Sciatic Nerve and Sciatica

See also Assessment and Management of Low Back Pain and Sinister Causes of Back Pain.

'Sciatica' is a lay term for pain and sensations of tingling that travel into the buttocks, back of the thigh, and into the calf and heel (the distribution of the sciatic nerve and its roots).¹ Medical use of the term 'sciatica' varies, and it is sometimes used synonymously with nerve root pain (radicular pain) or with lumbar disc disease.²

This article will use the lay meaning of sciatica, i.e. pain or dysaesthesia in the sciatic nerve distribution.

Sciatica is a common problem. The lifetime prevalence of low back pain is around 70%, and it accounts for 4% of GP consultations. About 5% of people with low back pain have nerve root pain.¹

This is helpful for interpreting clinical findings. For an clear illustration of spinal anatomy and details of pain mechanisms, see reference.³

The sciatic nerve (L4–S2) arises from the lumbosacral plexus (nerve roots L4-S2) and exits the pelvis below the piriformis muscle. It innervates the hamstring muscles and all the muscles below the knee.⁴ Its sensory innervation is the foot and lateral lower leg.⁵

Sciatic pain may arise either from the nerve trunk or its spinal nerve roots. Spinal nerve root compression can occur either:

• In/around the intervertebral foramina - this will cause unilateral symptoms in the affected dermatome and muscles.
• To the cauda equina (from lesions in the spinal canal, such as a central disc protrusion); this is more likely to produce bilateral symptoms and may affect buttocks, genitalia, sphincters and lower limbs.

General points

Lumbar disc herniation is a common cause of sciatica, but is not the whole story, as various pathologies can affect this nerve or its roots. In many patients with sciatica, no lumbar disc herniations are present on scans, and disc herniation can be found on scans in people without sciatica symptoms.⁶

Causes in the spine

• Lumbar disc herniation⁷
  • Intervertebral discs comprise an outer annulus fibrosus and an inner nucleus pulposus. The disc may bulge and the nucleus may herniate or extrude.
  • The commonest primary pathology is degeneration of the nucleus within the disc; pain usually arises from nearby structures.⁸
  • The L5-S1 disc is the commonest site of disc herniation.
• Chemical inflammation:
  • There is evidence that inflammatory factors produced by the disc nucleus (perhaps in association with mechanical trauma) will irritate nerves - this would explain how sciatica can occur without disc herniation.⁹
• Spinal stenosis:^8,10 the intervertebral foramina and spinal canal may be narrowed by degenerative disease, Paget's disease or ankylosing spondylitis.This may cause leg pain akin to sciatica, but with "claudication" symptoms (see under assessment).
• Tumours: common examples are bony secondaries from breast or prostate, and myeloma.
• Infections: osteomyelitis - bacterial or tuberculous; discitis, epidural abscess or shingles.²
• Bone or joint disease: Paget's disease, ankylosing spondylitis.
• Spondylolithsesis: this is the forward displacement of one vertebra on its lower neighbour, often with a stress fracture in the pars interarticularis.
• Spinal trauma: fracture or displacement impinging on the spinal nerves. Remember that in osteoporotic patients fractures can occur with minor trauma.
• Epidural varicose veins (rare): these have been reported, caused by IVC obstruction due to pregnancy or abdominal mass.¹¹

Causes outside the spine¹

• Intra-abdominal pathology with referred pain, e.g. abdominal aortic aneurysm, abdominal lymphadenopathy.
• Pelvic lesions, e.g. psoas muscle abscess, pelvic tumours or inflammation; pelvic fractures.
• Piriformis syndrome (entrapment of the sciatic nerve in the piriformis muscle) and other pelvic nerve entrapments.^12,13
• Trauma around the sciatic nerve, e.g. misplaced gluteal muscle injection or gluteal muscle compartment syndrome.¹⁴

This should be focused so that patients can be categorized into:

• Immediate or urgent referral required (red flag features)
• Non-urgent: other nerve root pain, spinal stenosis and non-specific low back pain

