2026 Volume 13 Pages 49-54
Spinal arachnoid cysts are relatively uncommon lesions, broadly classified as either congenital or acquired. Acquired cysts may develop following trauma, hemorrhage, or post-infectious inflammation. Depending on their location, these cysts can cause a range of symptoms, including quadriplegia, pain, sensory disturbances, and gait impairment. Surgical resection may be indicated in cases with significant neurological symptoms. We report the case of a 77-year-old woman who presented with persistent bilateral leg numbness and pain. Magnetic resonance imaging revealed a suspected intradural spinal arachnoid cyst or arachnoid web at the Th7-8 level, and surgery was planned to relieve the symptoms.
A laminectomy was performed, revealing a cystic, membrane-like structure in the subdural space. By injecting a crystal violet staining solution into the cyst, we enhanced the visibility of the boundary between the normal arachnoid membrane and the cyst, allowing for complete resection. Postoperatively, the patient experienced improvement in leg pain and numbness. This case demonstrates that crystal violet staining can facilitate clear boundary recognition during arachnoid cyst resection, enabling more precise cyst removal.
Spinal arachnoid cysts are relatively rare, accounting for approximately 1% of all spinal tumors.1-4) These cysts are classified as extradural, intradural extramedullary, or intramedullary based on their positional relationship to the dura mater and spinal cord. Their causes include congenital and acquired factors such as meningitis, spinal cord trauma, postoperative effects, and idiopathic cases of unknown origin.1) Laminectomy and cyst excision are typically performed for a single lesion spanning up to 3 vertebral segments.5) Given that residual arachnoid cysts can lead to recurrence, complete resection is ideal. However, because these cysts are filled with cerebrospinal fluid (CSF) and may exhibit strong adhesions, visually distinguishing the cyst wall from the normal arachnoid membrane can be challenging, complicating complete resection.
Crystal violet is a common skin marker. In neurosurgery, it is also used to stain suture lines in bypass surgery, aiding the suturing process.6) Additionally, crystal violet has been used during the resection of malignant cystic brain tumors to visualize mural damage.7) More recently, it has been applied to the resection of spinal arachnoid cysts,8,9) enhancing tissue recognition and allowing for more precise surgery. In the present case, we injected crystal violet into the arachnoid cyst during surgery to stain the cyst wall, facilitating identification of the boundary between the cyst and the normal arachnoid membrane. This approach enabled easier and complete cyst removal. We believe this method can potentially improve the safety and effectiveness of spinal arachnoid cyst surgery.
A 77-year-old woman presented to the clinic with a chief complaint of persistent numbness and pain in both lower limbs for the past 2 years. The symptoms were localized to the region between the knees and ankles. Neurological examination revealed sensory abnormalities in both lower limbs, including decreased tactile sensation and diminished vibration sense. Vibration sense in both legs was reduced to approximately 7 seconds. Bilateral hyperreflexia was observed in the patellar and Achilles tendon reflexes, and the Babinski sign was positive. No apparent muscle weakness was detected. Magnetic resonance imaging (MRI) of the entire spine revealed a CSF flow artifact at the T7-T8 level on T2-weighted images (Fig. 1A). Ventral displacement of the spinal cord at T7 and a cystic structure with CSF-equivalent signal intensity were observed on the dorsal side of the spinal cord. Both dorsal nerve roots were displaced anterolaterally (Fig. 1B). Computed tomography myelography (CTM) of the entire spine was then performed. A 23-gauge needle was inserted at the L4-5 level, and 10 mL of CSF was drained. Subsequently, 10 mL of the contrast agent iotrolan 240 (Isovist®) was slowly injected into the spinal canal. CTM scans were obtained at 3 and 24 hours post-injection. At 3 hours, the entire cyst was opacified, appearing similar to the spinal subarachnoid space on CTM, suggesting communication with the subarachnoid space. Based on the imaging findings, differential diagnoses included a spinal arachnoid cyst and an arachnoid web.
