Neuropathology is the discipline that interprets disease of the nervous system through the lens of tissue. The brain, spinal cord, muscle, and peripheral nerve all have characteristic and limited reactions to injury, and the patterns those reactions take — at gross examination, on histology, with immunohistochemistry, and now increasingly with molecular profiling — are what allow specific diagnoses to be made. For the clinical neurologist, neuropathology is not an obscure subspecialty: it provides the substrate for every disease the bedside examination identifies, and many of the most rapidly evolving areas of modern neurology (glioma classification, neurodegenerative proteinopathies, autoimmune encephalitis, muscle and nerve biopsy interpretation) are fundamentally driven by neuropathologic insight. This page covers the framework that organizes the discipline.

The Levels of Neuropathologic Analysis

Neuropathology operates at several levels simultaneously, and each contributes information that the others cannot:

Gross (Macroscopic) Examination

The brain is examined externally for atrophy patterns, surface vascular abnormalities, herniation marks, contusions, and discoloration. It is then sectioned in coronal slabs (the most informative single technique in brain dissection) — typically about 1 cm thick — and the cut surfaces are examined for:

  • Atrophy patterns (focal, diffuse, lobar, regional).
  • Ventricular size and shape.
  • Discoloration of cortex, white matter, deep gray.
  • Cysts, cavities, mass lesions.
  • Hemorrhage, infarction.
  • Demyelinated plaques (sometimes visible grossly).
  • Loss of normal landmarks.

Many neurologic diseases produce gross findings that are essentially diagnostic — bilateral mammillary body atrophy in Korsakoff syndrome, hippocampal atrophy in mesial temporal sclerosis, putaminal pallor in Wilson disease, caudate atrophy in Huntington disease, depigmentation of substantia nigra in Parkinson disease.

Microscopic (Histologic) Examination

Routine stains and special stains reveal the cellular reactions to disease:

  • Hematoxylin & eosin (H&E): the workhorse stain. Identifies neurons, glia, blood vessels, inflammation, necrosis. Most lesions can be characterized in broad strokes from H&E alone.
  • Luxol fast blue: stains myelin blue. Identifies demyelination and white matter pallor.
  • Bielschowsky silver stain: classical for senile plaques and neurofibrillary tangles in Alzheimer disease, and for axons.
  • Bodian, Holmes silver stains: axons.
  • PAS: glycogen, fungal organisms, basement membranes.
  • Trichrome (Gomori): muscle biopsy fiber-type and connective tissue; ragged-red fibers in mitochondrial disease.
  • Congo red: amyloid (apple-green birefringence under polarized light).
  • Prussian blue: hemosiderin (old hemorrhage).
  • Acid-fast stains (Ziehl-Neelsen, Fite): mycobacteria.
  • Gram stain: bacteria.
  • Methenamine silver (GMS): fungi, Pneumocystis.

Immunohistochemistry (IHC)

IHC uses antibodies against specific proteins to identify cell types, infectious agents, and pathologic inclusions. Modern neuropathology relies heavily on IHC for diagnosis. Common targets:

  • GFAP: astrocytes (and astrocytic tumors).
  • Synaptophysin, chromogranin, NeuN, NF: neurons and neuroendocrine markers.
  • S100, SOX10: Schwann cells (and schwannomas).
  • EMA: meningiomas, ependymomas.
  • CD68, IBA1: macrophages / microglia.
  • CD3, CD20, CD45: T cells, B cells, leukocytes (inflammation, lymphoma).
  • Ki-67 (MIB-1): proliferation index (tumor grade).
  • p53: many tumors and dysplasias.
  • Disease-specific antibodies: tau, beta-amyloid, alpha-synuclein, TDP-43, FUS, ubiquitin, p62.
  • IDH1 R132H: most common IDH1 mutation in adult glioma (now a routine first-pass marker).
  • ATRX: loss of nuclear staining identifies ATRX-mutated tumors (mostly diffuse astrocytomas).
  • H3K27me3, H3 G34: pediatric high-grade gliomas.
  • BRAF V600E: pleomorphic xanthoastrocytoma, ganglioglioma, some pilocytic astrocytomas.

