:

  • Herniated discs. A disc in the lower back may bulge or herniate, causing irritation, and/or compression of a sciatic nerve root.

    Watch: Lumbar Herniated Disc Video

  • Foraminal stenosis. Stenosis, or narrowing of the intervertebral opening through which the sciatic nerve roots travel, may compress or irritate the sciatic nerve. Other degenerative changes in the spine, such as thickening of facet joint capsules and/or ligaments, may also directly compress the sciatic nerve.

    Watch: Lumbar Spinal Stenosis Video

  • Segmental instability. Instability of a vertebral segment that occurs if one vertebra slips over the one below it (spondylolisthesis), vertebral defects (spondylolysis), or complete dislocation of one or more vertebrae may directly compress the nerve root(s) of the sciatic nerve.

Rarely, tumors, cysts, infection, or abscesses in the lower spine or pelvic region may compress the sciatic nerve

See Sciatica Causes

Chemical inflammation
Research indicates that chemical irritants may cause inflammation and/or irritation of the sciatic nerve. These chemical irritants are thought to include substances like hyaluronic acid and/or fibronectin (protein) fragments that leak out of degenerated or herniated discs.2-4 Sometimes, degenerated discs may cause nerve tissues to grow into the disc, penetrating the outer and inner layers of the disc, causing sciatica pain.5

See Diagnosing the Cause of Sciatica

Some studies suggest that immune system responses may play a role in causing sciatica pain when there is exposure of disc material from herniated discs. Certain substances such as glycosphingolipids (fats) and neurofilaments (protein polymers) that are secreted by the immune system are particularly high in people with sciatica. It is believed that these substances are released due to the reaction between nerve roots and exposed disc material and may cause inflammation of the sciatic nerve.4

Physical characteristics
Certain physical characteristics of an individual may affect the sciatic nerve, causing sciatica. Research indicates a higher risk of sciatica may be present in:

  • Obese or overweight individuals6
  • Older individuals who are tall (typically over 180 cm for men and 170 cm for women)7

See A Healthy Weight for a Healthy Back

Research also shows that sciatica tends to occur more often in women, who may experience slower recovery in severe cases. Men, however, are reported to have more hospitalizations and surgery due to sciatica.8

See Sciatica Surgery

Occupational causes People with certain occupations may be at a higher risk of developing sciatica pain. A few examples include truck drivers, machine workers, carpenters, and weight-lifting athletes. In general, sitting for long periods of time without proper posture, frequently bending the spine forward or sideways, and/or regularly lifting the arms above shoulder level are risk factors for sciatica.9-10

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Vitamin B12 deficiency
An adequate level of vitamin B12 is important for maintaining nerve health.11-12 Vitamin B12 helps in synthesizing the fatty (myelin) sheath that covers the nerves, which is important in nerve function and conduction of impulses. Sciatica may occur due to vitamin B12 deficiency and may be more common in elderly people, above 60 years of age.13-14

See Food for Thought: Diet and Nutrition for a Healthy Back

Diabetic individuals who are treated with metformin may also be at risk of vitamin B12 deficiency due to malabsorption.14-15

Genetic factors
Research suggests that genetic factors may play a role in the degeneration of intervertebral discs, which in turn may cause sciatica. The genetic factors may alter the structure and function of certain collagen proteins within the disc, which weaken and deteriorate the function of the disc.16 These degenerated discs may eventually cause compression or irritation of the sciatic nerve.

See The Truth About Sciatica: Genetics, Low Back Problems, Piriformis, Arthritis, and More

When the sciatic nerve is affected, the symptoms of sciatica are typically felt along the skin and muscles supplied by the nerve. Depending on the specific nerve root affected, pain and/or neurological symptoms may be felt in different areas of the thigh, leg, and/or foot.

See Sciatica Symptoms

References:

  1. Giuffre BA, Jeanmonod R. Anatomy, Sciatic Nerve. [Updated 2018 Dec 16]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2019 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK482431/.
  2. Allegri M, Montella S, Salici F, et al. Mechanisms of low back pain: a guide for diagnosis and therapy. F1000Res. 2016;5:F1000 Faculty Rev-1530. Published 2016 Oct 11. doi:10.12688/f1000research.8105.2.
  3. Quero L, Klawitter M, Schmaus A, et al. Hyaluronic acid fragments enhance the inflammatory and catabolic response in human intervertebral disc cells through modulation of toll-like receptor 2 signalling pathways. Arthritis Res Ther. 2013;15(4):R94. Published 2013 Aug 22. doi:10.1186/ar4274.

Complete Listing of References

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