Meninges

The brain and the spinal cord are enveloped by 3 overlapping layers of connective tissue called the meninges. The layers are, from the most external layer to the most internal layer, the dura mater, arachnoid mater, and pia mater. Between these layers are 3 potential spaces called the epidural, subdural, and subarachnoid spaces. The meninges function to protect the contents of the brain and spinal cord. Infection of the CNS presents with inflammation of the meninges, and the etiology can be elicited by examining CSF, which is contained within the subarachnoid space.

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Layers of the Meninges

The meninges are layers of connective tissue that protect the brain and spinal cord.

The meninges consist of 3 layers of connective tissue, with potential spaces between them:

  • Dura mater: outermost layer
  • Arachnoid mater: middle layer
  • Pia mater: innermost layer
Table: Layers of the meninges from superficial to deep
LayerOriginCharacteristics
Epidural spaceNA
  • Potential space between the dura mater and skull/vertebral column
  • Contains blood vessels and fat
  • Site of blood collection in cases of middle meningeal artery injury
Dura materMesoderm
  • Divided into 2 layers:
    • Superficial periosteal layer
    • Inner meningeal layer
  • Grows adhered to the periosteum of the calvaria
  • Blood supply: middle meningeal artery
  • Nerve supply:
    • Trigeminal nerve branches: innervate supratentorial structures
    • Cervical nerves (C2 and C3): innervate infratentorial structures
Subdural spaceNA
  • Potential space between the arachnoid mater and the dura mater
  • Contains “bridging veins” that run between the cerebral hemispheres and dural venous sinuses
  • This space typically exists only under pathologic conditions, such as with a subdural hematoma.
Arachnoid mater (leptomeninges)Neural crest
  • The outer layer of the subarachnoid space
  • Avascular
  • Arachnoid trabeculae: web-like strands that separate the arachnoid mater and pia mater
Subarachnoid spaceChoroid plexus
  • Arachnoid/pacchionian granulations: allow CSF to enter from the subarachnoid space into the venous system
  • CSF: produced by the choroid plexus and contained in the subarachnoid space
  • The major arteries of the brain run through the subarachnoid space.
Pia mater (leptomeninges)Neural crest
  • The inner layer of the subarachnoid space
  • Adherent to the brain
  • Shiny appearance grossly
  • Highly vascularized
NA: not applicable
Meninges and subarachnoid space

Layers of the meninges and their relationships beneath the skull

Image: “Diagram of section of top of brain showing the meninges and subarachnoid space” by OpenStax. License: CC BY 4.0

Dural Foldings, Reflections, and Sinuses

The dura mater is the thickest layer of the meninges and provides structure to the brain.

Dural foldings and dural sinuses come from the dura mater.

  • Dural foldings separate the right and left hemispheres:
    • Falx cerebri
    • Falx cerebelli
    • Tentorium cerebelli
    • Diaphragma sellae
  • Dural sinuses (venous sinuses) form between layers of dura mater. CSF flows through the dural sinuses.
Table: Characteristics of dural foldings, reflections, and sinuses
StructureCharacteristics
Falx cerebriSeparates the right and left cerebral hemispheres
Falx cerebelliSeparates the right and left cerebellar hemispheres
Tentorium cerebelliTent, or roof, over the cerebellum
Diaphragma sellaeThe roof over the pituitary gland
Dural sinusesThe 2 layers of dura mater run together throughout most of the skull. Where they separate, the gap between them is called a dural venous sinus. These sinuses drain blood and CSF from the brain and empty into the internal jugular vein.
Dural venous sinuses

Dural venous sinuses

Image: “Dural venous sinuses” by Jmarchn. License: CC BY-SA 3.0, edited by Emma C. Cheshire et al. (2017).

Clinical Relevance

Neoplastic disorders:

  • Meningiomas are the most common cranial neoplasm, arising arise from arachnoidal cells that are found in the arachnoid villi. Meningiomas are typically benign, slow-growing tumors, but they can sometimes present in an atypical or malignant fashion. Symptoms of meningiomas depend on their location. The symptoms occur as a result of the tumor compressing nearby structures. The most common treatment is surgical excision. 

