ICO Visual Sciences cheat sheet

Snell's law: n₁sinθ₁ = n₂sinθ₂; refractive index n = c/v, with n(air)=1.000, n(cornea)=1.376, n(lens core)=1.406, n(vitreous)=1.336.

Gullstrand reduced eye: single refracting surface, power +60 D, radius 5.55 mm, refractive index 1.333, axial length 22.22 mm.

Total eye power ≈ +60 D; corneal power ≈ +43 D (anterior surface +48.8 D, posterior −5.8 D); crystalline lens ≈ +19 D at rest.

Thin-lens/vergence: 1/f = 1/u + 1/v in diopters; surface power F = (n₂ − n₁)/r with r in meters.

Far point = 1/ocular refraction (D); myope −4.00 D has far point 25 cm; hyperope +4.00 D has far point 25 cm behind eye.

Spectacle-to-cornea vertex distance correction: F_contact = F_spectacle / (1 − d·F_spectacle), d in meters.

Astigmatism by power cross: with-the-rule = steeper vertical meridian (more plus at 90°); against-the-rule = steeper horizontal.

Spherical equivalent = sphere + ½ cylinder; Jackson cross-cylinder flips sphere by +½ cyl and cylinder axis by 90° per reversal.

Diffraction (Airy disk) angular radius = 1.22λ/a; Rayleigh criterion limits resolution as pupil aperture shrinks.

Photopic (cone) acuity peaks at ~555 nm; scotopic (rod) sensitivity peaks at ~507 nm - the Purkinje shift.

Weber fraction for luminance discrimination ≈ 1–2% at photopic levels; contrast sensitivity peaks near 2–6 cycles/degree.

Visual acuity conversions: 20/20 = 6/6 = 1.0 decimal = logMAR 0.0; logMAR = log₁₀(Snellen denominator/6).

Depth cues: stereopsis (binocular disparity) is finest at ~2–4 arcsec; monocular cues include motion parallax, texture gradient, occlusion.

Dark adaptation: cone branch completes in ~5–7 min; rod branch continues to ~20–30 min with ~3–4 log unit threshold drop.

Critical flicker fusion ≈ 50–60 Hz in fovea; Ferry–Porter law: CFF rises with log luminance.

Simultaneous contrast and Mach bands arise from lateral inhibition in retinal receptive fields.

Accommodation amplitude declines with age (Duane): ~14 D at 8 y, ~8 D at 40 y, ~2 D at 50 y; near point recedes.

Colour vision: trichromatic (Young–Helmholtz) plus opponent-process (Hering) stages; anomalous trichromacy = shifted cone pigment.

Cornea layers (outside-in): epithelium, Bowman's layer, stroma, Descemet's membrane, endothelium; central thickness ~0.52 mm, peripheral ~0.65 mm.

Retinal layers (inner to outer): nerve fibre, ganglion cell, inner plexiform, inner nuclear, outer plexiform, outer nuclear, external limiting membrane, photoreceptor, RPE.

Optic nerve axons ≈ 1.0–1.2 million; exit at lamina cribrosa; partial decussation at chiasm - nasal fibres cross.

Post-chiasmal pathway: optic tract → LGN (6 layers) → optic radiations (Meyer's loop temporal, Baumgarten parietal) → V1 at occipital pole.

Pupillary light reflex: afferent = optic nerve, efferent = CN III parasympathetic via ciliary ganglion; near reflex adds accommodation and convergence.

Extraocular muscles: 4 recti + 2 obliques; all CN III except superior oblique (CN IV) and lateral rectus (CN VI).

Tear film: mucin (goblet cells), aqueous (lacrimal gland, main + accessory Krause/Wolfring), lipid (meibomian glands).

Ciliary body: pars plicata secretes aqueous (active Na⁺/K⁺/Cl⁻ transport, carbonic anhydrase); trabecular meshwork → Schlemm's canal drains ~90%.

Fovea: avascular, cone-only, no rods; foveola ~0.35 mm; highest ganglion cell density at parafovea.

Aqueous humour: production ~2–2.5 µL/min; IOP = (F_outflow resistance × formation) + episcleral venous pressure (Goldmann equation).

