Neurology - Raised ICP and herniation syndromes

Question 1

Definition of intracranial pressure (ICP)

Answer 1

= Pressure within intracranial compartment, it is distributed evenly throughout the intracranial cavity and corresponds to pressure in the brain tissue and CSF

Lumbar CSF pressure in a supine patient equates to the ICP

Question 2

How is ICP measured?

Answer 2

Measured relative to the foramen of Monroe (usually approximated to the external auditory meatus)

5-15mmHg in normal supine adults (can be much higher in coughing, straining or valsalva)

Infants

Question 3

CBF

Answer 3

CBF = Cerebral blood flow
Raised ICP is only a problem if CBF is compromised

Flow = Pressure/ Resistance
So, CBF = Cerebral perfusion pressure (CPP)/ Cerebral vascular resistance (CVR)

CBF is reduced if CPP decreases or if CVR increases

Question 4

What controls CVR?

Answer 4

i) Autoregulation (changes in vessel calibre in response to changes in cerebral perfusion pressure; myogenic response)
- Increased pressure = CONSTRICTION
- Decreased pressure = DILATION

ii) Metabolic changes:
- Hypoxia, hypercapnia, acidosis, increased metabolic products = DILATION
- Hypocapnia, alkalosis, and decreased metabolic by products = CONSTRICTION

Question 5

CPP

Answer 5

CPP = Cerebral perfusion pressure
= MABP - ICP

Normally > 50mmHg

Therefore, if ICP increases CPP decreases (due to loss of pressure gradient) leading to reduced cerebral blood flow causing ischaemic damage

Question 6

How far does CBF need to fall before brain damage occurs?

Answer 6

Brain damage occurs when CBF falls to

Question 7

When does auto regulation of cerebral blood flow begin to fail?

Answer 7

Autoregulation maintains cerebral blood flow over certain limits despite changes in cerebral perfusion pressure (MABP - ICP)

Autoregulation only works between a CPP of 60-160mmHg after which blood flow is determined directly by perfusion pressure

Question 8

How is auto regulation affected during brain injury?

Answer 8

In the damaged brain autoregulation is impaired
Decreased CPP is more likely to cause reduced CBF and ischaemia
High CPP may raise CBF, break down the BBB and produce cerebral oedema

Question 9

Causes of raised ICP

Answer 9

1) Intracranial mass lesion
- Tumor
- Infection: Abscess/ subdural empyema/ tuberculoma
- Haematoma: Traumatic or spontaneous

2) Increased CSF volume
- Obstructive or communicating hydrocephalus

3) Increase in cerebral blood volume
- Venous outflow obstruction (e.g. venous sinus thrombosis)
- Vasodilation (e.g. reactive hyperaemia due to loss of auto regulation following head injury)

4) Cerebral oedema

5) Prevention of normal skull growth (children)
- Multisutural synostosis (premature fusion of skull sutures) - e.g. Crouton’s syndrome

Question 10

How does oedema appear on CT?

Answer 10

Affected brain is hypotenuse (darker) on CT

2 types of oedema - vasogenic and cytotoxic (both are often present in variable portions in trauma and schema)

Question 11

Vasogenic oedema

Answer 11

• Secondary to breakdown of BBB, leakage of intravascular proteins and fluid into interstitial space
• Primarily affects white matter
• Seen around tumours or abscesses

Question 12

When causes vasogenic oedema?

Answer 12

• Can occur in late stages of cerebral ischaemia due to ischaemic damage to cerebral blood vessels
• PRES = posterior reversible encephalopathy syndrome; type of vasogenic oedema that arises secondary to malignant hypertension
• Altitude sickness (HACE = high altitude cerebral oedema)

Question 13

Appearance of vasogenic oedema on MRI

Answer 13

Elevated apparent diffusion co-efficient (ADC) - i.e. increased water diffusion

Question 14

Cytotoxic oedema

Answer 14

Increased intracellular fluid in glia (particularly astrocytes) and neurones due to failure of cellular metabolism (ATP dependent ion channels)

Predominantly affects grey matter

Question 15

What are the common causes of cytotoxic oedema?