History

• Red flag symptoms as above.
• Acute or chronic problem? (Acute is arbitrarily defined as <6 weeks.)
• Distinguish referred pain from nerve root pain.
  • Referred pain is common with low back pain. It is usually a dull, poorly localized pain, and can affect both legs.
  • In contrast, nerve root pain is sharp and well localized, following a dermatome quite closely. People with nerve root pain often have a sensation of numbness or tingling.
  • Nerve root pain from L5/ S1 (the most common level) usually extends to the foot or toes.
  • When nerve root pain is present it is usually the chief complaint.¹
• Distinguish spinal stenosis features:⁸
  • Leg pain on walking, eased by leaning forward or sitting, but not by standing still (unlike vascular claudication, where pain does improve after standing still).
  • Normal peripheral circulation; normal straight leg raising (nerve root signs appear late).
  • More likely in over-60s or ankylosing spondylitis.

Examination (targeted from the history)

• Ask the patient to walk with upper clothes off to assess gait and posture.
• Tip-toe walking assesses the integrity of the S1 root.
• Palpate the painful area for spasm, any masses or unexpected tenderness.
• Check the power of the hamstrings. Test the ability to dorsiflex the foot (L4,5), the great toe (L5) and the function of the intrinsic muscles of the foot (S1,2). Check sensation if appropriate.
• The passive straight leg raise test (test of Lasegue) is widely used to help diagnose nerve root pain.¹
  • It has high sensitivity (about 90%) but low specificity (about 20%) for diagnosing nerve root pain due to herniated discs.
  • With the person lying flat on their back with both legs straight, raise one leg until limited by pain and/or tight hamstrings. Slightly lower the leg to provide relief.
  • In this position, increase tension on the sciatic nerve by dorsiflexion of the foot (or flexion of the neck or compression of the nerve in the popliteal fossa). This will aggravate or elicit pain radiating down the raised leg if there is nerve root irritation. The pain should be relieved by flexion of the knee.
• The crossed straight leg test is said to be more specific for radicular pain. It is performed by raising the unaffected leg in a similar manner to the straight leg test, looking for the reproduction of the pain.^15,6
• Check perineal sensation; if cauda equina syndrome suspected, check anal sphincter tone.
• Examine abdomen for enlarged bladder, aneurism or masses.
• Check lower limb circulation if claudication-type symptoms.
• Check hip rotation if hip joint pain suspected.¹

• Femoral nerve root pain (L1-4 nerve roots: symptoms will be in the distribution of the femoral nerve, i.e. higher on the limb and anteriorly).
• Hip pain: painful or reduced hip rotation is an early sign¹
• Nonspecific low back pain
• Facet joint arthrosis
• Sacroiliac pain

Typically, the pain of true sciatica will be sharp and in a nerve or dermatomal pattern; and pain in a single nerve root distribution is more likely due to a disc herniation.⁸

• If no red flags are present, investigations are not required initially. Lumbar spine XRays should be avoided unless indicated as their radiation dose is high.
• If red flags are present, first line investigations are:
  • Plain XRay of lumbosacral spine (and thoracic spine thoracic pain also present)
  • Blood tests: FBC, ESR, U&E and any other tests suggested by the possible differential diagnosis, e.g. alkaline phosphatase, PSA, serum calcium, serum protein electrophoresis/urinary Bence-Jones protein
  • Urinalysis if red flags for cancer or infection
• Second-line investigations are:
  • CT or MRI (equally effective⁶)
  • Bone scan

Prognosis

Most attacks (90%) of acute sciatica settle with conservative management.¹⁶ Of patients with acute sciatica, 50% report improvement within 10 days and 75% within 4 weeks. By one year, about 70% are recovered, while 30% continue to have pain.⁶ However, improvements may continue beyond 1 year. Decisions about invasive management should be balanced against this natural history.¹⁶

Initial management¹

Refer patients with red flag features (as above)
If no red flags, initial management is conservative for the first 6 weeks, comprising:

• Advice that s/he can remain active if feasible. A cochrane review on bedrest vs. staying active for sciatica gave neutral results (in contrast to non-speciific low back pain, where remaining active seems to confer benefit.)¹⁷
• Analgesia as needed. CKS recommends paracetamol first line; if required add a NSAID (ibuprofen/naproxen/diclofenac) if not contraindicated and/or codeine 30mg; consider diazepam 2-4 mg tds for 3-7 days for muscle spasm.
• Explanation and encouragement that initial symptoms usually improve within weeks.

