Misspelling of subarachnoidal.

(anatomy, relational) Located or occurring below the arachnoid mater, often specifically between the arachnoid membrane and the pia mater.

(anatomy) Alternative form of subarachnoid

(neuroanatomy) The space between the arachnoid mater and the pia mater that surrounds the brain and spinal cord, through which the cerebrospinal fluid circulates and major blood vessels pass, and across which extend delicate trabeculae of connective tissue.

plural of subarachnoid space