10.2.1 Raised ICP

Question 1

What contributes to intracanial pressure?

Answer 1

Brain 80%
Blood 10%
CSF 10%

Question 2

What is the normal range of ICP?

Answer 2

5-15 mmHg

Varies between different ages

Question 3

What is the Monro-Kellie Doctrine?

Answer 3

Sum of volumes (brain,blood,CSF) must remain constant to avoid a raised ICP

Increased volume of one, or volume addition, must be offset by equal decrease in volume of the others

Question 4

What is first to reduce in an attempt to prevent raised ICP?

Answer 4

CSF then venous blood

Question 5

Describe the relationship between intracranial pressure and volume

Answer 5

Non-linear

May have large volume change before ICP increases

Compensatory mechanisms of CSF and venous blood prevent pressure rising too much

Question 6

After compensatory mechanisms deplete what happens to ICP?

Answer 6

Rapid rises in ICP for a given increase in volume

Question 7

What can caused raised ICP?

Answer 7

Too much CSF (hydrocephalus)
- Congenital
- Acquired

Too much blood

Outside Cerebral Vessels
- Intracranial haemorrhage
- Haemorrhagic stroke

Inside Cerebral Vessels (rare)
- Increased arterial pressure (malignant hypertension)
- Increased venous pressure (SVC obstruction)

Cerebral oedema
- Secondary to:
- Trauma
- Infection
- Ischaemia and infarct

Mass lesion
- Space occupying lesion e.g. tumour, cerebral abcess

Other
- Idiopathic intracranial hypertension, affects overweight women of childbearing age (can cause headaches, blurred vision, they are alert and conscious)

Question 8

What happens in hydrocephalus in infants?

Answer 8

Increased head circumfrence due to unfused bones

Depending on age fontanelles will bulge

Fontanelles fuse at :
Posterior 1-3 months
Anterior 18-24 months

Question 9

What sign occurs in hydrocephalus when eyes are displaced downwards in infants?

Answer 9

Sunsetting sign

Question 10

How can hydrocephalus be treated?

Answer 10

Acutely- removal of CSF or external ventricular drain

Long term- shunts form ventricular system to peritoneum or right atrium

Question 11

How can hydocephalus appear on a CT scan?

Answer 11

Dilation of the lateral ventricles

Question 12

What are the two major conssequences of raised ICP?

Answer 12

Brain ischaemia- due to imparied cerebral perfusion

Compression and herniation of the brain, leading to death

Question 13

What is cerebral blood flow dependent on?

Answer 13

Cerebral perfusion pressure

Question 14

What is the equation for cerebral perfusion pressure?

Answer 14

CPP= Mean arterial pressure - ICP

Normal values
MAP~90mmHg
ICP~10mmHg

CPP >70mmHg

Question 15

How is cerebral perfusion pressure maintained?

Answer 15

Cerebral autoregulation

Ensures CPP and cerebral blood flow can be steadily maintained despite MAP variations

Question 16

What changes occur when MAP is decreased and increased?

Answer 16

Reduced MAP
Maximal vasodilation of cerebral arterioles

Increased MAP
Maximal vasoconstriction of cerebral arterioles

Question 17

Outline the limitations of autoregulation

Answer 17

MAP can be too low or high and the CPP cannot be maintained

Leads to reduced blood flow

Question 18

What can happen to cerebral autoregulation in damaged brain tissue?

Answer 18

Impaired or absent

Question 19

What would happen without autoregulation?

Answer 19

CPP directly dependent and responsive to changes in MAP

Therefore changes in MAP would have direct effects on the brain

Question 20

What happens to cerebral prefusion pressure in increased ICP?

Answer 20

CPP will decrease as
CPP= MAP-ICP

Question 21

What mechanisms help to mitigate CPP reduction in increased ICP?

Answer 21

Cerebral arterioles vasodilate to increase cerebral blood flow to maintain CPP

Elevation of MAP by increasing systemic BP

Question 22

Why is increased cerebral blood flow to maintain CPP problematic?

Answer 22

Increasing cerebral blood volume does not help rising ICP

Question 23

What happens when an expanding mass causes rising ICP?

Answer 23

1. Compensation by extrusion of CSF and venous blood to decrease volume
2. Rising ICP starts to reduce CPP, which reduces cerebral blood flow, this causes cerebral vasodilation and increase increased blood pressure to oppose decreased CPP
3. ICP continues to rise, CPP unable to compete, hypoxic brain
4. Cerebral hypoxia causes cerebral oedema, further rise in intracranial volume and pressure
5. Brain and brainstem compression