Many spinal complaints can result in pain referred to the leg, and in some cases back pain may not be noted. The distribution of leg pain and associated signs indicates if the pain is due to a trapped nerve root (radiculopathy). True sciatica radiates below the ankle, and pain is exacerbated by stretching the nerve.
The commonest cause of true sciatica is prolapse of intervertebral discs. Intervertebral discs make up about 25% of the height of the spine. Each disc consists of an outer fibrocartilagenous annulus fibrosus, which facilitates torsional movements, and an inner nucleus pulposus. The fibres of the annulus are kept under tension by the nucleus pulposus, which is 70-90% water; this allows it to change shape in response to compressive force.
Nerve roots lie in the immediate path of laterally prolapsing intervertebral discs. In the lumbar spine, such prolapses can compress the lower emerging root to produce pain and dysaesthesiae in the associated dermatome and other features typical of a lumbar root syndrome.
The straight leg raise test is positive if pain in the sciatic distribution is reproduced between 30° and 70° passive flexion of the straight leg. Dorsiflexion of the foot exacerbates the pain
Spinal stenosis—Narrowing of the spinal canal is relatively common in elderly people. It can have several causes, including facet joint arthrosis, ossification of the longitudinal ligament, Paget's disease, and ankylosing spondylitis. Symptoms typically include low back pain, leg pain, and pseudoclaudication. The spinal canal should be examined by magnetic resonance imaging or computed tomography, and surgery is indicated for severe symptoms.
Cauda equina syndrome—Compression of the cauda equina can occur with central disc herniation, epidural masses (for example, abscesses), or tumours. Low back pain, bilateral sciatica, saddle anaesthesia, loss of sphincter tone, and bladder and bowel incontinence may result. Urgent magnetic resonance imaging is needed to find the cause and direct management.
Red flags in patients with low back pain
Facet arthrosis or syndrome—Pain from facet joints is common, particularly in patients with degenerative disc disease. Local or diffuse back pain may be associated with leg pain but without localising neurological signs. Treatment includes analgesia, modification of activities, and rehabilitation. Local corticosteroid injections are used occasionally.
Mechanical stresses on spine
Non-specific back pain—In 85% of cases, pain in the low back results from the presumed effects of mechanical and postural stresses on spinal and paraspinal structures (“non-specific” back pain), although the pathophysiology is poorly understood. Muscles, particularly those of the abdomen (the obliques, transversus, and recti) provide dynamic stability and fine control to the spine. This is important when deciding on the appropriate management of non-specific back pain. Pain can be worse with movement in any plane, may radiate to one or both legs, and is relieved by lying with the hips and knees flexed.
In the absence of serious pathology, management should focus on education and encouragement about pain control so that rehabilitation can be started. In the most acute cases, in which muscle spasm is severe, one or two doses of diazepam can be useful. When a facet joint problem is suspected, some doctors advocate facet joint injections for diagnostic and therapeutic use.
Rehabilitation is through manual and exercise therapy to mobilise and strengthen the supporting structures of the spine, correction of postural and biomechanical irregularities, and educating patients about their back. A multidisciplinary team may be required, particularly if ergonomic issues are thought to be contributing to the pain. Most patients with a prolapsed disc respond to such measures, but surgery may be necessary.
Short term management of pain