We made an important distinction in the last post: that of the far lateral disc herniation. We’ve just discussed the more common central herniated nucleus pulposus (HNP) in which the disc herniates into the center of the spinal canal (see figure 3 of this post.) This centrally herniated fragment hits the traversing nerve that is still within the spinal canal (e.g. a central L4/5 disc herniation causes a radiculopathy of the L5 nerve.) In a far lateral HNP, occurring only in about 10% of cases, the piece of disc herniates on the side of the spine and compresses the nerve along its course within the neural foramen as it exits the spine (e.g. a far lateral L4/5 disc causes a radiculopathy of the L4 nerve.) See figure 1 for an MRI showing a far lateral HNP.
Figure 1: Axial MRI showing large right far lateral HNP (outlined in pink.) Note the displaced nerve root (pink arrow) as compared to the normal nerve root free in its neural foramen (green arrow).
Often I can identify a patient with a far lateral HNP right when I enter the room because they’re MISERABLE. The pain associated with far lateral HNPs is typically much worse than that seen with central HNPs. Just as it exits from within the neural foramen, the nerve dilates into an important junction point called the dorsal root ganglion (DRG). It’s this exquisitely sensitive part of the nerve that is compressed by a far lateral HNP. Couple that with the fact there’s a very limited amount of space within the bony neural foramen for both the herniated disc fragment and the DRG and one understands why this type of disc herniation is so debilitating (see figure 2).
Figure 2: Image of the right side of the lumbar spine showing the nerve roots exiting via the bony neural foramina. Note the dilation of the DRG within the confines of the foramen (red arrow.)
A far lateral HNP requires a different approach than the standard discectomy we discussed in the last post. In the far lateral discectomy, I’ll typically employ an “outside-in” approach to find the fragment under the nerve as it exits the foramen. First, the incision for a far lateral discectomy is made a few more centimeters off of midline compared to that of a standard discectomy. Next, I’ll dock a tubular retractor in between the transverse processes at the level in question (see figure 3). I’ll then work my way into the foramen and look for exiting nerve within the soft tissue of the intertransverse space. One benefit of using an outside-in approach is that usually I don’t have to drill away any of the facet joint and avoid potentially destabilizing the spine. Once I’ve found the nerve (the DRG is usually what I see first) I move it out of the way and the piece of herniated typically found right underneath. In order to mitigate some of the pain caused by my manipulation of the DRG I will apply some steroids to the nerve when I’m done removing the disc.
Figure 3: Image depicting the docking points for discectomy in relation to bony anatomy at the right L4/5 level. The blue circle illustrates the docking point for the tubular retractor in a standard central discectomy. The green circle illustrates the docking point for a far lateral discectomy.
Recovery from far lateral discectomies is typically rougher than after standard discectomies. The DRG is already inflamed and manipulating it to get to the herniated fragment can often make the patient’s pain and numbness worse before it gets better. Thus, I always have to get my patients with far lateral HNP mentally prepared for a tough couple of weeks after their discectomy. In the end, though, patients do very well after a far lateral discectomy.
Thanks for reading!
J. Alex Thomas, M.D.