Neurophysiology - ICP and head injury

Question 1

What is normal ICP

Answer 1

5 - 15 mmHg

During straining can reach 50 mmHg

Question 2

List and describe the methods of ICP measurement

Answer 2

1. EVD (External Ventricular Device)
   - Gold standard
   - Rx + Dx purposes
   - Difficult surgical placement
   - Blockage
   - Infection
2. Intraparenchymal probe
   - Small burr hole - fibre-optic tipped catheter within brain parenchyma
   - Easier insertion (vs EVD)
   - Lower infection
   - almost as accurate
   - low accuracy if prolonged (no recalibration in vivo)
   - may not represent global ICP (only local where prope is)
3. Subarachnoid probe
   - Obsolete
   - Poor accuracy
   - Easier insertion
4. Subdural probe
   - Obsolete
   - Easier to insert
   - Less accurate + blockage + flushing

Question 3

What is the Monroe Kellie Hypothesis

Answer 3

The cranium is a rigid box of fixed volume that contains:

1. Brain tissue: 1400 g (80% of volume)
2. CSF: 150 mL(10%)
3. Arterial + venous blood (10%)

An increase in the volume of any one of these components will increase the ICP unless there is a corresponding decrease in one or both of the other components.

End stage

1. Reduction in arterial blood volume –> ischaemia
2. Parenchyma herniation through foramen magnum

Question 4

Describe the symptoms suggesting raised ICP

Answer 4

1. Headache
   - Worse morning, lying flat, straining
   - N and V
2. Bulge fontanelle
3. Papilloedema
4. Altered level consciousness

Cranial nerve palsies (abducens CN 6)
Pupillary dilation (Compression CN 3)
Cushing Triad
- HPT
- Bradycardia
- Abnormal respiratory pattern

Question 5

Describe the physiology of the Cushing reflex

Answer 5

CPP = MAP - ICP

ICP increases and CPP falls. Autoregulation maintains a CBF of 50 ml / 100g / minute over a CPP range 50 - 150 mmHg.

As ICP increases CPP falls below 50 and CBF falls below 50 ml / 100g / min –> cellular ischaemia –> emergency hypertension (Vasomotor center) increases SNS output –> sudden rise MAP with reflex (baroreceptor bradycardia)

Herniation –> compresses brainstem and resp center leading to abnormal breathing

GCS 3 - 5 usual
Hypotension will result as Vasomotor center is compressed