NBEO Part I cheat sheet

Corneal layers anterior to posterior: epithelium (nonkeratinized stratified squamous, ~5-7 layers), Bowman's layer (acellular, nonregenerating), stroma (~90% thickness, type I collagen lamellae), Descemet's membrane (regenerates, produced by endothelium), endothelium (single layer, nonregenerating).

Corneal innervation derives from the ophthalmic division (V1) via long ciliary nerves; the subepithelial nerve plexus is densest at the limbus and declines toward the apex, so epithelial defects are most painful peripherally.

Aqueous outflow: trabecular meshwork -> Schlemm's canal -> collector channels -> episcleral veins; uveoscleral outflow is trabecular-independent and accounts for roughly 5-15% of total outflow depending on age.

Retinal layers inner to outer: nerve fiber layer, ganglion cell layer, inner plexiform, inner nuclear, outer plexiform, outer nuclear, external limiting membrane, photoreceptor inner/outer segments, RPE, Bruch's membrane, choroid.

Fovea centralis contains only cone photoreceptors, lacks rods and inner retinal layers (foveal avascular zone), and has the highest acuity; the optic disc lacks photoreceptors and creates the physiologic blind spot.

Ciliary muscle contraction (parasympathetic, via short ciliary nerves from the ciliary ganglion) releases zonular tension and increases lens power for near accommodation.

Lateral geniculate nucleus has six layers: parvocellular (1-2, ipsilateral; 3-6, contralateral) for fine detail/color, magnocellular for motion; Meyer's loop carries inferior retinal fibers through the temporal lobe.

Optic nerve head is supplied largely by the short posterior ciliary arteries; the central retinal artery supplies the inner retina, while the outer retina depends on the choriocapillaris.

Tear film: lipid layer (meibomian glands) retards evaporation, aqueous layer (lacrimal and accessory glands) contains lysozyme, lactoferrin, and IgA, mucin layer (goblet cells) provides wettability.

Aqueous humor is secreted by the ciliary epithelium via active Na+/K+ ATPase and carbonic anhydrase; secretion is relatively independent of intraocular pressure, unlike outflow.

Corneal transparency requires uniform collagen spacing, relative dehydration maintained by endothelial Na+/K+ ATPase pumps, and absence of blood vessels and myelinated nerves.

Phototransduction: light isomerizes 11-cis retinal to all-trans retinal, activating rhodopsin -> transducin -> phosphodiesterase, which lowers cGMP and closes cyclic nucleotide-gated channels, hyperpolarizing the photoreceptor.

Rods contain rhodopsin with peak sensitivity near 498 nm; cones have three opsins peaking near 420, 534, and 564 nm, underlying trichromatic color vision.

Lens growth is lifelong; lens proteins (crystallins) are largely synthesized prenatally, so oxidative and glycation damage accumulates with age, producing presbyopia and nuclear sclerosis.

Tear osmolarity above roughly 308 mOsm/L and matrix metalloproteinase elevation are objective markers of dry eye disease; tear breakup time under 10 seconds suggests instability.

Retinal pigment epithelium performs the visual cycle (regenerating 11-cis retinal), phagocytoses shed outer segments, and forms the outer blood-retinal barrier via tight junctions.

Bacterial conjunctivitis: Staphylococcus aureus and Streptococcus pneumoniae most common; purulent discharge with glued lashes; adenovirus gives watery discharge with follicles.

Neisseria gonorrhoeae causes hyperacute purulent conjunctivitis with corneal perforation risk; treat with systemic ceftriaxone plus saline lavage and topical antibiotic.

Pseudomonas aeruginosa is the leading cause of contact lens-related microbial keratitis; hallmark is a ring infiltrate with rapid stromal melting; fluoroquinolone monotherapy is first-line.

Acanthamoeba keratitis presents with severe pain out of proportion to signs, radial keratoneuritis, and ring infiltrate; risk factor is contact lens wear with tap water exposure; culture on non-nutrient agar with E. coli.

Herpes simplex keratitis: dendritic ulcer with terminal bulbs, positive fluorescein; topical corticosteroid alone is contraindicated because it promotes viral replication.

Fungal keratitis (Fusarium, Aspergillus, Candida) shows feathery margins, satellite lesions, and hypopyon; KOH preparation and culture on Sabouraud agar confirm.

Immunoglobulins: IgA is the dominant secretory antibody in tears; IgE mediates type I hypersensitivity (allergic conjunctivitis); IgG and IgM fix complement and mediate type II/III reactions.

Corneal graft rejection is primarily a cell-mediated (type IV) response against donor endothelium; the cornea's immune privilege derives from lack of lymphatics and anterior chamber-associated immune deviation.

Inflammation: acute response is neutrophil-predominant; chronic is lymphocyte/macrophage-predominant with fibrosis; granulomatous inflammation shows epithelioid histiocytes and giant cells.

Neoplasia: benign lesions are well circumscribed and noninvasive; malignancy shows invasion, pleomorphism, and metastasis; basal cell carcinoma is the most common eyelid malignancy and rarely metastasizes.

