What Causes Sciatica?

Sciatica refers to low back pain that is often accompanied by leg pain, weakness, or numbness. This condition is caused by compression or irritation of one or more nerve roots in the lumbosacral (lower back) area. The most common cause of sciatica is a herniated (or "slipped") intervertebral disc at the L5/S1 level, which accounts for about 90% of cases.

Additionally, age-related changes in the spine can narrow the passage through which the nerve roots exit the spinal canal, leading to sciatica. Individuals may experience pain that radiates down both legs, which tends to improve when sitting or bending forward but worsens with standing or walking. This pattern is often associated with spinal canal stenosis, a condition characterized by the narrowing of the central and/or lateral recesses of the spinal canal. This narrowing can temporarily compress and irritate the nerve roots. Stenosis can be congenital (present at birth) or result from degenerative thickening and bulging of bones and ligaments, commonly known as degenerative joint disease.

Less common causes of sciatica include spondylolisthesis, where one vertebra slides forward over another, potentially compressing the posterior nerve roots. Although rare, infections and cancers can also lead to nerve root compression.

Prevalence

The percentage of people who experience sciatica at some point in their lives ranges from 13% to 40%. The risk of developing sciatica increases with age, and it is most common among individuals aged 45 to 64. While sciatica can also occur in younger people, it is rarely seen in those under the age of 20.

Intervertebral disc herniations

The spine is made up of 24 segments called vertebrae, which have a small degree of movement when considered individually, but together create a strong and mobile structure. Each vertebra, starting from C2 downwards, is separated by an intervertebral disc (IVD). These discs provide structural support to the spine and function as shock absorbers. You can think of the intervertebral disc as being similar to a jam donut. It has a soft, jelly-like interior that holds a significant amount of water, known as the nucleus pulposus. This nucleus is surrounded and contained by the annulus fibrosus, which is the fibrous outer layer of the disc.

What happens when the intervertebral disc fails?

The term "slipped disc" is misleading. In reality, discs don't actually slip. Instead, when discs are exposed to abnormal or sustained loads, the outer layer, known as the annulus, can become damaged and may start to tear. As this tear widens, some of the inner, jelly-like material can begin to push out into the space between the vertebrae. You can think of it like squeezing a jelly-filled donut until the jelly starts to ooze out. This condition is referred to as a disc herniation.

 Disc herniations can lead to significant lower back spasms and pain. When the nucleus of the disc herniates, the inner disc material may compress or irritate nearby spinal nerves, resulting in secondary pain, weakness, or numbness that can radiate down the leg. In very rare cases, large herniations can compress the nerves that control the bladder and bowel, potentially causing urinary retention or incontinence, a condition known as cauda equina syndrome. If cauda equina syndrome is suspected, an urgent referral is necessary. Approximately 90% of sciatica cases are attributed to herniated intervertebral discs.

What causes pain from sciatica?

It was previously believed that ALL leg pain resulted from direct compression of a nerve root due to a herniated nucleus pulposus. However, current understanding suggests that while the nerve root is often inflamed, it is not always compressed. The nucleus pulposus has little to no blood supply, which means that the body’s immune system does not recognize it. When the nucleus herniates, it triggers the release of irritating substances that stimulate an inflammatory immune response.

A herniated disc, the corresponding inflamed nerve root, or compression of the nerve root by the disc are all potential sources of pain.

What else might it be?

Degenerative disc disease

Degenerative disc disease (DDD) is typically characterized by the thinning of intervertebral discs (IVDs), along with the thickening of spinal ligaments and bony structures. This thickening can encroach upon the central or lateral recesses of the spinal canal, leading to irritation of the nerve roots. The exact cause of DDD is unknown, but it is often considered a natural part of the aging process. However, it can also affect younger individuals and is one of the primary causes of low back pain. DDD is believed to be influenced by a combination of genetic, environmental, traumatic, inflammatory, and other factors.

Piriformis syndrome

Piriformis syndrome, now frequently referred to as "deep gluteal syndrome," is another common cause of sciatica. In this condition, instead of the nerve being compressed at the nerve root level, it becomes trapped further along its pathway within the deep hip muscles, specifically in the piriformis muscle. This syndrome may arise from anatomical variations (such as the nerve passing through the muscle rather than underneath it), trauma, overuse, or muscle hypertrophy. Additionally, factors like scarring of either the nerve or muscle have also been considered potential contributors. When sciatica pain is primarily concentrated in the buttock area before radiating down the leg, piriformis syndrome is often the more likely diagnosis.

 Conclusion

In conclusion, sciatica is a multifaceted condition primarily resulting from nerve root irritation or compression in the lower back. Understanding the diverse causes of sciatica is essential for effective management and treatment.

Chiropractic care offers a holistic approach to managing sciatica, focusing on spinal adjustments, therapeutic exercises, and lifestyle modifications. These interventions aim to alleviate nerve compression, reduce inflammation and muscle spasms, and address the symptoms and underlying causes of sciatica.

References

1.       NICE (National Institute for Health and Care Excellence) CKS Clinical Knowledge Summaries: Sciatica (lumbar radiculopathy). https://cks.nice.org.uk/topics/sciatica-lumbar-radiculopathy/background-information/causes/

2.       Patient.info “Spinal disc problems” https://patient.info/doctor/spinal-disc-problems-including-red-flag-signs#degenerative-disc-disease

3.       Sharma S, Kaur H, Verma N, Adhya B. Looking beyond Piriformis Syndrome: Is It Really the Piriformis? Hip Pelvis. 2023 Mar;35(1):1-5. doi: 10.5371/hp.2023.35.1.1. Epub 2023 Mar 6. PMID: 36937215; PMCID: PMC10020728.

4.       Souza T.A., Differential Diagnosis and Management for the Chiropractor: Protocols and Algorithms, 5th ed, Burlington, MA: Jones & Bartlett Learning, 2016. P. 167, 212-14