CNS Pathology — Neurons and Glial Reactions

Acute Neuronal Injury and Degeneration

• Ischemic neuronal change (acute neuronal injury): Results from energy depletion due to hypoxia/ischemia; characterized by cytoplasmic hypereosinophilia, pyknotic nuclei, and loss of Nissl substance (rough endoplasmic reticulum). Morphologically appears as shrunken, brightly staining neurons with triangular cell body shape. Occurs within minutes to hours of ischemic insult and peaks at 12-24 hours.
• Axonal degeneration (Wallerian degeneration): Follows axonal transection or severe axonal injury; results from loss of axoplasmic transport and calcium dysregulation. Distal axon fragments into axonal spheroids (swollen, eosinophilic profiles containing organelles), followed by myelin breakdown and infiltration by macrophages. Can be anterograde (distal to lesion) or retrograde (affecting proximal axon and soma).
• Apoptotic neuronal death: Programmed cell death triggered by intrinsic (mitochondrial) or extrinsic (death receptor) pathways; involves activation of caspase cascades, DNA fragmentation, and formation of apoptotic bodies. Results in scattered, fragmented neurons without inflammatory response.

Astrocytic Reactions (Gliosis)

• Reactive astrogliosis: Hypertrophic response to CNS injury characterized by increased GFAP (glial fibrillary acidic protein) expression, cell body enlargement, and prominent branching. Astrocytes become activated within hours to days of injury through cytokine signaling (IL-6, TNF-α, TGF-β) and neuronal death. Results in gemistocytic astrocytes (plump cells with eccentric nuclei and abundant eosinophilic cytoplasm) visible on H&E staining. Chronic gliosis produces fibrillary gliosis with dense networks of GFAP-positive processes replacing damaged neural tissue.
• Scar formation: In severe, chronic injuries, astrocytes produce collagen and form glial scars (composed of astrocyte processes and extracellular matrix), which physically impede axonal regeneration. Subpial gliosis occurs at brain surfaces, creating dense astrocytic scarring.
• Loss of astrocytes: Severe injury can result in astrocyte death, reducing neuroprotection and neurotrophic support, and impairing the blood-brain barrier.

Microglial Activation and Neuroinflammation

• Microglial response: Resident CNS macrophages derived from yolk sac precursors; normally resting ("ramified") with fine, branched processes. Upon activation, transform to amoeboid morphology with enlarged cell bodies, retracted processes, and increased surface area for phagocytosis. Express CD11b, CD68 (KP1), and ionized calcium-binding adaptor molecule 1 (Iba1).
• Functions: Phagocytose pathogens, dead neurons, myelin debris, and protein aggregates; produce pro-inflammatory cytokines (TNF-α, IL-1β, IL-6), chemokines, and reactive oxygen species (ROS). Activation occurs within minutes to hours of injury and peaks at days to weeks.
• Macrophage infiltration: In severe inflammation or infections, peripheral macrophages penetrate the damaged blood-brain barrier, contributing to inflammatory burden and often identified by CD68+ immunostaining.

Oligodendrocyte Degeneration and Demyelination

• Primary oligodendrocyte injury: Results in demyelination and impaired myelin maintenance. Oligodendrocytes are metabolically active and vulnerable to ischemia, toxic insults, and inflammation.
• Secondary demyelination: Follows axonal loss; oligodendrocytes lose axonal support signals and undergo apoptosis. Results in myelin pallor and eventual demyelinated plaques (areas of axonal loss with preserved or demyelinated axons).
• Remyelination: Limited capacity in the CNS; mediated by oligodendrocyte progenitor cells (OPCs) and mature oligodendrocytes. Remyelinated segments are shorter with thinner myelin sheaths compared to normal myelin.

Neuronal Soma Changes

• Central chromatolysis: Axotomy-induced response; nucleus moves peripherally ("eccentric"), Nissl substance disperses centrally, and cell swells. Represents stress response and attempt at regeneration. Reversible if axon regenerates; irreversible changes progress to cell death.
• Granulovacuolar degeneration: Accumulation of granular material within cytoplasmic vacuoles; associated with aging and neurodegenerative diseases (Alzheimer's disease). Non-specific finding.
• Hirano bodies: Paracrystalline inclusions composed of actin and associated proteins; found in aging and Alzheimer's disease. Represent aberrant cytoskeletal organization.

