Radiology Resident Case of the Week, May 15, 1997

Lumbar disk herniation and conjoined nerve root sleeve (lumbar)

Eric Fitzcharles, M.D.
Peer Review Status: Not Internally Reviewed

Clinical Sx:
53-year-old male with several-month history of increasing right lower extremity weakness and numbness.

Etiology/Pathophysiology:
Disk degeneration is noticeable by the age of 20. The process consists of desiccation, or water loss in the nucleus pulposus and decreased tissue resiliency with decrease in the height of the disk space. With age, the initially soft and gelatinous nucleus pulposus is replaced by fibrocartilage and the distinction between nucleus pulposus and annulus fibrosis becomes less distance. The annulus becomes fissured and negative pressures bring nitrogen out of solution causing vacuum phenomenon. Disk height loss leads to malalignment and all of these processes permit disk material to bulge and subsequently herniate.

The second finding in this patient was a conjoined nerve root sleeve. This is a congenital anomaly that is considered a normal variant and is found in 1-3% of the population.

Pathology:
The terminology of disk disease is at times muddy, however the following definitions are helpful: bulge: concentric smooth circumferential expansion of softened disk material beyond the confines of endplates. Protrusion = herniation: focal protrusion of disk material maintaining broad base with parent disk due to weakened or ruptured annulus fibrosus but intact posterior longitudinal ligament (herniation implies ruptured annulus fibrosus but protrusion doesn't necessarily). Extrusion: prominent focal extrusion of disk material with only an isthmus of connection with the parent disk due to ruptured annulus and intact or ruptured posterior longitudinal ligament. Free fragment: frank separation of disk material from parent disk (may migrate). There was no resected pathologic specimen from this patient.

Conjoined nerve roots are one pair of unilateral nerve roots in a unique dural sleeve that leaves the spinal canal through a single intervertebral foramen or at two separate levels. L5-S1 is the most common level, with S1-2 the next most likely. Affected levels are prone to compression by disks and lateral bony stenosis, as they run close to bony structures along the lateral recess.

Imaging:
Plain film myelographic images reveal decreased contrast within the thecal sac adjacent to the L4-5 disk, more prominently on the right, corresponding to a right paracentral disk herniation. Mild disk bulges are also noted at L3-4 and L5-S1. Also noted is a wide nerve root sleeve appearing to contain two nerve roots on the right, passing just medial to the left L5 nerve root which exits under the L5 left pedicle. This is a conjoined left S1-2 nerve root sleeve.

CT myelographic images show the right paracentral L4-5 disk herniation obliterating a significant portion of the thecal sac at that level and filling the lateral recess.

DDX:
Osteophyte, facet hypertrophy, and ligamentous hypertrophy can all narrow the thecal sac, but the location and imaging features of the process in this case are highly suggestive of a disk herniation.

Key References:
1. Dahnert, Radiology Review Manual, 3rd ed, pp.146-47. Grossman, Yousem, Neuroradiology: the Requisites, pp. 460-61. Manelfe, Imaging of the Spine and Spinal Cord,

ACR Code:
363.78313

Keywords:
disk herniation, conjoined nerve root sleeve, degenerative disk disease, disk protrusion

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See related Provider Topics Bones, Joints and Muscles or Slipped Disk.

See related Patient Topics or Bones, Joints and Muscles.

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