Could a small arachnoid cyst on frontal lobe and arachnoiditis from spinal block be related? don't have arachnoiditis but do have back painafterspinal

Not at all: Arachnoiditis from spinal blocks has to do with painful swelling or inflammation of the arachnoid layer of a covering called the meninges which envelopes the spinal cord. This can be caused by the trauma from a puncture wound such as a spinal block would cause. Arachnoid cysts are believed to be congenital cysts formed by the arachnoid layer of the meninges within the brain itself. No relationship

Arachnoid cyst: Primary arachnoid cysts are the result of developmental abnormalities in the brain and spinal cord that arise during the early weeks of gestation. Secondary arachnoid cysts develop as a result of head injury, meningitis, or tumors, or as a complication of brain surgery. They are unrelated to your spinal tap. If the pain continues more than a week see the doctor who performed the tap. Good luck.