NBEO Part I flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Bowman's layer

Acellular anterior corneal layer beneath the epithelium that does not regenerate after injury; scarring here produces permanent subepithelial opacity and irregular astigmatism.

Foveal avascular zone

Central retinal area lacking capillaries, containing only cones and no inner retinal layers; it provides the highest visual acuity and is vulnerable to vascular occlusion.

Trabecular meshwork

Porous tissue at the anterior chamber angle through which aqueous humor drains into Schlemm's canal; increased resistance here raises intraocular pressure in open-angle glaucoma.

Endothelial pump

Na+/K+ ATPase activity of corneal endothelial cells that removes stromal fluid and maintains corneal deturgescence and transparency; failure causes corneal edema.

Phototransduction cascade

Light-activated rhodopsin stimulates transducin, lowering cGMP and closing cation channels, which hyperpolarizes the photoreceptor and reduces neurotransmitter release.

Aqueous humor formation

Active secretion by ciliary epithelium driven by carbonic anhydrase and Na+/K+ ATPase; it is largely independent of intraocular pressure, unlike outflow.

Pseudomonas keratitis

Rapidly progressive corneal infection with ring infiltrate and stromal melting, most often in contact lens wearers; treated with intensive topical fluoroquinolone.

Dendritic ulcer

Branching corneal epithelial lesion with terminal bulbs seen in herpes simplex keratitis; fluorescein stains it, and topical steroids alone are contraindicated.

Secretory IgA

Predominant immunoglobulin in tears, produced by plasma cells near the lacrimal gland; it neutralizes pathogens on the ocular surface without complement activation.

Acanthamoeba keratitis

Painful corneal infection with radial keratoneuritis, linked to contact lens use with tap water; diagnosed by culture on non-nutrient agar with E. coli.

Flexner-Wintersteiner rosettes

Histologic structures of cells arranged around a central lumen seen in retinoblastoma; they reflect photoreceptor differentiation and support the diagnosis.

Uveal melanoma spread

Choroidal melanoma metastasizes hematogenously, most commonly to the liver; monosomy 3 and larger tumor size predict poorer survival.

Metaplasia

Reversible replacement of one mature epithelial cell type by another, such as squamous change in chronic dry eye; it differs from dysplasia, which is disordered growth.

Latanoprost mechanism

Prostaglandin F2-alpha analog that increases uveoscleral outflow to lower intraocular pressure; causes iris darkening and eyelash lengthening.

Timolol contraindications

Nonselective beta-blocker that reduces aqueous secretion; avoid in asthma, COPD, and heart block because of systemic beta-adrenergic effects.

Cycloplegic versus mydriatic

Antimuscarinics such as tropicamide paralyze the iris sphincter and ciliary muscle, while alpha agonists such as phenylephrine dilate without cycloplegia.

Snell's law

n1 sin(theta1) equals n2 sin(theta2) at an interface; it governs refraction and underlies the calculation of corneal and lens power.

Prism diopter

Unit of prism power equal to 1 cm of deviation at 1 m; calculated as 100 times the tangent of the deviation angle.

Reduced eye

Simplified schematic eye with about 60 D total power, corneal power near 43 D, nodal point about 7 mm behind the cornea, and focal length near 22.2 mm.

Weber's law

The just-noticeable difference in stimulus intensity is a constant fraction of the background intensity, holding across many sensory modalities including brightness.

Dark adaptation curve

Biphasic threshold decline after light exposure: cone branch recovers in 5-10 minutes and rod branch in about 30 minutes, separated by the rod-cone break.

Contrast sensitivity function

Curve relating threshold contrast to spatial frequency; it peaks near 2-6 cycles per degree and detects deficits that Snellen acuity misses.

Hering's law

Yoked extraocular muscles of the two eyes receive equal innervation, so a palsy produces secondary deviation in the nonparetic eye when the paretic eye fixates.

AC/A ratio

Change in accommodative convergence per diopter of accommodation; normal is about 4-6 prism diopters per diopter, with higher values suggesting convergence excess.

Superior oblique action

Innervated by cranial nerve IV; its actions are intorsion, depression, and abduction, making it the depressor of the adducted eye.

Spherical equivalent

Sphere power plus half the cylinder power, used to summarize a spherocylindrical refraction as a single value for comparison and contact lens selection.

Retinoscopy working distance

The distance from examiner to patient; its dioptric value is subtracted from the neutralization finding, so 50 cm requires subtracting 2.00 D.

Giant cell arteritis

Granulomatous large-vessel inflammation causing headache, jaw claudication, and ischemic optic neuropathy; ESR and CRP rise, and high-dose steroids are urgent.

Ectopia lentis

Lens dislocation; superotemporal in Marfan syndrome and inferonasal in homocystinuria, helping distinguish the two connective tissue disorders.

Positive predictive value

Proportion of positive test results that are truly positive; unlike sensitivity, it changes with disease prevalence in the tested population.

Type II error

Failure to reject a false null hypothesis; its probability is beta, and statistical power equals 1 minus beta, conventionally set at 0.80.

Negligence elements

A malpractice claim requires duty, breach of that duty, causation linking breach to harm, and damages; failure to prove any element defeats the claim.