The mechanism of dural ectasia and dural diverticula formation in patients with long-standing AS is still not clear. A possible theory is that the meninges and the dural sac expand promptly in response to the CSF pulse pressure under normal conditions, allowing absorption of CSF and dampening of the transmitted pressure variations. In patients with long-standing AS, the inflammatory process may extend from the paraspinous ligaments to the dural matter, resulting in adhesion of the dural matter to the surrounding structures that reduce the compliance and elasticity of the lower dural sac and its ability to dampen the fluctuations in CSF pressure [3
]. The inflamed dura matter may be weakened, further encouraging the development of dural ectasia and bony erosion. In our case, a large dural defect was found in surgery and the pathological findings showed chronic inflammation of the meninges, which is compatible with this theory.
There are several possible mechanisms of CES–AS syndrome, including small vessel angiitis involving the vasa vasorum of the nerve roots, previous spinal irradiation for treatment of AS, nerve root damage owing to dural ectasia with decreased elasticity of the dural sac and increased arterial pulsatile forces transmitted through the CSF, and chronic inflammatory process [6
]. However, most reported cases including our patient did not have a history of spinal irradiation. The inflammatory process may spread from the peridural soft tissues to the nerve roots of the cauda equina, which in turn causes arachnoiditis and CES [3
]. Our patient's clinical course showed slowly progressive sensory and motor disorders in the bilateral lower limbs and saddle region as well as bowel and urinary bladder dysfunction. The NCV and EMG revealed lumbosacral radiculopathy, polyneuropathy and disuse myopathy in the bilateral lower limbs. The MRI also showed tethering of the conus medullaris to the dorsal aspect of the spinal canal and adhesion of the nerve roots of the cauda equina to the wall of the dural sac, suggesting an adhesive arachnoiditis. All of the above findings are consistent with the chronic inflammatory theory of CES–AS syndrome, which usually develops between 17 and 53 years (average, 35 years) after the onset of AS [9
There is no effective medical or surgical treatment for dural ectasia, presumably an end result of the chronic inflammatory process. Ahn et al [11
] performed a meta-analysis study on the treatment effect of CES–AS syndrome. They found steroids are not effective in the chronic stage, while non-steroidal anti-inflammatory drugs (NSAIDs) decrease back pain but do not improve neurological deficit, probably owing to no active inflammation in the chronic stage of CES–AS syndrome. Lan et al [12
] reported a case with partial clinical improvement after intrathecal injection of methotrexate and decadron, and lumboperitoneal (LP) shunting. The CSF study (lymphocytosis and elevated protein level) and MRI features (acute transverse myelitis and enhancement of the leptomeninges) in their case suggested an acute episode of inflammation. Cornec et al [13
] also reported a case with dramatic clinical improvement in anal sphincter function and sensation at anus and buttocks after treated with infliximab, a monoclonal antibody to tumour necrosis factor α (a major pro-inflammatory cytokine) that is used for the treatment of active AS. Surgical intervention, such as LP shunting or laminectomy may improve or arrest the progress of pain, sensory, motor, bowel and bladder dysfunction in a few patients based on the theory of attributing nerve root damage to excessive pulse pressure in CSF and to nerve root compression from the dural diverticula [6
]. However, some reported cases, including our patient, revealed limited or no clinical improvement after surgical treatment, and this theory is still controversial [11
]. Arslanoglu et al [17
] proposed a combined theory that an initial inflammatory process results in adhesion of the nerve roots of the cauda equina to the dural sac, and the nerve roots are progressively stretched by the enlarging diverticula. We assume that the effect of medical and surgical treatments may be determined on the timing of the intervention, whether there is active inflammation or irreversible nerve root damage has developed, such as in our case. MRI and CSF studies may provide valuable information in the diagnosis and treatment planning in patients with CES–AS syndrome.