Conjoined nerve root

A conjoined root is a type of developmental anomaly involving a nerve root. It is the most common nerve root developmental anomaly of the cauda equina being twice as common as two roots in the same foramen, the next most common anomaly.

Epidemiology

The incidence in cadaveric studies is about~10 (range 8-14)%². In 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 malformations ³, which may cause low back pain. The nerve root anomaly itself does not cause symptoms but may be confused with disc herniasasymptomatic or lead to complications, if overlooked prior to spinal surgerycause spontaneous pain (i.e. sciatica) ².

Gross anatomy

The term conjoined nerve root actually refers to the roots of two adjacent segments, arising at the same level from the thecal sac, enveloped by a common root sleeve. Of the affected neural exit foramina, they may be empty or contain a, sometimes additional, nerve root ². 

Location

The L5 and S1 are most frequently involved.

Associations

Vertebral anomalies ³, such as:

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

Radiographic features

Conventional x-ray myelography, CT myelography and MRI can be used to demonstrate this anomaly. Coronal T2WI and x-ray myelography have been shown to have equal sensitivity with MRI considered the gold standard CT myelography is useful in indeterminate cases ². 

Plain radiograph

May

• may show associated vertebral anomalies or malformations.

CT

• may show the anomaly but often fails to do so, if confined to the intervertebral disc spaces
• CT myelography will also demonstrate the anomaly

MRI

The most accurate non-invasive imaging study ². Axial slices should be continuous over several segments, and coronal slices should be included. Typical signs include ⁴:

• corner sign: asymmetry of the anterolateral corners of the dural sac
• fat crescent sign: extradural fat between the asymmetric dura and the conjoined nerve root sleeve
• parallel sign: visualization of the parallel course of the entire affected nerve root at the disc level

Differential diagnosis

Imaging differential considerations include:

• disc hernia
• post-arachnoiditis adhesions of the cauda equina
• other congenital root anomalies