PA34.1-3,PA35.1 | Nervous System & Eye — Glossary

Deletion of chromosomal arms 1p and 19q; the defining molecular alteration of oligodendroglioma (along with IDH mutation); associated with better chemosensitivity to PCV regimen and improved prognosis.

Two tissue patterns in schwannoma: Antoni A = compact, organised spindle cells forming Verocay bodies (palisaded nuclei); Antoni B = loose, hypocellular, myxoid tissue; the alternating pattern is characteristic.

Hypercellular component of schwannoma. Spindle-shaped Schwann cells arranged in compact interlacing fascicles with nuclear palisading. Verocay bodies (parallel nuclear palisades flanking anuclear eosinophilic zones) are found within Antoni A areas.

Hypocellular component of schwannoma. Loose, myxoid background with scattered stellate cells, disorganised arrangement, and frequent hyalinised vessels. Represents degenerative change in the tumor. Alternates with Antoni A areas within the same tumor.

Gelatinous inflammatory exudate concentrated at the base of the brain (around interpeduncular cistern, Sylvian fissures, and posterior fossa) in tuberculous meningitis. Predisposes to communicating hydrocephalus, cranial nerve palsies (II, III, VI), and arteritis with cerebral infarction.

A selective structural and functional barrier formed by tight junctions between cerebrovascular endothelial cells, astrocyte end-feet, and pericytes; restricts entry of pathogens, immune cells, and large molecules into the CNS.

'Ox eye' — pathological enlargement of the globe in infancy and early childhood due to chronically elevated intraocular pressure. In retinoblastoma, caused by secondary glaucoma from tumor infiltration of the angle or neovascularisation of the iris.

A type of coagulative necrosis with a cheese-like (caseous) gross appearance; the hallmark of granulomatous inflammation in tuberculosis, resulting from macrophage death and accumulation of lipid-rich cell debris.

Systemic chemotherapy (typically vincristine, carboplatin, etoposide) used in retinoblastoma to reduce tumor volume before focal consolidation (laser photocoagulation, cryotherapy, thermotherapy). The primary globe-preserving strategy in bilateral hereditary retinoblastoma.

Enlargement of all ventricles due to impaired CSF absorption at the arachnoid granulations (e.g., blocked by fibrinous exudate in bacterial or tubercular meningitis); CSF circulation between ventricles is maintained but reabsorption is impaired.

An encapsulated yeast causing meningitis primarily in immunocompromised hosts (HIV/AIDS); the polysaccharide capsule inhibits phagocytosis; diagnosed by India ink preparation showing clear capsule halo.

Seeding of tumour cells via CSF along the spinal canal, producing nodular deposits on the spinal cord surface; characteristic of medulloblastoma (and other posterior fossa tumors with access to CSF); requires spinal MRI staging.

Transcription factor that, when unbound, drives expression of genes required for DNA synthesis and S-phase entry. In the normal cell cycle, pRb sequesters E2F in G1; phosphorylation of pRb releases E2F, allowing S-phase entry. Loss of RB1 → constitutively free E2F → uncontrolled proliferation.

Growth pattern where the tumor grows inward toward the vitreous cavity. White nodular masses project into the vitreous, and tumor cell seeding of the vitreous ('snowballs') can occur. Makes local (focal) treatment more difficult.

An activated macrophage with abundant pale eosinophilic cytoplasm and indistinct cell borders resembling epithelial cells; the principal cell of granulomas in tuberculosis and other granulomatous diseases.

Growth pattern where the tumor grows outward between the retina and the retinal pigment epithelium, causing retinal detachment. Subretinal seeding can occur. May be misdiagnosed as Coats disease (a non-neoplastic cause of retinal detachment and leukocoria).

Highly differentiated structural unit in retinoblastoma where tumor cells project photoreceptor-like bouquet processes outward from a cluster, resembling a flower. Associated with less aggressive, more differentiated variants; also seen in retinocytoma (benign counterpart).

Rosette specific to retinoblastoma. A ring of columnar tumor cells arranged around a central true lumen, which is PAS-positive and bounded by apical junctions (terminal bars). Represents photoreceptor (cone-like) differentiation. Distinguishing feature: true central lumen.

Histological artefact in oligodendroglioma: formalin fixation causes perinuclear oedema producing a clear halo around round uniform nuclei, resembling fried eggs; characteristic but not visible on frozen section.

Artefactual histological appearance of oligodendroglioma cells. Round central nucleus surrounded by a clear cytoplasmic halo (caused by swelling during formalin fixation). Arranged in monotonous sheets with a delicate branching 'chicken-wire' capillary network in the background.

The most common and most malignant primary brain tumor (WHO Grade 4) arising from astrocytes; characterised histologically by pseudopalisading necrosis and microvascular proliferation; median survival ~15 months.

Tufts of proliferating endothelial and smooth muscle cells within blood vessel walls, forming complex structures resembling renal glomeruli. A hallmark of glioblastoma (WHO Grade 4 astrocytoma). Driven by VEGF secreted by hypoxic tumor cells.

Dissemination of microorganisms via the bloodstream; the most common route by which bacteria and fungi reach the meninges from a primary infection site (e.g., nasopharynx, lung).

A pseudorosette in medulloblastoma (and other embryonal tumors): tumour cells arranged radially around a central tangle of neuropil (fibrillary material), without a true central lumen; distinguishes medulloblastoma from ependymoma.

Mutation in isocitrate dehydrogenase 1 or 2 genes, producing the oncometabolite 2-hydroxyglutarate; present in secondary GBM and lower-grade gliomas; confers better prognosis compared to IDH-wild type primary GBM.

