2,701 articles
Sciatica
Sciatica: pain radiating from the buttock down the leg from sciatic nerve irritation or compression, usually a herniated lumbar disc; up to 43% lifetime prevalence, mostly self-limiting, with surgery offering only short-term benefit.
Scimitar syndrome
Scimitar syndrome: rare congenital anomaly with right pulmonary venous drainage into the inferior vena cava, right lung hypoplasia, and systemic arterial supply; named by Neill in 1960; survival 88% at age 30.
Sclera
The sclera is the tough white fibrous outer coat of the eye, about 85% of the outer tunic, spanning cornea to optic nerve; type I collagen-rich, strain-stiffening, linked to myopia, glaucoma, and scleritis.
Scleral buckle
Scleral buckle (scleral buckling): silicone explant sutured to the sclera to indent the eyewall and close retinal breaks in rhegmatogenous retinal detachment; introduced by Custodis in 1949, refined by Schepens and Lincoff, still used alone or with vitrectomy.
Scrotum
Scrotum: the pouch of skin and dartos muscle below the penis containing the testes, epididymides, and spermatic cords; keeps testes 2–4 °C below core temperature for sperm production.
Sebaceous gland
Sebaceous gland: skin exocrine gland secreting sebum (squalene, wax esters, triglycerides) by holocrine rupture of sebocytes; absent on palms and soles, dense on face; androgen-driven, central to acne and seborrhoeic dermatitis.
Secondary intestinal lymphangiectasia
Secondary intestinal lymphangiectasia (ORPHA:90363, ICD-10 I89.0) is acquired intestinal lymphangiectasia causing protein-losing enteropathy from another disorder such as Crohn's disease, congestive heart failure, or the Fontan operation; treatment targets the underlying disease.
Secondary myocardial involvement of systemic disease
Secondary myocardial involvement of systemic disease is heart muscle dysfunction caused by an identifiable systemic condition, including endocrine, nutritional, electrolyte, iron-overload, and storage disorders rather than primary heart disease.
Secondary prevention after myocardial infarction
Secondary prevention after myocardial infarction is the combination of medications, lifestyle change, and structured follow-up used to prevent recurrent cardiovascular events in heart attack survivors.
Secretin
Secretin: 27-amino acid peptide hormone, the first hormone discovered (Bayliss and Starling, 1902). Released by duodenal S cells on acid entry; stimulates pancreatic bicarbonate secretion, inhibits gastric acid, used in gastrinoma and pancreatic function tests.
Segmental arterial mediolysis
Segmental arterial mediolysis is a rare, non-inflammatory arterial disease that weakens medium-sized artery walls, causing dissections, aneurysms, and hemorrhage, most often in abdominal visceral arteries.
Segmental venous malformation
Segmental venous malformation, also called Bockenheimer syndrome or genuine diffuse phlebectasia, is an ultra-rare congenital, progressive venous malformation of the limbs affecting veins of all sizes, with pain, swelling, and ulceration.
SELECT trial (NCT03574597)
SELECT (NCT03574597), the Semaglutide Effects on Cardiovascular Outcomes in People with Overweight or Obesity trial, was Novo Nordisk's phase 3 trial showing once-weekly semaglutide 2.4 mg cut major adverse cardiovascular events 20% in 17,604 adults with cardiovascular disease and overweight or obesity but no diabetes.
Sella turcica
Sella turcica, or Turkish saddle, is the saddle-shaped depression in the sphenoid bone housing the pituitary gland; named by Spigelius in 1627, with tuberculum sellae, hypophyseal fossa, and dorsum sellae.
Semantic memory
Semantic memory: Tulving's 1972 memory system for general world knowledge, facts, and word meanings, distinct from episodic memory; hub-and-spoke ATL brain basis, K.C. and semantic dementia evidence.
Semaphorin
Semaphorins are a family of secreted and membrane proteins with a conserved Sema domain, first described in 1993 as axon guidance repellents; they signal via plexins and neuropilins and act in immunity, angiogenesis, and cancer.
Semen
Semen, or seminal fluid, is the whitish-gray ejaculate made of sperm from the testicles plus seminal vesicle and prostate secretions (65-75% vesicle fluid, 25-30% prostate, 1-5% sperm). WHO reference values and standardized semen analysis define its clinical use.
Semicircular canals
Semicircular canals: three fluid-filled inner-ear loops (anterior, posterior, horizontal) sensing angular head acceleration via endolymph-cupula mechanics; basis of the VOR, Ewald's laws, BPPV, and vHIT/caloric testing.
Sense
A sense, also known as the five senses, is a biological system that detects stimuli through receptors and sends signals to the brain; humans have an estimated 22 to 33 senses.
Sensitization
Sensitization is a form of non-associative learning in which behavioral responses are amplified after an aversive or noxious stimulus, classically studied in the Aplysia gill- and siphon-withdrawal reflex.
Sensorineural hearing loss
Sensorineural hearing loss (SNHL): inner ear hearing loss from damaged cochlear hair cells or auditory nerve fibers, caused by aging, noise, ototoxic drugs, Ménière's disease, or genetics; permanent, managed with hearing aids, cochlear implants, and now OTOF gene therapy.
Sensory Feedback
Sensory feedback: a organism's own sensory-system information about a task, movement, or skill (MeSH Feedback, Sensory). Covers proprioceptive and cutaneous sources, spinal sensorimotor circuits, intrinsic versus augmented feedback in motor learning, and wearable haptic feedback devices.
Sensory nervous system
The sensory nervous system detects external and internal stimuli through receptors and relays them to the brain, generating touch, pain, vision, hearing, and other senses.
Sensory substitution
Sensory substitution delivers information from a lost sense (vision, hearing, balance) through another modality such as touch or sound; pioneered by Bach-y-Rita's 1969 TVSS, now including devices like BrainPort.
Sentinel lymph node
Sentinel lymph node: first node receiving drainage from a tumor, key to cancer staging. Examined via dye or radiotracer biopsy in breast cancer and melanoma; trials show lower morbidity than axillary dissection with equivalent survival.
Septic shock
Sepsis with septic shock (ICD-11 1G41) is the severe form of sepsis defined by Sepsis-3: vasopressor-dependent hypotension with lactate above 2 mmol/L, carrying over 40% hospital mortality.
Sequelae of trachoma
Sequelae of trachoma: trachomatous conjunctival scarring, trichiasis, and corneal opacity following repeated Chlamydia trachomatis infection; WHO grading, entropion mechanism, surgery outcomes, and declining global burden.
Serotonergic neuron
Serotonergic neurons synthesize and release serotonin (5-HT) from hindbrain raphe nuclei (B1–B9 groups), about 0.015% of rat brain neurons, with TPH2/SERT/VMAT2 phenotype, rostrocaudal subtypes, and forebrain-to-spinal cord projections.
Serous fluid
Serous fluid, or serosal fluid, is the clear, serum-like fluid secreted by serous membranes to lubricate the pleural, pericardial, and peritoneal cavities; its pathological buildup is called an effusion.
Serous retinal detachment
Serous retinal detachment (exudative retinal detachment, ICD-11 9B73.3): subretinal fluid separates the retina from the RPE without a break, caused by CSC, inflammation, tumors, or hypertension; treated medically.