Spinal Arachnoid Cysts: Presentation, management and pathophysiology

•Spinal arachnoid cysts are rare entities and they are often missed on radiographic imaging.•Microsurgical removal of the thickened arachnoid cap heralds resolution of symptoms in the majority of cases.•Myelopathy may develop, but not necessarily.•They may arise as a result of a dissection within the septum positicum.•Turbulent CSF flow within the dissection is suspected to be the cause of the focal cord ischaemia. Evaluation of the presentation and outcomes associated with surgical marsupialisation of spinal arachnoid cysts and formulation of a putative hypothesis explaining their pathogenesis. Cases were identified from electronic and theatre records at a single centre. All patients underwent pre-operative assessment and radiographic evaluation with subsequent spinal multidisciplinary discussion. Following surgery patients were reviewed at 6, 12 weeks, 6-months and beyond. A total of 17 patients with dorsal thoracic arachnoid cysts with a mean age at time of surgery of 58 years with a male to female ratio of 1.8:1 were identified. Paraesthesia (76%), neuropathic pain (76%), weakness (47%) and unsteadiness (53%) were the commonest presenting complaints. Abnormal gait (76%), altered sensation (71%) and weakness (47%) were the most commonly observed signs. Average cyst volume was observed to be 2570 mm3 (sd ±1682, range 544 to 7644 mm3), spanning a median of 2 thoracic levels, with a resultant reduction of cord volume of 33% (sd 12%). A syrinx was associated with 35% of SAC. All cases underwent marsupialisation of the arachnoid cyst. Six months following surgery all patients experienced improvement in at least of one their presenting symptoms and or clinical signs. Weakness, gait and paraesthesia were most likely to improve following surgery. Only 29% of cases had resolution of neuropathic pain, with 13% of the rest reporting an improvement in the sensitivity component of their pain. Clinical improvements correlated with an average 45% (sd 18%) volume increase in previously compressed cord. Intradural arachnoid cysts commonly present with paraesthesia, neuropathic pain and gait disturbance. Marsupialisation of the SAC heralds immediate and long-term improvement in symptoms. Cysts putatively arise within a dissection in the septum posticum and give rise to both dynamic and static compression of cord parenchyma secondary to the complex CSF flow dynamics within the thoracic spine.