COT cheat sheet

Visual acuity notation: Snellen fraction = test distance / distance at which the letter subtends 5 arc minutes; 20/40 means the patient needs 20 ft to see what a standard eye sees at 40 ft.

Tonometry by Goldmann applanation: normal IOP 10-21 mmHg; central corneal thickness affects readings (thick cornea overestimates, thin underestimates); fluorescein plus topical anesthetic required.

Pupil dilation: tropicamide 0.5-1% lasts 4-6 h; phenylephrine 2.5% lasts 3-5 h; cyclopentolate 1% paralyzes accommodation for 24-72 h; atropine 1% lasts 1-2 weeks.

Amsler grid: 10 cm square, 20-degree field, tested at 33 cm with near correction; metamorphopsia or scotoma suggests macular disease.

Instillation technique: one drop per instillation (conjunctival sac holds ~30 microliters), punctal occlusion for 1-2 min reduces systemic absorption.

Anesthetic abuse: topical proparacaine/tetracaine cause corneal epithelial toxicity; never dispense for home use.

Ocular emergency triage: chemical burn = immediate copious irrigation before acuity; central retinal artery occlusion and acute angle closure are same-day emergencies.

Sterile field: instrument tips and cannula never touch lashes or lids; used needles go to sharps container without recapping.

Keratometry measures anterior corneal curvature over the central ~3 mm; converts radius (mm) to dioptric power by K = 337.5 / r(mm).

A-scan ultrasound axial length: average adult eye ~23.5-24 mm; sound velocity assumed 1532 m/s in phakic, 1530 m/s in pseudophakic eyes.

OCT provides cross-sectional retinal imaging by low-coherence interferometry; central macular thickness reference ~250-270 microns.

Humphrey visual field 24-2 tests 54 points at 6-degree spacing; 10-2 tests 68 points at 2-degree spacing for macular/paracentral defects.

Pachymetry: average central corneal thickness ~540-550 microns; used to adjust IOP interpretation and screen for keratoconus.

Slit lamp: diffuse illumination for general survey, optic section for depth/localization, retroillumination for corneal edema and cysts, sclerotic scatter for subtle opacity.

Indirect ophthalmoscopy: 20D lens gives ~3x magnification and ~45-degree field; 28D gives wider field with less magnification.

Corneal topography maps curvature (Placido ring or elevation-based); keratoconus pattern = inferior steepening with asymmetric bow-tie.

Tear breakup time: normal >10 s; fluorescein instilled, interval to first dry spot recorded under cobalt blue.

Corneal layers (anterior to posterior): epithelium, Bowman layer, stroma, Descemet membrane, endothelium; endothelium does not regenerate and maintains deturgescence via Na+/K+ ATPase pumps.

Aqueous flow: ciliary epithelium -> posterior chamber -> pupil -> anterior chamber -> trabecular meshwork -> Schlemm canal -> episcleral veins; also uveoscleral outflow.

Retinal layers: photoreceptors (rods ~120 million, cones ~6 million) synapse to bipolar cells then ganglion cells whose axons form the optic nerve.

Fovea has no rods and no retinal vessels; highest cone density and 1:1 cone-to-ganglion convergence give best acuity.

Cataract types: nuclear sclerotic (yellowing, index myopia), cortical (spokes, glare), posterior subcapsular (rapid, near-vision loss, steroid-associated).

Diabetic retinopathy: nonproliferative shows microaneurysms, dot-blot hemorrhages, hard exudates; proliferative shows neovascularization driven by VEGF.

Glaucoma: open-angle shows optic disc cupping (cup-to-disc >0.6, neuroretinal rim thinning) and nerve fiber layer loss; angle closure shows shallow anterior chamber and mid-dilated fixed pupil.

Lens and accommodation: crystalline lens loses elasticity with age (presbyopia onset ~40-45 y); zonules suspend lens from ciliary body.

Macula is supplied by choroidal circulation; central retinal artery supplies inner retina, so CRAO spares the fovea (cherry-red spot).

Snellen to logMAR: logMAR = log10(denominator/20) for 20-ft notation; 20/20 = 0.0, 20/40 = 0.3, 20/200 = 1.0.

Spherical equivalent = sphere + (cylinder / 2); a -2.00 -1.00 x 180 lens has SE of -2.50 D.

Vertex distance conversion: Fc = F / (1 - dF), where d is vertex distance in meters; clinically significant above +/-4.00 D.

Tear film break-up and lens fit: soft lens base curve typically 8.3-8.8 mm; steeper (smaller) BC tightens fit; fluorescein pattern shows apical bearing vs edge lift.

Oxygen transmissibility: Dk/t determines corneal oxygen supply; silicone hydrogel materials have high Dk (often >100) allowing extended wear.

Prism: 1 prism diopter deviates a ray 1 cm at 1 m; Prentice rule P = cF (c in cm, F in diopters).

Magnification of a simple magnifier: M = 25 cm / f(cm) for relaxed eye at infinity; near addition for reading at 40 cm is +2.50 D.

Astigmatism correction: with-the-rule axis near 180, against-the-rule near 90; rigid lenses correct irregular astigmatism by creating a tear lens.

Contact lens care: multipurpose solutions require rub-and-rinse; hydrogen peroxide systems need neutralization time before insertion.