Subarachnoid space

Spatium subarachnoidale

  • Latin synonym: Spatium subarachnoideum; Spatium leptomeningeum
  • Synonym: Leptomeningeal space
  • Related terms: Subarachnoid space; Leptomeningeal space

Definition

The subarachnoid space is the area between the arachnoid membrane and the pia mater in the central nervous system. It contains cerebrospinal fluid (CSF) and the brain's blood vessels, including the circle of Willis. In certain areas, the arachnoid membrane is widely separated from the pia mater, forming enlarged subarachnoid spaces called cisterns. Examples of important cisterns include the cerebellomedullary cistern, pontine cistern, and interpeduncular cistern

The subarachnoid space has a network of spider-web like septa that connect the arachnoid membrane to the underlying pia mater. These are knowns as arachnoid trabeculae.

The subarachnoid space surrounds the brain, spinal cord, olfactory bulbs and tracts, and optic nerves, which are all part of the central nervous system. This is why the optic disc can appear cupped when there is increased intracranial pressure inside the skull and cranial cavity. The subarachnoid space ends at the S2 vertebral level inferiorly. The lumbar cistern, located between the conus medullaris at the L1 vertebral level and the S2 vertebra, is used for performing lumbar puncture as it is a safe site with cerebrospinal fluid but no spinal cord. The subarachnoid space also surrounds the brain’s arteries as they enter the brain tissue and extends till the arteriole-level. The arachnoid and dura mater merge with the epineurium of the spinal and cranial nerves when they exit through the foramina in the vertebral canal or cranial cavity, thus ending the subarachnoid space in those areas.

The arachnoid membrane extends through the inner layer of the dura mater at specific points along the superior sagittal sinus, creating openings into the sinus. These extensions are called arachnoid villi or arachnoid granulations. Each arachnoid villus acts as a one-way valve, allowing CSF to flow from the subarachnoid space into the dural venous sinus, but not in the opposite direction.

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Text written by Muhammad A. Javaid, MD, PhD © 2023 IMAIOS.

  • Snell, R.S. (2010). ‘Chapter 16: The ventricular system, the cerebrospinal fluid, and the blood-brain and blood cerebrospinal fluid barriers’, in Clinical Neuroanatomy. (7th ed.) Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins, pp. 457.


Comparative anatomy in animals

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