Physiology

Cranial Vault

• Cranial Vault Volume: 1400-1700 mL (this volume is fixed in an individual)
• Intracranial Components
  • 80%: Brain
  • 10%: Cerebrospinal Fluid (CSF)
  • 10%: Blood
• Monro-Kellie Doctrine: intracranial pressure is a function of the compliance of each component of the intracranial compartment
  • Based on Rigid Structure of the Skull and Inability of Cranial Vault Volume to Change: increased volume in any of the three intracranial components may result in intracranial hypertension
  • Relationship Between Intracranial Volume and Intracranial Pressure is Exponential: with initial increase in volume, pressure rises only slightly, but when the buffering capacity of the system is exceeded, intracanial pressure rises rapidly -> this explains the rapid clinical deterioration that may occur in the setting of a traumatic intracranial hematoma
• Normal Intracranial Pressure (Adult): <15 mm Hg
  • Intracranial Pressure Fluctuates with Cardiac and Respiratory Cycles
  • Transient Increases in ICP May Occur During Coughing/Sneezing
  • Pathologically Increased ICP is Defined as Sustained ICP >20 mm Hg
  • ICP is Normally Lower in Children (and May Be Subatmospheric in Newborns)

Cerebral Blood Flow

• Hypercapnia and Hypoxia Increase Cerebral Blood Flow

Cerebral Perfusion Pressure (CPP)

• Cerebral Perfusion Pressure = MAP-ICP

Cerebrospinal Fluid (CSF) Dynamics

• Cerebrospinal Fluid is Produced in the Choroid Plexus and Other Locations Within the Central Nervous System: CSF is produced at a rate of 20 mL/hr
• Cerebrospinal Fluid is Normally Absorbed by the Arachnoid Granulations into the Venous System

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Etiology of Increased Intracranial Pressure (by Predominant Mechanism)

Cerebral Edema

• Anoxic/Ischemic Encephalopathy (see Hypoxic-Ischemic Brain Injury, [[Hypoxic-Ischemic Brain Injury]])
  • Physiology
    • Cerebral Edema
• Cerebral Venous Thrombosis
  • Physiology
    • Altered Cerebrospinal Fluid Circulation
    • Cerebral Edema
• During the Course of Therapy for Diabetic Ketoacidosis/Hyperosmolar Hyperglycemic State (see Diabetic Ketoacidosis and Hyperosmolar Hyperglycemic State, [[Diabetic Ketoacidosis and Hyperosmolar Hyperglycemic State]])
  • Physiology
    • Cerebral Edema
• Fulminant Hepatic Failure (FHF) (see Fulminant Hepatic Failure, [[Fulminant Hepatic Failure]])
  • Etiology
    • Acetaminophen Intoxication (see Acetaminophen, [[Acetaminophen]])
    • Toxic Mushroom Intoxication (see Toxic Mushrooms, [[Toxic Mushrooms]])
  • Physiology
    • Cerebral Edema
    • Cerebral Vasodilation
• High-Altitude Cerebral Edema (HACE) (see High-Altitude Cerebral Edema, [[High-Altitude Cerebral Edema]])
  • Physiology
    • Cerebral Edema
• Hypertensive Encephalopathy (see Hypertension, [[Hypertension]])
  • Physiology
    • Cerebral Edema
• Hyponatremia (see Hyponatremia, [[Hyponatremia]])
  • Physiology
    • Cerebral Edema
• Large Intracerebral Hemorrhage (see Intracerebral Hemorrhage, [[Intracerebral Hemorrhage]])
  • Physiology
    • Cerebral Edema
    • Mass Effect
• Large Ischemic Cerebrovascular Accident (CVA) (see Ischemic Cerebrovascular Accident, [[Ischemic Cerebrovascular Accident]])
  • Physiology
    • Cerebral Edema
• Malaria (see Malaria, [[Malaria]])
  • Physiology
    • Cerebral Edema
• Meningitis (see Meningitis, [[Meningitis]])
  • Physiology
    • Cerebral Edema
• Subarachnoid Hemorrhage (SAH) (see Subarachnoid Hemorrhage, [[Subarachnoid Hemorrhage]])
  • Physiology
    • Altered Cerebrospinal Fluid Circulation
    • Cerebral Edema
    • Mass Effect
• Traumatic Brain Injury (TBI) (see Traumatic Brain Injury, [[Traumatic Brain Injury]])
  • Physiology
    • Cerebral Edema
    • Mass Effect
    • Cerebral Vasodilation

