TEACHING POINTS
■ Arachnoid cysts are lesions within the spinal canal that are a rare but serious cause of spinal cord compression. These lesions can be intradural or extradural.
■ A patient who presents with a history of progressive spastic or flaccid paralysis, backache, paresthesias, and difficulty with ambulation may have a compressing lesion.
■ Diagnosis is mainly by MRI, but plain radiographs may also be used.
■ While extradural arachnoid cysts are often symptomatic and require surgical removal, intradural arachnoid cysts, which are more commonly seen, do not have a tendency to enlarge and most often do not require surgery.
CASE
A 19-year-old female presented to the emergency department, accompanied by her mother, with complaints of back pain and paresthesias. The patient stated that she had fallen down the night before and developed paresthesias in both lower extremities. She was also having difficulty with ambulation. She noted that she had fallen down a short flight of stairs on two different occasions within the previous month. After one of the falls, she developed midthoracic level pain and sought evaluation and treatment with a chiropractor. Plain radiographs taken of the thoracic spine failed to show any abnormalities. The patient's pain had continued, and she began to notice weakness in the lower extremities. She also experienced increasing difficulty with ambulation. No bowel or bladder dysfunction was present. The patient's history was significant only for anxiety and sinus infection, and she denied any trauma or surgery to the spine.
Physical examination demonstrated a female of normal weight, development, and tone. Cardiopulmonary and GI examinations were both unremarkable. Neurologic examination revealed muscle strength of 5+/5 in the upper extremities; however, proximal muscle strength in the lower extremities, particularly the hip flexors and quadriceps, was 4/5 bilaterally. The patient was alert and oriented to time, person, and place. Speech and language were intact. Plantar flexion and dorsiflexion of the ankles were normal. Cranial nerves II to XII were grossly intact. Sensory testing in both the upper and lower extremities was normal to light touch and pinprick. Plantar responses were downgoing. Deep tendon reflexes were brisk, especially in the lower extremities. Pain was elicited while palpating the midthoracic spine.
MRI of the cervical, thoracic, and lumbar spine without gadolinium was performed. A mass lesion approximately 1.91.6 cm was found dorsal to the spinal cord at the T6-T7 level, causing about 50% stenosis of the canal (Figure 1). It appeared to be an extradural lesion. The patient was admitted to the hospital for further evaluation. Thoracic MRI with contrast revealed the same nonenhancing lesion at T6-T7 (Figure 2). The imaging results were discussed with the patient and her family. Given the size and location of the lesion as well as her deteriorating clinical symptoms, surgical removal was recommended. Based on the MRI images, the differential diagnosis included schwannoma, meningioma, arachnoid cyst, and Tarlov's perineurial cyst.
A complete laminectomy at T6 and T7 revealed a cystic, cerebrospinal fluid (CSF)-filled lesion dorsal to the spinal cord. The lesion was gradually dissected off the dura of the spinal canal and was found to have a small pedicle attaching it to the dural sac of the nerve root exiting the spinal cord between T6 and T7. There appeared to be communicating flow of CSF through the pedicle opening. This connection was ligated with a surgical clip and disconnected. The lesion was then removed en bloc. The patient tolerated the surgical procedure well. The pathology report confirmed an epidural arachnoid cyst.
The patient had an uncomplicated postoperative course. The lower extremity paresthesias resolved within the first day after surgery. She experienced mild incisional and thoracic back pain postoperatively and received inpatient physical therapy for 3 days prior to being discharged from the hospital. No detectable lower extremity weakness or pain was present at her 4-week follow-up visit.
DISCUSSION
Arachnoid cysts are lesions within the spinal canal that are a rare but serious cause of spinal cord compression. These lesions can be intradural or extradural. Extradural arachnoid cysts are meningeal outpouchings that arise either from congenital defects in the dura mater or as a result of previous trauma, infection, or inflammation. They are CSF-filled lesions that are most commonly found on the dorsal aspect of the spinal canal. Congenital extradural cysts often appear at the dural sleeve of the nerve root and can protrude through the neural foramen. Extradural lesions have been associated with congenital pigmented nevus, diastematomyelia, multiple sclerosis, Marfan syndrome, neural tube defects, spinal dysrhaphism, and syringomyelia.1-3 Noncongenital extradural cysts have been reported in patients with previous arachnoiditis, surgery, or trauma.1,4
Extradural arachnoid cysts are seen most frequently in the thoracic spine (65-70%), followed by the lumbosacral, thoracolumbar, and cervical spine.3 Twice as many extradural cysts occur in males than in females, and they manifest most often in the second decade of life.5 Patients typically present with painless progressive spastic or flaccid paraparesis and difficulty walking. Motor weakness is more pronounced than sensory loss, and sphincter tone is typically spared. Back pain is generally uncommon in thoracic lesions but more common in the lumbar cysts. Symptoms may be intermittent and can sometimes be exacerbated by Valsalva maneuvers. Slowly expanding cysts may result in osseous erosion and remodeling of the spinal canal, and kyphoscoliosis is seen on plain radiographs.1-3