Normal IOP 10–21 mmHg; diurnal variation typically 3–5 mmHg, highest in early morning.

Corneal transparency depends on stromal collagen lattice regularity and relative deturgescence maintained by endothelial Na⁺/K⁺-ATPase pump.

Corneal endothelium: ~2500–3000 cells/mm² in youth; lost ~0.5%/year; cannot regenerate - cell spread (polymegethism, pleomorphism).

Phototransduction: light isomerises 11-cis retinal to all-trans → rhodopsin → transducin → PDE6 → cGMP falls → CNG channels close → hyperpolarisation.

RPE regenerates 11-cis retinal (retinoid cycle) and phagocytoses outer segment discs daily.

Lens: highest protein concentration in body (~35%); crystallins; anaerobic glycolysis supplies most lens ATP; glutathione defends against oxidative damage.

Tear film osmolarity >308 mOsm/L supports dry eye diagnosis; fluorescein staining, TBUT <10 s abnormal.

Retinal metabolism: dual supply - choroid (outer retina, high flow) and central retinal artery (inner retina).

Photoreceptors: rods ~120 million, cones ~6 million; cone subtypes S (~420 nm), M (~530 nm), L (~560 nm).

Rod outer segment disc turnover ~10–14 days; discs are not continuous with plasma membrane.

Bipolar cells: ON-centre depolarise to light (mGluR6, sign-inverting), OFF-centre hyperpolarise (ionotropic glutamate).

Horizontal cells mediate lateral inhibition; amacrine cells mediate temporal/contrast processing; ~30+ amacrine types.

Ganglion cell classes: midget (P, parvocellular, high acuity/colour), parasol (M, magnocellular, motion), small bistratified (S-cone/koniocellular).

Receptive field centre–surround antagonism produces contrast enhancement and Mach bands.

Retinal ganglion cell axons are unmyelinated within the retina and myelinated after the lamina cribrosa.

Primary visual cortex (V1): orientation-selective simple and complex cells (Hubel & Wiesel), ocular dominance columns, retinotopic map.

Dorsal ('where/how') stream = motion/spatial; ventral ('what') stream = form/object/colour recognition.

Ductions (monocular), versions (conjugate), vergences (disjunctive); Sherrington's law of reciprocal innervation, Hering's law of equal innervation.

Yoke muscle pairs: LR/SR, MR/IO, SR/IO (for vertical), SO/IR, and corresponding contralateral muscles.

Cover test: cover-uncover detects manifest (tropia); alternate cover reveals total deviation; prism neutralisation quantifies.

Near point of convergence normal ≤6 cm; AC/A ratio normal 4–6 Δ/D (heterophoria method or gradient method).

Stereopsis tests: Titmus (contour, 40–800 arcsec), Randot, TNO (random dot, truly binocular), Lang.

Amblyopia types: strabismic, anisometropic, deprivation; critical period ~first 7–8 years; penalisation and occlusion therapy.

Suppression, ARC (anomalous retinal correspondence), and eccentric fixation are sensory adaptations to strabismus.

Nystagmus: jerk (latent, manifest latent, gaze-evoked), pendular; optokinetic response tests pursuit pathways.

Parks–Bielschowsky three-step test localises vertical muscle palsy: head tilt, laterality, version.

Mydriatics/cycloplegics: tropicamide (short, ~4–6 h), cyclopentolate (~24 h), atropine (7–14 days), phenylephrine (α₁, mydriasis only, no cycloplegia).

Topical anaesthetics: proparacaine, oxybuprocaine, tetracaine - block voltage-gated Na⁺ channels; corneal toxicity with chronic use.

Anti-glaucoma: prostaglandin analogues (latanoprost - uveoscleral outflow), β-blockers (timolol - avoid in asthma/heart block), α₂-agonists (brimonidine), CAIs (dorzolamide), pilocarpine (M3).

Fluorescein angiography: sodium fluorescein excited ~465–490 nm, emits ~520–530 nm; contraindicated in known severe allergy.

Steroid side effects: raised IOP (steroid responder), cataract, delayed healing, infection risk.

Cholinergic crisis (organophosphate) - miosis, salivation, bradycardia; atropine + pralidoxime.