Molecular Profiling

The 2021 WHO classification of CNS tumors and the modern classification of many neurodegenerative diseases rely on molecular markers as much as on histology. Specific tests include:

  • IDH1/IDH2 mutation status: defines adult diffuse glioma categories.
  • 1p/19q codeletion: required for oligodendroglioma diagnosis.
  • CDKN2A/B homozygous deletion: grade 4 IDH-mutant astrocytoma.
  • MGMT promoter methylation: predicts temozolomide response in glioblastoma.
  • BRAF V600E, BRAF fusions, FGFR fusions, MYB/MYBL1 fusions: low-grade gliomas.
  • H3 K27M, H3 G34: pediatric high-grade gliomas.
  • SHH, WNT, Group 3, Group 4: medulloblastoma molecular groups.
  • C9orf72, SOD1, TARDBP, FUS: familial ALS / FTD.
  • MAPT, GRN, C9orf72: familial frontotemporal dementia.
  • APP, PSEN1, PSEN2, APOE: Alzheimer disease genetics.
  • HTT CAG repeat: Huntington disease.
  • SCA gene panels: hereditary ataxias.
  • Mitochondrial DNA sequencing: mitochondrial myopathies.

Ultrastructure (Electron Microscopy)

Electron microscopy is now used selectively rather than routinely, but it remains essential in a few areas:

  • Muscle biopsy: mitochondrial morphology, glycogen accumulation, nemaline rods, central cores.
  • Nerve biopsy: axonal vs demyelinating distinction, amyloid fibrils, onion bulb formations.
  • Kidney biopsy adjuncts when neurologic disease is suspected (Fabry inclusions).
  • Viral particles (some encephalitides).

The Major Reaction Patterns of the Nervous System

The CNS has a limited repertoire of cellular reactions. Almost any neuropathologic diagnosis is built from recognizing these reactions and interpreting their distribution:

  • Neuronal injury: acute (red neuron, ischemic change), chronic (neuronal loss with gliosis), specific (inclusions, atrophy patterns).
  • Astrocyte reaction: reactive astrogliosis (most common; surrounds almost any chronic lesion); Alzheimer type II (hepatic encephalopathy); gemistocytic; Rosenthal fiber formation; corpora amylacea.
  • Microglial reaction: rod cells, microglial nodules, neuronophagia, foamy macrophages (phagocytosing myelin or other debris).
  • Oligodendrocyte / myelin reaction: demyelination, remyelination, axonal damage with secondary myelin loss.
  • Vascular reaction: inflammation, thrombosis, fibrinoid necrosis, amyloid deposition.
  • Inflammatory infiltrate: perivascular cuffing (lymphocytic — viral, autoimmune); neutrophils (acute bacterial); granulomas (TB, sarcoid, fungi); eosinophils (parasitic, hypersensitivity).
  • Necrosis: coagulative (infarct), liquefactive (later infarct, abscess), caseating (TB), fibrinoid (vasculitis).
  • Hemorrhage: acute, evolving, chronic (hemosiderin).
  • Neoplastic: cellular crowding, atypia, mitoses, necrosis, microvascular proliferation, infiltrative vs circumscribed growth.

The Anatomic Pattern of the Lesion

Just as in clinical localization, the anatomic distribution of pathology is a powerful diagnostic clue. Many diseases have remarkably specific topographic patterns:

  • Mesial temporal lobe (especially hippocampus, amygdala): HSV encephalitis, limbic encephalitis, mesial temporal sclerosis, Alzheimer disease (earliest).
  • Substantia nigra pars compacta: Parkinson disease, MSA, MPTP.
  • Caudate (head): Huntington disease, anterior choroidal infarct.
  • Subthalamic nucleus: hemiballism (small infarct).
  • Globus pallidus (bilateral): carbon monoxide, manganese, kernicterus, neurodegeneration with brain iron accumulation.
  • Mammillary bodies: Wernicke-Korsakoff.
  • Dorsal columns + corticospinal tracts: subacute combined degeneration (B12 deficiency).
  • Central pons: osmotic demyelination (“trident” or “bat-wing” appearance).
  • Watershed zones (cortical or deep): hypoperfusion, severe hypotension.
  • Periaqueductal gray + medial thalamus: Wernicke encephalopathy.
  • Cingulate, paracentral, and watershed cortex: anoxic brain injury.
  • Anterior horn cells: ALS, SMA, poliomyelitis, West Nile.
  • Cortex with random cortical involvement plus periodic EEG: CJD.

Recognition of these patterns moves the diagnosis forward dramatically — sometimes from “looks abnormal” to a specific disease in one cut of one section.