A rapid diagnostic test for cryptococcal meningitis: India ink is mixed with CSF; the dark particulate background outlines the clear polysaccharide capsule of Cryptococcus as a white halo around budding yeast cells.

A multinucleated giant cell formed by fusion of epithelioid macrophages in granulomatous inflammation; characterised by nuclei arranged in a horseshoe or peripheral ring pattern; a microscopic hallmark of tuberculosis.

The inner two layers of the meninges — the arachnoid mater and pia mater — that directly surround the brain and spinal cord; collectively the primary site of meningeal infection.

White pupillary reflex ('cat's eye' reflex). The most common presenting sign of retinoblastoma (~60% of cases). Caused by a white retinal mass reflecting light back through the pupil. Any leukocoria in a child requires urgent ophthalmology referral.

Mechanism by which the second allele of a tumor suppressor gene is inactivated. The chromosome containing the remaining normal allele is lost (monosomy) or the mutant chromosome duplicates at the expense of the normal copy (mitotic recombination). Most common mechanism of Hit 2 in retinoblastoma.

Inflammation of the leptomeninges (arachnoid and pia mater) and subarachnoid space, classified as purulent (bacterial), aseptic (viral), or chronic (TB/fungal) based on the predominant CSF cell type.

Epigenetic silencing of the O-6-methylguanine-DNA methyltransferase gene promoter; prevents tumour cells from repairing alkylating-agent-induced DNA damage; predicts significantly better response to temozolomide chemotherapy in GBM.

Glomeruloid tufts of multilayered, thickened blood vessels in glioblastoma, driven by VEGF secretion from hypoxic tumour cells; a WHO Grade 4 feature alongside pseudopalisading necrosis.

Extension of retinoblastoma cells along the optic nerve toward the chiasm and subarachnoid space. The single most important adverse pathological prognostic factor. Invasion beyond the lamina cribrosa significantly increases risk of intracranial extension and requires systemic adjuvant chemotherapy.

Swelling of the optic disc on fundoscopy due to raised intracranial pressure transmitted along the optic nerve sheath; a clinical sign of raised ICP; blurring of disc margins, obliteration of the central optic cup.

The most common microscopic feature of ependymoma: tumour cells arranged radially around blood vessels with an anucleate (cell-free) fibrillary zone intervening between the vessels and the nuclei.

Protein product of the RB1 gene. In its hypophosphorylated (active) form, it binds and sequesters E2F transcription factors, preventing S-phase entry. Phosphorylation by cyclin D-CDK4/6 inactivates pRb, releasing E2F and allowing cell cycle progression.

A laminated, concentrically calcified spherical structure formed by calcification of whorled meningothelial cell nests; virtually pathognomonic of meningioma when seen abundantly; also found in papillary thyroid carcinoma and serous ovarian carcinoma.

A microscopic hallmark of glioblastoma — areas of serpentine/geographic necrosis rimmed by a dense layer of tumour cells arranged in a palisade pattern, reflecting cell migration away from the hypoxic necrotic centre.

A thick, pus-like inflammatory exudate composed predominantly of neutrophils, necrotic cell debris, and bacteria; characteristic of acute bacterial (pyogenic) meningitis.

Tumor suppressor gene located at chromosome 13q14; encodes the retinoblastoma protein (pRb), which regulates the G1/S checkpoint of the cell cycle. Biallelic inactivation leads to retinoblastoma and contributes to other cancers.

Benign, highly differentiated counterpart of retinoblastoma. Composed primarily of fleurettes. Occurs in individuals with RB1 mutations who achieve spontaneous differentiation rather than malignant proliferation. May undergo spontaneous regression (phthisis bulbi).

A subpial or subependymal tuberculous lesion in the brain (small tuberculoma) formed during haematogenous dissemination; rupture of a Rich focus into the subarachnoid space initiates tubercular meningitis.

Descriptive category for tumors composed of small, undifferentiated cells with hyperchromatic nuclei, scant cytoplasm, and high N:C ratio appearing blue on H&E. Includes retinoblastoma, medulloblastoma, neuroblastoma, Ewing sarcoma/PNET, Wilms tumor, and rhabdomyosarcoma. Site of origin and ancillary studies (IHC, molecular) are essential for classification.

Gelatinous pseudocysts seen in brain parenchyma (especially basal ganglia) in cryptococcal meningitis; formed by expanding clusters of encapsulated yeast with minimal inflammatory reaction in immunocompromised hosts.

Downward displacement of cerebellar tonsils through the foramen magnum; compresses the medulla → respiratory arrest; a feared complication of raised ICP; reason LP is contraindicated without prior CT in suspected raised ICP.

Displacement of the medial temporal lobe (uncus) through the tentorial notch due to raised ICP; compresses the ipsilateral CN III causing a fixed dilated pupil ('blown pupil'), and may compress the cerebral peduncle causing contralateral hemiplegia.

Knudson's 1971 statistical model proposing that both alleles of a tumor suppressor gene must be inactivated for tumor formation. In hereditary retinoblastoma, one hit is germline (present in every cell); the second occurs somatically. In sporadic cases, both hits occur somatically in a single cell.

A microscopic feature of schwannoma — two parallel rows of palisaded nuclei separated by eosinophilic acellular material, within compact (Antoni A) tissue areas.

Bilateral adrenal haemorrhage and infarction complicating overwhelming septicaemia, most classically from Neisseria meningitidis. Leads to acute adrenal insufficiency, DIC, purpuric skin lesions, and cardiovascular collapse. Historically fatal; early antibiotics and steroids have improved outcomes.

A life-threatening complication of meningococcal septicaemia characterised by bilateral adrenal haemorrhage, overwhelming septic shock, DIC, and petechial/purpuric skin lesions.