OPTE cheat sheet

Vergence L = n/l, in diopters (m⁻¹); light diverging from a point 0.5 m away in air has L = −2.00 D, converging toward a point 0.5 m ahead has L = +2.00 D.

Thin lens: U + F = V, so image vergence equals object vergence plus lens power; image distance l' = n'/V.

Focal length f' = n'/F and f = −n/F; a +5.00 D lens in air has f' = +0.20 m.

Surface power F = (n' − n)/r, with r in meters and sign by the incident-to-emergent direction convention.

Snell's law n₁sinθ₁ = n₂sinθ₂; small-angle (paraxial) approximation n₁θ₁ ≈ n₂θ₂ underlies all Gaussian optics formulas.

Critical angle θc = arcsin(n₂/n₁) for n₁ > n₂; for cornea (n≈1.376) to air, θc ≈ 46.5°.

Magnification: transverse m = l'/l (with reduced vergences m = L/L'), so a real image formed by a plus lens is inverted when m is negative.

Diffraction limit: Rayleigh criterion θ ≈ 1.22λ/D radians; a 3 mm pupil at 550 nm resolves about 0.77 arcmin.

Prism deviation for thin prisms δ ≈ (n − 1)A; prism diopter = 100·tan δ, so 1Δ displaces an object 1 cm at 1 m.

Corneal layers (anterior to posterior): epithelium, Bowman layer, stroma (~90% thickness, ~78% water), Descemet membrane, endothelium.

Corneal endothelium is nonregenerating in humans; cell density ~2,500–3,000/mm² at birth and declines ~0.5%/year; pump failure causes stromal edema.

Tear film: lipid (meibomian glands), aqueous (main and accessory lacrimal glands), mucin (goblet cells); turnover ~10–15% per minute.

Aqueous humor is secreted by the ciliary epithelium at ~2–3 µL/min; outflow via trabecular meshwork–Schlemm canal (~80–90%) and uveoscleral route.

Normal IOP ~10–21 mmHg; it is determined by secretion rate, outflow facility, and episcleral venous pressure (Goldmann equation).

Retinal layers: photoreceptors (rods ~120 million, cones ~6 million), outer nuclear, outer plexiform, inner nuclear, inner plexiform, ganglion cell layer, nerve fiber layer.

Fovea has no rods and no overlying retinal vessels; cones are densest at the foveola (~150,000–200,000/mm²).

Crystalline lens grows throughout life by adding cortical fibers; nucleus hardens, reducing accommodative amplitude (Helmholtz mechanism).

Optic nerve head has ~1.0–1.2 million axons; cup-to-disc ratio >0.6 or asymmetry >0.2 warrants glaucoma evaluation.

Spherical equivalent = sphere + ½ cylinder; a −2.00/−1.00×180 prescription has SE −2.50 D.

Myopia: far point is finite and in front of the eye; far point distance (m) = 1/|Rx|, so −4.00 D gives 0.25 m.

Hyperopia: far point is virtual behind the eye; latent hyperopia is unmasked by cycloplegic refraction, especially in children.

Astigmatism types: with-the-rule (plus cylinder axis near 90°), against-the-rule (near 180°), oblique (axis 30–60° or 120–150°).

Jackson cross-cylinder: power axis at 45° to the tested meridians; use ±0.25 D JCC steps, refine axis before sphere.

Duochrome (red-green) test: with best sphere, red and green appear equally clear; if red is clearer add minus, if green clearer add plus.

Snellen acuity fraction = test distance / letter size distance; 6/6 m equals 20/20 ft; logMAR = log₁₀(denominator/6).

Amplitude of accommodation (D) ≈ 18.5 − 0.3 × age (Hofstetter minimum); near add ≈ amplitude − half the working distance demand.

Lensmeter measures back vertex power; compensate when the refracting vertex distance differs from the fitting vertex distance using Fc = F/(1 − dF).

Vertex distance effect: minus lenses gain plus power as they move closer to the eye; plus lenses gain plus power as they move away.

Lens thickness: edge thickness grows with decentration and minus power; center thickness grows with plus power; use minimum blank size = ED + 2×decentration + 2 mm.

Prentice's rule: prism (Δ) = decentration in cm × power in D; a −4.00 D lens decentered 3 mm induces 1.2Δ.

Base curve selection: steeper base curves for plus lenses, flatter for high minus; wrap and pantoscopic tilt require compensation.

Pantoscopic tilt reduces effective sphere and induces cylinder; face-form (wrap) induces oblique astigmatism corrected with compensated Rx.

Transmittance/refractive index: higher index (1.67, 1.74) reduces thickness but raises chromatic aberration (Abbe value falls).

Anti-reflective coating reduces surface reflections (~4% per uncoated air-glass surface); photochromic and polarized lenses serve different functions.

Keratometry values convert: steeper K means shorter radius; average K ~7.8 mm (43.00 D); 0.05 mm change ≈ 0.25 D.

Soft lens sagittal depth and base curve: flatter BC tightens fit; steeper BC loosens; evaluate movement, centration, and push-up test.

RGP fit: apical clearance 10–20 µm, mid-peripheral clearance, edge lift 60–80 µm; fluorescein pattern guides adjustment.

Oxygen permeability Dk/t: daily wear minimum ~24, extended wear ~87 (×10⁻⁹ units); silicone hydrogels exceed these.

Overnight wear risk: microbial keratitis incidence ~20 per 10,000 wearer-years for extended wear vs ~2 per 10,000 for daily wear.

Toric soft lens: stabilize axis with prism ballast or thin zones; assess rotation and apply LARS (left add, right subtract) for axis adjustment.

