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Neuroradiology Case of the Week

Case 115

Clinical Presentation:The patient is a 47-year-old female who presents with a history of left leg weakness, back pain, and left leg pain. It began the day prior to presentation when she sneezed while she was shampooing her hair.

Radiological Findings: MRI of the lumbar and sacral spine shows a far lateral focal disc protrusion at the level of the L4-5 disc. The protrusion extends superiorly and impinges upon the exiting L4 nerve root. There is also a slight broad based disc bulge indenting the anterior aspect of the thecal sac without significant spinal stenosis.

Discussion: The intervertebral disc is composed of an outer ligamentous annulus and an inner gelatinous nucleus pulposus. Disc herniation occurs when a tear in the tough annulus allows the inner nucleus pulposus to prolapse through the annulus. Neurologic symptoms of pain or dysesthesia may occur if the prolapsed disc presses against a nerve root.
     A specific type of disc herniation is the far lateral disc herniation (FLDH), also known as a foraminal disk herniation. FLDHs account for 3-10% of all disc herniations. In this situation, the herniated disc may cause nerve root compression within the foramin or extraforaminally, as the nerve root continues its extraforaminal course. They differ from classic, more medial herniations because FLDHs compress the exiting root at that level, whereas classic herniations compress the root at the level below. For example, an L4-5 FLDH will impinge upon the L4 root, while a classic herniation at L4-5 impinges upon the L5 root.
     Most FLDHs occur in older individuals and originate from upper lumbar levels - the L3-4 and L4-5 discs. In contrast, classic disc herniations usually originate from the lower lumbar levels - the L4-5 and L5-S1 discs. Because of the anatomical location of FLDHs, their clinical presentation often differs from that of classic herniations. Classic herniations usually produce lower back pain and pain in the posterolateral thigh with radiation to the foot. On the other hand, FLDHs can cause sudden onset of pain in the anterolateral thigh. Additionally, the patient will likely have weakness in the quadriceps, decreased patellar reflex, decreased sensation in the associated dermatome, and referred pain to the knee.
     Before the advent of CT and MRI, FLDH was rarely diagnosed. This underdiagnosis is attributable to the fact that myelography is typically negative in all cases of FLDH. MR is helpful in differentiating a FLDH from a classic disc herniation as well as differentiating it from the other primary diagnosis on the differential - a schwannoma. On MR, FLDH appears as a protrusion that is contiguous with the intervertebral disc. It will show peripheral contrast enhancement while a schwannoma shows homogenous enhancement.
     Treatment of FLDH usually requires surgical intervention. In the past, one surgical approach was to create an interlaminar window with loss of the facet joint; however, this approach often left the patient with postoperative instability. Recently, a far lateral surgical approach has proven to be safe, effective, and with no disruption of spinal stability.

References:

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