Uveal melanoma arises from choroidal melanocytes, spreads hematogenously (liver most common), and carries poor prognosis with monosomy 3 and larger basal diameter.

Retinoblastoma is caused by biallelic RB1 mutation; leukocoria and strabismus are typical; histology shows Flexner-Wintersteiner rosettes and calcification.

Diabetic retinopathy: pericyte loss and capillary basement membrane thickening cause microaneurysms; VEGF-driven neovascularization follows retinal ischemia and can cause tractional detachment.

Apoptosis is programmed cell death with cell shrinkage and no inflammation; necrosis is passive swelling with membrane rupture and inflammation; the two are distinguished histologically.

Amyloid appears as eosinophilic, apple-green birefringent material under polarized light with Congo red stain; it can deposit in the conjunctiva and eyelid.

Metaplasia is reversible replacement of one mature cell type by another (e.g., squamous metaplasia of conjunctival epithelium in chronic dry eye); dysplasia is disordered growth that may progress to carcinoma in situ.

Pilocarpine is a direct muscarinic agonist that increases trabecular outflow and causes miosis; side effects include induced myopia, brow ache, and dim vision in cataract.

Timolol (beta-blocker) reduces aqueous secretion; contraindicated in asthma, heart block, and severe COPD; systemic absorption is reduced by punctal occlusion.

Latanoprost (prostaglandin F2-alpha analog) increases uveoscleral outflow; adverse effects include iris darkening, eyelash lengthening, and periorbital fat atrophy.

Atropine and tropicamide are antimuscarinic cycloplegics; phenylephrine is an alpha-1 agonist mydriatic without cycloplegia; avoid phenylephrine in infants due to systemic hypertension risk.

Topical anesthetics (proparacaine, tetracaine) block voltage-gated sodium channels; they are diagnostic only and chronic use causes corneal epithelial toxicity and melting.

Fluorescein angiography uses excitation near 465-490 nm and detects emission above ~520 nm; sodium fluorescein is excreted renally and can cause anaphylaxis.

Benzalkonium chloride is the most common preservative and is toxic to corneal epithelium; it is avoided in patients with dry eye or frequent dosing.

Corticosteroids suppress inflammation by inhibiting phospholipase A2 and NF-kB; chronic topical use raises intraocular pressure and cataract risk and can reactivate herpes simplex.

Snell's law: n1 sin(theta1) = n2 sin(theta2); refractive index of air is 1.00, cornea ~1.376, aqueous and vitreous ~1.336, crystalline lens ~1.42.

Thin lens equation: 1/f = 1/do + 1/di, with object distance positive for real objects; magnification m = -di/do.

Gullstrand reduced eye: total power ~60 D, with corneal power ~43 D and lens ~19 D; the eye's nodal point is ~7 mm behind the cornea and focal length ~22.2 mm.

Vergence formula: L' = L + F, where L = n/l (object vergence) and L' = n'/l' (image vergence); use it to compute image position through a refracting surface.

Prism diopter: 1 prism diopter deviates a ray 1 cm at 1 m; prism power P = 100 tan(theta).

Diffraction limit of resolution: theta = 1.22 lambda / D; smaller pupil improves resolution until diffraction dominates below ~2 mm.

Rayleigh criterion and Fresnel equations govern image quality and reflection; antireflective coatings exploit quarter-wave optical thickness.

Spherical aberration increases with pupil size and is the dominant higher-order aberration; chromatic aberration arises from dispersion (Abbe number).

Weber's law: delta I / I = constant; the just-noticeable difference scales with background intensity.

Fechner's law: perceived magnitude S = k log(I); Stevens' power law: S = k I^n, with n < 1 for brightness.

Visual acuity 20/20 corresponds to 1 minute of arc per letter stroke; Snellen fraction = testing distance / distance at which the letter subtends 5 minutes of arc.

Dark adaptation is biphasic: cone branch completes in ~5-10 minutes, rod branch in ~30 minutes; the rod-cone break reflects the cone threshold.

Contrast sensitivity peaks near 2-6 cycles per degree and declines at high and low spatial frequencies; it detects deficits missed by Snellen acuity.

Color vision: protanopia lacks red cones, deuteranopia lacks green cones, tritanopia lacks blue cones; Ishihara plates screen red-green defects.

Critical flicker fusion frequency rises with luminance and stimulus size; it reflects temporal resolution of the visual system.

Stereopsis threshold in normal observers is ~10-20 seconds of arc; random-dot stereograms isolate binocular disparity from monocular cues.

Six extraocular muscles: medial, lateral, superior, and inferior recti, plus superior and inferior obliques; superior oblique is innervated by CN IV, lateral rectus by CN VI, all others by CN III.

Actions of the superior oblique are intorsion, depression, and abduction; inferior oblique performs extorsion, elevation, and abduction.

Hering's law: yoked muscles receive equal innervation; Sherrington's law: increased innervation to an agonist is accompanied by reciprocal decrease to its antagonist.

