11.8.b.Meninges and Ventricular system.txt

  1. 11.8.b.Meninges and Ventricular system
    • Development of meninges
    • The meninges develop from a single menbrane, the “PRIMARY MENINX”, which differentiates into two layers: the external pachymeninges (Dura); and the leptomeninges (arachnoid and pia)
  2. Dura Mater
    • Dense fibrous connective tissue, tightly attached to the periosteum of the skull
    • Within the dura see meningeal arteries which are tightly attached to the dura: they supply the blood to the calvarium
    • Middle meningeal artery enters the scull through: foramen spinosum
  3. Foding and Reflection of Dura Mater:
    • Falx cerebri: between hemispheres
    • Tentorium cerebelli: between cerebrum and cerebellum
    • Gives rise to terms: supratentorial and infratentorial
  4. Herniations defined by dural compartments
    • Subfalcine: side to side beneath the falx cerebri; also called the CINGULATE herniation
    • Transtentorial: through the opening in the tentorium cerebelli; also called UNCAL herniation
    • Tonsillar (medulla) or brain stem (cerebellar) herniation: through the foramen magnum
  5. Inner and outer layers of the dura separate
    • To form venous sinuses.
    • Cerebral veins drain into the venous sinuses
    • Veins cross the potential subdural space
  6. Dural venous sinuses
    • Superior and inferior sagittal sinuses
    • Straight sinus
    • Transverse sinuses, and sigmoid sinuses
  7. Hemorrhage within the cranial cavity is defined by location, often with reference to the meninges:
    • Epidural
    • Subdural
    • Subarachnoid
    • Intraventricular
    • Intracerebral
  8. Epidural Hematoma
    • Surgical emergency!
    • A hemorrhage between the dura and the inner table of the skull
    • Caused by injury to a meningeal artery following trauma with skull fracture
    • Most often affects the middle meningeal artery or one of its branches
    • Since it’s high arterial pressure – causes hemorrhage to enlarge very rapidly: surgical EMERGENCY!
  9. Subdural Hematoma
    • The arachnoid closely underlies the dura
    • Bleeding between the dura and the arachnoid: subdural hematoma
  10. Bleeding in the subdural space is from “
    • bridging” veins: connect the cerebral veins with the dural venous sinuses
    • They may be damaged by minor trauma, especially in the elderly
    • Almost always result from trauma, but don’t need a scull fracture here to cause a bleed
    • Often develop slowly, so are usually not a surgical emergency.
  11. Subdural hematoma
    • Blood between the dura and arachnoid
    • Usually related to closed head injury
    • Bleeding is due to tearing of small veins
    • Hematoma enlarges slowly: b/c of low pressure
    • Gradually displaces brain
    • Often bilateral
  12. Chronic subdural hematoma
    • Is often asymptomatic, until it becomes very large
    • Layer of organizing hematoma is adherent to dura; it can be peeled off as a “membrane”
  13. Subarachnoid Hemorrhage
    • Blood accumulates in the space normally occupied by CSF
    • Trauma – most frequent cause
    • Spontaneous SAH is usually due to rupture of a “berry” aneurysm
  14. Most common cause of a spontaneous subarachnoid hemorrhage
    Rupture of a “berry” aneurism
  15. Most common cause of a subarachnoid hemorrhage
  16. Arachnoid villi (granulations)
    • Invaginations of the arachnoid membrane into the overlying dural venous sinuses
    • CSF is reabsorbed into the venous system through the arachnoid villi
  17. “Syncytial” arachnoid cells
    • Light microscopy – many nuclei appear to occupy a cytoplasmic mass
    • E.M. of two adjacent arachnoid cells: their cytoplasm interdigitates so intimately, that the light microscope can’t resolve the cell membrane
  18. Meningioma
    Tumor of arachnoid cells
  19. The brain is protected from infection by the skull and meninges, but ways in which infection can gain access are:
    • Sinuses and middle ear
    • Hematogenous dissemination
    • Direct extension
    • Peripheral nerve
  20. Meningitis
    • Inflammation in the subarachnoid space
    • Subarachnoid fluid supposed to be clear, but it’s not
    • Inflammatory cells are distributed throughout the subarachnoid space