Acute Neuronal Injury

• Ischemic stroke (thrombotic, embolic, hypoperfusion)
• Hypoxia (cardiac arrest, severe hypoxemia, carbon monoxide poisoning)
• Traumatic brain injury (TBI)
• Acute intracranial hemorrhage (epidural, subdural, subarachnoid, intraparenchymal)
• Status epilepticus (excitotoxic injury via glutamate release)
• Metabolic derangements (hypoglycemia, hyperammonemia, hepatic encephalopathy)

Chronic Neuronal Loss

• Neurodegenerative diseases (Alzheimer's disease, Parkinson's disease, ALS, Huntington's disease)
• Chronic infection (HIV-associated dementia, progressive multifocal leukoencephalopathy, neurosyphilis)
• Chronic inflammation (multiple sclerosis, chronic meningitis)
• Dementia with Lewy bodies (DLB)
• Frontotemporal dementia (FTD)

Astrocytic Gliosis (Reactive)

• Any CNS injury or disease causing neuronal loss
• Infection (meningitis, encephalitis, abscess)
• Demyelinating disease (multiple sclerosis)
• Repetitive trauma (chronic traumatic encephalopathy, CTE)
• Toxic/metabolic insults (alcohol, heavy metals)

Oligodendrocyte Damage

• Demyelinating diseases (MS, acute disseminated encephalomyelitis, neuromyelitis optica spectrum disorder, ADEM)
• Ischemic injury (particularly vulnerable due to high metabolic demand)
• Toxic exposure (chemotherapy agents, ethanol)
• Viral infection (progressive multifocal leukoencephalopathy caused by JC virus)
• Inherited leukodystrophies

Microglial Activation

• Infection (bacterial meningitis, viral encephalitis, fungal infection)
• Trauma
• Ischemia
• Inflammatory/autoimmune disease
• Neurodegeneration
• Exposure to danger signals (pathogen-associated molecular patterns, PAMPs; damage-associated molecular patterns, DAMPs)

Acute Neuronal Injury (Ischemic Stroke Model)

• Acute focal neurological deficit: Sudden onset weakness, speech disturbance (Broca's or Wernicke's aphasia), sensory loss, or visual field defect corresponding to vascular territory. Reflects acute loss of neuronal function in distribution of affected vessel.
• Loss of consciousness: If large territory involved or subcortical structures affected (thalamus, brainstem).
• Seizures: May occur acutely due to excessive neuronal firing or cortical irritation at infarct margins.
• Cerebral edema: Cytotoxic edema peaks at 24-72 hours; manifests as increased intracranial pressure, headache, and neurological deterioration. Vasogenic edema may develop later (72+ hours).
• Imaging correlate: CT shows hypodensity (cytotoxic edema); diffusion-weighted imaging (DWI) shows restricted diffusion within minutes of stroke onset.

Chronic Neuronal Loss (Neurodegenerative Disease Model)

• Progressive cognitive decline: Insidious onset over months to years; manifests as memory loss (Alzheimer's disease), language impairment (primary progressive aphasia in FTD), or executive dysfunction.
• Motor signs: Bradykinesia, rigidity, resting tremor (Parkinson's disease); progressive weakness and fasciculations (ALS).
• Behavioral changes: Apathy, disinhibition, personality changes (FTD).
• Imaging correlate: MRI shows progressive cortical atrophy; brain weight progressively decreases (normal ~1400 g; severely atrophied ~1000 g or less).

Astrocytic Gliosis and Scar Formation

• Focal scarring: Follows stroke, trauma, or infection; appears as dimpling or cystic cavitation on imaging.
• Generalized gliosis: Diffuse gliosis produces slight brain stiffness and may cause traction on overlying meninges.
• Seizure predisposition: Chronic glial scarring increases seizure risk through altered neuronal excitability and impaired inhibition.
• Imaging: T2/FLAIR hyperintensity representing gliotic tissue (increased water content and astrocyte processes).

