The principle of adaptation shown in the following diagram (depicting the skull containing venous blood, arterial blood, brain, mass, and CSF within a fixed volume) is explained by
Monro-Kellie doctrine
The principle is the Monro-Kellie doctrine: the adult cranium is a rigid, non-expansile box of fixed volume containing three incompressible components - brain (about 80 per cent), blood (about 10 per cent) and CSF (about 10 per cent). Since total volume is constant, a fourth component such as a tumour, haematoma or oedema must be offset by displacement of an equal volume of CSF into the spinal thecal sac and of venous blood out of the skull. This keeps intracranial pressure near normal at first; once buffering is exhausted the pressure-volume curve rises steeply, so a small further increase causes a large ICP rise, falling cerebral perfusion pressure and herniation.
Moynihan's aphorism is a surgical dictum on decisive operating and the gallstone masquerade, not intracranial dynamics. Goodsall's rule predicts the internal opening of a fistula-in-ano. Cushing's reflex - hypertension, bradycardia and irregular respiration - is the physiological response to raised ICP, a consequence rather than the explanation of volume adaptation.
Key point: Compensation occurs first by CSF and venous blood displacement; Cushing's triad signals that compensation has already failed.
Important Questions from Brain & Nervous System Surgery
Which of the following chromosomal abnormalities are associated with brain tumours?
1. Neurofibromatosis type 1
2. Neurofibromatosis type 2
3. Peutz-Jeghers syndrome
4. Hereditary nonpolyposis colorectal cancer
Select the correct answer using the code given below.
Consider the following clinical features:
1. Raised ICP
2. Seizures
3. Focal deficit
4. Headache
Which of the above clinical features are related to most brain tumours?
- The most common type of brain tumours associated with neurofibromatosis type 1 brain tumour syndrome is
Upper motor neuron lesions are characterized by which of the following?
1. Increased tone spasticity
2. Hyperreflexia
3. Absent plantar response
4. Motor weakness
Select the correct answer using the code given below.
A comatose patient after sustaining severe head injury has been admitted in neurosurgical ICU. Which of the following parameters should ideally be maintained in this patient ?
1. $pCO_2 = 4.5 – 5.0$ kPa ($33-38$ mm Hg)
2. MAP = $80 – 90$ mm of Hg
3. $pO_2 > 11$ kPa ($> 80$ mm Hg)
4. $Na^+ < 130$ meq/L
Select the correct answer using the code given below :
