Three membranes, the dura mater, pia mater and the arachnoid surround the spinal cord and nerves. The Dura mater, which is the outer most membrane, contains the cerebral spinal fluid (CSF) and nerves or spinal cord depending upon the level of the spine. Some individuals may have heard of a dural leak. This occurs when the spinal fluid (CSF) leaks out of a small hole in the dural membrane and causes a headache. A dural leak can be caused by an injection in the spinal column such as a spinal block or surgery. Arachnoiditis has nothing to do with a dural leak.
Arachnoiditis is a combination of two words taken from the root Latin. “Arachnoid” means spider and “itis” means inflammation of. The arachnoid membrane has hundreds of small spider web type attachments from nerve to nerve and this explains why it was named as such. This is similar to the term “appendicitis” where this interprets to inflammation of the appendix.
Arachnoiditis is the inflammation of the arachnoid membrane. The nerves are coated by this membrane within the CSF and inflammation can cause scarring and the nerves to “stick together” in clumps. Interestingly enough- like many conditions of the spine, some people have this condition and never develop symptoms.
Arachnoiditis is caused by a number of conditions. Arachnoiditis was more common in the 1970s and early 1980s when radiologists used an older type of oil based myelogram dye. This dye was used back then to visualize the spinal canal to look for canal compression and disc herniations. Thankfully- this dye is no longer used.
Arachnoiditis can be caused by the compression of spinal stenosis associated with degenerative spondylolysthesis (see section on this website). Epidural steroids can be used to manage the pain of spinal stenosis and individuals can now live with this nerve compression associated with canal narrowing. There is however, a certain small percentage of these individuals living with stenosis that will develop arachnoiditis.
Arachnoiditis can also be caused by trauma, infection, inflammation, and even surgery. If there is a larger dural tear during surgery, there is a chance of arachnoiditis. If trauma to the spine occurs such as in a motor vehicle accident and bleeding in the spinal canal results, arachnoiditis can form. Fractures of the spine that disturb the nerves in the canal can cause this condition. Obviously infection can cause injury to the structures that have become infected and the arachnoid is no exception to this rule. Meningitis (the arachnoid is a meningie just like the dura and pia) can cause arachnoiditis. Tuberculosis is famous for causing arachnoiditis. Cancer in the central nervous system can cause arachnoiditis.
The pathology of arachnoiditis is that the small spider web connections scar, contract and stick the nerves together. This reduces the nerve membrane exposure to the CSF and injures the small blood vessels that line and feed these nerves. This lack of blood circulation and the direct scar injury to the nerve can cause chronic nerve discharge or blocked signals. The nerves also don’t move normally when scarred. A normal nerve moves back and forth in the canal up to an inch when the leg is flexed or extended just like a cable over a pulley. If the nerve is tethered, it can’t stretch and the mechanical deformation with movement can cause symptoms.
Chronic nerve membrane injuries cause spontaneous discharges, blocking of the signal or nerve tethering. The symptoms generated depend upon the function of the damaged nerve. If it is a proprioceptive nerve, pins and needles will result (paresthesias). If it is a motor nerve, weakness can occur. If it is a sensory nerve, numbness or even bizarre sensations can occur (hot water dripping down the leg). If it is a bowel or bladder nerve, dysfunction can occur. When aggravated, pain nerves will deliver aberrant pain signals. “Burning pain” is one of the characteristics of arachnoiditis but can be seen in any chronic nerve injury such as peripheral neuropathy or chronic radiculopathy.
Unfortunately, there is no cure for arachnoiditis- only management. Thankfully, many patients who have arachnoiditis don’t even know it is present and need no treatment. However, if treatment is needed, treatment will not be curative but is designed to manage the symptoms using therapy and medications. For severe flair-ups, epidural steroids can occasionally be temporarily effective.
A chronic pain specialist normally manages patients with arachnoiditis. This expert can originate from many different specialties such as internal medicine, occupational medicine or physical medicine.
A surgically implantable spinal cord stimulator can reduce the symptoms in some patients. If a patient decides to go that route, consultation with an implant specialist that has significant experience with this condition is important.
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| Axial MRI of severe central spinal stenosis- white arrow points to severe compression of canal |
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| Axial MRI post surgery - white arrow points to opened canal and nerve _clumping_- nerves stuck together |
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