Objectives: Intradural spinal arachnoid cysts are rare, sometimes symptomatic with cord compression, canal stenosis or radiculopathy. Only 5% occur in the lumbar region. Etiology remains uncertain, some being related with the development of arachnoiditis. Our aim is to show the natural evolution of a symptomatic lumbar arachnoid cyst without surgery.

Methods: 50 years old patient with chronic lumbar pain, diagnosed in 2011 by MRI of chronic lumbar discopathies. In November 2015, the patient suffered lumbar and radicular pain of a month and a half duration, MRI (Dic 2015) showed, at the level of the vertebral body L3, the presence of an expansive process of intradural-extramedullary appearance, which is introduced in the left foraminal foramen of L3-L4, with dishomogeneous enhancement, suggesting as a first diagnostic possibility a schwannoma-neurofibroma.

Results: Patient was referred for neurosurgery consultation, asymptomatic after 3 months of evolution, reason why surgery was not considered but MRI control (June 2016) was performed: Absence of the expansive formation of intradural-extramedullary appearance, and a small enhancement of the emergent root L3-L4. From these findings and evolution, we believe that the diagnostic possibility of a cyst or an arachnoid diverticulum with inflammatory changes should be considered. Electroneuromyographic study obtained mild pre-ganglionic neurogenic disorder L4-L5.

Conclusions: This pathology traditionally has been treated using microsurgical techniques, nevertheless, spontaneous vanishing of an arachnoid cyst seems to be very rare: just a few head cases have been reported but in the spine we only know one case reported in 2014, and this is the second one. This shows how important is the patient's clinical and evolutionary control by images, before surgery.