Space Occupying Lesions and Aspects of Cerebral Perfusion Flashcards
What is the Munro-Kellie Doctrine?
States that the cranial compartment is incompressible and that the volume inside the cranium is fixed. The cranium and its constituents (blood, CSF, and brain tissue) create a state of volume equilibrium, such that any increase in volume of one of the cranial constituents must be compensated by a decrease in volume of another.
Vcsf+Vblood+Vbrain+Vother = Vintracranial space = constant
How would you calculate cerebral perfusion pressure?
CPP = MAP - ICP
How do you calculate mean arterial pressure?
Diastolic BP + 1/3 pulse pressure
What is normal cerebral perfusion pressure?
80 mmHg
What is normal MAP?
90 mmHg
What is normal ICP?
5-15 mmHg
What proportion of cardiac output does the brain receive?
15%
How do neurones produce energy?
Entirely oxidative metabolism of gluocse or ketone bodies. There is little capacity for anaerobic metabolism
What happens if the brain is starved of oxygen?
Energy dependent processes cease, leading to irreversible damage within 3-8 minutes
What processes contribute to the control of blood flow into the brain?
Autoregulation
- Myogenic mechnism - effect of transmural blood pressure changes, which are directly detected by the vascular smooth muscle in arterioles. Then, the calibers are adjusted accordingly to keep blood flow constant.
- Metabolic mechanism - driven by metabolic demand and oxygen delivery through cerebral blood flow and acts by means of a vasoactive substance.
- Autonomic mechanism - vascular smooth muscle actuators in the resistance arterioles are controlled via sympathetic innervation, receiving the input from the appropriate brainstem autonomous control center.
What is the most important autoregulation mechanism implicated in cerebral perfusion?
Myogenic mechanism
What happens to cerebral perfusion below a blood pressure of 50 mmHg systolic?
Arteries collapse and brain tissue becomes ischaemic
What happens to cerebral perfusion at blood pressures above 150 mmHg systolic?
Blood flow increases, causing force mediated dilatation of vessels, and vasogenic oedema to form
What happens to cerebral vessel diameter as blood pressure approaches 150 mmhg systolic?
Vessels reach maximal point of constriction in an attempt to maintain CBF and ICP
What are mass effect causes of raised ICP?
- Tumours (1o or mets)
- Infarction with oedema
- Abscess
- Contusion
- Chronic subdural/epidural haematoma
- Granuloma
- Cyst
What are specific features of the headache seen in individuals with raised ICP?
Worse on:
- Waking
- Lying down
- Bending forward
- Coughing
- Vomiting
What are symptoms seen in someone with raised ICP?
- Headache
- Seizures
- Nausea and vomiting
- Subtle personality change
- Drowzy
What are signs of increased ICP?
- Evolving focal neurology
- Papilloedema
- False localising VI nerve palsy
- Decreased GCS
- Seizures
What focal neurological signs would point to a lesion in the temporal lobe?
- Dysphasia/Wernicke’s Aphasia
- Confusional states
- Contralateral homonymous haemianopia/superior quadrantinopia
- Amnesia
- Odd/inexplicable phenomena
What focal neurological signs would point to a lesion in the frontal lobe?
- Hemiparesis
- Personality change
- Release phenomena (e.g. grasp reflex)
- Broca’s dysphasia
- Unilateral anosmia
- Perseveration
- Executive dysfunction
- Decreased verbal fluency