2,701 articles
Hemispatial neglect
Hemispatial neglect, also called unilateral spatial neglect or hemineglect, is a post-stroke attention disorder in which patients ignore the side of space opposite a brain lesion, usually the left after right hemisphere stroke.
Hemispherectomy
Hemispherectomy: neurosurgical removal or disconnection of one cerebral hemisphere to treat drug-resistant epilepsy. Includes anatomic hemispherectomy, functional hemispherectomy, and hemispherotomy; about 73% pooled seizure freedom, hydrocephalus the main complication.
Hemopericardium
Hemopericardium, or hemorrhagic pericardial effusion, is blood accumulating in the pericardial sac, risking cardiac tamponade. Leading causes: invasive cardiac procedures (~31%), aortic dissection, trauma, malignancy, infection, anticoagulants; diagnosed by echocardiography and high-attenuation CT fluid.
Hemostasis
Hemostasis, the arrest of bleeding from injured vessels, involves vascular constriction, platelet plug formation, and coagulation; explained by the classic cascade and the modern cell-based model of Hoffman and Monroe.
Henry Molaison
Henry Gustav Molaison (H.M., 1926–2008), American epilepsy patient whose 1953 bilateral medial temporal lobe resection caused profound amnesia; decades of study by Brenda Milner revealed distinct memory systems and the hippocampus's role.
Hepatic portal system
Hepatic portal system (portal venous system): veins draining the stomach, intestines, pancreas, spleen, and gallbladder into the hepatic portal vein, which supplies about 75% of liver blood flow before blood returns to the inferior vena cava.
Hepatopulmonary syndrome
Hepatopulmonary syndrome (HPS): hypoxemia from intrapulmonary vascular dilatations in liver disease or portal hypertension; diagnosed by A-a gradient and contrast echocardiography; cured only by liver transplantation.
Hereditary atrial fibrillation
Hereditary atrial fibrillation, also called familial or early-onset AF, is atrial fibrillation driven by inherited variants, from monogenic KCNQ1, SCN5A, TTN, and LMNA mutations to polygenic risk, with 2023 guideline Class IIb genetic testing for onset before age 45.
Hereditary benign intraepithelial dyskeratosis
Hereditary benign intraepithelial dyskeratosis, or HBID, is a rare inherited disorder causing white plaques on the conjunctiva and mouth, first described in the Haliwa-Saponi tribe of North Carolina in 1960.
Hereditary hemorrhagic telangiectasia
Hereditary hemorrhagic telangiectasia, also called Osler–Weber–Rendu disease, is an autosomal dominant genetic disorder causing recurrent nosebleeds, anemia, and arteriovenous malformations in the lungs, brain, and liver.
Hereditary lymphedema
Hereditary (primary) lymphedema is a group of genetic disorders in which lymphatic vessels develop abnormally or function poorly, causing swelling, most often of the legs, from birth or later.
Hereditary spastic paraplegia
Hereditary spastic paraplegia (HSP), or Strümpell-Lorrain syndrome, is a group of inherited disorders causing progressive leg spasticity and weakness from corticospinal tract degeneration; over 80 genes, SPG4 most common, pure form in about 90%.
Heritability of autism
Heritability of autism: the share of autism risk variance attributable to genetic factors, estimated at roughly 80% via twin, family, and SNP-based studies, with common variation explaining most risk.
Herpes simplex keratitis
Herpes simplex keratitis (HSK, herpetic keratitis) is HSV-1 corneal infection, a leading infectious cause of blindness. Dendritic epithelial disease heals with trifluridine or acyclovir; stromal disease needs steroids plus antivirals.
Herpes simplex virus stromal keratitis
Herpes simplex virus stromal keratitis (HSK, herpes stromal keratitis) is immune-mediated corneal stromal inflammation after HSV-1 infection, causing scarring and vision loss; treated with tapered topical corticosteroids plus oral antivirals, as established by the HEDS trials.