Microglial Activation and Neuroinflammation

• Fever: If infectious trigger (bacterial meningitis, viral encephalitis).
• Meningeal signs: Neck stiffness, photophobia, headache (if meningeal involvement).
• Altered mental status: Encephalitis produces confusion, behavioral changes, and sometimes seizures.
• CSF pleocytosis: Elevated WBC count (neutrophils if bacterial; lymphocytes if viral).
• Brain imaging: Cortical enhancement, edema, or ring-enhancing lesions depending on etiology.

Demyelination

• Acute demyelinating episode (MS relapse): Sudden or subacute onset of focal neurological deficit (optic neuritis causing vision loss, transverse myelitis causing paraplegia, brainstem syndrome causing internuclear ophthalmoplegia).
• Chronic progressive symptoms: Progressive disability with repeated relapses and incomplete recovery (relapsing-remitting MS, RRMS) or steady progression (primary progressive MS, PPMS).
• Imaging: MRI shows T2/FLAIR hyperintense demyelinating plaques perpendicular to ventricular surfaces (Dawson's fingers); plaques enhance transiently on post-contrast imaging.
• CSF findings: Oligoclonal bands, elevated IgG index, lymphocytic pleocytosis.

Histological Findings on Light Microscopy

Acute Ischemic Neuronal Change

• Morphology: Shrunken neurons with dense cytoplasmic hypereosinophilia (loss of basophilia due to rough ER dispersion), pyknotic nuclei, and loss of Nissl substance. Described as red neurons or pale neurons (depending on staining).
• Timeline: Difficult to detect in first 6-12 hours; becomes evident by 12-24 hours; peaks at 24-72 hours; begins to clear by 1-2 weeks as neurons are phagocytosed.
• H&E staining: Neurons appear distinctly different from normal neurons in adjacent tissue; counterstaining with H&E highlights contrast.
• Distribution: Laminar pattern in cortical infarcts (layers 3, 5, 6 most vulnerable); specific vascular territories in subcortical infarcts.

Reactive Astrogliosis

• Acute phase (hours to days): Mild astrocyte swelling, increased GFAP expression.
• Intermediate phase (days to weeks): Gemistocytic astrocytes—large cells with abundant eosinophilic cytoplasm, eccentric nuclei, and prominent perinuclear halos. Appear as isolated cells or in clusters.
• Chronic phase (weeks to months): Fibrillary gliosis—dense networks of GFAP-positive astrocyte processes with mature nuclei; replaces damaged neural tissue.
• GFAP immunostaining: Gold standard for demonstrating astrocytic response; highlights astrocyte processes and identifies gemistocytes.

Microglial Activation

• Morphology: Transform from resting (ramified) to activated (amoeboid) forms. Activated microglia are larger with rounded cell bodies and shorter, retracted processes.
• Immunostaining: CD11b, CD68 (KP1), Iba1 highlight activated microglia. Increased density and staining intensity reflect degree of activation.
• Phagocytic activity: Microglia contain debris (myelin fragments, neuronal material, pathogens) visible as cytoplasmic inclusions or vacuoles. Microglial nodules (clusters of activated microglia) occur around dying neurons.
• Distribution: Concentrate at site of injury or infection.

Demyelination

• Myelin loss: Reduced or absent myelin staining on Luxol fast blue (LFB) or proteolipid protein (PLP) immunostaining. Demyelinated plaques appear pale or unstained.
• Relative axonal preservation: Axons remain intact within demyelinated plaques on silver stains (Bielschowsky, Bodian) or neurofilament immunostaining. Distinguishes demyelination from axonal degeneration.
• Plaque characteristics: Well-demarcated borders; irregular shape; commonly located near ventricular surfaces and in brainstem (MS).
• Chronic plaques: Gliotic scarring with astrocytic infiltration; oligodendrocytes reduced or absent.