MRI. (A) The T2-weighted image shows a flow artifact at the T7-T8 level (white arrow). (B) At the T7 level, ventral displacement of the spinal cord (white arrow), a cystic structure on the dorsal side of the spinal cord, and lateral anterior displacement of the bilateral dorsal root nerves were observed (white arrowhead).
MRI: magnetic resonance imaging
In this case, a right-sided hemilaminectomy at the Th7-8 level was planned based on the T2-weighted MRI findings, which showed CSF flow voids converging at this level. The goal was to address the suspected arachnoid pathology at the lesion site.
The surgery was performed with the patient in the left lateral decubitus position, using motor evoked potentials and somatosensory evoked potentials to monitor for intraoperative neurological changes. After completing the laminectomy, the dura was incised, revealing a cyst-like membranous structure continuous with the arachnoid membrane and located just beneath the dura (Fig. 2A). To enhance visualization of the boundary between the normal arachnoid and the cystic structure, 0.5 mL of fivefold-diluted crystal violet solution was injected into the cyst (Fig. 2B). The dye remained within the cyst, staining its membranes and distinguishing them from the normal arachnoid membrane. Based on these intraoperative findings, the lesion was identified as a spinal arachnoid cyst. This allowed clear identification of the cyst's upper and lower boundaries, enabling careful and complete resection (Fig. 2C and D). Following cyst resection, an endoscopic inspection was performed to ensure that no residual lesion remained and to confirm complete excision. No abnormalities were observed in the motor evoked potentials or somatosensory evoked potentials throughout the procedure. Regarding the use of crystal violet, patients were informed preoperatively about its purpose and potential carcinogenic risks, and informed consent was obtained.
(A) A cyst-like membrane structure, continuous with the arachnoid membrane, was observed beneath the dura mater. (B) 0.5 mL of crystal violet solution was injected into the cyst. (C, D) The clear delineation of the superior and inferior boundaries of the cyst through staining enabled complete resection.
Histological examination revealed that the cyst wall was composed of fibrous connective tissue, with flat, non-atypical covering cells observed in certain areas. These findings were consistent with those of arachnoid cysts (Fig. 3A and B). The patient reported reduced lower-limb pain and numbness the day after surgery. By postoperative day 4, the pain had improved significantly, with the Numerical Rating Scale score decreasing from 10 preoperatively to 2, and the range of symptoms had diminished. While there was no change in vibration sensation in the right foot, the left foot showed improvement, with vibration sense increasing from 7 to 9 seconds. Additionally, the previously noted hyperreflexia of the tendon reflexes had resolved. The patient was discharged on postoperative day 8. Outpatient follow-ups continued, and 6 months passed without any symptom recurrence. Follow-up MRI confirmed the disappearance of the flow void and improvement in the anterior displacement of the spinal cord (Fig. 4A and B).
Pathological image. The cyst wall is composed of fibrous connective tissue, with some areas showing flat, non-atypical covering cells. (A) ×20 magnification. (B) ×100 magnification.
(A, B) An MRI conducted 4 months later revealed the disappearance of the flow void and improvement in the ventral displacement of the spinal cord.
