CEREBRAL EDEMA, HYDROCEPHALUS, RAISED ICP & HERNIATION Neuropathology MKU Pathology — Dr. Lilian Bosire --- OVERVIEW The brain and spinal cord are enclosed in a
CEREBRAL EDEMA, HYDROCEPHALUS, RAISED ICP & HERNIATION Neuropathology MKU Pathology — Dr. Lilian Bosire --- OVERVIEW The brain and spinal cord are enclosed in a rigid, non-expansile compartment (skull + dural reflections + bony spinal canal). Any increase in intracranial volume raises ICP. Three main causes of raised ICP: - Generalized brain edema - Increased CSF volume - Focal expanding mass lesions (tumor, abscess, hemorrhage) Consequences range from subtle neurological deficits to death, depending on degree and rapidity of pressure rise. --- I. CEREBRAL EDEMA Excess fluid accumulation within brain parenchyma, due to increased vascular leakage or cell membrane injury. 1. Vasogenic Edema - Mechanism: Blood-brain barrier (BBB) disruption → increased vascular permeability → fluid shifts from intravascular compartment into intercellular spaces - Key feature: Brain has few lymphatics, so resorption of excess extracellular fluid is severely impaired - Distribution: Localized (adjacent to inflammation/neoplasms) or generalized (e.g., global ischemic injury) 2. Cytotoxic Edema - Mechanism: Direct injury to neuronal, glial, or endothelial cell membranes → failure to maintain ionic gradients → intracellular fluid accumulation - Causes: Global hypoxia/ischemia, metabolic derangements - Key distinction from vasogenic: BBB remains intact; the problem is intracellular, not extracellular Gross Morphology of Generalized Edema - Gyri are flattened - Sulci are narrowed - Ventricles are compressed - Severe cases progress to herniation --- II. HYDROCEPHALUS Excessive accumulation of CSF within the ventricular system. Normal CSF pathway: Choroid plexus (production) → Ventricular system → Foramina of Luschka & Magendie → Cisterna magna → Subarachnoid space over cerebral convexities → Arachnoid granulations (resorption) Most cases result from impaired flow or resorption, not overproduction. Types Type Description Causes --- --- --- Non-communicating (Obstructive) Focal obstruction within the ventricular system; ventricles upstream of obstruction enlarge Aqueduct stenosis, third ventricle mass Communicating Entire ventricular system enlarged; ventricles remain continuous with subarachnoid space Choroid plexus tumor (overproduction), arachnoid fibrosis post-meningitis Hydrocephalus ex vacuo Compensatory ventricular enlargement due to loss of brain parenchyma (e.g., neurodegeneration, infarct) Not true hydrocephalus — no raised ICP Age-Dependent Consequences - Before suture closure (infancy): Head enlarges (macrocephaly) - After suture closure: Ventricles expand + ICP rises; no change in head circumference --- III. RAISED INTRACRANIAL PRESSURE & HERNIATION Pathophysiology - Rising intracranial volume first displaces CSF as compensation - Once compensatory capacity is exceeded, ICP rises sharply - Elevated ICP compresses vasculature → reduced cerebral perfusion → ischemia → more edema (vicious cycle) - Herniation occurs when brain tissue is displaced past rigid dural folds (falx, tentorium) or through skull openings Mass effects causing herniation: - Diffuse: generalized cerebral edema - Focal: tumor, abscess, hemorrhage Herniation Syndromes 1. Subfalcine (Cingulate) Herniation - Unilateral/asymmetric cerebral expansion displaces the cingulate gyrus under the falx cerebri - Compresses the anterior cerebral artery → secondary infarcts in ACA territory 2. Transtentorial (Uncal/Mesial Temporal) Herniation - Medial temporal lobe (uncus) is compressed against the free margin of the tentorium - Consequences with increasing displacement: - CN III compression → ipsilateral pupillary dilation ("blown pupil") + impaired ocular movements - Posterior cerebral artery compression → infarct of PCA territory (includes primary visual cortex) - Contralateral cerebral peduncle compression → hemiparesis ipsilateral to the lesion (false localizing sign — Kernohan's notch) - Duret hemorrhages — linear/flame-shaped hemorrhages in the midline and paramedian midbrain/pons, caused by distortion or tearing of penetrating vessels supplying the upper brainstem 3. Tonsillar Herniation - Cerebellar tonsils displaced through the foramen magnum - Most life-threatening pattern - Compresses the medulla → compromises vital respiratory and cardiac centers - Can cause sudden death --- KEY CONCEPTS SUMMARY Entity Core Defect ICP Effect --- --- --- Vasogenic edema BBB disruption → extracellular fluid Raises ICP Cytotoxic edema Cell membrane failure → intracellular fluid Raises ICP Non-communicating hydrocephalus CSF flow obstruction within ventricles Raises ICP Communicating hydrocephalus Impaired resorption or overproduction Raises ICP Hydrocephalus ex vacuo Parenchymal loss → compensatory expansion Does NOT raise ICP Herniation Brain displaced past dural/bony barriers Consequence of raised ICP --- Note: Herniation syndromes can occur simultaneously if pressure rise is severe enough.