Intracranial Pressure Homeostasis

Intracranial pressure (ICP) homeostasis is a crucial physiological process that maintains a stable environment for the brain within the closed cranial vault. The balance between cerebrospinal fluid (CSF) production and absorption, cerebral blood volume, and brain tissue compliance ensures that ICP remains within a normal range (5-15 mmHg in adults). Disruptions in this homeostasis can lead to conditions such as intracranial hypertension or hypotension, both of which have serious neurological consequences.

ICP is governed by three primary intracranial components, as outlined by the Monro-Kellie hypothesis:

 Cerebrospinal Fluid (CSF)
   Produced mainly by the choroid plexus in the ventricles at a rate of ~500 mL/day.
   Circulates through the ventricular system and subarachnoid space.
   Absorbed by the arachnoid granulations into the venous system.
   Acts as a buffer to compensate for transient changes in ICP.

 Cerebral Blood Volume (CBV)
   The brain receives ~15% of cardiac output.
   CBV is tightly regulated by cerebral autoregulation, which ensures constant cerebral perfusion pressure (CPP) despite fluctuations in systemic blood pressure.
   Vasodilation or vasoconstriction of cerebral arterioles adjusts blood flow as needed.

 Brain Parenchyma
   Composed of neurons, glial cells, and interstitial fluid.
   Changes in tissue volume due to tumors, edema, or hemorrhage can impact ICP.

 The brain maintains constant cerebral blood flow (CBF) between mean arterial pressures of 50-150 mmHg.
 Mechanisms involved:
   Myogenic response: Arterioles constrict or dilate in response to changes in pressure.
   Neurogenic control: Sympathetic and parasympathetic influences on vascular tone.
   Metabolic factors: CO₂ levels influence vasodilation (hypercapnia) or vasoconstriction (hypocapnia).

 The CSF system is dynamic, with continuous production, circulation, and absorption.
 Increased CSF production or reduced absorption (e.g., in hydrocephalus) leads to ICP elevation.
 Lumbar puncture opening pressures reflect CSF dynamics and ICP status.

 The compliance of the intracranial compartment determines the ability to buffer volume changes.
 Compensatory mechanisms:
   CSF displacement to the spinal canal.
   Venous blood displacement to extracranial veins.
   Limited expansion of the cranial vault in infants due to open sutures.

 Causes:
   Traumatic brain injury (TBI)
   Intracranial hemorrhage
   Brain tumors
   Hydrocephalus
   Meningitis or encephalitis
 Consequences:
   Reduced CPP → ischemia and neuronal injury
   Herniation syndromes (e.g., uncal, tonsillar)

 Causes:
   CSF leaks (post-lumbar puncture, spontaneous intracranial hypotension)
   Over-drainage from shunts
 Symptoms:
   Orthostatic headaches
   Brain sagging on MRI

 Invasive Methods:
   External ventricular drain (EVD)
   Intraparenchymal ICP monitors
 Non-Invasive Methods:
   Transcranial Doppler (TCD)
   Optic nerve sheath diameter (ONSD) measurement
   MRI/CT-based estimations

 Elevating head of bed (30°)
 Osmotherapy (mannitol, hypertonic saline)
 Controlled hyperventilation (reducing CO₂ to cause vasoconstriction)
 CSF drainage (EVD, lumbar puncture in selected cases)
 Surgical decompression (craniotomy, decompressive craniectomy)

ICP homeostasis is a dynamic balance between CSF, cerebral blood volume, and brain tissue. Understanding its regulation is critical for managing neurological conditions that threaten brain function. Advances in ICP monitoring and intervention strategies continue to refine our ability to maintain cerebral homeostasis and prevent secondary brain injury.