Answer 15

1) Early cerebral ischaemia (failure of oxygen delivery to the brain) - e.g. stroke, near drowning, cardiac arrest, carbon monoxide poisoning
2) Drugs and toxins - e.g. hepatic encephalopathy (secondary to elevated ammonia), isoniazid

Question 16

Appearance of cytotoxic oedema on MRI

Answer 16

Reduced ADC (decreased water diffusion)

Question 17

Which type of oedema is most responsive to therapy?

Answer 17

Vasogenic oedema, not cytotoxic, is helped with dexamethasone therapy

Question 18

3 stages in pathogenesis of ICP

Answer 18

1) Stage of compensation
2) Stage of decompensation
3) Vasomotor paralysis

Question 19

What happens during the stage of compensation in raised ICP?

Answer 19

An expanding lesion (e.g. tumour, oedema, haematoma etc) leads to a potential increase in ICP

Initially there is:

• Vasoconstriction - reduces intracranial blood volume (predominantly venous)
• Increased CSF absorption and decreased formation - decreases CSF volume

This allows the brain to expand without serious effects but is only temporary

Question 20

What happens during decompensated raised ICP?

Answer 20

At this stage there are herniations (with associated complications) and raised ABP with bradycardia (Cushing reflex)

Question 21

Vasomotor paralysis

Answer 21

Raised ICP leads to reduced CPP this leads to reduced CBF and hypoxia and hypercapnia. This damages neurones and causes vasodilation with further increases ICP - vicious cycle

Question 22

Monro-Kellie doctrine

Answer 22

Cranial compartment is of fixed volume. Any increase in one of the intracranial contents (blood, CSF or brain tissue) must be compensated by a decrease in another

Question 23

What happens in infants with raised ICP?

Answer 23

Infants are the exception because their skull sutures have not fused

When volume increases their sutures separate (diastasis) and the cranial vault enlarges to accommodate increased volume

Question 24

History of raised ICP

Answer 24

• Headache: global headache; usually persistent; may be relieved on standing or by vomiting; may wake from sleep in early morning due to headache; worse on coughing or straining
• Vomiting
• Drowsiness
• Confusion

Question 25

Examination findings in raised ICP

Answer 25

• Reduced consciousness - GCS (correlates with severity of raised ICP)
• Eye signs - papilloedema if present is a reliable indicator of elevated ICP causing enlarged blind spot, constricted visual fields, decreased acuity
• Cushings triad - Hypertension, bradycardia and respiratory irregularity (only seen in severely elevated ICP usually in comatose patients)
• Infants - full or tense fontanelle, dilated scalp veins

Question 26

What cranial nerve provides a “false localising sign” in raised ICP?

Answer 26

CN IV (Abducens nerve) - it has a long intracranial course and is therefore susceptible to stretch with raised ICP

Question 27

Foster Kennedy Syndrome

Answer 27

A mass lesion (usually an olfactory groove meningioma) compresses one optic nerve - causing atrophy, with papilloedema on the contralateral side due to raised ICP from the tumour

Question 28

Investigations in raised ICP

Answer 28

• Symptoms OR signs of raised ICP should prompt neuroimaging (CT is best modality in cases of trauma and depressed GCS)
• In non traumatic cases, if NO mass lesion is identified on imaging and the cerebral aqueduct is patent an LP can be performed to measure CSF pressure
• ICP monitoring

Question 29

Key treatment goals in managing raised ICP

Answer 29

i) Reduce ICP (based around Monro-Kellie doctrine)
- Reduce volume of intracranial contents
- Increase volume of intracranial compartment
ii) Protect the brain from damage due to impaired cerebral blood flow

Question 30

Reducing the volume of intracranial contents in raised ICP

Answer 30

(Monro-Kellie)

i) Blood:
- Optimise venous drainage (i.e. elevate the head, thrombolysis if raised ICP secondary to CVST)
- Prevent hyperaemic vasodilation: maintain pCO2 at 4.0-4.5kPA

ii) CSF:
- surgical drainage of CSF from ventricles (even if hydrocephalus is not present)

iii) Brain:
- Surgical removal of expanding mass (e.g. tumour, haematoma, abscess)
- “Shrink” the brain

