Subarachnoid Space

The subarachnoid space is the interval between the arachnoid mater and pia mater within the meninges that surround the brain and spinal cord. It is filled with cerebrospinal fluid (CSF) and serves as a protective cushion for the central nervous system (CNS). The space also contains blood vessels, cranial nerves, and arachnoid trabeculae that span the gap between the two meningeal layers. It plays a critical role in CSF circulation, nutrient delivery, and waste removal.

Location and Boundaries

• Superficial boundary: Arachnoid mater — a thin, avascular membrane lining the inside of the dura mater
• Deep boundary: Pia mater — a delicate, vascular membrane tightly adhering to the surface of the brain and spinal cord

The subarachnoid space is continuous around the brain and spinal cord and extends into the cranial and spinal nerve root sheaths. It is widest in specific areas known as subarachnoid cisterns.

Contents

• Cerebrospinal fluid (CSF): Circulates within the subarachnoid space, cushioning and supporting the CNS
• Blood vessels: Arteries and veins supplying the brain and spinal cord travel through this space, especially the cerebral arteries
• Cranial nerves: Portions of cranial nerves traverse the subarachnoid space after exiting the brainstem
• Arachnoid trabeculae: Delicate strands of connective tissue connecting the arachnoid and pia mater

Subarachnoid Cisterns

Cisterns are expanded regions of the subarachnoid space that contain larger volumes of CSF and are often used as landmarks in neuroimaging and clinical procedures:

• Cisterna magna (cerebellomedullary cistern): Located between the cerebellum and medulla; receives CSF from the fourth ventricle via the foramen of Magendie
• Interpeduncular cistern: Located between the cerebral peduncles of the midbrain; contains the basilar artery
• Prepontine cistern: Anterior to the pons; contains the basilar artery and abducens nerve
• Quadrigeminal cistern: Posterior to the midbrain; contains the great cerebral vein and pineal gland
• Suprasellar cistern: Above the sella turcica; contains the optic chiasm and circle of Willis components
• Lumbar cistern: Located in the lower spinal canal (L2–S2); site of CSF sampling via lumbar puncture

CSF Flow and Absorption

CSF enters the subarachnoid space from the fourth ventricle through the median (Magendie) and lateral (Luschka) apertures. It then circulates over the brain and spinal cord and is absorbed into the venous system through arachnoid granulations, primarily located in the superior sagittal sinus. Minor absorption also occurs through lymphatics and perineural sheaths.

Clinical Significance

• Subarachnoid hemorrhage (SAH): Bleeding into the subarachnoid space, often from a ruptured cerebral aneurysm; presents with sudden severe headache, photophobia, and meningeal signs
• Meningitis: Inflammation of the meninges causes leukocyte infiltration into the subarachnoid space; CSF analysis shows elevated white cells, protein, and altered glucose
• Hydrocephalus: Impaired CSF absorption in the subarachnoid space (e.g., after meningitis or hemorrhage) can lead to communicating hydrocephalus
• CSF leak: Loss of CSF through a dural defect (often in the spinal region) reduces subarachnoid pressure, resulting in orthostatic headache
• Lumbar puncture: Performed in the lumbar cistern to collect CSF for diagnostic or therapeutic purposes

Imaging

• CT scan: Useful for detecting subarachnoid hemorrhage; hyperdense blood may accumulate in cisterns and sulci
• MRI: Provides high-resolution views of the subarachnoid space, cisterns, and CSF flow; FLAIR sequences are sensitive for detecting abnormal CSF content
• MR cisternography: Specialized imaging to visualize CSF spaces and leaks

Development

The subarachnoid space forms during embryonic development as a result of cavitation within the meninx primitiva. The arachnoid and pia mater separate, creating the space filled with CSF by the end of the first trimester. Subarachnoid cisterns become prominent in fetal development and expand with continued CSF production.