Wallerian Degeneration

• Axonal spheroids: Swollen axonal segments containing organelles and neurofilaments; appear as round, eosinophilic profiles larger than normal axons. Identified on H&E or with silver stains.
• Myelin debris: Fragmented myelin sheaths surrounding degenerating axons; appear as concentric rings on electron microscopy.
• Macrophage infiltration: Foamy macrophages containing myelin/axonal debris; identified with CD68 staining.
• Timeline: Axonal fragments appear within hours to days; macrophagic infiltration peaks at 1-3 weeks.

Gross Pathology Findings

Acute Infarction

• Initially unremarkable (first 6-12 hours; "red dead brain")—tissue appears hyperemic due to loss of autoregulation and blood stasis.
• By 24-48 hours: Subtle softening and pale discoloration (necrotic tissue).
• By 3-7 days: Liquefactive necrosis progresses; tissue becomes increasingly soft and discolored (yellow-tan at margins representing macrophagic infiltration).
• Chronic infarct: Cystic cavity or loss of tissue volume in affected territory; brain atrophy in corresponding region.

Demyelinating Lesions (MS Plaques)

• Acute plaques: Gray, slightly elevated, granular appearance; hyperemic; may show edema and mild hemorrhage at margins.
• Chronic plaques: Brown or yellowish discoloration; firm; well-demarcated borders; depressed surface (atrophic); located near ventricles and corpus callosum.
• Distribution pattern: Periventricular (60%), juxtacortical (30%), infratentorial (30%).

Neurodegenerative Atrophy

• Cortical atrophy: Thinned cortex with widened sulci; loss of gyral volume.
• Ventricular dilation: Secondary d

Immediate stabilisation

• Airway, oxygenation, glucose: Correct hypoxemia and hypoglycemia first — both directly drive the energy failure that produces red neurons. Avoid hypotonic fluids, which worsen cytotoxic edema.
• Non-contrast head CT before anything else: The AHA/ASA acute ischemic stroke guideline makes exclusion of hemorrhage the gating step before reperfusion.

First-line therapy by mechanism of injury

• Acute ischemic stroke — IV thrombolytic (alteplase; tenecteplase is an accepted alternative) within the established early window, plus mechanical thrombectomy for large-vessel occlusion, extended into the late window when perfusion imaging shows a favorable core-to-penumbra mismatch (AHA/ASA).
• Bacterial meningitis — empiric third-generation cephalosporin (ceftriaxone) plus vancomycin, with **ampicillin added whenever Listeria monocytogenes is a plausible pathogen: age >50 years, pregnancy, immunocompromise (including chronic alcohol use, corticosteroids, malignancy). Neonates (<1 month) are covered differently — ampicillin plus cefotaxime (or an aminoglycoside), since ceftriaxone displaces bilirubin and interacts with calcium-containing fluids. Adjunctive dexamethasone is given before or with the first antibiotic dose** to blunt microglial cytokine release (IDSA). Vancomycin is dosed to a 24-hour AUC targeting AUC/MIC 400–600 per the 2020 IDSA/ASHP consensus — the old 15–20 mcg/mL trough goal is retired.
• Status epilepticus — benzodiazepine (lorazepam) first, then a second-line agent (levetiracetam, fosphenytoin, or valproate) to stop glutamatergic excitotoxic neuronal loss (American Epilepsy Society).
• MS relapse — high-dose IV methylprednisolone, with long-term relapse reduction by disease-modifying therapy (AAN).

Escalation and definitive management

• Malignant cerebral edema: hyperosmolar therapy (hypertonic saline or mannitol), head elevation, brief hyperventilation as a bridge only, then decompressive hemicraniectomy.
• Surgical: hematoma evacuation, abscess drainage, external ventricular drain for obstructive hydrocephalus.

Contraindicated

• Glucocorticoids in traumatic brain injury — the Brain Trauma Foundation recommends against them (increased mortality).
• Thrombolysis with active hemorrhage or recent intracranial surgery; prophylactic prolonged hyperventilation; routine antiepileptics for asymptomatic gliotic scar.