Multifocal soft design: center-near or center-distance; add power and pupil size determine near performance.

Orthokeratology flattens central cornea by ~1–4 D; requires overnight wear and topographic monitoring for decentration.

Tropicamide 0.5–1%: cycloplegia ~20–30 min, mydriasis 4–6 h; cyclopentolate 1%: cycloplegia ~30–60 min, mydriasis 24 h.

Atropine 1% produces 7–14 days of cycloplegia; used for penalization and amblyopia therapy; risk of systemic anticholinergic effects.

Phenylephrine 2.5% is a direct alpha-1 agonist mydriatic without cycloplegia; avoid 10% in hypertension and the elderly.

Proparacaine 0.5% and tetracaine 0.5% are topical anesthetics; do not dispense for home use (corneal toxicity, abuse).

Fluorescein sodium (0.25% with benoxinate) stains corneal epithelial defects; cobalt blue light enhances visibility.

Lissamine green stains devitalized epithelium and conjunctiva; rose bengal stains healthy cells lacking mucin protection.

Timolol 0.5% (beta-blocker) lowers IOP by reducing aqueous production; contraindicated in asthma and heart block.

Pilocarpine 1–2% (miotic) increases trabecular outflow; causes miosis, brow ache, and induced myopia.

Preservative benzalkonium chloride (BAK) disrupts the tear film and epithelium; consider preservative-free drops in dry eye or frequent dosing.

Nine diagnostic positions of gaze test each extraocular muscle; ductions test one eye, versions test both together.

Hirschberg test: 1 mm corneal light reflex decentration ≈ 7° or ~15Δ of deviation.

Cover-uncover test detects tropia; alternate cover test measures total deviation (tropia + phoria).

Prism alternate cover test: neutralize movement; endpoint is reversal; record distance and near separately.

AC/A ratio normal ~4–6 Δ/D; high AC/A suggests convergence excess, low AC/A suggests divergence excess.

Amblyopia treatment: patching or atropine penalization; critical period ends around age 7–9, earlier intervention improves outcomes.

Esotropia with onset after 6 months and hyperopia >+2.00 D: consider accommodative esotropia; full cycloplegic correction first.

Anomalous retinal correspondence and suppression develop in long-standing strabismus; test with Worth 4-dot and Bagolini lenses.

Stereopsis: Titmus fly (gross), Randot circles (fine) ~40–60 arcsec normal; absence suggests monofixation or suppression.

Glaucoma diagnosis: IOP >21 mmHg, optic disc cupping (C/D >0.6, rim thinning, disc hemorrhage), and visual field defects (arcuate, nasal step).

Diabetic retinopathy: microaneurysms, dot-blot hemorrhages, hard exudates; refer for panretinal photocoagulation in proliferative disease.

Age-related macular degeneration: dry (drusen, geographic atrophy) vs wet (choroidal neovascularization, subretinal fluid); anti-VEGF for wet.

Cataract grading: nuclear sclerosis, cortical spokes, posterior subcapsular; surgery indicated when acuity or function is impaired.

Keratoconus: inferior steepening, irregular astigmatism, Vogt striae, Fleischer ring; corneal topography and crosslinking for progression.

Blepharitis: anterior (staphylococcal, seborrheic) vs posterior (meibomian gland dysfunction); lid hygiene and warm compresses.

Conjunctivitis: viral (watery, follicular), bacterial (purulent, papillae), allergic (itching, chemosis); avoid steroids in undiagnosed red eye.

Retinal detachment: flashes, floaters, curtain; fundus exam for tears; refer emergently for laser or surgery.

Anterior uveitis: cells and flare in anterior chamber, photophobia, ciliary flush; treat with topical steroids and cycloplegics.

Case history: chief complaint, ocular history, medical history, medications, allergies, family history, occupational/visual demands.

Visual acuity: Snellen or logMAR at 6 m/20 ft; pinhole improves refractive blur, not pathology.

Cover test and motility precede refraction; record unaided and aided acuities.

Slit lamp: diffuse, direct focal, indirect, retroillumination, sclerotic scatter; record findings in anterior-to-posterior order.

Goldmann applanation tonometry: 3.06 mm diameter applanation; average IOP 15–16 mmHg; avoid in corneal edema or irregularity.

Non-contact tonometry and rebound tonometry: useful screening; calibrate and compare with Goldmann.

Ophthalmoscopy: direct (15×, no stereopsis, small field) vs indirect (2–3×, stereopsis, wide field, requires condensing lens).

Visual field: 24-2 or 30-2 SITA; reliability indices fixation losses <20%, false positives/negatives <33%.

Pachymetry: central corneal thickness ~540 µm; thinner corneas underestimate Goldmann IOP.

Informed consent: disclose diagnosis, proposed treatment, alternatives, risks, and expected outcomes; document discussion.

Record keeping: retain patient records per jurisdictional law (often 7–10 years; longer for minors); include test results and advice given.

Confidentiality: disclose only with patient consent or legal requirement (e.g., court order, public health reporting).

Scope of practice: perform only procedures within licensure and training; refer when outside competence.

Billing and coding: use correct CPT/ICD codes; upcoding and unbundling are fraud; document medical necessity.

Conflict of interest: disclose financial interest in products or referrals; avoid self-referral where prohibited.

Advertising: must be truthful, not misleading; avoid superlatives and unsubstantiated claims.

Infection control: hand hygiene, instrument disinfection, single-use items; follow standard precautions for all patients.

Patient abandonment: do not terminate care without notice and opportunity to transfer; provide records on request.