Heterosynaptic plasticity
Heterosynaptic plasticity: lasting synaptic strength changes at synapses not active during induction, also called modulatory input-dependent plasticity. It counteracts Hebbian runaway dynamics, stabilizes synaptic weights, and supports long-term memory maintenance.
Hexosaminidase (β-N-acetylhexosaminidase)
β-N-acetylhexosaminidase (Hex A αβ, Hex B ββ, Hex S αα; HEXA/HEXB) is a lysosomal GH20 enzyme; only Hex A with GM2 activator degrades GM2 ganglioside, and its deficiency causes Tay-Sachs and Sandhoff diseases.
Hickman line
Hickman line, also called Hickman catheter, a tunneled silicone central venous catheter with a Dacron cuff, invented by Robert Hickman in 1979 by enlarging the Broviac catheter for bone marrow transplantation; used for chemotherapy, parenteral nutrition, and blood sampling.
High-altitude cerebral edema
High-altitude cerebral edema (HACE), the severe end-stage form of acute mountain sickness, is hypoxia-induced brain swelling with ataxia and altered mental status above roughly 4,000 m, treated by descent, oxygen, and dexamethasone.
High-output heart failure
High-output heart failure (HOHF): heart failure with cardiac output >8 L/min or CI >4 L/min/m2, low systemic vascular resistance; caused by obesity, cirrhosis, AV shunts, anemia; vasodilators worsen it.
High-STEACS trial
High-STEACS (NCT01852123) was a Scottish stepped-wedge cluster-randomized trial of high-sensitivity troponin I with sex-specific 99th centile thresholds in 48,282 patients; it reclassified many women but did not reduce 1-year MI or cardiac death.
Hilar mossy cell
Hilar mossy cell: glutamatergic principal neuron of the dentate gyrus hilus, marked by thorny excrescences, axons reaching up to 35,000 granule cells; key in pattern separation, vulnerable in epilepsy.
Hip
Hip (hip joint, coxofemoral joint): the ball-and-socket joint between the femoral head and the acetabulum; multiaxial, weight-bearing, stabilized by labrum and iliofemoral, pubofemoral, and ischiofemoral ligaments.
Hip bone
Hip bone (os coxae, innominate bone): large irregular pelvic bone of three fused parts, ilium, ischium, pubis, forming the acetabulum; the skeleton's most sexually dimorphic bone, used in forensic sex determination.
Hippocampal granule cell
Hippocampal granule cell, dentate gyrus principal neuron (dentate granule cell). Receives entorhinal input, fires ultra-sparsely, sends mossy fibers to CA3 as detonator synapses for pattern separation; generated by adult neurogenesis.
Hippocampal pyramidal neuron
Hippocampal pyramidal neuron (CA pyramidal cell): glutamatergic principal neuron of hippocampal fields CA3, CA2, CA1 and subiculum; receives ~30,000 CA3 synapses in CA1, fires at ~1.4 Hz, and shows marked within-type heterogeneity.
Hippocampus
Hippocampus: seahorse-shaped medial temporal lobe brain structure essential for forming new declarative memories and spatial navigation; subfields DG, CA3, CA2, CA1; defined by H.M. amnesia, place cells, and cognitive map theory.
Histaminergic neuron
Histaminergic neurons, the brain's only histamine-releasing neurons, sit in the tuberomammillary nucleus of the posterior hypothalamus, fire as wake-promoting pacemakers, project throughout the CNS, and signal via H1–H4 receptors.
HLA-B27
HLA-B27, an HLA-B gene allele encoding an MHC class I antigen, is the strongest known common-variant disease association, linked to ankylosing spondylitis, reactive arthritis, and anterior uveitis since its 1973 discovery.
Hoffmann's reflex
Hoffmann's reflex (Hoffmann sign, finger flexor reflex): bedside flick of the middle finger causing thumb and finger flexion, an upper-limb corticospinal tract sign; distinct from Paul Hoffmann's electrically evoked H-reflex.