Lower Back and Lower Limb Pain

Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) commonly is performed to assess the lumbosacral spine and lumbosacral plexus. It also can be used to evaluate the peripheral nerves in the pelvis and lower limbs.

MRI of the lumbosacral spine has the highest yield when the patient has back pain associated with radicular distribution of pain. Isolated back pain with no clinical symptoms or signs in the leg seldom is associated with significant findings on MRI. Intraspinal disorders that may not be revealed by MRI without contrast enhancement include neoplastic meningitis, epidural abscess, diskitis, and some chronic infectious meningitides ( ).

Techniques for MRI of the lumbosacral plexus and peripheral nerves have greatly improved, so this modality can reveal masses, infiltration, and some inflammatory lesions, but MRI can miss disorders that are without a structural defect.

Myelography and Postmyelographic Computed Tomography

With the advent of MRI, myelography has been performed less commonly. If adequate information is not obtained from noninvasive studies, myelography occasionally may be indicated. Modern uses of myelography are not only for patients who cannot have MRI due to size or implanted devices ( ).

For myelography, lumbar puncture is performed, and radiopaque dye is infused into the cerebrospinal fluid (CSF). Conventional radiographs are obtained as the dye is manipulated through the CSF pathways. Postmyelographic computed tomography (CT) is performed in most instances.

Nerve Conduction Studies and Electromyography

Nerve conduction studies (NCS) and electromyography (EMG) are performed for four principal purposes:

• Assist localization of the lesion(s)

• Assist in evaluating the severity of the lesion(s)

• Determine whether the lesion is acute, subacute, or chronic

• Determine whether the lesion is neuropathic, axonal, or demyelinating

Axonal damage seen with radiculopathy or entrapment neuropathy suggests consideration of surgical decompression. Of note, signs of denervation may not appear on EMG until up to 4 weeks after onset of axonal damage.

Entrapment neuropathy, or nerve root compression which can be responsible for lower limb pain, is likely to slow conduction velocity across the region of compression. Conduction velocities proximal and distal to the compression are usually normal, so conduction across the affected nerve segment must be studied.

Radiculopathy typically is associated with normal NCS findings in the peripheral branches of the nerves but with slowing of the F-wave. Asymmetry of tibial H-reflexes can suggest S1 radicular process on the side with the prolonged H-reflex Absence of abnormalities on NCSs and EMG does not rule out the presence of a radiculopathy.

Mechanical lower back pain is associated with no EMG or NCS alterations, so these studies are not usually indicated unless symptoms or signs of neural involvement are present.

Radiography

Plain radiographs are obtained in patients with acute skeletal trauma and in almost all patients with isolated lower back pain. Among the potential findings are degenerative joint disease, vertebral body collapse, bony erosion, subluxation, and fracture. Radiographs of the pelvis and long bones also are obtained and may show fractures and destructive lesions.

Bone Scan

Bone scan is especially important for examining multiple bone regions in cases of suspected neoplastic bone involvement. Multifocal involvement makes a neoplastic cause more likely than an infectious cause for the destruction.

Lumbar Spine Stenosis

Lumbar spine stenosis is a disorder that affects mainly late middle-aged and older adults. The cause is multifactorial, with disk disease, bony hypertrophy, and thickening of the ligamentum flavum being the most important. Some of the symptoms are undoubtedly caused by direct pressure of these tissues on the cauda equina and exiting nerve roots, but a major contributor appears to be compression of the vascular supply of the nerve roots. Standing is associated with extension of the lumbar spine, which causes anterior bulging of the ligamentum flavum that lies posteriorly. Compression of the vascular supply creates nerve root ischemia, which can produce severe pain and weakness with exertion.

A diagnosis of lumbar spine stenosis should be suspected in patients with leg pain that is exacerbated by standing and walking and relieved promptly by sitting. Lying down, especially in the prone position, may exacerbate the low back pain, again through lumbar extension, a feature that helps differentiate lumbar spine stenosis from lumbar radiculopathy.

