Arachnoiditis (ORPHA:137817)
A chronic inflammation of the arachnoid layer of the meninges, of which adhesive arachnoiditis is the most severe form, characterized by debilitating, intractable neurogenic back and limb pain and a range of other neurological problems.
Epidemiology
The prevalence is unknown. About 25,000 cases of arachnoiditis occur each year, mostly in North and South America, Asia and Europe, where spinal operations are more prevalent.
Clinical description
Patients present with chronic, persistent deafferentiation pain in the lower back, limbs and trunk that is increased by activity, hyporeflexia, loss of temperature sensation, numbness, and often widespread allodynia, dysesthesia and hyperpathia. Patients may also experience proprioception alterations (including loss of balance, tinnitus and reduced hearing and vision), motor weakness, muscle cramps, fasciculation, anhidrosis, and bladder, bowel and sexual dysfunction. Arachnoiditis may, in a minority of cases, involve the brain as well as the spinal cord, possibly causing communicating hydrocephalus.
Etiology
Arachnoiditis can be mechanically (localized) or chemically (diffuse) induced, and is often associated with spinal operations (60% of cases), neuroaxial (spinal and epidural) anesthesia (22%), spinal taps (7%), myelography (3%), pain relief procedures and secondary infections. It can also be caused by bacterial and viral spinal infections (7%) and repeated subarachnoid injections of anticancer drugs or antimetabolites. Syringomyelia, cauda equina syndrome, pseudomeningoceles, intrathecal cysts or tethering of the spinal cord and nerve roots (NR) may complicate lumbosacral arachnoiditis. Arachnoiditis occurs as a progression of inflammatory changes. In the early (inflammatory) phase NR are edematous (enhanced), while in the late proliferation stage (adhesive arachnoiditis) NR are clumped and asymmetric. The flow of cerebrospinal fluid from the distal dural sac to the brain is impeded, intrathecal pressure increases and this causes back pain and postural headache. In some cases the scar tissue calcifies (arachnoiditis ossificans).
Diagnostic methods
Diagnosis is based on patient history, clinical presentation and a causative event, and can be confirmed by MRI with contrast. When MRI is not possible, myelogram followed by CT scan is indicated. The adhesions generally occur on the dorsal segments, are arranged peripherally, and have been described as looking `like the bark of a tree' when viewed by myelography.
Differential diagnosis
Differential diagnoses include intra-spinal hematoma or dislodged disc fragment if the condition presents immediately after surgery, and Failed Back Surgery Syndrome (FBSS). Some patients are diagnosed with fibromyalgia (see this term), but these symptoms are likely to occur as a secondary feature due to the altered spinal dynamics.
Management and treatment
In the early phase, treatment includes large doses of IV methylprednisolone for five days, preferably within three months of the causative injury, followed by a protocol directed to control neuropathic pain using a multimodal approach that includes an anti-inflammatory, an anticonvulsant and an antidepressant. Large doses of opiates, that can cause hyperalgesia, hypersensitivity and tachyphylaxis, and lead to dependence, should be discouraged. If necessary, to treat exacerbation ``flare-ups'', IV infusions of NMDA receptors antagonists (including lidocaine, MgSO4, ketamine) can be given.
Prognosis
Once the proliferative stage has begun, arachnoiditis will be permanent and is complicated by the aging process of the spine. Operations, injections or any other invasions of the spine may exacerbate the disease significantly.

                            Last update: 
                            March 2010


                            - Expert reviewer(s): 
                            Pr Antonio ALDRETE