Cover test: uncover test detects tropia; alternate cover test detects total deviation including phoria; prism neutralization quantifies the deviation.

AC/A ratio normal range is about 4-6 prism diopters per diopter of accommodation; high AC/A suggests convergence excess.

Amblyopia classification: strabismic, anisometropic, deprivation; critical period for treatment is early childhood, with reduced response after age 7-9.

Saccades are rapid conjugate eye movements with velocities up to ~500 deg/s; smooth pursuit requires a moving target and is impaired in deep sleep and cerebellar lesions.

Near triad: convergence, accommodation, and miosis occur together; the near point of convergence is normally under 6 cm.

Myopia: concave (minus) lens; hyperopia: convex (plus) lens; astigmatism: cylindrical lens; the spherical equivalent equals sphere + half the cylinder.

Jackson cross-cylinder technique refines cylinder axis first, then power; the duochrome (red-green) test uses chromatic aberration, with red clearer for under-minus and green clearer for over-minus.

Retinoscopy: with motion indicates neutrality approached by adding plus; against motion by adding minus; working distance of 50 cm requires subtracting 2.00 D.

Accommodative amplitude declines with age; average amplitude roughly equals 18.5 - 0.3 x age in diopters, reaching presbyopia near age 40-45.

Keratometry measures anterior corneal curvature; average corneal power is ~43-44 D; corneal astigmatism is inferred from the difference in meridional power.

Amsler grid screens for macular disease; confrontation visual fields screen for gross defects; pupil testing compares direct and consensual responses to detect afferent defects.

Intraocular pressure normal range is 10-21 mmHg; central corneal thickness affects tonometry, with thicker corneas overestimating pressure.

Anisometropia above 1.00 D increases amblyopia risk; aniseikonia arises from unequal retinal image sizes, typically exceeding 5% tolerance.

Diabetes mellitus: nonproliferative retinopathy shows microaneurysms, dot-blot hemorrhages, and hard exudates; proliferative disease shows neovascularization; cataract and cranial nerve palsies also occur.

Hypertension causes arteriolar narrowing, AV nicking, flame hemorrhages, and cotton-wool spots; malignant hypertension adds papilledema.

Graves' disease causes lid retraction, proptosis, restrictive myopathy (inferior rectus most often), and optic neuropathy; smoking worsens it.

Marfan syndrome causes ectopia lentis (superotemporal dislocation) and aortic root dilation; homocystinuria causes inferonasal dislocation.

Giant cell arteritis presents with headache, jaw claudication, and sudden vision loss from anterior ischemic optic neuropathy; ESR and CRP are elevated; treat immediately with high-dose corticosteroids.

Multiple sclerosis commonly causes optic neuritis with pain on eye movement, afferent pupillary defect, and color desaturation; MRI shows white matter lesions.

Sarcoidosis produces granulomatous uveitis, conjunctival nodules, and lacrimal gland enlargement; chest radiograph shows bilateral hilar lymphadenopathy.

Sickle cell disease causes comma-shaped conjunctival vessels and proliferative retinopathy with sea-fan neovascularization; avoid hypoxia and dehydration.

Sensitivity = true positives / (true positives + false negatives); specificity = true negatives / (true negatives + false positives); both are properties of the test, not prevalence.

Positive predictive value depends on prevalence: PPV = sensitivity x prevalence / (sensitivity x prevalence + (1-specificity) x (1-prevalence)).

Relative risk compares incidence in exposed versus unexposed; odds ratio approximates relative risk for rare outcomes and is used in case-control studies.

Cohort studies measure incidence and allow relative risk; case-control studies are efficient for rare diseases but cannot directly yield incidence.

Type I error (alpha) is rejecting a true null hypothesis; type II error (beta) is failing to reject a false null; power = 1 - beta, conventionally 0.80.

p-value is the probability of observing data at least as extreme as observed, assuming the null hypothesis is true; it is not the probability the null is true.

Confidence interval width decreases with larger sample size and increases with greater variability; a 95% CI excluding the null value corresponds to p < 0.05.

Randomized controlled trials reduce selection bias; randomization balances known and unknown confounders, unlike observational designs.

Informed consent requires disclosure of diagnosis, proposed treatment, alternatives, risks, and benefits, plus patient comprehension and voluntariness.

Standard of care is the degree of care and skill a reasonably prudent practitioner in the same specialty would provide under similar circumstances.

Negligence requires four elements: duty, breach of duty, causation, and damages; all four must be proven.

Patient confidentiality may be breached only with consent or where legally mandated, such as reporting communicable diseases or suspected abuse.

Documentation should be contemporaneous, legible, and complete; alterations should be dated and initialed; records are generally discoverable.

Scope of practice is defined by state law and licensure; performing beyond it exposes the practitioner to licensure discipline and liability.

Mandatory reporting includes child abuse, elder abuse, and certain communicable diseases; failure to report can carry criminal and civil penalties.

Conflict of interest must be disclosed; financial relationships with optical suppliers or referral sources can violate anti-kickback and Stark-type laws.