Confirmation of the diagnosis is by MRI or CT of the lumbar spine, which shows obliteration of the subarachnoid space at the level of the lesion. The hypertrophied ligamentum flavum and osteophyte formation are usually evident on these studies. If doubt about the diagnosis exists, myelography with postmyelographic CT scanning can be performed, but this is seldom needed.

Treatment can be conservative in the absence of neurological deficits. Physical therapy and medications can help, but surgical decompression may be required. Weakness of the legs or sphincter disturbance indicates a need for decompression. Although good evidence supports the benefit of surgical decompression at least in the short term, it is not clear that complex spine surgery with instrumentation produces substantial improvement in outcome especially when the etiology of the pain is not related to fracture or instability ( ).

Cauda Equina Syndrome and Conus Medullaris Syndrome

Lesions of the lumbar spine can result in damage to the conus medullaris, cauda equina, or both. Cauda equina syndrome is compression of the nerve roots below the termination of the spinal cord. Nerve root dysfunction is due to direct compression by surrounding structures. Important causes include acute trauma; chronic degenerative bony disease with retropulsion of fragments into the spinal canal; lumbar disk disease; infections such as abscess; intraspinal and meningeal tumor; and intraspinal hematoma. This syndrome can be a rare complication of minor and major spinal procedures. Cauda equina syndrome usually develops as an insidious chronic process unless due to acute trauma. Symptoms can include back pain, leg pain, and weakness and cramps in the legs. Sensory symptoms can be sensory loss as well as neuropathic pain. Sphincter disturbance is common, especially with progression.

Conus medullaris syndrome is due to damage to the terminus of the spinal cord above most of the cauda equina and therefore at a higher spinal level. Etiology can be compression from all the conditions listed above plus occasional infiltrating lesions of the conus medullaris itself, especially by tumor. Conus medullaris syndrome is usually more rapidly progressive, is associated with earlier back pain and sphincter disturbance, and is more likely to be associated with preservation of some lower extremity reflexes, usually patellar.

MRI is the preferred diagnostic imaging method. If MRI cannot be performed, many causes of both syndromes can be identified on CT of the spine but contrast may be required.

Lumbosacral Radiculopathy

Lumbosacral radiculopathy is usually caused by infringement on the neural foramen by either herniated disk material or osteophytes. Herniated disk is more common in young patients; osteophyte formation is more common in older patients.

Patients present with back pain radiating down the leg in a distribution appropriate to the involved nerve root. The most common lumbosacral radiculopathy is of the S1 nerve root, produced by a lesion at the L5–S1 interspace. Table 33.7 presents the typical motor, sensory, and reflex deficits associated with lumbosacral radiculopathy at individual levels.

The presence of lower back pain with radiating pain in a nerve root distribution points to a diagnosis of radiculopathy. Motor, sensory, and reflex deficits are not always present, so the diagnosis is suspected on the basis of symptoms without objective signs.

Confirmation of the diagnosis is by MRI, which can show disk protrusion or osteophyte encroachment with nerve root compression. MRI is the diagnostic procedure of choice for most surgeons, although postmyelographic CT is still occasionally used. Myelography with CT also may be used, especially in patients who cannot undergo MRI because of certain implanted electronic devices and metallic heart valves.

NCS findings are usually normal in patients with lumbosacral radiculopathy, although F-waves may be delayed in the affected root. EMG can reveal evidence of denervation in a nerve root distribution and can usually differentiate peripheral neuropathic processes from radiculopathy. This study also can determine whether denervation is present with radiculopathy.

Management of lumbosacral radiculopathy depends on the severity of symptoms, including pain and weakness. If the symptoms are mild, antiinflammatory agents may suffice. Muscle relaxants can produce short-term relief of muscle spasm and pain. Surgical options for lumbosacral radiculopathy are considered when the patient has intractable pain refractory to conservative care; when weakness is prominent, especially if it is unresponsive to conservative management; and when sphincter disturbance is present. Sphincter disturbance caused by lumbar disk disease or spondylosis necessitates consideration of urgent surgery. Patients with such deficits should not be given a trial of conservative therapy.