Radiological anatomical consideration of conjoined nerve root with a case review
A 71-year-old man presented with acute low back pain and radiating pain in the right leg, with a positive straight-leg raising sign at 30°. He had a history of minor trauma suggestive of a lumbar sprain. His symptoms had gradually worsened, and he was unable to walk due to aggravation of pain in the back and leg upon standing or walking. On manual muscle testing, muscle weakness was not detected in the extremities. His patellar tendon reflexes were normal. On neurological examination, L5 nerve root compression was identified. On plain lumbar spine radiographs, spinal abnormalities, except for spondylolisthesis, were difficult to detect. Slight disc space narrowing at L4-5 was noted in lateral view (). Computed tomography showed no definite bone abnormalities. MRI revealed disc herniation at both root exits at the L4-5 intervertebral disc level (). Based on these observations, we diagnosed the patient with spondylolisthesis and lumbar disc herniation at L4-5.\nIntraoperatively, disc sequestration was noted, and the herniated disc was found to be incarcerated to the adjacent conjoined L5 nerve roots. Upon further investigation, we discovered that the L5 nerve root originated from the caudal level of the L5 pedicle and was conjoined with the S1 nerve root (). The disc herniation at L4-5 migrated bilaterally and was found beneath the abnormal conjoined nerve roots. After removal of the disc herniation and unroofing of the nerve root, we did not observe any obvious mobility of the conjoined nerve root. After surgery, the patient's leg pain immediately disappeared and no muscle weakness was recorded. Upon retrograde imaging review, we observed several signs of conjugated nerve roots on routine MRI images (): the "sagittal shoulder sign," a vertical structure connecting two consecutive nerve roots and the overlying herniated disc on the parasagittal MRI which represents a combination of a protruded or extruded disc adjacent to a conjoined nerve root; "corner sign," asymmetric structure of the anterolateral corner of the dural sac with one side being angulated compared with the other; "fat crescent sign," the presence of extradural fat between the conjoined nerve root and the asymmetric dural sac; and "parallel sign," an unusual course of the entire nerve root at the disc level, running parallel to the disc plane.