An MRI report might list a disc herniation, but the report often adds “clinical correlation suggested.” What are the clinical correlations of a herniated disc said to compress the L4, L5 or S1 nerve roots?
The wise physician always demands “clinical correlation” for spinal MRI findings because abnormalities in the discs can be present, yet not relevant (so-called incidental findings). In point of fact, in older people, incidental findings are more likely than not.
Do not succumb to the wrong idea that a patient with symptoms and an MRI abnormality is necessarily experiencing symptoms because of that abnormality. Indeed, symptoms should be attributed to an MRI abnormality if, and only if, they “correlate” with the patient’s presenting symptoms.
One form of correlation is that the patient’s complaints in the extremities, if present, should match the spinal level of the disc abnormality.
For example, a patient may present with radiating leg pain that is due to a diseased disc compressing a spinal nerve root: a so-called radiculopathy.
Abnormalities of the spinal nerve root produce characteristic patterns of weakness and numbness. About 95% of herniations involve L4/5 or L5/S1 levels and therefore the nerve roots of interest are L4, L5, and S1. Table 1 displays the characteristic findings of abnormalities of the L4, L5, and S1 roots, with Figure 1 illustrating the sensory deficits that can be expected with impingement at these levels.
The exam findings are correlated to the MRI by noting two facts regarding the described disc herniation: first, the level of the herniation, as denoted by the vertebral bodies above and below; and second, the location of the herniation, denoted as central or lateral. If the disc herniation is lateral, the side affected, left or right, should be noted as well.
(Simplification alert: technically, a “central” herniation is rare; what is called “central” here is more accurately called “para-central.”)
To briefly review anatomy, the spinal cord terminates at roughly L2. In the canal distal to this termination, the nerve roots continue, before exiting through their neural foramen.
In the lumbar spine (as contrasted with the cervical region), the numbering system is that the Lx nerve root exits between Lx and Lx+1. That is, a given lumbar root exits under that numbered pedicle. For example, L4 exits between L4 and L5 (see Figure 2).
At the level of the L4/5 disc, the L4 nerve roots are lateral, on their journey out of the right and left foramina. At this level, however, the L5 roots are still pretty central, having not yet taken their steps right and left to their respective foramina.
Thus, it should be pretty clear: at the level of L4/5, a central herniation will compress L5, whereas a far lateral herniation will compress L4, as shown in Figure 2.
One additional form of necessary correlation is laterality: the patient’s finding in the extremities must match the side of the disc abnormality. For example, a compression of the left L4 nerve root does not (perhaps needless to say) correlate with right-sided medial knee symptoms.
It is possible that a very large central disc herniation (or other mass) can compress multiple nerve roots within the central canal and produce a so-called cauda equina syndrome. This can present as more diffuse extremity signs and symptoms as well as bowel/bladder dysfunction. This is a medical emergency. See other question for additional information on cauda equina syndrome.
Additional Points to Consider
- Osteophytes can also put pressure on the nerve roots. Thus, extremity pain can be present from degenerative spondylosis too.
- The symptoms of radiculopathy might also be designated as a “wign” see other question for additional information on wigns), and needless to say, strength testing is affected by effort, and thus is not strictly objective either.
- The straight leg raise test is an exam maneuver that places tension on the nerve roots to help diagnose compression. The patient lies prone and the leg is passively raised by the examiner flexing the patient's hip, with the patient's knee maintained in full extension and the opposite leg is kept flat on the table in extension. A positive test result is the reproduction of radiating leg symptoms when the leg is raised. Note that one exception to the "laterality rule" above is that a straight leg raise test of the asymptomatic lower extremity can reproduce the symptoms in the symptomatic extremity when there is a lumbar disk herniation. This is known as a positive "contralateral straight leg raise test" and is considered a highly specific finding.