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Meninges & CSF - Comprehensive Review

Notes

Meninges & CSF - Comprehensive Review

Meninges & CSF - Essentials

Meningeal Layers

See: Brain Atlas: Meninges

Layers

From outside to inside

Meningeal spaces

Spaces

From outside to inside

  • Epidural (above the dura)
  • Subdural (below the dura)
  • Subarachnoid (below the arachnoid mater)

Anatomy of the cerebral ventricles

  • Produce cerebrospinal fluid (CSF).
  • C-shaped appearance.
  • Horns
  • Body is the long superior bend.
  • Atrium (aka trigone) is the convergence of the body, and temporal and occipital horns.
  • 3rd ventricle in midline.
  • Diamond-shaped 4th ventricle.
  • Narrow cerebral aqueduct (of Sylvius) connects the 3rd and 4th ventricles.
  • The 4th ventricle empties into the central canal of the spinal cord.
    • Floor of the fourth ventricle is its anterior border: an anatomically important site because of its proximity to numerous key brainstem structures (eg, the locus coeruleus and the area postrema). Pressure on the floor of fourth ventricle triggers vomiting via the area postrema.
    • Superior–posterior border of the fourth ventricle is the superior medullary velum (aka anterior medullary velum), often the site of medulloblastoma.
    • Inferior–posterior border is the inferior medullary velum (aka posterior medullary velum).
    • Cerebellar peduncles form the lateral borders of the fourth ventricle and the cerebellum helps form the rest of the posterior border (the roof).

Physiological flow of CSF through the ventricular system.

  • The choroid plexus is the highly vascularized secretory epithelial tissue that produces cerebrospinal fluid (CSF).
  • Choroid plexus lies centrally: in the body, atrium, and temporal horn of the lateral ventricle, and the third and fourth ventricles.
    • It comprises tight junctions within its cuboidal epithelium that form an important blood–cerebrospinal fluid barrier.
  • There is a lack of choroid plexus in the frontal and occipital horns.
  • This makes them the ideal site for intraventricular drain placement because the drain won't accidentally tear the highly vascularized choroid plexus.
    • The choroid plexus is formed where invaginations of vascularized meninges, called tela choroidea, merge with ventricular ependyma. The tela choroidea are variably defined histologically as either combinations of pia and ependyma or double pial layers.

CSF Flow Summary

  • Empties from the lateral ventricles through the paired foramina of Monro into the third ventricle, down the cerebral aqueduct, and then into the fourth ventricle.
  • From there, CSF empties into the subarachnoid space through the foramen of Magendie (in midline) and the bilateral foramina of Luschka (laterally), and also show that it empties down the central canal.
    • The obex is where the 4th ventricle transitions into the central canal of the spinal cord. It lies at the inferior angle of the fourth ventricle, at the level of the gracile tubercle (the swelling formed by the gracile nucleus in the posterior wall of the medulla).

anatomy of the meninges

Meninges

From outside to inside

Terminology

  • Pachymeninges refers to the dura mater.
  • Leptomeninges refers to the combined arachnoid and pia mater.
  • A key disease process that commonly causes exclusively pachymeningitis is intracranial hypotension.
  • One that commonly causes exclusively leptomeningitis is infectious meningitis, specifically bacterial or viral.

Meningeal spaces

From outside to inside

  • Epidural (above the dura)
  • Subdural (below the dura)
  • Subarachnoid (below the arachnoid mater)

Clinical correlations

dural folds (aka septa)

  • Falx cerebri
    • The reflection in the midline vertex is the falx cerebri; it separates the cerebral hemispheres; indicate that it houses the superior sagittal sinus.
  • Tentorium cerebelli
    • The reflection along the lateral base of the brain as the tentorium cerebelli; it separates the cerebellum from the overlying occipital and postero-medial temporal lobes, and house the transverse sinuses.
  • Falx cerebelli
    • The falx cerebelli (not shown here) separates the cerebellar hemispheres.

key anatomic and clinical points

  • The cranial epidural space is only a potential space because the dura tightly adheres to the skull.
  • The spinal epidural space is an actual space; it separates the vertebral column, externally, from the dura mater, internally.
    • As a result, epidural processes, such as infection, hemorrhage, or spread of neoplastic disease occur more commonly in the spinal epidural space than the cranial space.

Clinical Correlation - Spinal Cord Compression

Middle Meningeal Artery

  • The meningeal arteries (notably the middle meningeal artery) run between the skull and dura, thus epidural hematoma classically occurs from skull fracture and middle meningeal artery rupture.

Bridging Veins

  • Bridging veins connect the brain surface to the superior sagittal venous sinus: they are easily torn within the subdural space and, thus, are the key cause of subdural hematoma.

Physiology of CSF return

  • Dural venous channels are mostly filled with blood because the rate of CSF production and reabsorption is far slower than the rate of blood entry and reabsorption into and out of the cranial vault (about 20ml per hour).
  • Arachnoid granulations comprise arachnoid villi, which drain from the subarachnoid space into the venous sinuses.
  • Neoplastic arachnoid villi cells form meningiomas; thus meningiomas are typically found where there are the greatest concentration of arachnoid villi: at the cerebral convexity (falx cerebri) and base of the skull.

Histology of the dura mater and its sublayers.

  • Underneath the skull lies the periosteal dural sublayer, which is vascular, as we've seen with the location of the meningeal arteries.
    • It ends within the cranium.
  • The meningeal sublayer is avascular and continues within the spinal canal.
  • The dural border cell sublayer internal to the meningeal sublayer; thus, the subdural space is actually filled with this loosely arranged sublayer.

Cerebral ventricle development

  • The shape of the lateral ventricles resembles many major cerebral structures — the cerebral hemispheres, the caudate–putamen, and the fornix–hippocampus.
  • During embryogenesis, all of these structures undergo a backward, downward, and forward migration, which we can demonstrate with our arms as follows:
    • First, create a coronal view of the developing brain as follows.
    • Hold your arms together with your elbows bent and extend your wrists so you could set a plate on your palms.
    • Your hyper-extended palms represent the flat surface of the brain when it first forms.
    • Next, curl your fingertips to demonstrate that during early development, there is inrolling of the walls of the hemispheres.
    • Then, continue to curl your fingers in so that they touch your palms to form the bilateral lateral ventricles and the small midline third ventricle.
    • Next, initiate the backward, downward, and forward evagination of the ventricles.
    • Bring your forearms back toward your chest, then fan your elbows apart as you bring your hands downward, and then extend your arms forward.
    • We can imagine how each of the horns takes shape during the different steps of lateral ventricular development: the frontal horns take shape during the origination of the ventricular system, the occipital horns are created during the backward migration, and the temporal horns form during the downward and forward migration.