Methanol toxicity → formate accumulation → optic neuropathy and metabolic acidosis; treat with ethanol/fomepizole.

Preservatives: benzalkonium chloride causes corneal epithelial toxicity and punctal stenosis with chronic use.

Drug-induced: hydroxychloroquine (bull's eye maculopathy), ethambutol (optic neuritis), amiodarone (corneal verticillata).

Normal conjunctival flora: coagulase-negative Staphylococcus, Corynebacterium, Propionibacterium; low virulent load.

Bacterial keratitis: Pseudomonas (contact lens), Staphylococcus aureus, Streptococcus pneumoniae (ulcer serpens), Moraxella.

Herpes simplex keratitis: dendritic ulcer with terminal bulbs; rose bengal/fluorescein stains; acyclovir/trifluridine.

Acanthamoeba keratitis: contact lens + tap water; ring infiltrate, radial keratoneuritis; culture on non-nutrient agar with E. coli.

Fungal keratitis: Fusarium, Aspergillus, Candida; feathery margins, satellite lesions; KOH prep, culture on Sabouraud.

Endophthalmitis: post-cataract most commonly Staphylococcus epidermidis; intravitreal antibiotics (vancomycin + ceftazidime).

Immune privilege: anterior chamber-associated immune deviation (ACAID); blood–retinal barrier, absence of lymphatic drainage.

Hypersensitivity types: I (IgE, allergic conjunctivitis), II (antibody-mediated), III (immune complex, Arthus), IV (delayed, e.g., phlyctenular).

HLA associations: HLA-B27 (anterior uveitis), HLA-B51 (Behçet), HLA-DR4 (Vogt–Koyanagi–Harada).

Cataract: most common cause of reversible blindness; age-related nuclear/cortical/posterior subcapsular; oxidative stress and crystallin aggregation.

Glaucoma: open-angle (trabecular outflow) vs angle-closure; optic disc cupping, RNFL loss, not just IOP.

Diabetic retinopathy: microaneurysms, dot-blot haemorrhages, hard exudates (NPDR), neovascularisation (PDR); VEGF-driven.

AMD: dry (drusen, geographic atrophy) vs wet (CNV); complement factor H (CFH) polymorphism is a major risk locus.

Retinitis pigmentosa: rod-cone dystrophy; RPGR, RHO, USH2A mutations; bone-spicule pigment, ring scotoma, ERG reduction.

Leber hereditary optic neuropathy: mitochondrial DNA mutations (m.11778G>A most common); maternal inheritance, painless bilateral loss.

Retinoblastoma: RB1 tumour suppressor (chromosome 13q14); two-hit hypothesis; leucocoria, calcification on imaging.

Marfan syndrome: FBN1 mutation; ectopia lentis, aortic root dilatation; autosomal dominant.

Albinism: OCA types (TYR mutation in OCA1); foveal hypoplasia, misrouting of optic nerve fibres (abnormal VEP).

Visual acuity: Snellen (6 m), logMAR (each line 0.1 log unit, 5 letters), ETDRS gold standard; pinhole reduces refractive blur.

Visual field: Humphrey 24-2/30-2, 10-2 for macula; glaucoma pattern = nasal step, arcuate scotoma, paracentral defects.

OCT: retinal nerve fibre layer and ganglion cell layer thinning in glaucoma; macular oedema and CNV detection.

Fluorescein angiography: hypofluorescence (blockage/non-perfusion) vs hyperfluorescence (leakage, staining, pooling, window defect).

Tonometry: Goldmann applanation (gold standard, calibrated 1.5 mm diameter), non-contact, rebound; central corneal thickness affects reading.

Gonioscopy: Shaffer grading (0–4) or Spaeth; angle closure when grade ≤2 or 3 mirror view.

ERG: a-wave (photoreceptors), b-wave (bipolar/Müller); pattern ERG and multifocal ERG for localised function.

Pupil testing: relative afferent pupillary defect (Marcus Gunn) in optic nerve disease; swinging flashlight test.

Colour vision: Ishihara (screening), Farnsworth–Munsell 100 hue (discrimination), anomaloscope (gold standard for red-green).