Clinical Presentation

The patient is a 65-year-old male with a 2-year history of progressive right thigh pain radiating down the lateral aspect of the right leg. The pain is exacerbated by walking but also present when sitting. The patient walks with a limp. Straight leg raising sign is negative. There is a diminished right knee jerk ( Figs. 21-1 and 21-2 ) .

Right L4-5 Foraminal Disc Herniation.

Herniated disc material (arrows) obscures the fat in the right L4-5 neural foramen on T2-weighted MR image A and T2-weighted MR image B. On contrast enhanced image C , the herniated disc (long arrow) does not enhance, but there is subtle contrast enhancement (short arrow) adjacent to the nonenhancing disc fragment.

Right L4-5 Foraminal Disc Herniation.

Same patient as in Figure 21-1 . The herniated disc (arrow) is relatively hypo intense on unenhanced axial T2-weighted MR image A and on T1-weighted image B. On T1-weighted contrast enhanced MR image C, note the subtle contrast enhancing tissue- (short arrows) adjacent to the nonenhancing disc fragment (long arrow) .

Imaging Presentation

Magnetic resonance (MR) imaging of the lumbar spine reveals a large herniated disc extending into the right L4-5 neural foramen obscuring the intraforaminal fat adjacent to the right L4 nerve root. Subtle contrast enhancement is demonstrated adjacent to the nonenhancing intraforaminal disc fragment.

Discussion

Foraminal or extraforaminal disc herniations comprise approximately 10% of lumbar disc herniations. The L4 nerve root is most commonly involved. Foraminal disc herniations tend to occur in older patients, often with little evidence of disc degeneration. Posterolateral vertebral osteophytes and facet hypertrophy may contribute to the neural foraminal stenosis associated with foraminal disc herniation. The predisposing factors for lateral disc herniations are similar to other lumbar disc herniations. (Refer to discussion in Chapter 17 on lumbar disc herniation).

With foraminal or far lateral (extraforaminal) disc herniations, characteristic clinical findings are related primarily to radiculopathy in the distribution of the involved nerve root. The radicular pain with foraminal and extraforaminal disc herniations may be extremely intense, which is believed to be due to compression of the dorsal root ganglion. The pain is often located in the groin or anterior thigh, but may radiate more distally in the lower extremity. The pain may be accentuated by lateral bending. Low back pain may be present but is frequently absent or less severe than the radicular pain. The patient may experience dysesthesias or sensory loss. The knee jerk may be absent. A positive straight-leg raising sign is often present.

Imaging Features

Herniated discs that extend into the neural foramen (intervertebral foramen) usually are not detected by myelography alone, because they cause minimal or no root pouch deformity and therefore require computed tomography (CT) or MR for imaging detection ( Figs. 21-1 to 21-7 ) . Foraminal and extraforaminal (far lateral) disc herniations are not uncommonly overlooked even when CT and MR imaging studies are interpreted.

Left L3-4 Foraminal (Lateral) Disc Herniation.

On axial post myelogram CT image A, the herniated disc (arrows) in the left neural foramen is less dense than the contrast in the thecal sac and adjacent vertebra but is nearly isodense relative to the paraspinal muscles. On T1-weighted axial MR image B, the herniated disc fragment (arrows) obscures fat in the left L3-4 neural foramen.

Right L3-4 Foraminal Disc Herniation.

The herniated disc (arrow) on T1-weighted MR image A and T2-weighted MR image B is in continuity with the intervertebral portion of the disc. The foraminal disc herniation superiorly displaces the right L3 nerve root (small arrow in image A and image B ) .

Right L3-4 Foraminal Disc Herniation.

Same patient as in Figure 21-4 . A broad based right L3-4 foraminal disc protrusion/herniation (arrow) obscures the foraminal fat adjacent to the right L3 nerve root on axial T1-weighted MR image A and T2-weighted MR image B. The herniated disc (arrow) is slightly T1 hyperintense relative to the intervertebral disc on image A and T2 isointense relative to the intervertebral disc on image B.

Left L1-2 Foraminal Disc Herniation.

On axial T2-weighted MR image A, the herniated disc (arrows) is slight T2 hyperintense relative to the vertebra but T2 hypointense relative to intrathecal CSF. On axial T1-weighted MR image B, the herniated disc (arrows) is nearly isointense relative to CSF. Note the intense contrast enhancement (arrows) surrounding the nonenhancing central core of the herniated disc fragment on contrast-enhanced T1-weighted MR image C.

Left L3-4 Foraminal Disc Herniation.

Herniated disc material (arrows) obscures the fat in the neural foramen on unenhanced axial T1-weighted MR image A. Intense contrast enhancement (arrows) is present along the margins, the nonenhancing central core of the herniated disc material on axial contrast enhanced T1-weighted MR images B and C.

Lateral disc fragments compress the nerve roots as they course below the pedicle at a given lumbar disc level. For example, a lateral disc herniation at the L4-5 level will produce L4 radiculopathy, whereas a posterolateral disc herniation compressing the thecal sac at the same level causes L5 radiculopathy. A foraminal disc herniation may be visible on CT as an isointense or slightly hyperintense mass within the neural foramen (see Fig. 21-3 ). MR imaging has advantages over CT for detection of lateral disc herniation. The foraminal disc herniation is usually well demonstrated on axial T1-weighted images, which show excellent contrast between the disc material and the fat (see Figs. 21-1 to 21-6 ).On MR imaging, the foraminal disc herniation is usually contiguous with the parent intervertebral disc, a finding that is best demonstrated on parasagittal MR images obtained through the neural foramen (see Figs. 21-1 and 21-4 ). The parasagittal T1-weighted or T2-weighted Spin Echo (SE) MR images obtained through the foramen are also valuable for assessing the degree of nerve root compression (see Figs. 21-1 and 21-4 ). The herniated disc fragment typically does not enhance, but enhancement is nearly always demonstrated along the margin of the intraforaminal disc fragment (see Figs. 21-6 and 21-7 ).

Most foraminal and extraforaminal disc (far lateral) herniations tend to have a wide base where they extend beyond the vertebral body margin, and therefore the majority of foraminal disc herniations are disc protrusions by definition (see Figs. 21-5 and 21-6 ). However, lateral and far lateral foraminal disc extrusions also occur. Double disc fragments can also occur in the neural foramen or lateral to the neural foramen.