Anatomy

The arachnoid mater is an avascular region that lies between two vascularized membranes: the pia mater and the dura mater. The arachnoid is attached to the underlying pia by numerous trabeculae, which create a space between the arachnoid and the pia. This space, or potential space in some instances, transmits arterioles and is referred to as the subarachnoid space. The arachnoid is composed of layers of squamous cells held together by a network of connective tissue. The arachnoid contains intercellular pores that allow for the passage of molecules.

Pathogenesis

A chronic infection or irritation can cause the arachnoid membrane to become thickened and adherent to both the overlying dura mater and the subjacent pia mater. The pia-arachnoid carries the blood vessels to the spinal cord, and this layer contains mesenchymal cells. In 1951, Smolik and Nash recognized that when the outer arachnoid layer is injured, both the blood vessels and mesenchymal cells lend themselves to extensive proliferation. The ensuing reaction between the pia-arachnoid and the dura mater leads to obliterative arachnoiditis.

When the arachnoid membrane is exposed to an insult, an inflammatory response ensues, which is characterized by fibrinous exudates, neovascularization, and a relative paucity of inflammatory cellular exudates. Vascular occlusive changes can occur, which can lead to spinal cord ischemia. The small perforating blood vessels that supply portions of the white matter may be obliterated, with resultant necrosis and cavitation of the spinal cord parenchyma. In addition to ischemia, blockage of venous return from the spinal cord or occlusion of cerebrospinal fluid (CSF) pathways may occur.

Burton described the stages of progressive inflammation of the arachnoid that occur in lumbosacral arachnoiditis. The initial stage, radiculitis, consists of an inflamed pia-arachnoid with associated hyperemia and swelling of the nerve roots. The second stage, arachnoiditis, is characterized by fibroblast proliferation and collagen deposition. During this stage, nerve root swelling decreases, and the nerve roots adhere to each other and to the pia-arachnoid. The final stage, adhesive arachnoiditis, is the resolution of the inflammatory process and is characterized by dense collagen deposition. There is marked proliferation of the pia-arachnoid layers as well as complete nerve root encapsulation, hypoxemia, and progressive atrophy. For reasons that are not fully understood, the adhesions occur preferentially on the dorsal segments. The exact time course of these three phases has not been elucidated. Furthermore, it is not known how the specific causative insult for the development of arachnoiditis might affect the time course of each of the three phases.

Yamagami and colleagues postulated that the pathologic changes in arachnoiditis may be secondary to diminished nutritional supply. They found that in an experimental rat model, the development of arachnoiditis and neural degeneration directly corresponded to the magnitude of extradural inflammation and wound-healing processes that occurred after laminectomy, with or without foreign bodies. Furthermore, adhesions of the arachnoid cause the nerve roots to lump together, and in the process, these nerve roots are isolated from contact with the CSF, with resultant nutritional compromise.

Etiology

In the first half of the 20th century, arachnoiditis was most often attributed to infectious causes. Furthermore, arachnoiditis had been described mainly in the cervical and thoracic regions. Since the 1950s, there has been a trend toward a higher incidence of arachnoiditis of noninfectious origin affecting the lumbar region. The precise causes of spinal arachnoiditis are not clear; likewise, the incidence and prevalence of spinal arachnoiditis in the general population are unknown ( Table 205-1 ).

As was stated previously, the etiology of arachnoiditis was mainly of infectious origin in the first half of the 20th century. Syphilis, tuberculosis, and gonorrhea were the most prevalent causes. Less common infectious causes include parasitic diseases and viral meningitis. These infectious causes are important to differentiate from noninfectious causes of arachnoiditis because, in most cases, effective treatment is available. However, in some cases, even despite adequate treatment of the causative agent, scarring of the arachnoid membrane may still occur and lead to permanent damage.

Arachnoiditis has a number of important noninfectious etiologies. In the 1940s, blood in the CSF following subarachnoid hemorrhage or surgery became the most prevalent cause. Spinal arachnoiditis following subarachnoid hemorrhage continues to be common and is usually treated in a conservative fashion. The breakdown products of hemoglobin has been postulated to be the causative agents, which result in neural damage. Evidence supporting this claim comes from animal experiments. Investigators who have injected blood and blood breakdown products into the subarachnoid space have demonstrated more meningeal inflammation compared to the injection of fresh blood. Interestingly, injection in the epidural space does not uniformly result in the same condition. Digiovanni and colleagues demonstrated that the placement of an autologous blood patch into the epidural space produced no more inflammation than a standard lumbar puncture. Abouleish and coworkers described 118 cases of epidural blood patches over a 2-year period. This group found 19 cases of axial back pain, 2 cases of radiculopathy, and no cases of arachnoiditis. Other authors, however, have reported cases in which the epidural blood patch had allegedly been responsible for arachnoiditis.

