Although spinal arachnoid cysts are relatively rare, the frequent use of magnetic resonance imaging (MRI) has led to an increase in the number of cases reported in recent years. A spinal arachnoid cyst is formed with an extremely thin membrane and is filled internally with cerebrospinal fluid (CSF), which communicates with the external surrounding CSF. This conformation hinders the visualization of the cyst on conventional MRI in the absence of a local spinal cord deformity or syringomyelia. Anterior deviation of the spinal cord and dorsal depression may indicate the presence of arachnoid cysts.
In this study, we report the case of a patient with gait disturbance and no significant findings on conventional MRI; cine MRI was performed because of suspected compression on the dorsal aspect of the spinal cord.
The patient was informed that data concerning the case would be submitted for publication, and she provided consent.
A 28-year-old woman first noticed muscle weakness in both lower limbs, predominant on the left side, and consequently started developing a gait disorder 3 years before visiting our hospital. She underwent several orthopaedic and neurological visits and various medical examinations; however, no abnormalities were noted. She had not received any treatment before the visit to our hospital, and her symptoms gradually worsened. At the physical examination in our hospital, muscle weakness was confirmed in the left lower extremity, accompanied by hypesthesia in the lower legs, increased patellar tendon reflex in both knees, and spastic walking. The manual muscle test was scored as follows (R/L): iliopsoas 4/4, quadriceps 4/4, tibialis anterior 4/2, extensor hallucis longus 4/2, and gastrocnemius 4/2. The patient had no history of trauma, encephalitis, myelitis, or spinal surgery.
An MRI of the thoracic spine showed a slight narrowing of the spinal cord at the T3-T4 level (Fig. 1), and axial images suggested a ventral deviation of the spinal cord. Computed tomography myelography showed similar findings (Fig. 2). Because the spinal cord appeared separated from the dorsal side, we performed cine MRI, which confirmed the presence of a cyst on the dorsal aspect of the spinal cord. The cyst pulsated in synchrony with the heartbeat and compressed the spinal cord (Videos 1 and 2). We performed a laminectomy from T3 to T6, and no obvious abnormalities were found superficially. The dura mater was incised, and a band-like, thickened white arachnoid membrane was observed at T5-T6, which expanded like a septum within the cyst (Fig. 3). We removed both the thickened membrane and the cyst. A year after the surgery, the sensory impairment and spasticity of the lower extremities decreased, and the muscle strength in the left lower limb improved. The patient could walk with a cane.
Spinal arachnoid cysts are divided into 3 categories, and intradural arachnoid cysts are classified as type 31. These cysts develop because of a congenital malformation, trauma, or inflammation. As a congenital anomaly, it is possibly caused by the septum posticum on the posterior aspect of the thoracic spinal cord or by a congenital defect of the arachnoid trabeculae2,3. Approximately 80% of intradural arachnoid cysts develop in the thoracic spine, causing impairment of the spinal cord and nerve roots4. In addition, the CSF flow may change depending on the size of the cyst access holes to the exterior. A typical cyst with no external access appears as a uniform mass, and the spinal cord compressed by a typical cyst with no external access can be visualized on MRI. However, it is difficult to visualize a cyst with large access holes using conventional MRI because the compression on the spinal cord is mild5. In our case, the arachnoid cyst was pulsating and the compression on the spinal cord was intermittent. Therefore, the cyst was considered to have external access holes and could not be visualized on conventional MRI.
A thickened arachnoid membrane may block the CSF recirculation. Some recent reports have suggested that a thickened arachnoid lesion, as observed in our case, could be defined as a spinal arachnoid web (SAW)6. In cases where a SAW cannot be directly visualized, an MRI may show depressions and ventricular shapes on the dorsal spinal cord and changes in intramedullary signals. Readon et al. reported that a local depression of the spinal cord could be defined as a “scalpel sign,” with a shape similar to a round-edged knife7,8. In our case, a multilocular arachnoid cyst was confirmed. Since the cyst volume decreased when the SAW was removed, it was estimated that the CSF recirculation was inhibited by the SAW. Previous reports described various types of SAWs characterized only by linear web structures or including cystic regions8. These findings support the hypothesis that SAWs may be a variant of arachnoid cysts9,10.
In this case, the dorsal side of the spinal cord was subject to mild pressure. We suspect that the myelopathy was caused by repeated compression or alterations of the blood and CSF circulation. Although the Romberg's sign could not be adequately evaluated because of the significant difficulty in standing even with the eyes open, we speculate that the area mainly affected by the cyst was the left pyramidal tract. In fact, the motor disturbance was more pronounced than the sensory disturbance. The arachnoid cyst was slightly unevenly distributed, affecting mostly the left side, and consequently, the symptoms were predominant on the left lower limb.
Although MRI has a high diagnostic ability, it may be inadequate for the visualization of a moving target. In recent years, the number of magnetic resonance images obtained per unit of time has drastically increased, and cine MRI has been developed, allowing the clear visualization of a moving target by observing the continuous cinematic images of a single section11-13. Cine MRI can depict the presence and dynamics of intradural arachnoid cysts noninvasively. Although previous reports suggested that cine MRI may be useful for the diagnosis of intradural arachnoid cysts, no reports have clearly presented a pulsatile arachnoid cyst.
It is possible that arachnoid cysts may remain unrecognized during daily medical examinations. Cine MRI should be considered for an unexplained leg paralysis or weakness.
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