Cerebral Vasodilation

• Fever (see Fever, [[Fever]])
  • Physiology
    • Cerebral Vasodilation
• Hypoxemia (see Hypoxemia, [[Hypoxemia]])
  • Physiology
    • Cerebral Vasodilation
• Hypercapnia (see Hypercapnia, [[Hypercapnia]]): hypercapnia causes cerebral vasodilation
  • Physiology
    • Cerebral Vasodilation
• Hypotension (see Hypotension, [[Hypotension]])
  • Physiology
    • Cerebral Vasodilation
• Seizures (see Seizures, [[Seizures]])
  • Physiology
    • Cerebral Vasodilation

Intracranial Mass Lesion

• Brain Abscess (see Brain Abscess, [[Brain Abscess]])
  • Physiology
    • Cerebral Edema
    • Mass Effect
• Brain Tumor
  • Physiology
    • Cerebral Edema
    • Mass Effect

Decreased Cerebrospinal Fluid (CSF) Absorption

• Post-Bacterial Meningitis Arachnoid Granulation Adhesions (see Meningitis, [[Meningitis]])

Increased Cerebrospinal Fluid (CSF) Production

• Choroid Plexus Papilloma

Obstructive Hydrocephalus (see Hydrocephalus, [[Hydrocephalus]])

• xxxx

Increased Arterial Blood Pressure/Hypertension

• Bladder Distention
• Pain

Obstruction of Venous Outflow from Brain

• Abdominal Compartment Syndrome (see Abdominal Compartment Syndrome, [[Abdominal Compartment Syndrome]])
  • Physiology: increased venous pressure
• Jugular Vein Compression
  • Physiology
    • Neck Position
    • Restrictive Neck Dressing
• Neck Surgery
• Pneumothorax (see Pneumothorax, [[Pneumothorax]])
  • Physiology: increased venous pressure
• Sagittal Sinus Thrombosis (see Cerebral Venous Thrombosis, [[Cerebral Venous Thrombosis]])
• Other Venous Sinus Thrombosis
• Patient-Ventilator Dyssynchrony
  • Physiology: increased venous pressure

Idiopathic Intracranial Hypertension (Pseudotumor Cerebri) (see Pseudotumor Cerebri, [[Pseudotumor Cerebri]])

• Physiology
  • Altered Cerebrospinal Fluid Circulation: probable mechanism

Transiently Increased Intracranial Pressure

• Coughing (see Cough, [[Cough]])
• Sneezing
• Valsalva Maneuver (see Valsalva Maneuver, [[Valsalva Maneuver]])

Other

• Craniosynostosis
  • Physiology: inadequate skull growth
• Reye’s Syndrome
  • Physiology: vasodilation
• Succinylcholine (see Succinylcholine, [[Succinylcholine]])
  • Physiology: xxx

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Diagnosis

Head CT (see Head Computed Tomography, [[Head Computed Tomography]])

• xxx

Brain MRI (see Brain Magnetic Resonance Imaging, [[Brain Magnetic Resonance Imaging]])

• xxx

External Ventricular Drain (EVD) (see External Ventricular Drain, [[External Ventricular Drain]])

• Indications for Monitoring in the Setting of Traumatic Brain Injury (Guidelines for the Management of Severe Traumatic Brain Injury, 2007) [MEDLINE]
  • All Salvageable Patients with Severe TBI, GCS 3-8 After Resuscitation, and an Abnormal CT Scan (Hematoma, Contusion, Edema, Herniation, Compressed Basal Cisterns) (Level II Recommendation)
  • Severe TBI with Normal CT Scan with Two or More Criteria at Admission: Age >40 y/o, Unilateral or Bilateral Motor Posturing, SBP <90 mm Hg (Level III Recommendation)
• Contraindications
  • Coagulopathy (see Coagulopathy, [[Coagulopathy]]): relative contraindication
  • Thrombocytopenia with Platelets <100k (see Thrombocytopenia, [[Thrombocytopenia]]): relative contraindication
• Adverse Effects/Complications
  • Intracerebral Hemorrhage: rate of hemorrhage with ventricular catheter is 1-7%
    • Rate of Hemorrhage for Intraparenchymal Monitor is Less Than That of Ventricular Catheter
    • Hemorrhages Rarely Require Surgical Evacuation
  • Infection/Ventriculitis (see Central Nervous System Device Infection, [[Central Nervous System Device Infection]])
    • Culture of Tip of External Ventricular Drains May Demonstrate Bacterial Colonization: however, the rate of invasive infection is lower
    • Risk of Infection is Higher with Ventricular Catheter (1-27%) Than with Parenchymal Monitor
    • Risk Factors for Infection
      • Leakage Around the Ventriculostomy Site
      • Longer Duration of Catheter Placement
      • Presence of Open Skull Fracture with Leakage of Cerebrospinal Fluid

