I. INTRACRANIAL PRESSURE: MASS LESIONS
A. CHARACTERISTICS OF INTRACRANIAL SPACE-OCCUPYING EXPANDING MASS LESIONS
- 1. General Considerations
- Since the brain is enclosed in a rigid cranium, the free space to expand
is minimal. When the mass of brain intracranial contents increases in the
presence of disease, intracranial pressure (ICP) increases. When ICP exceeds
a critical point, displacements and herniations occur. The nature of herniation
is determined by the location of the lesion.
- Intracranial pressure is usually estimated by measuring CSF pressure
through lumbar puncture. The normal value is less than 200 mm water. CSF
pressure depends on cerebral blood volume (varies with systole/diastole
and respiration), volume of brain tissue and volume of CSF.
- 2. Compensatory Mechanisms
- a. Reduction in cerebrospinal fluid (CSF) volume
- b. Reduction in intracerebral blood volume
- c. Loss of brain tissue, e.g. necrosis and atrophy
- 3. Factors Governing Increased ICP
- a. Size of lesion and size of remaining space, e.g. more space may
be available due to atrophy in elderly brain, or in brain with previous
infarct.
- b. Severity of edema
- c. Vasoregulatory mechanisms: Autoregulation involves the maintenance
of constant cerebral blood flow (CBF) over wide range of perfusion pressures.
When systemic blood pressure decreases, brain vessels dilate; with more
decrease, CBF decreases and Cushing's phenomemon occurs (systemic hypertension
to maintain CBF). When systemic blood pressure increases, brain vessels
constrict. With loss of autoregulation, vasoparalysis and edema occur.
- d. Patency of CSF pathway: e.g. block of CSF leads to hydrocephalus
- e. Speed of expansion: slowly expanding lesions are more readily accommodated
- f. Age of patient; e.g. in infants, skull can increase in size and
accommodate expansion; in elderly, more space may be available due to atrophy.
- 4. Clinical Features of Increased ICP
- Headache, nausea, vomiting, diminished level of consciousness, hypertension,
bradycardia, respiratory and postural changes, "rostrocaudal decompensation"
- 5. Types of Diseases Commonly Associated with Mass Lesions