    • Collection of pus and congestion of vessels is apparent, vessels are seen though a thin layer of pus, so appear “out of focus”
  21. Subarachnoid space is larger in
    the lower lumbar region below the conus medullaris: so CSF may safely be sampled by lumbar puncture
  22. CSF normal and in acute meningitis
    • Normally: clear, colorless, no cells seen
    • In acute meningitis: proteinacious material, neutrophils can be seen and other inflammatory cells, and may see an organism
  23. CSF functions and amount
    • Provides mechanical buoyancy and cushioning
    • Removes brain metabolites and acts as a chemical reservoir to buffer local changes
    • Average person has 80-150 ml of CSF in subarachnoid space and ventricular system
    • Choroid plexus makes ~500 ml per day
  24. Circulation of CSF
    Flows through the ventricular system into subarachnoid space and is absorbed into the venous system through arachnoid villi (granulations)
  25. Ventricular system
    • Cavities within the brain derived from the neural canal
    • Contiguous with the central canal of the spinal cord
    • Lined by ependymal cells
  26. Ependymal cells
    • Line the ventricles and the central canal of the spinal cord
    • Simple columnar epithelial cells many are ciliated
  27. Lateral ventricles (2) consist of
    • Frontal (anterior) horn
    • Central part
    • Temporal (inferior) horn
    • Occipital (posterior) horn
  28. Lateral ventricles connect to the third ventricle via
    • Left interventricular foramen
    • Aka foramen of Monro
  29. Choroid plexus
    • Vascular organ that produces CSF
    • Present in body and temporal horns of lateral ventricles
    • Also present in roofs of 3rd and 4th ventricles:
  30. Third Ventricle
    • Thalamus and hypothalamus: form the lateral walls
    • Choroid plexus forms the roof
  31. Ventricles and foramina/apertures
    Image Upload 2
  32. Cerebral Aqueduct
    • Aka aqueduct of Sylvius: passage through the midbrain
    • Narrowest part of the system and most vulnerable to occlusion
  33. Fourth Ventricle
    • Cerebellum: forms the roof
    • Pons and medulla: form the floor
    • Choroid plexus: lies in the roof and extends to the lateral apertures
  34. Flow of CSF through the ventricles
    Image Upload 4
  35. Foramen of Magendie (median aperture) and Foramen of Luschka (lateral apertures)
    • In the fourth ventricle
    • Allow the CSF to flow from the ventricles into the subarachnoid space surrounding the brain and spinal cord
  36. Subarachnoid Cisterns
    • Enlarged “reservoirs” of CSF in the subarachnoid space
    • Named for anatomical locations, e.g:
    • Cerebello-medullary cistern (cisterna magna)
    • Suprachiasmatic
    • Interpeduncular
    • Quadrigeminal
    • Paramesencephalic
    • Pontine
    • Lumbar
    • Etc.
  37. Increased CSF Pressure
    • Results from either blockage of flow or impaired reabsorption
    • Symptoms: may include headache, nausea, vomiting, and/or bradycardia
    • Due to pressure on the vagal centers on the floor of the 4ths ventricle: !
  38. Increased CSF pressure often causes papilledema
    Papilledema: swelling of the optic disk by increased CSF pressure surrounding the optic nerve which impedes the venous drainage from the eye
  39. Hydrocephalus
    • Increased volume of CSF within the cranial cavity with or without an increase in pressure
    • Enlarged ventricles
    • Caused by blockage of CSF flow and/or decreased resorption
  40. Non-communicating (obstructive) Hydrocephalus
    • CSF flow is blocked within the ventricular system
    • e.g. by aqueductal stenosis
    • Reabsorption of CSF is blocked outside ventricles, e.g. due to scarring from meningitis
  41. Hydrocephalus in infants
    The skull can expand, so this may lead to enlargement of the head
  42. Rx for Hydrocephalus
    Treated by inserting a shunt from ventricle to jugular vein or peritoneum
  43. Compensatory hydrocephalus
    • (Hycrocephalus ex vacuo)
    • occurs in brain atrophy as CSF fills space of lost brain tissue volume
    • no increase in CSF pressure: !
    • Can be gross or focal enlargement of ventricles following tissue destruction by infarct (stroke)
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11.8.b.Meninges and Ventricular system.txt
11.8.b.Meninges and Ventricular system