Lateral Disc Herniations

and Clement K. Jones, M.D.

The Elusive Lateral Disc Herniation is a lesion that is commonly missed by radiologists and clinicians since it is difficult to visualize on the MRI scan. The Myelo Disco-CT, although not always necessary, is the definitive test for the lateral disk herniation when the MRI scan is normal.

DEFINITION

The lateral disc herniation distinguishes itself from the postero-lateral herniation in that the disc ruptures outside the spinal canal, lateral to the root foremen. The disc, instead of tethering the traversing nerve root compresses the more rostral nerve root which has already exited its more rostral nerve root foremen. The neurologic symptoms therefore, correspond to a lesion at the upper disc level often leading to confusion in the diagnosis.

(fig.1) Lateral disc herniation on the more proximal nerve root.

(fig. 2) Posterolateral disc herniation impinges on the traversing nerve root.

THE PROBLEM

A patient with a lateral disc herniation can sometimes be a true diagnostic challenge and therapeutic dilemma. Not uncommonly he presents with symptoms of a clear cut disc herniation, with pain radiating below the knee, made worse by sitting, standing, and walking and relieved by rest with the knees flexed. Yet when the clinician consults the MRI report no herniated disc is seen. For the patient with private insurance this is a frustrating experience, but for the patient with a worker's compensation injury it is doubly frustrating, since his credibility is challenged and his benefits may be terminated.
The clinician is also placed on the spot. He must explain to his patient that he has symptoms of a herniated disc yet none appears on the MRI report. He must also explain this fact to the insurance carrier who will immediately refer the patient to another physician since by this time both the carrier and the patient have lost confidence in the treating doctor.

What are the two reasons for the confusion? One, the symptoms of the lateral disc do not correspond to the proper symptomatic level on the MRI scan, and two, the lateral disc zone can be difficult to visualize . As we have mentioned in prior reports the problem can be solved by the clinician, if he takes a careful history and performs a meticulous physical examination. His suspicion will then be heightened and he will seek other avenues to prove his diagnosis. He must also be critical of the quality of the MRI scan, and ask himself is this test sufficient. The following is a case seen recently which illustrates these points.

CASE REPORT

HISTORY: P.J. is a 44 year old woman who worked as a receptionist for a veterinary clinic. She was seen with a chief complaint of pain radiating to the medial aspect of the left thigh and knee as well as groin and buttock pain for 6 months. Six months previously she bent over to catch a small dog and noticed a sharp pain in her back. The next day she had difficulty getting up. She sought the help of a chiropractor who helped her through her acute illness, but then referred her to the spinal surgeon because of weakness in her left leg. She felt numbness along the medial aspect of her left knee. Bending increased the pain as did standing for 5 minutes. Sitting for 10 minutes and walking several blocks also increased the pain. She noticed buckling of her left knee. Driving a car and sleeping were all difficult.

EXAMINATION: The patient has difficulty bending forward or leaning to the left. She walks with a noticeable limp and cannot bear full weight on her left leg. Heel walking and toe walking are difficult on the left but she is able to perform both. There is decreased sensation along the medial aspect of the left thigh when tested to pin prick. There is normal strength of the foot and ankle dorsiflexors and plantarflexors, but the left quadriceps is weak. There is a diminished left knee jerk, and the straight leg raise and bow string sign are normal. An MRI was performed and was read as follows:

MRI REPORT: "There is normal development and alignment with no evidence of fracture or signal abnormality. Disc spaces are normally-maintained in height and signal, and there is no evidence of disc protrusion or significant bulge of disc or annular ligament. The spinal canal and neural foramina are normal in size and contour.

CONCLUSION: Normal examination of the lumbosacral spine by magnetic resonance imaging."
The patient eventually was taken to surgery where a large lateral disc herniation was removed compressing the L3 root at the L3-4 level (fig. 5).

DISCUSSION

Lateral disc herniations represent between 1 and 11% of all disc herniations. Abdullah1 is often quoted that whenever neurologic findings suggest upper lumbar nerve root compression, the chances are 4 to 1 that the responsible lesion is an extremelateral disc herniation. This patient had all the signs and symptoms of such a lesion i.e., thigh numbness, quadriceps weakness, and loss of the patellar reflex. In addition she had a confirmatory aversion toward leaning to the symptomatic side.

(fig.3) Lateral disc herniation is not seen on sagittal view of MRI since the lesion is lateral to plane of the scan.

The straight leg raising test was normal since it was the femoral nerve roots and not the sciatic nerve roots compressed by the herniated disc. In this case the femoral stretch test would have been more helpful than the straight leg raising test in diagnosing the herniated disc.
With such a convincing history and physical the clinician must not rely solely on the MRI scan. The lateral zone is not well seen on a myelogram or the sagittal view of the MRI (fig. 3). The lateral disc does appear on the axial view but can be missed if the CUt does not transect the precise level of the disc herniation. Furthermore, the signal given off by the disc herniation is often similar to that given off by the pedicle, and the psoas muscle making it difficult to visualize (fig.4).

(fig.4) Comparison of axial view of MRI (above) and Disco-CT with contrast injected into the disc (below). In upper image (MRI), note normal appearance of the lateral border of the disc (large white arrow). In lower image (Disco-CT), note the obvious lateral disc herniation ( black arrowhead).

The clinician may also be misled since the symptoms do not correlate with the MRI. In a case where the patient has symptoms of L5 root involvement i.e., pain and numbness along the anterolateral leg, the top of the foot and the great toe, the L5 root is normally compressed in the spinal canal at the L4-5 level, by an extruded disc at L4-5 or by proximal migration of a sequestered fragment from the L5-S1 level. With a lateral herniation and L5 root entrapment, an axial view of both levels may reveal a normal scan if the lateral portion of the L5-S1 disc is not clearly distinguished from the pedicle, marginal osteophytes, and psoas muscle.

(fig. 5) Photograph of lateral disc in surgery. L3 root is retracted laterally (small white arrows). Facet joint (fac), Disc herniation (large white arrow)

When you see a patient age 30-60 with dear cut symptoms of nerve root irritation or malfunction, made worse with sitting, bending, and walking, and relieved by rest, think herniated disc! If the patient is reliable do not only depend on the radiologists report. Remember, he does not have the advantage of seeing the patient. He only sees the study. Obtain the scan for yourself. Determine which nerve root is involved by taking an accurate history, and performing a meticulous physical examination. Map out the course of the affected root on the scan for yourself. If you still do not see the lesion, and you are convinced that the patient has true radicular pain, a Myelo-Disco-CT will often show the herniated disc missed by the other study.

REFERENCES

1. Abdullah AF, Ditto EW: Extreme-lateral lumbar disc herniations. J. Neurosurgery 41 229-34, 1974.
   
   
2. Armstrong GW, O'Neil DJ: Far Lateral disc herniation treated with an anterolateral retroperitoneal approach. Report of two cases. Spine 17(3): 363-5, 1992.
   
   
3. Epstein NE: Evaluation of varied surgical approaches used in the management of 170 far-lateral lumbar disc herniations: indications and results. Journal of Neurosurgery 83(4): 648-56, 1995.
   
   
4. Hood RS: Far lateral lumbar disc herniations. Neurosurgery Clinics of North America4(1): 117-24, 1993.
   
   
5. Kurobane Y., Takeshi T., et.al.: Extraforaminal Disc Herniation. Spine 11: 260-268, 1986.

Material provided by the San Francisco Spine Center
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