Emergencies

• Malignant cerebral edema with herniation: Cytotoxic edema peaks at 24–72 hours; astrocyte swelling and blood–brain barrier failure add vasogenic edema. Signaled by declining level of consciousness, new anisocoria, or a blown pupil with contralateral hemiparesis (uncal herniation). Requires emergent hyperosmolar therapy and consideration of decompressive craniectomy.
• Hemorrhagic transformation after thrombolysis or reperfusion: Reperfused, protease-damaged capillaries leak. Signaled by acute headache, vomiting, neurologic deterioration, or hypertension during infusion — stop the thrombolytic and obtain immediate CT.
• Orolingual angioedema with alteplase, potentiated by ACE inhibitors — an airway emergency.
• Acute obstructive hydrocephalus from mass effect or intraventricular blood; and communicating hydrocephalus after meningitis or subarachnoid hemorrhage, from subpial/leptomeningeal fibrosis and gliosis blocking arachnoid granulations.

Disease-related sequelae

• Post-injury epilepsy: The chronic glial scar alters extracellular potassium and glutamate handling, producing hyperexcitable cortex — late-onset seizures months to years after stroke, trauma, or abscess.
• Cystic encephalomalacia: Liquefactive necrosis is cleared by macrophages, leaving a fluid-filled cavity rimmed by dense fibrillary gliosis; CT shows focal volume loss with ex vacuo ventricular dilation.
• Failed axonal regeneration: Chondroitin sulfate proteoglycans in the glial scar plus myelin inhibitors make CNS deficits permanent, unlike peripheral nerve.
• Chronic traumatic encephalopathy after repetitive head injury: perivascular tau at sulcal depths with gliosis; behavioral and cognitive decline.

Treatment-related

• Vancomycin nephrotoxicity (rising creatinine, especially with concurrent piperacillin-tazobactam) and infusion reaction from histamine release.
• Cephalosporin use in penicillin allergy: true cross-reactivity is roughly 1–3%, driven by shared R1 side chains rather than the beta-lactam ring; the historical 10% figure is obsolete.
• Corticosteroid complications: hyperglycemia, psychosis, opportunistic infection; natalizumab carries risk of PML from JC virus reactivation, heralded by new subcortical white-matter lesions.

• "Red neurons" are the earliest light-microscopic marker of irreversible ischemic injury: shrunken, hypereosinophilic cytoplasm with pyknotic nucleus and loss of Nissl substance. They are not visible before roughly 6 hours and are reliably evident at 12–24 hours after the insult. A stem describing a patient who died a few hours after cardiac arrest with a normal-looking brain is testing exactly this lag.
• The CNS scars with astrocytes, not fibroblasts: repair is gliosis, not collagenous fibrosis and not granulation tissue. The classic distractor is "fibroblast proliferation with collagen deposition" — that answer is wrong except where the dura or a penetrating wound is involved.
• Microglia are the only CNS glial cell of mesodermal (yolk sac) origin. Neurons, astrocytes, oligodendrocytes, ependymal cells, and choroid plexus epithelium are all neuroectodermal — only the choroid plexus stroma and its vessels are mesenchymal. This origin question is a frequent Step 1 item.
• Microglial nodules with neuronophagia = viral encephalitis; add multinucleated giant cells and think HIV encephalitis. Cowdry A intranuclear inclusions in the temporal lobe point to HSV-1.
• Gemistocytic astrocytes (plump, eccentric nucleus, abundant eosinophilic cytoplasm) mean reactive gliosis; the exam trap is calling them a low-grade astrocytoma. Reactive astrocytes are evenly spaced with a lesion to explain them; neoplastic astrocytes are hypercellular, atypical, and infiltrative.
• Alzheimer type II astrocytes — swollen, pale, glassy nuclei — indicate hyperammonemia (hepatic encephalopathy, urea cycle defects), not Alzheimer disease. Pure name-based distractor.
• Central chromatolysis (round swollen soma, peripheral nucleus, dispersed Nissl) follows axotomy and signals an attempt at regeneration — distinguish from ischemic neuronal change.
• Single best next step in suspected acute stroke: non-contrast head CT to exclude hemorrhage before any thrombolytic, per the AHA/ASA acute ischemic stroke guideline. In suspected bacterial meningitis with focal deficits or papilledema, obtain CT and give antibiotics plus dexamethasone before lumbar puncture — never delay antibiotics for imaging (IDSA).