Conjoined nerve root

A conjoined root is a type of developmental anomaly involving a nerve root.

It is the commonest nerve root developmental anomaly of the cauda equina being twice as common as two roots in the same foramen, the next most common anomaly¹.

Epidemiology

The incidence in cadaveric studies is about 8%. In a recent prospective MRI study, it was 6%². In retrospective studies, the incidence is reported much lower.

Clinical presentation

There seems to be an association with vertebral malformations³, which may cause low back pain. The nerve root anomaly itself does not cause symptoms, but may be confused with disk hernias or lead to complications, if overlooked prior to spinal surgery.

Pathology

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.

Location

The L5 and S1 are most frequently involved.

Associations

Vertebral anomalies³, such as

• vertebral arch defects
• spondylolisthesis
• spina bifida
• absence of the ipsilateral facet joints

Radiographic features

Plain film

May show associated vertebral anomalies or malformations

CT

May show the anomaly, but often fails to do so, if confined to the intervertebral disk spaces.

MRI

The most accurate non-invasive imaging study.

Axial slices should be contiguous over several segments, and coronal slices should be included. Typical signs include ⁴:

• asymmetry of the anterolateral corners of the dural sac
• excess extradural fat between the asymmetric dura and the nerve root (fat crescent)
• parallel course of the affected nerve roots at the disc level

Myelography, post-myelographic MSCT

Probably the most accurate imaging study in this type of anomaly⁵.

Differential diagnosis

Imaging differential considerations include