Many patients with spinal arachnoid cysts complain of symptoms suggesting spinal cord compression, and are often initially evaluated by their primary physicians. However, these cysts are often discovered incidentally.
This article discusses how to manage spinal arachnoid cysts, whether found incidentally or during an evaluation for symptoms of spinal cord compression.
PRESENTATIONS CAN VARY WIDELY
A patient with a clinically relevant spinal arachnoid cyst is most likely to be a boy in his teens, but these cysts can occur in either sex and have been reported in patients as young as a few months and as old as nearly 80 years.1–6
In their typical presentation, spinal arachnoid cysts cause progressive signs and symptoms suggesting spinal cord compression. But because a cyst can occur at any spinal level and in a patient of any age, no one clinical presentation is pathognomonic, and the clinical sequelae can differ drastically from patient to patient. Nevertheless, we can make certain generalizations: a spinal arachnoid cyst that compresses the spinal cord typically causes waxing and waning pain and progressive spastic or flaccid paraparesis, which often are exacerbated by Valsalva maneuvers.1,6 Spinal arachnoid cysts can also present with symptoms suggestive of an isolated radiculopathy.
Less typical presentations include noncardiac chest pain, isolated gait difficulty, and isolated urinary urgency.2–4
Missed diagnosis is common
Because the symptoms are so variable and nonspecific, the diagnosis of spinal arachnoid cysts is often missed. For example, a sacral extradural arachnoid cyst can cause pain in the low back and perineal region, which is often relieved by lying flat and aggravated by Valsalva maneuvers.7
Complicating the picture, spinal arachnoid cysts can also coexist with other disorders of the central nervous system. Cases have been reported of sacral extradural arachnoid cysts coexisting with lumbar disk prolapse7 and of spinal arachnoid cysts located near a syrinx (a tube-shaped cavity in the spinal cord).3,8 A patient can have more than one spinal arachnoid cyst, or both a spinal arachnoid cyst and a concurrent intracranial arachnoid cyst or a tumor.9
EXTRADURAL VS INTRADURAL CYSTS
Like other types of spinal meningeal cysts, spinal arachnoid cysts can be broadly characterized as either extradural or intradural.10
Extradural cysts are extradural outpouchings of arachnoid that are contiguous with the spinal subarachnoid space via a small dural defect. They typically occur in the thoracic spine dorsal to the spinal cord, although they may be found elsewhere.
Intradural cysts are outpouchings of arachnoid that, regardless of size, lie entirely within the dural space. Intradural arachnoid cysts are more common than extradural cysts.
Either type of cyst may or may not communicate with the subarachnoid space.1–3
Other cystic lesions of the spine exist. One of the most common is the Tarlov cyst, which may look similar to a spinal arachnoid cyst, as both types of cysts are collections of cerebrospinal fluid. But, unlike typical spinal arachnoid cysts, Tarlov cysts occur only in the sacral spine and appear solely within the sacral root on radiographic imaging.
HOW DO CYSTS FORM?
How spinal arachnoid cysts start to form is open to conjecture, and several theories exist.1,2,7 They are often attributed to congenital defects. Another possibility is that arachnoid adhesions develop secondary to inflammation, which may arise from infection (meningitis), hemorrhage, or an iatrogenic cause such as injected contrast media or anesthetics or from the intraoperative contaminants of fibrin glue.11 Some cysts are due to trauma from lumbar puncture, anesthetic procedures, or intradural surgery. Other cysts are idiopathic.
WHY DO CYSTS ENLARGE?
Several mechanisms have been proposed to explain why spinal arachnoid cysts enlarge.2 The cells in the cyst wall probably do not secrete fluid: many spinal arachnoid cyst walls are composed primarily of simple connective tissue, and many completely lack an inner arachnoid lining—the cells that normally secrete spinal fluid—or have only a sparse lining.6 A unidirectional “valve” might let fluid in but not out. Another mechanism is pathologic distribution of arachnoid trabeculae, leading to fluid shifts within the cyst, thereby causing an increase in size.
DIAGNOSIS IS OFTEN INCIDENTAL
Spinal arachnoid cysts are rare, so an algorithm to diagnose them solely on the basis of common presenting symptoms would be impractical.
Whenever an arachnoid cyst is discovered, one must determine whether the cyst—or another problem—is actually causing the symptoms. If treatment is to succeed, the clinical presentation must correspond to the radiographic findings. For example, removing a cervical arachnoid cyst is unlikely to relieve low back pain.