MRI: magnetic resonance imaging
Arachnoid cysts most commonly occur in the mid to lower thoracic spine and are more frequently located on the dorsal rather than the ventral side of the spinal cord. They present with various symptoms, including paralysis, pain, sensory disturbances, and gait abnormalities. When symptoms are severe, treatment is indicated; however, the therapeutic approach depends on the size and number of lesions. In the surgical removal of arachnoid cysts, complete resection is generally recommended, as residual cysts may lead to recurrence. In one report, post-laminectomy spinal arachnoid cysts caused progressive compressive myelopathy.10) However, complete excision can be challenging due to factors such as arachnoid adhesions. In such cases, partial resection, fenestration, or percutaneous drainage may be considered.11) Additionally, when the lesion spans more than 3 vertebral bodies or multiple lesions are present, the risk of spinal instability following surgery must be considered.12) In these scenarios, cyst removal or fenestration at the spinal level with the most severe symptoms may be prioritized. From a pathophysiological perspective, the goal is not merely the complete resection of the arachnoid cyst wall. Rather, the objective is to improve CSF circulation by removing the portion of the cyst wall that has formed a closed cavity obstructing CSF flow. Resecting this structure as a single unit is considered more effective in restoring CSF circulation. In this context, staining for closed cavities is regarded as useful. The arachnoid membrane is a delicate connective tissue that can be challenging to manage during resection because it tends to collapse and fragment with age.13) Arachnoid cysts have thicker walls than normal arachnoid membranes, and although there is a lack of trabecular projections,14,15) the cyst wall is predominantly composed of fibrous connective tissue, occasionally lined by a single layer of arachnoid epithelial-like cells.16,17) Inflammatory cell infiltration is typically absent, though chronic thickening and hyalinization of the cyst wall may be observed.18) Electron microscopy reveals cellular microvilli and pinocytotic vesicles, with intercellular junctions present, but true cilia are generally lacking.19) An arachnoid web may be considered in the differential diagnosis. By definition, an arachnoid web is an intradural, extramedullary transverse band of arachnoid tissue that extends to the dorsal surface of the spinal cord and is thought to cause a dorsal indentation.20) Pathologically, an arachnoid web is not clearly recognized as a distinct entity and may be regarded as a variant of a spinal arachnoid cyst.21) Although it is sometimes distinguished based on intraoperative findings or preoperative MRI, in this case, we consider it an arachnoid cyst based on a comprehensive assessment. Improved visualization of the cyst membrane facilitates the identification of boundaries, enabling total removal even within a confined space. In this case, the lesion was localized and spanned 2 vertebral bodies; therefore, we aimed for complete resection. During surgery, the cyst was stained with crystal violet, which allowed the dye to accumulate within the cyst and make it visible. This enhanced visualization made it easier to identify the upper and lower margins of the cyst and distinguish the boundary between the normal arachnoid membrane and the cyst, enabling safer and more precise dissection. Consequently, normal CSF dynamics were restored. A previous report indicated that this technique also helps reduce resection time.8) In the present case, no significant arachnoid adhesions were observed. However, in situations where adhesions complicate dissection or en bloc resection is difficult, requiring segmental removal, crystal violet staining can help delineate the extent of dissection and confirm any residual cystic tissue. In addition, if the dural incision is insufficient, staining can guide further extension of the incision. Although crystal violet staining is used to stain the cyst wall during resection, it can also be applied to the internal fluid during partial removal or fenestration to confirm the elimination of fluid stagnation. This suggests that removing the obstructive cavity caused by the arachnoid web may also improve CSF circulation, making the procedure highly beneficial. There has been considerable discussion regarding the safety of crystal violet. It has been used for many years in the medical field, particularly in surgical applications such as skin marking and visualization of blood vessel walls in microvascular anastomosis. However, reports have also indicated its carcinogenic potential and its ability to inhibit the migration and proliferation of venous cells.22-25) The United States Food and Drug Administration and the Pharmaceuticals and Medical Devices Agency have stated that there are insufficient data regarding the safety of crystal violet and, as a result, cannot guarantee its safety. The product has been discontinued in Japan, and general guidelines recommend avoiding its routine use. However, in situations where no alternatives are available and the benefits outweigh the risks, its use (with the patient's consent after a thorough explanation of the risks) is recommended. In this surgery, efforts were made to minimize the amount used and restrict its administration within the cyst, thereby minimizing systemic effects. Thorough postoperative irrigation was performed to ensure that no staining solution remained.
Staining with crystal violet during surgery for spinal arachnoid cysts can facilitate complete and safer resection while minimizing surgical invasiveness.
Author Nobuhiro Mikuni is one of the Editorial Board members of the Journal. This author was not involved in the peer-review or decision-making process for this paper.
All authors have no conflict of interest.
The authors consented to the submission of this case report to the journal.