The Modern Diagnostic Workflow

A contemporary neuropathologic diagnosis is built in layers:

  1. Clinical history and imaging: the bedside picture and MRI/CT context the tissue findings. Reading slides “blind” without clinical context misses information.
  2. Gross examination: external and cut surface.
  3. Routine histology: H&E plus selected special stains.
  4. Immunohistochemistry: targeted to the differential generated by H&E.
  5. Molecular profiling: required for tumor classification, often informative for neurodegenerative disease, sometimes definitive for hereditary disorders.
  6. Integration: the diagnosis is the synthesis of clinical, imaging, histologic, and molecular data — not any one layer alone.

Working With the Clinician

The neuropathology consultation is most useful when the clinician supplies:

  • Age, sex, broad clinical context.
  • Tempo (acute, subacute, chronic).
  • Key clinical findings (cognitive, motor, sensory, autonomic).
  • Imaging description and location.
  • Specific diagnostic question.
  • Relevant systemic findings (cancer history, infection, immunosuppression).

Conversely, the clinician gets the most from a pathology report when it specifies:

  • The diagnosis with supporting features.
  • The differential considered and excluded.
  • The reactions present (gliosis, demyelination, neuronal loss).
  • The anatomic regions involved.
  • The molecular findings, if relevant.
  • Recommendations for further tissue or molecular workup.

🔍 Did You Know?

The 2021 WHO classification of tumors of the central nervous system is now built around an explicit integrated diagnosis — histologic name + grade + critical molecular markers — combined into a single line on the pathology report. The shift began with the 2016 update and accelerated with 2021, and it reflects the recognition that two tumors that look identical under the microscope can behave very differently depending on their molecular profile. The most important example is diffuse astrocytic tumors in adults: an IDH-mutant tumor has a fundamentally different biology and prognosis from an IDH-wildtype glioblastoma even when the histology is similar. The new system requires that an integrated diagnosis include the histologic name, the CNS WHO grade, and the critical molecular alterations — e.g., “Astrocytoma, IDH-mutant, CNS WHO grade 3,” or “Glioblastoma, IDH-wildtype, CNS WHO grade 4.” For practicing neurologists, this means tumor reports are now denser and more informative — and that the diagnosis influences treatment selection (temozolomide for MGMT-methylated GBM, PCV chemotherapy for 1p/19q-codeleted oligodendroglioma, vorasidenib for IDH-mutant lower-grade gliomas) more directly than ever before.

Pitfalls and Pearls

  • Read tissue in clinical context. The same gliosis in a patient with epilepsy, a patient with stroke, and a patient with vasculitis means three different things.
  • The CNS has a limited reaction repertoire. Almost any chronic injury produces gliosis; almost any acute injury produces ischemic neuronal change. The specifics — distribution, additional findings, molecular profile — make the diagnosis.
  • Always section coronally. Anatomy is preserved, comparison is easy, atrophy patterns are visible.
  • Molecular profiling is now standard in tumor diagnosis (2021 WHO) and increasingly in neurodegenerative disease.
  • IHC is the most powerful single tool for identifying cell types, infectious agents, and disease-specific inclusions.
  • The anatomic distribution of the lesion is as informative as the histology. Mammillary bodies → Wernicke; substantia nigra → PD; central pons → osmotic demyelination.
  • Bidirectional communication between clinician and pathologist dramatically improves diagnostic yield.
  • Electron microscopy is now selective, not routine — but remains essential for muscle, nerve, and selected viral diagnoses.
  • Frozen-section diagnosis in the operating room is for triage (lesion adequate? infectious vs neoplastic? margins?), not final classification — which depends on permanent sections and ancillary studies.

References

  1. Love S, Budka H, Ironside JW, Perry A, eds. Greenfield’s Neuropathology. 9th ed. CRC Press; 2015.
  2. Ellison D, Love S, Chimelli L, et al. Neuropathology: A Reference Text of CNS Pathology. 3rd ed. Mosby; 2013.
  3. Louis DN, Perry A, Wesseling P, et al. The 2021 WHO Classification of Tumors of the Central Nervous System: a summary. Neuro Oncol. 2021;23(8):1231-1251.
  4. Dubowitz V, Sewry CA, Oldfors A. Muscle Biopsy: A Practical Approach. 5th ed. Elsevier; 2020.
  5. Vallat J-M, Weis J, eds. Peripheral Nerve Disorders: Pathology and Genetics. Wiley; 2014.
  6. Burger PC, Scheithauer BW, Vogel FS. Surgical Pathology of the Nervous System and Its Coverings. 4th ed. Churchill Livingstone; 2002.