9.1 Pathophysiology of Intracranial Pressure, Monro-Kellie Doctrine & Herniation Syndromes
Key Takeaways
Once intracranial compensation through shifts of CSF and venous blood is exhausted, small increases in volume can cause large increases in ICP.
An open fontanelle and patent sutures allow limited expansion. A tense fontanelle, progressive head growth, splayed sutures or a setting-sun sign can prompt urgent evaluation for increased ICP.
For severe pediatric TBI, CPP is MAP minus ICP. BTF suggests a minimum CPP of 40 mmHg and a target of 40–50 mmHg, with age influencing where to aim.
Cerebral autoregulation can be impaired after severe TBI. Prevent hypotension and recognize that hypocapnia can lower ICP while reducing cerebral blood flow.
Hypertension, bradycardia and irregular respirations form the late Cushing triad. New anisocoria and declining consciousness can warrant herniation treatment before the triad appears.
Intracranial Pressure, Cerebral Perfusion and Herniation
Clinical core. Rising intracranial pressure (ICP) can reduce cerebral perfusion, and differences in pressure between compartments can displace brain tissue. Follow the neurologic exam and the pressure trend together. A child with acute anisocoria and declining consciousness needs immediate assessment and treatment even before invasive monitoring is available.
Intracranial Volume and Compliance
The Monro–Kellie model treats the mature skull as a nearly rigid container for brain, blood and cerebrospinal fluid (CSF). When a hematoma, edema or obstructed CSF circulation adds volume, intracranial venous blood and CSF can shift out of the cranial compartment. That reserve helps keep pressure from climbing early. Once reserve is depleted, further volume produces a much steeper rise in pressure.
On the flat part of the pressure–volume curve, a small additional volume causes little change in ICP. On the steep part, the same volume can cause a substantial pressure rise. Draining CSF or removing a mass can move the patient back down the curve. The degree of pressure change depends on the starting compliance. Normal ICP reference values are difficult to establish in healthy children, and a treatment target for severe TBI is not a general reference value. Experimental compliance study · ICP and age study
Infants. Patent sutures and fontanelles allow some expansion, particularly when volume accumulates over time. Progressive hydrocephalus can present with increasing head circumference, splayed sutures, a tense fontanelle and sustained downward eye deviation (the setting-sun sign, with impaired upgaze). Irritability, feeding changes, vomiting and drowsiness can accompany the pressure rise. Open sutures provide finite buffering. A tense fontanelle or worsening exam after significant trauma needs urgent assessment for intracranial injury. Head circumference trends and fontanelle findings cannot quantify ICP. Infant hydrocephalus study · Pediatric increased ICP cohort · Setting-sun case
Cerebral Perfusion Pressure and Autoregulation
At the bedside in severe TBI, calculate cerebral perfusion pressure (CPP) as the difference between mean arterial pressure (MAP) and ICP, with pressure measurements referenced consistently. A high venous outflow pressure can also compromise perfusion. When a measured MAP is unavailable, is an estimate. Use a reliable measured arterial MAP when available.