Historically, oil-based contrast media have been an important cause of arachnoiditis. Iophendylate (Myodil, Pantopaque) is an oil-based contrast medium used in diagnostic myelograms. It was first used in the United States in 1944, and its use continued for 40 years. Interestingly, in Sweden, iophendylate was banned from clinical use in 1948 because of animal studies that identified it as a causative agent for arachnoiditis. The incidence of arachnoiditis after the use of iophendylate is dose dependent and has been stated to be approximately 1%. Iophendylate has a long half-life; therefore, classically, it was aspirated from the thecal sac at the conclusion of the myelogram. Often, this removal process is not entirely successful; in fact, incomplete removal of the contrast dye may produce further trauma and cause bleeding into the CSF, which may increase the incidence of arachnoiditis.

Guyer and colleagues listed the following factors as influencing the development of arachnoiditis after myelography: the type of contrast agent used (the risk is greater with oil-based than with water-soluble media and greater with ionic than with nonionic media), the dosage of contrast medium, and the observation time after myelography ( Fig. 205-1 ).

Myelogram (anteroposterior view, oil-based contrast medium [Pantopaque]) of severe adhesive arachnoiditis. This myelogram demonstrates a marked lack of filling of the nerve roots throughout the lumbar spine with blunting of the thecal sac.

The use of intrathecal medications, either steroids for multiple sclerosis or anesthetic agents, has also been implicated as a cause of arachnoiditis. It is less clear if there is similar risk with epidural injections, such as epidural corticosteroids, which has become exceedingly common for the management of degenerative spine disease. One of the most commonly used agents in the epidural space is methylprednisolone acetate (MPA). Cases of arachnoiditis have been reported with the use of MPA; however, it remains unclear if the causative agent is MPA or its carrier, polyethylene glycol. Both polyethelene glycol and MPA may easily cross into the intrathecal space and cause arachnoiditis. The evidence remains conflicted in that other studies have failed to demonstrate arachnoiditis with MPA use.

The use of intrathecal bupivacaine, with or without epinephrine, has also been reported to cause arachnoiditis as described by Boiardi and associates. Gemma and coworkers described a case of arachnoiditis after an intrathecal administration of bupivacaine without epinephrine. It is unclear in these cases whether the arachnoiditis was triggered by the bupivacaine or other preservatives or whether epinephrine plays a role in the pathogenesis as well.

Spine surgery is an independent risk factor for the development of arachnoiditis. In particular, some investigators have specifically stated that surgery for a herniated intervertebral disc may lead to the condition. Carroll and Wiesel demonstrated that a postoperative pain-free interval lasting between 1 and 6 months, followed by the gradual onset of leg pain, increases the likelihood that some scar tissue is responsible for the symptoms. Smolik and Nash showed that simple dural retraction for the visualization of a ruptured intervertebral disc may trigger arachnoiditis. Haughton and colleagues showed that in monkeys, the nucleus pulposus of an intervertebral disc was able to cause focal arachnoiditis.

Clinical Features

The diagnosis of arachnoiditis requires a detailed medical history and physical examination as well as a review of confirmatory radiographic imaging studies. In obtaining a medical history, the clinician should seek three major characteristics of the pain: (1) burning pain, (2) constant in nature, and (3) worsened by activity. The pain may be located in the back, the lower limbs, or both. It is often diffuse and poorly localized. Moreover, patients may present with radiculopathy or myelopathy. The pain symptoms of chronic arachnoiditis may be similar to those of other chronic pain syndromes, such as complex regional pain syndrome. The exact relationship of these pain syndromes has not been fully elucidated. In many patients, arachnoiditis is asymptomatic and is discovered as an incidental radiographic finding.

The physical examination findings have been reviewed in two large clinical series. Burton followed 100 patients with arachnoiditis and found little motor weakness to be present. These patients were commonly found to have a positive straight-leg raise sign, a tender sciatic notch, limited range of motion of the trunk, and paravertebral muscle spasms. Guyer and coworkers followed 51 patients over more than 10 years and found that a decreased range of motion of the trunk was the most common finding on physical examination. In cases of chronic arachnoiditis with resultant syrinx formation, physical examination findings of syringomyelia are present. These include dissociative sensory loss and variable long tract signs.