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| | ==[[General CNS Responses to Injury]]== | | ==[[General CNS Responses to Injury]]== |
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| − | ===Ischaemic Damage===
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| − | * The CNS is particularly sensitive to ischaemia, because it has few energy reserves.
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| − | * The CNS is protected by its bony covering.
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| − | ** Despite offering protection, the covering also makes the CNS vulnerable to certain types of damage, for example:
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| − | *** Damage due to fractures and dislocation.
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| − | *** Damage due to raised intracranial pressure.
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| − | **** Raised intracranial stimulates a compensatory increase in blood flow, further raising intracranial pressure. This stimulates a further increase in blood flow, and the cycle continues until intracranial pressure is so high that blood flow is impeded.
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| − | ***** The result of this is '''ischaemia'''.
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| − | * Survival of any cell is dependent on having sufficient energy.
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| − | ** Ischaemia causes cell death by impeding energy supply to cells.
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| − | *** Cells directly affected by ischamia die rapidly.
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| − | **** For example, those suffering a failure of pefusion due to an infarct.
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| − | *** Neurons surrounding this area of complete and rapid cell death exist under sub-optimal conditions and die over a more prolonged period.
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| − | **** This area of gradual death is known as the '''lesion penumbra'''.
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| − | **** There are several mechanisms implicated in cell death in the penumbra:
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| − | ****# Increase in intracellular calcium
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| − | ****# Failure to control free radicals
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| − | ****# Generation of nitrogen species (e.g NO and ONOO) are the main damaging events.
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| − | ===Oedema===
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| − | * There are three types of cerebral oedema:
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| − | *# '''Vasogenic oedema'''
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| − | *#* Vasogenic oedema follows vascular injury.
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| − | *#* Oedema fluid gathers outside of the cell.
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| − | *#* This is the most common variation of cerebral oedema.
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| − | *# '''Cytotoxic oedema'''
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| − | *#* Cytotoxic oedema is due to an energy deficit.
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| − | *#** The neuron can’t pump out sodium and water leading to swelling within the cell.
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| − | *# '''Interstitial oedema'''
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| − | *#* Associated with hydrocephalus.
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| − | *#* This type of cerebral oedema is of lesser importance.
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| − | * One serious consequence of oedema is that the increase in size leads to the brain trying to escape the skull.
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| − | ** This causes herniation of the brain tissue.
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| − | ** The most common site of herniation is at the foramen magnum.
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| − | *** The medulla is compressed at the site of the respiratory centres, leading to death.
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| − | ===Demyelination===
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| − | * Demyelination is the loss of initially normal myelin from the axon.
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| − | * Demyelination may be primary or secondary.
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| − | ====Primary Demyelination====
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| − | * Normally formed myelin is selectively destroyed; however, the axon remains intact.
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| − | * Causes of primary demyelination:
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| − | ** Toxins, such as hexachlorophene or triethyl tin.
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| − | ** Oedema
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| − | ** Immune-mediated demyelination
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| − | ** Infectious diseases, for example canine distemper or caprine arthritis/encephalitis.
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| − | ====Secondary Demyelination====
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| − |
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| − | * Myelin is lost following damage to the axon.
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| − | ** I.e. in [[CNS Response to Injury - Pathology#Wallerian Degeneration|wallerian degeneration]]
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| − |
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| − | ===Vascular Diseases===
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| − |
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| − | * Vascular diseases can lead to complete or partial blockage of blood flow which leads to ischaemia.
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| − | ** Consequences of ischaemia depend on:
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| − | **# Duration and degree of ischaemia
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| − | **# Size and type of vessel involved
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| − | **# Susceptibility of the tissue to hypoxia
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| − | * Potential outcomes of vascular blockage include:
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| − | ** Infarct, and
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| − | ** Necrosis of tissue following obstruction of its blood supply.
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| − | * Causes include:
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| − | ** Thrombosis
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| − | *** Uncommon in animals but may be seen with DIC or sepsis.
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| − | ** Embolism. e.g.
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| − | *** Bone marrow emboli following trauma or fractures in dogs
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| − | *** Fibrocartilaginous embolic myelopathy
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| − | ** Vasculitis, e.g.
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| − | *** Hog cholera (pestivirus)
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| − | *** Malignant catarrhal fever (herpesvirus)
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| − | *** Oedema disease (angiopathy caused by E.coli toxin)
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| − | ===Malacia===
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| − |
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| − | * Malacia may be used:
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| − | ** As a gross term, meaning "softening"
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| − | ** As a microscopic term, meaning "necrosis"
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| − | * Malacia occurs in:
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| − | ** Infarcted tissue
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| − | ** Vascular injury, for example vasculitis.
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| − | ** Reduced blood flow or hypoxia, e.g.
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| − | *** Carbon monoxide poisoning, which alters hemoglobin function
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| − | *** Cyanide poisoning, which inhibits tissue respiration
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| − | [[Category:CNS Response to Injury]]
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