10b.) Intracranial Pressure Flashcards
(45 cards)
What 3 things determine normal intracranial pressure?
ICP is determined by volume of:
- Blood
- Brain
- CSF
… all enclosed in a rigid box
State some possible causes of raised intracranial pressure
- Intra-cranial mass lesions
- Blood
- Brain
- Tumour
- CSF
- Disorders of CSF
- Diffuse intracranial pathological processes
Roughly, what volume of each should you have:
- Brain
- Blood
- CSF
- Brain: 1300-1700ml
- Blood: 100-150ml
- CSF: 100-150ml
*Main idea is to know normal amounts of blood and CSF
State the normal intracranial pressures for each of the following categories:
- Children
- Adults
- Term infants
- Children: 5-7mmHg
- Adults: 5-15mmHg
- Term infants: 1.5-6mmHg
*Good rue of thumb= if >20mmHg then ICP is raised
Describe the Monro-Kellie doctrine
- If there is an increase in volume in any one of the intracranial constituents that contributes to intracranial pressure (brain, blood, CSF) then this must be compensated by a decrease in volume of one of the others
- The first components to be pushed out of the intracranial space are constituents with the lowest pressure= CSF and venous blood
How do you calculate cerebral perfusion pressure?
CPP= MAP - ICP
- *MAP= mean arterial pressure*
- *ICP= intracranial pressure*
What is the normal CPP?
>70mmHg
What is the normal MAP?
~90mmHg
What is normal ICP?
~10mmHg
*But remember, it is different for different categories of people and general rule is if it is above 20mmHg it is raised
Describe the cerebral myogenic autoregulation if MAP increases
- MAP increases
- Increase in MAP causes increase in CPP
- Vasoconstriction of cerebral vasculature to prevent too much blood going to brain and increasing ICP
Describe the cerebral myogenic autoregulation when ICP increases
- ICP increases
- Will cause CPP to decrease
- Hence get vasodilation of cerbral vasculature to ensure adqequete perfusion to brain
Below what pressure does cerebral myogenic autoregulation fail and why?
Cerebral myogenic regulation fails below 50mmHg because at a CPP of 50mmHg the cerebral arterioles are maximally dilated
This will consequently lead to imparied perfusion of brain
Can ICP be maintained as an intracranial mass expands?
ICP can be maintained up to a cerain point; beyond this point ICP rises very rapidly/exponentially
Can brain damage impair cerebral myogenic autoregulation?
Brain damage can impair or even abolish cerebral autoregulation
Describe Cushing’s reflex
- Increase in intracranial pressure
- Leads to decrease cerebral blood flow
- Decreased cerebral blood flow means less CO2 is removed hence there is a regional increase in CO2
- High CO2 sensed by vasomotor centre in brain
- Activate sympathetic nervous sytem to cause vasoconstriction
- Vasoconstriction increases MAP which then increases cerebral perfusion pressure (remember CPP= MAP-ICP)
- Baroreceptors in aortic arch & carotid sinus then detect the increase in MAP
- Baroreceptors send signals to vasomotor centre to increase vagal activity (parasympathetic) to decrease HR
- This results in Cushing’s triad: bradycardia, hypertension and irregular breathing
State Cushing’s triad
- Hypertension
- Bradycardia
- Irregular respiration/bradypnea
What triad is indicative of raised ICP?
Cushing’s triad
The increased vagal activity which is seen in Cushing’s reflex can cause stomach ulcers as a side effect; explain how it can cause stomach ulcers
CNX (vagus nerve) can stimulate:
- G cells to produce gastrin. Gastrin then goes on to stimulate ECL cells which release histamine which causes parietal cells to release HCL
- ECL cells directly to produce histamine which stimulates parietal cells
- Parietal cells directly to produce HCL
Why do you get irregular breathing in Cushing’s reflex?
Due to continuing compression of brainstem which leads to damage to respiratory centres in brainstem
State 6 possible broad causes of raised ICP
- Too much blood in cerebral vessels (RARE)
- Too much blood outside of cerebral vessels e.g. haemorrhage
- Too much CSF
- Acquired e.g. meningitis, tumours
- Too much brain e.g.cerebral oedema
- Soemthng else e.g. cerebral abcess, idiopathic
Raised ICP can be caused, RARELY, by too much blood in cerebral vessels; state 2 possible reasons for too much blood in cerebral vessels
- Raised arterial pressure
- Malignant hypertension (very high blood pressure that comes on suddenly)
- Raised venous pressure
- SVC obstruction e.g. by lung tumour
Raised ICP can be caused by too much blood outside of cerebral vessels a.k.a. a haemorrhage; state 5 possible haemorrhages
- Extradural
- Subdural
- Subarachnoid
- Haemorrhagic stroke
- Intraventricular stroke
Raised ICP can be caused by too much CSF a.k.a hydrocephalus; state some possible causes of hydrocephalus
Congenital
-
Obstructive (drainage impaired):
- Neural tube defects
- Aqueduct stenosis
- As part of a larger syndrome
-
Communicating (drainage not impaired):
- Increased prodution
- Decreased absorption
Acquired
- Infection or kinking ofa ventricular shunt which has been put in place
- Meningitis
- Trauma
- Haemorrhage (e.g post subarachnoid)
- Tumours (which could compress e.g. cerebral aqueduct)
Describe the clinical signs of hydrocephalus in infants and young children
- Bulging head with head circumference increasing faster than expected
- Bulging fontanelles
- Sunsetting eyes (up-gaze paresis with the eyes appearing driven downward. The lower portion of the pupil may be covered by the lower eyelid, and sclera may be seen between the upper eyelid and the iris. Due to direct compression of the orbit aswell as involvment of occulomotor nerve as it exits midbrain)