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Clinical Manifestations

Neurologic Manifestations

Coma (see Obtundation-Coma, [[Obtundation-Coma]])

• xxx

Brain Herniation

• Routes of Brain Herniation
  • Hemisphere is Displaced Medially Against the Falx, Resulting in Falcine Herniation
  • Unilateral Pressure Gradient Pushes the Medial Edge of the Temporal Lobe (Uncus) Through the Tentorial Foramen, Resulting in Uncal Herniation
    • Third Cranial Nerve and Posterior Cerebral Artery are Compressed -> Unilateral Pupillary Dilation, Lack of Response to Light, and Infarction
    • Brainstem is Distorted and Compressed with Early Impairment of Consciousness
  • Bilateral Homogeneous Increase in Intracranial Pressure in the Supratentorial SPace Displaces the Brain Downward Through the Tentorial Foramen, Resulting in Transtentorial Herniation
    • Brainstem is Compressed and Displaced Downward without Signs of Lateralization -> Bilateral Pupillary Abnormalities

Other Manifestations

• xxx

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Management of Increased Intracranial Pressure (ICP)

General Measures

• Avoid Increases in ICP >20-25 cm H2O
• Body Position: keep head at 30 degree elevation

Sedation

Barbiturates (see Barbiturates, [[Barbiturates]])

• Pharmacology

  • Decrease Cerebral Blood Flow: decreasing intracranial pressure
  • Decrease Cerebral Metabolism
    

    Propofol (Diprivan) (see Propofol, [[Propofol]])

• Pharmacology

  • Onset: rapid
  • Half-Life (with Infusion): 30-60 min
    • Longer Half-Life is Observed After Prolonged Infusion, Due to Redistribution from Fat Stores
    • However, the Duration of the Clinical Effect is Typically Minutes, as Propofol is Rapidly Distributed into Peripheral Tissues
• Properties
  • Amnestic Effect (see Amnesia, [[Amnesia]])
  • Anti-Emetic Effect
  • Anxiolytic Effect
  • Decreases Intracranial Pressure (see Increased Intracranial Pressure, [[Increased Intracranial Pressure]])
  • Increases Seizure Threshold
  • Sedative Effect
  • No Analgesic Effect
  • Large Lipid Load: requiring adjustment of enteral/parenteral nutritional support
• Administration
  • Dose: 10-60 μg/kg/min
  • Decrease Dose in Elderly by 20%
  • Slow Administration in Elderly

Other Pharmacologic Interventions

Mannitol (see Mannitol, [[Mannitol]]): indicated for

• Indications
  • xxxx
• Pharmacology: osmotic diuretic
• Adverse Effects
  • xxxx

Hyperosmolar Fluids

• Hypertonic Saline (see Hypertonic Saline, [[Hypertonic Saline]])

Corticosteroids (see Corticosteroids, [[Corticosteroids]])

• Indications
  • xxxx

Respiratory Support

• Mechanical Ventilation (see General Ventilator Management, [[General Ventilator Management]])
  • Avoid Hypoxemia (see Hypoxemia, [[Hypoxemia]]): hypoxemia causes cerebral vasodilation
  • Avoid Hypercapnia (see Hypercapnia, [[Hypercapnia]]): hypercapnia causes cerebral vasodilation
  • Avoid Patient-Ventilator Dyssynchrony
• Treat Airway Obstruction: airway clearance, intubation, etc

