Approach Considerations
After plain radiographs are obtained, further imaging studies (eg, magnetic resonance imaging [MRI], computed tomography [CT], and CT myelography) may be indicated to assess degenerative disk disease, loss of disk height, and facet deterioration, such as sclerosis or hypertrophy. MRI clearly provides the most information—perhaps too much, in that it has a 25% false-positive rate (asymptomatic herniated nucleus pulposus [HNP]).
An HNP that is noted on imaging studies must be correlated with objective examination findings; otherwise, it must be presumed to be an asymptomatic HNP if there is no correlation between the imaging findings and pain or clinical symptoms. Therefore, imaging studies should perhaps be reserved for cases in which positive physical findings have been documented.
Other causes of significant back pain in the absence of neurologic findings should be considered. Sciatic nerve irritation may result from sacroiliac dysfunction or degenerative joint disease caused by the proximity of the sciatic notch to the sacroiliac joint or peripheral entrapment, including piriformis syndrome. Careful examination with an adequate differential for the diagnosis may prevent prolonged ineffective empirical care for presumed lumbar disk disease.
The facet syndrome has been controversial, but neurophysiologic studies have shown discharges from the capsule consistent with pain, as well as inflammation and degenerative joint disease. [29] However, large numbers of patients have reported significant relief after facet-joint injections for nonspecific low back pain (LBP); as a result, the facet syndrome has become more widely accepted. Clinically, patients usually have pain only to the knee, not below, as would be expected from an HNP.
Imaging Studies
Plain radiography
Plain radiographs cannot show a disk herniation directly. Their main utility in the workup of a patient suspected of having disk herniation is to show indirect evidence of disk degeneration, such as disk-space narrowing, endplate changes, osteophytes, facet-joint degeneration, and alteration of sagittal balance. In fact, in most young patients with disk herniations, plain radiographs may be completely normal, except insofar as they show loss of lordosis due to muscle spasm or a sciatic tilt. As far as planning for surgery is concerned, radiography can rule out more serious underlying causes of back pain, such as infections or tumors; it can also show sacralization or lumbarization of vertebrae.
Magnetic resonance imaging
MRI is the gold standard in the evaluation of a suspected lumbar disk herniation. It provides accurate and detailed information regarding disk morphology, hydration, herniations, endplate changes, and nerve-root and cord status. (See the image below.) MRI with gadolinium contrast enhancement is often used to evaluate patients who have already undergone decompression surgery and are suspected of having recurrent or residual disk herniation. The postgadolinium T2-weighted images can be used to differentiate between scar tissue (which enhances) and disk fragments (which do not).
MRI may be useful for predicting the likelihood that a patient with lumbar disk herniation will require microdiskectomy. [30]
Myelography
Myelography, once considered the investigation of choice, is now rarely used. Its main disadvantage is that whereas it is capable of showing the level at which the pathology exists, it cannot define the nature or morphology of the lesion or the precise locationof the lesion in the anatomic segment. Current application of myelography is restricted to the performance of CT myelography in patients who have a contraindication for MRI, such as those who have a pacemaker in place or those who are claustrophobic.
Other Tests
Several other blossoming modalities exist that have been sparingly used in the evaluation of patients with suspected disk herniation, such as the following:
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Magnetic resonance (MR) spectroscopy
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MR neurography
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Electromyography (EMG) and nerve conduction tests
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Dynamic MRI
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Open/standing MRI
Procedures
Diagnostic selective nerve root blocks using bupivacaine have been used to pinpoint a particular nerve root as the culprit behind a patient's symptoms. This can prove useful when imaging shows disk herniations at two levels and there is confusion as to which of the two levels is contributing to the patient's symptoms.
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Hyaluronan long chains form backbone for attracting electronegative or hydrophilic branches, which hydrate nucleus pulposus and cause swelling pressure within anulus to allow it to stabilize vertebrae and act as shock absorber. Deterioration within intervertebral disk results in loss of these water-retaining branches and eventually in shortening of chains.
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Nuclear material is normally contained within anulus, but it may cause bulging of anulus or may herniate through anulus into spinal canal. This commonly occurs in posterolateral location of intervertebral disk, as depicted.
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Spinal nerves exit spinal canal through foramina at each level. Decreased disk height causes decreased foramen height to same degree, and superior articular facet of caudal vertebral body may become hypertrophic and develop spur, which then projects toward nerve root situated just under pedicle. In this picture, L4-5 has loss of disk height and some facet hypertrophy, thereby encroaching on room available for exiting nerve root (L4). Herniated nucleus pulposus within canal would embarrass traversing root (L5).
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Appearance of lumbar disk herniation on MRI.