Infectious disorders:

  • Meningitis: inflammation of the meninges that surround the brain and spinal cord. In general, meningitis causes symptoms such as headache, fever, and stiff neck. This disease is subdivided into bacterial (most common cause), viral, and parasitic meningitis. Infectious etiologies are diagnosed with a lumbar puncture and must be treated aggressively with antimicrobial medications.

Traumatic disorders:

  • Epidural hemorrhage: bleeding in the space external to the dura mater—right beneath the skull. Epidural hemorrhage most commonly occurs after direct trauma to the head. Patients present with loss of consciousness after a direct blow to the head and, occasionally, altered mental status. If symptomatic, patients should undergo craniotomy with evacuation of the hematoma.
  • Subdural hemorrhage: occurs beneath the dura mater and is usually more chronic than epidural hemorrhages. In infants, subdural hemorrhage typically occurs as a result of child abuse. In young adults, common causes include bike or car accidents; and in elderly adults, subdural bleeds are usually the result of falls. If symptomatic, patients should undergo craniotomy with evacuation of the hematoma.
  • Dural sinus thrombosis: blood clots can form in the dural sinuses as a result of trauma or infection. Dural sinus thrombosis can lead to hemorrhagic infarction and cerebral edema. A cavernous sinus thrombosis is a subtype of dural sinus thrombosis that occurs because of seeding of an infection from the nasal area.

References

  1. Greenberg RW, Lane EL, Cinnamon J, Farmer P, Hyman RA. (1994). The cranial meninges: anatomic considerations. Semin Ultrasound CT MR. https://www.ncbi.nlm.nih.gov/pubmed/7880562
  2. Rai R, Iwanaga J, Shokouhi G, Oskouian RJ, Tubbs RS. (2018). The tentorium cerebelli: a comprehensive review including its anatomy, embryology, and surgical techniques. Cureus. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6168052/
  3. Louveau A, Smirnov I, Keyes TJ, et al. (2015). Structural and functional features of central nervous system lymphatic vessels. https://pubmed.ncbi.nlm.nih.gov/26030524/
  4. Aspelund A, Antila S, Proulx ST, et al. (2015). A dural lymphatic vascular system that drains brain interstitial fluid and macromolecules. J Exp Med. https://rupress.org/jem/article/212/7/991/41853/A-dural-lymphatic-vascular-system-that-drains
  5. Apra C, Peyre M, Kalamarides M. (2018). Current treatment options for meningioma. Expert Rev Neurother. https://pubmed.ncbi.nlm.nih.gov/29338455/
  6. Bi WL, Dunn IF. (2017). Current and emerging principles in surgery for meningioma. Chin Clin Oncol. https://pubmed.ncbi.nlm.nih.gov/28758410/
  7. Hoffman O, Weber RJ. (2009). Pathophysiology and treatment of bacterial meningitis. Ther Adv Neurol Disord. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3002609/
  8. Khairat A, Waseem M. (2020). StatPearls StatPearls. https://pubmed.ncbi.nlm.nih.gov/30085524/
  9. Mehta V, Harward SC, Sankey EW, Nayar G, Codd PJ. (2018). Evidence-based diagnosis and management of chronic subdural hematoma: A review of the literature. J Clin Neurosci. https://pubmed.ncbi.nlm.nih.gov/29428263/
  10. Ziu E, Mesfin FB. (2020). Subarachnoid hemorrhage. StatPearls. https://pubmed.ncbi.nlm.nih.gov/28722987/
  11. Wiemels J, Wrensch M, Claus EB. (2010). Epidemiology and etiology of meningioma. Journal of Neuro-Oncology 99:307–314.
  12. Starr CJ, Cha S. (2017). Meningioma mimics: five key imaging features to differentiate them from meningiomas. Clinical Radiology 72:722–728.
  13. Lee JH. (2008). Meningiomas: diagnosis, treatment, and outcome. Springer Science & Business Media, pp. 3–13.

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