MRI Lumbar spine
In first section, only right side L5 nerve root has emerged out. There is no corresponding L5 nerve root on left.
In second section an ovoid out pouching from thecal sac on left side due to combined exit of L5 and S1 nerve root. Same separate and single L5 nerve root in lateral recess on right.
In last section on left side both L5 and S1 nerve root seen intra spinally in lateral recess. The same separate L5 nerve root in right lateral recess. The right S1 nerve root is about to emerge seen as a small bulge from thecal sac on right side.
Conjoined nerve root
Syn: Composite nerve root sleeve.
A type of developmental anomaly involving nerve root.
The term conjoined nerve root actually refers to the roots of 2 adjacent segments, arising at the same level from the thecal sac, enveloped by a common root sleeve.
Nerve roots usually exit separately at expected neural foraminal levels. Occasionally both the nerve roots exit through the same usually the lower foramen.
It is the commonest nerve root developmental anomaly of the cauda equina.
Most common location is lumbar spine in that commonly involves L5 and S1 nerve root.
The incidence in cadaveric studies is about 8% and 6% in MRI study.
The nerve root anomaly itself does not cause symptoms.
Symptomatic patient present with radiculopathy. Conjoined nerve roots are more prone to compression by degenerative processes of disc or facetal joint.
It is important to know about this normal anatomical variation as it may be confused with disk herniations, possible with poor quality MR images. Axial slices should be contiguous over several segments. Typical signs include asymmetry of the anterolateral corners of the dural sac, excess extradural fat between the asymmetric dura and the nerve root, parallel course of the affected nerve roots at the disc level
MRI is the investigation of choice. Associated findings may be enlarged or asymmetric neural foramen or lateral recesses, hypoplastic or absent pedicle. Association mentioned with vertebral anomalies such as vertebral arch defects, spondylolisthesis, spina bifida, absence of the ipsilateral facet joints.
Natural history and prognosis is asymptomatic patients requires no treatment. Rare symptomatic patient with pain referable to conjoined nerve root may require surgery. Symptomatic undiagnosed cases of conjoined nerve root are one cause of failed back syndrome.