Question 31

Therapeutic strategies to “shrink” the brain in raised ICP

Answer 31

1) Osmotic agents (reduce brain interstitial fluid) - e.g. 20% mannitol or hypertonic saline
2) Steroids (dexamethasone) - reduce vasogenic oedema around tumours and cerebral abscesses (not helpful in trauma or ischaemic injury)
3) Brief (minutes) of hyperventilation (pCO2 3.5-4.0kPa) can transiently reduce cerebral blood volume to offset “spikes” in raised ICP in ventilated patients

Question 32

How can the volume of the intracranial compartment be increased in raised ICP?

Answer 32

• Surgical decompressive craniectomy - removing the skull vault and surgically expanding the dural sac to allow the brain to swell without increasing ICP
• Brain resection - in trauma, a non dominant partial temporal or frontal lobectomy can be considered (esp if those areas are badly contused)

Question 33

Cerebral protection in raised ICP

Answer 33

1) Intubation and ventilation - ensures adequate cerebral oxygenation and control of CO2

2) Reduce brain metabolism
i) High dose barbiturate therapy (thiopentone coma) - intractable raised ICP; given until burst suppression seen on EEG
ii) Cerebral cooling; moderate hypothermia reduces brain oxygen requirements

Question 34

2 main complications of raised ICP

Answer 34

1) Visual loss - severe and prolonged papilloedema can result in visual loss due to optic atrophy

2) Cerebral herniation - different types:
- Tentorial/ Uncal herniation (lateral)
- Tentorial herniation (central)
- Subfalcine herniation
- Tonsillar herniation
- Upward cerebellar

Question 35

What is an “uncal” herniation?

Answer 35

This is a lateral tentorial herniation
Unilateral supratentorial mass lesion causes the medial edge of the temporal lobe (known as the uncus) to herniate through the tentorial hiatus

If ICP continues to rise, can progress to central and then tonsillar herniation

Question 36

What are the clinical features of an uncal herniation?

Answer 36

NB - uncal herniation = lateral tentorial herniation

• Compression of IIIrd nerve causes ipsilateral pupil dilation that is unreactive to light (a “fixed” pupil)
• Compression of posterior cerebral artery causes homonymous hemianopia
• Pressure on midbrain reticular formation leads to reduced consciousness
• Pressure from the contralateral edge of the tentorium against the cerebral peduncle (Kernohan’s notch) can cause ipsilateral hemiparesis (a false localising sign)

Question 37

What causes a central tentorial herniation?

Answer 37

• Diffuse bilateral hemisphere swelling
• Hydrocephalus
• Midline mass lesions

They cause vertical displacement of the diencephalon through the tentorial hiatus

Question 38

Clinical features of a central tentorial herniation

Answer 38

• Distortion of diencephalic and midbrain structures and stretching of perforating vessels causes coma
• Pupils may initially be small later becoming fixed
• Pressure on the upper midbrain (tectum) may cause loss of upward gaze (Parinaud’s syndrome)
• Distortion of the pituitary stalk may cause diabetes insipidus

Question 39

Subfalcine herniation

Answer 39

Occurs early with unilateral supratentorial mass lesions causing herniation across the midline below the fall cerebri

Question 40

Clinical features of subfalcine herniation

Answer 40

• Often asymptomatic
• Can result in contralateral hydrocephalus of lateral ventricle
• If severe can cause kinking and infarction of distal ipsilateral anterior cerebral artery - causing contralateral leg weakness

Question 41

What is idiopathic intracranial hypertension?

Answer 41

(= pseudo tumour cerebri)
Consider in patients presenting as if with a mass (headache, raised ICP and papilloedema) where none is found!

Typical patients are obese women with narrowed visual fields, blurred vision +/- diplopia, 6th nerve palsy and an enlarged blind spot if papilloedema is present (it usually is)

Consciousness and cognition are preserved