| Clinical situation | MAP | ICP | CPP |
|---|---|---|---|
| Before intervention | 59 mmHg | 22 mmHg | 37 mmHg |
| Illustrative response to lower ICP and higher MAP | 63 mmHg | 17 mmHg | 46 mmHg |
From the first to the second row, lowering ICP by 5 mmHg and supporting MAP by 4 mmHg raises CPP by 9 mmHg. For severe pediatric TBI, BTF suggests treating to ICP below 20 mmHg and maintaining CPP at a minimum of 40 mmHg (Level III). A target of 40–50 mmHg gives a margin above the floor. Infants may be toward the lower end and adolescents may need the upper end or higher. In a child at CPP 37 mmHg, consider both sides of the equation. Evaluate the cause of low MAP and address elevated ICP while reassessing the exam and response. BTF pediatric guideline · Pediatric ICP/CPP burden study
Cerebral arterioles adjust their resistance to buffer changes in perfusion pressure. Severe TBI can impair this autoregulation, with variation among children and over time. When flow becomes pressure-passive, falling perfusion pressure increases ischemic risk. High pressure can also contribute to hyperemia or edema in injured tissue. Avoid assuming a fixed autoregulatory plateau for every age. Rising arterial carbon dioxide tends to dilate cerebral vessels and can increase ICP. Reducing carbon dioxide constricts those vessels and can lower ICP at the cost of reduced blood flow. The risk of ischemia explains the distinction between routine ventilation and brief rescue hyperventilation during active herniation. Pediatric autoregulation cohort · BTF pediatric guideline
Recognize Herniation Before the Full Triad
Herniation is tissue displacement across a dural partition or through the foramen magnum. It can develop from a focal pressure gradient. Cushing's triad consists of hypertension often with widened pulse pressure, bradycardia and irregular breathing or apnea. Rising arterial pressure can be a compensatory response to threatened brainstem perfusion, and bradycardia can follow through the baroreflex. Respiratory irregularity reflects brainstem dysfunction. The complete triad is late and may be absent. A declining exam, new pupil asymmetry or abnormal posturing can warrant action sooner.
| Pattern | Displacement | Clues and structures at risk |
|---|---|---|
| Uncal (lateral transtentorial) | Medial temporal lobe across the tentorial edge | A new ipsilateral fixed dilated pupil from CN III compression and contralateral weakness or posturing from ipsilateral corticospinal tract compression. |
| Central transtentorial | Downward displacement of midline structures through the tentorial opening | Progressive impaired arousal with bilateral pupillary and motor changes as the brainstem is compressed. |
| Subfalcine | Cingulate gyrus beneath the falx | Anterior cerebral artery compression can cause ischemia and contralateral leg weakness. |
| Tonsillar | Cerebellar tonsils through the foramen magnum | Medullary compression can cause abrupt respiratory and cardiovascular deterioration. |
False localizing sign. With severe lateral shift, the opposite cerebral peduncle can compress against the tentorial edge. This is the Kernohan–Woltman notch phenomenon, which can produce weakness on the same side as the lesion. Use the full exam and imaging to identify the lesion side. A new fixed pupil with declining responsiveness is a time-critical emergency. Mobilize neurosurgery, support oxygenation and perfusion, and use indicated temporizing therapy while preparing definitive care. ENLS herniation protocol · Pediatric Kernohan–Woltman case
A 9-month-old infant is assessed after a significant fall. The infant becomes irritable and has a tense anterior fontanelle, widened sutures and sustained downward gaze. What explains why important intracranial disease can be present even with patent sutures?
Patent sutures and fontanelles offer limited volume buffering, after which additional volume can sharply increase ICP.
Patent sutures allow enough expansion to rule out clinically important intracranial hypertension.
A tense fontanelle confirms that CPP is preserved regardless of intracranial volume.
The pressure–volume curve remains flat until the anterior fontanelle closes.
A 4-year-old with severe TBI has a reliable arterial MAP of 59 mmHg, ICP persistently 22 mmHg and CVP 8 mmHg. What is the CPP, and what should the team address?
CPP 51 mmHg. Subtract CVP and continue observation.
CPP 37 mmHg. Address elevated ICP and assess and support MAP to restore adequate perfusion.
CPP 81 mmHg. Reduce MAP to avoid hyperperfusion.
CPP 29 mmHg. Subtract both ICP and CVP and restrict fluid immediately.
A 16-year-old with a right temporal contusion suddenly becomes less responsive. The right pupil is fixed and dilated, and there is new left-sided extensor posturing. Which pattern best explains the findings?
Subfalcine herniation from cingulate displacement beneath the falx.
Tonsillar herniation from cerebellar descent through the foramen magnum.
Right uncal herniation with compression of the right oculomotor nerve and right cerebral peduncle.
Central herniation with symmetric downward displacement and bilateral brainstem involvement.
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