Treatment of Cerebral Vasodilation

• Treat Fever (see Fever, [[Fever]])
  • Rationale: fever causes cerebral vasodilation
  • Standard Therapy
• Treat Hypotension (see Hypotension, [[Hypotension]])
  • Standard Therapy
• Treat Seizures (see Seizures, [[Seizures]])
  • Rationale: seizures result in cerebral vasodilation
  • Agents
    • Levatiracetam (Keppra) (see Levatiracetam, [[Levatiracetam]])

Treatment of Increased Arterial Blood Pressure/Hypertension (see Hypertension, [[Hypertension]])

• Treat Bladder Distention
  • Foley Catheter (see Foley Catheter, [[Foley Catheter]])
• Treat Pain
  • Analgesics

Treatment Increased Venous Pressure

• Neck Repositioning/Removal of Restrictive Neck Dressings
• Treat Abdominal Compartment Syndrome (see Abdominal Compartment Syndrome, [[Abdominal Compartment Syndrome]])
  • Exploratory Laparotomy with Decompression
• Treat Abdominal Distention/Ileus
  • Nasogastric Tube (see Nasogastric Tube, [[Nasogastric Tube]])
• Treat Pneumothorax (see Pneumothorax, [[Pneumothorax]])
  • Chest Tube (see Chest Tube, [[Chest Tube]])

Treatment of Hydrocephalus/Disturbance in Cerebrospinal Fluid Flow (see Hydrocephalus, [[Hydrocephalus]])

• External Ventricular Drain (EVD) (see External Ventricular Drain, [[External Ventricular Drain]])
• Ventriculoperitoneal Shunt (see Ventriculoperitoneal Shunt, [[Ventriculoperitoneal Shunt]])

Treatment of Hyponatremia (see Hyponatremia, [[Hyponatremia]])

• Rationale: hyponatremia causes cerebral edema
• Standard Therapy

Renal Management

• Choice of Intermittent Hemodialysis vs Continuous Venovenous Hemodialysis (CVVHD)
  • If Hemodialysis is Required, CVVHD is Preferred Over Intermittent HD, Due to Rapid Solute Changes and Potential for Hypotension with Intermittent HD (Which May Exacerbate Cerebral Ischemia) (see Hemodialysis, [[Hemodialysis]])

Decompressive Craniectomy (see Decompressive Craniectomy, [[Decompressive Craniectomy]])

• Indications
  • Cerebral Contusion/Traumatic Brain Injury (see Traumatic Brain Injury, [[Traumatic Brain Injury]])
  • Epidural Hematoma (see Epidural Hematoma, [[Epidural Hematoma]])
  • Intracerebral Hemorrhage (see Intracerebral Hemorrhage, [[Intracerebral Hemorrhage]])
  • Subdural Hematoma (SDH) (see Subdural Hematoma, [[Subdural Hematoma]])

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References

• Hypertonic saline solutions in brain injury. Curr Opin Crit Care. 2004;10:126-131
• A comparision of albumin and saline for fluid resuscitation in the intensive care unit. N Engl J Med. 2004;350:2247-2256
• Injury to the cranium. In: Moore EE, Feliciano DV, Mattox KL, eds. Trauma. 5th ed. New York, NY: McGraw-Hill; 2004:385-404
• Effects of 23.4% sodium chloride solution in reducing intracranial pressure in patients with traumatic brain injury: a preliminary study. Neurosurgery. 2005;57(4):727 [MEDLINE]
• The use of hypertonic saline for treating intracranial hypertension after traumatic brain injury. Anesth Analg. 2006;102:1836-1846
• Guidelines for the management of severe traumatic brain injury. J Neurotrauma. 2007;24 Suppl 1:S1-106 [MEDLINE]
• Refractory intracranial hypertension and “second-tier” therapies in traumatic brain injury. Intensive Care Med. 2008;34:461–467 [MEDLINE]
• Sedation for critically ill adults with severe traumatic brain injury: a systematic review of randomized controlled trials. Crit Care Med. 2011 Dec;39(12):2743-51 [MEDLINE]
• Decompressive craniectomy in diffuse traumatic brain injury. N Engl J Med. 2011;364(16):1493 [MEDLINE]
• Hyperosmolar therapy for raised intracranial pressure. N Engl J Med. 2012;367:746–752 [MEDLINE]
• Traumatic intracranial hypertension. N Engl J Med. 2014;370:2121–2130. doi: 10.1056/NEJMra1208708 [MEDLINE]