Follow-up and further management¹

Assess response to treatment in about 4 weeks, looking at:

• Symptoms, distress, disability and employment
• Risk factors for chronic pain ('yellow flags')

Management strategies are:

• If nerve root pain has not responded to conservative treatment, consider referral for further imaging and possible surgery.
• Provide education and advice about back pain.
• Adjust analgesia as for initial management. In addition, consider other pain control methods such as tricyclic antidepressants, topical capsaicin, gabapentin, or strong opioids (the latter usually on specialist advice).
• If yellow flags present, consider a multidisciplinary approach or cognitive behavioural therapy.
• Occupational health issues advice if needed.
• Consider spinal manipulation treatments, exercise therapy or back school. Recent USA guidelines also suggest the options of yoga, acupuncture, massage therapy or progressive relaxation therapy.¹⁸

Invasive treatment options for patients with lumbar disc herniation

Epidural steroid injection:¹⁹

• There is limited evidence that this may improve pain and delay/prevent the need for surgery.
• Conservative management should be tried first.
• It should be performed under fluoroscopy with contrast for best results.

Surgery

The standard procedures are discectomy or microdiscectomy - the latter uses magnification to view the disc and nerves during surgery. A recent Cochrane review¹⁶ of these procedures found that:

• Results are comparable between discectomy and microdiscectomy; the choice mainly depends on local expertise.
• For relief of sciatica symptoms, surgery produces better results at one year compared with conservative treatment. Careful selection of patients is needed. In the longer term, the benefit of surgery is unknown.
• If there is uncertainty as to the need for surgery, postponing it in order to assess clinical progress is reasonable. This may delay recovery but does not cause long-term harm.

Drawbacks: there are small but important risks of disc surgery, and a lack of good data on complication rates.² Chronic pain ('failed back surgery syndrome') can occur in the longer term, perhaps due to scarring or recurrence of disc herniation (see discussion section of reference.²⁰)

Other treatments and research

Minimally invasive surgical techniques have been developed as alternatives to open discectomy. Currently these are regarded as research techniques, until more data is available on their safety and effectiveness:¹⁶

• Percutaneous intradiscal electrothermal therapy²¹
• Automated percutaneous mechanical lumbar discectomy²²
• Endoscopic laser foraminoplasty²³
• Laser lumbar discectomy²⁴

Injection of oxygen-ozone mixture gas into the disc and foramen is a new technique which shows promise. The mixture is thought to dry out the disc, reduce inflammation and improve local microcirculation.^20,25

Antibodies against tumour necrosis factor alpha have been researched, because of the possible causative role of this factor in sciatic pain. Some small trials have used infliximab²⁶ or etanercept,²⁰ but with no clear benefit so far.

Chemonucleolysis is the injection of an enzyme such as chymopapain into the disc in order to reduce the disc size. This has been used since the 1970s, but currently is not favoured and is not generally available. There are conflicting opinions about its usefulness.⁵

Traction for sciatica has been reviewed but the evidence is conflicting. (For patients with low back pain rather than sciatica the review found that traction was not effective.) It is not recommended.⁵

• Chronic pain and recurrent sciatica; medication side-effects
• Psychosocial problems
• Loss of employment

• Improved general fitness and posture
• 'Back school' – advice on how to cope with and ameliorate pain, advice on lifting techniques
• Occupational therapy and job retraining if these activities precipitate attacks

The author is grateful to Dr Sean Kavanagh for writing the original article.