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Definition of Subarachnoid

  • Medical Author:
    William C. Shiel Jr., MD, FACP, FACR

    Dr. Shiel received a Bachelor of Science degree with honors from the University of Notre Dame. There he was involved in research in radiation biology and received the Huisking Scholarship. After graduating from St. Louis University School of Medicine, he completed his Internal Medicine residency and Rheumatology fellowship at the University of California, Irvine. He is board-certified in Internal Medicine and Rheumatology.

Subarachnoid: Literally, beneath the arachnoid, the middle of three membranes that cover the central nervous system. In practice, subarachnoid usually refers to the space between the arachnoid and the pia mater, the innermost membrane surrounding the central nervous system.

The subarachnoid space is a potential space. It normally contains cerebrospinal fluid. A subarachnoid hemorrhage is a bleed into this space.

The arachnoid is named for its delicate, spider-web-like filaments that extend from its undersurface through the cerebrospinal fluid in the subarachnoid space to the pia mater. "Arachnoid" comes from the Greek "arachne" meaning spider or cobweb + "eidos" meaning resemblance = spider-web-like.

Reviewed on 12/11/2018

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