MRI Lumbar spine sagittal and axial T2w images:
E/o Multiple level laminectomies supports previous history of surgery.
Abnormal clumping of nerve roots of cauda equina, adhesion attributed to Arachnoiditis.
Arachnoiditis is a dynamic process involving a spectrum of collagen deposition and fibrosis, ranging from minimal changes such as contrast enhancement of the dural tube, clumping of two or more nerve roots to a soft tissue mass involving roots and meninges.
The pathogenesis is similar to the repair process of serous membranes, such as the penitoneum,with a negligible inflammatory cellular exudate and prominent fibrinous exudate. The fibrin covered roots stick to themselves as well as to the thecal sac. Eventually, dense collagenous adhesions are formed by proliferating fibrocytes during the repair phase.
Various etiologies categorized into infectious, non infectious inflammation and neoplastic.
Infectious includes bacterial, viral, fungal, and parasitic agents.
Noninfectious inflammatory etiologies include surgery, intrathecal hemorrhage, administration of intra thecal myelographic contrast, anesthetics and steroids.
Neoplastic includes the haematogenous spread or direct seeding from primary CNS tumors.
The investigation of choice for diagnosis of Arachnoiditis is MRI.
Best diagnostic clue is abnormal clumping of nerve roots of cauda equina and adhesion to the thecal sac. Normally nerve roots of cauda equina should fall freely in the dependent portions of thecal sac appreciated most easily against the background of high signal intensity Csf on Axial T2 images. Midline sagittal images shows nerve roots as a single band of intermediate signal intensity following the posterior thecal sac. The band of roots gradually tapered from the conus to the L4 level. Para sagittal images shows the roots dispensing in a fan-shaped manner as they traveled antero inferiorly.
The diagnosis of Arachnoiditis should not be made on one axial image, but necessitates the visual integration of the appearance of the roots over several levels. Focal clumping of lumbar nerve roots will be seen with spinal stenosis.
As per the severity Arachnoiditis can be grouped in to three.
Group 1 or 'milder form' is central clumping, conglomerations of roots at the center.
Group 2 or 'moderate form' is peripheral adhesions give rise to an empty sac appearance.
The 'more severe form' is group 3, showing soft tissue replacing the subarachnoid space due to fibrosis and adhesion.
As per the studies the most efficient sequence for imaging arachnoiditis is probably the axial Ti -weighted sequence. It allows confidence in defining all three groups of arachnoiditis. In my experience, T2-weighted axial study are helpful in defining the distribution of roots in the thecal sac with greater contrast than was provided by Ti -weighted images, at least for groups 1 and 2. However, the pathology of group 3 may potentially be masked on the T2-weighted study because of high signal from the fibrosis and adhesions mimicking normal CSF signal.