EBOD cheat sheet

Snell's law: n1 sin θ1 = n2 sin θ2; light bends toward the surface normal when entering a denser medium.

Thin-lens power F = 1/f (f in metres); two lenses separated by distance d combine as F = F1 + F2 − d·F1·F2.

Vergence L = n/l; across a refracting surface L' = L + F, with reduced vergence used inside the eye.

One prism dioptre deviates a ray by 1 cm at 1 m; Prentice rule: P (Δ) = c (cm) × F (D).

Four Purkinje images exist: I and II from the anterior and posterior cornea, III and IV from the anterior and posterior lens.

Rayleigh resolution limit: θ = 1.22 λ/D radians; a smaller pupil aperture degrades resolution by diffraction.

Longitudinal chromatic aberration: blue light is refracted more and focuses anterior to red light.

Positive spherical aberration makes peripheral rays focus anterior to paraxial rays; aspheric intraocular lenses are designed to offset corneal spherical aberration.

Myopia: parallel rays focus anterior to the retina at rest and are corrected with minus (concave) lenses.

Hyperopia: parallel rays would focus behind the retina and are corrected with plus (convex) lenses; young patients accommodate to compensate.

Corneal dioptric power is approximated as 337.5 divided by the corneal radius in millimetres.

Myopic LASIK/PRK ablates the central cornea to flatten it; hyperopic treatment ablates peripherally to steepen the centre.

LASIK raises a stromal flap typically 100–160 µm thick; PRK removes the epithelium and ablates superficial stroma without a flap.

With-the-rule astigmatism has the steeper meridian near 90°; against-the-rule astigmatism has the steeper meridian near 180°.

Spherical equivalent = spherical power + ½ cylindrical power, corresponding to the circle of least confusion on the retina.

Anisometropia and high uncorrected astigmatism in children are risk factors for amblyopia.

Post-refractive-surgery intraocular lens power uses adjusted methods (Haigis-L, Barrett True-K) because standard keratometry is unreliable.

Corneal layers: epithelium, Bowman layer, stroma, Descemet membrane and endothelium.

Endothelial cells do not regenerate; normal density is about 2500 cells/mm², and loss below roughly 500–1000 cells/mm² causes corneal oedema.

Bacterial keratitis: contact-lens-related Pseudomonas is common; central stromal infiltrate, overlying epithelial defect and hypopyon.

Fungal keratitis: feathery infiltrate margins, satellite lesions and endothelial plaques, often after vegetable-matter injury.

HSV epithelial keratitis shows a dendritic ulcer with terminal bulbs; topical steroid alone is contraindicated.

Keratoconus signs: inferior steepening, Fleischer iron ring, Vogt striae that disappear on pressure, apical thinning and Munson sign.

Acanthamoeba keratitis: contact lens use, severe pain out of proportion, ring infiltrate and radial keratoneuritis.

Dry eye disease: tear film breakup time under 10 seconds, reduced Schirmer test and raised tear osmolarity.

Corneal abrasion produces an epithelial defect that stains green with fluorescein and is viewed with cobalt blue light.

Nuclear sclerotic cataract increases refractive index and can cause a myopic shift ('second sight'); posterior subcapsular cataract causes disproportionate glare and near-vision loss.

Phacoemulsification uses a continuous curvilinear capsulorrhexis of about 5–5.5 mm with a foldable intraocular lens placed in the capsular bag.

The average phakic intraocular lens power is approximately +20 D.

Acute post-cataract endophthalmitis is most often due to coagulase-negative Staphylococcus epidermidis and presents within days with pain, hypopyon and vitritis.

Posterior capsular opacification from residual lens epithelial cells is treated with neodymium:YAG laser capsulotomy.

Marfan syndrome causes superotemporal ectopia lentis; homocystinuria causes inferonasal subluxation with thromboembolic risk.

Phacomorphic glaucoma arises from an intumescent lens closing the angle; phacolytic glaucoma from leakage of lens proteins obstructing the trabecular meshwork.

Congenital cataract causes leukocoria and requires exclusion of rubella and metabolic disease; dense unilateral cases are operated within the first weeks of life.

Normal intraocular pressure is approximately 10–21 mmHg; pressure is the only major modifiable risk factor.

Primary open-angle glaucoma shows optic disc cupping, neuroretinal rim loss, arcuate scotoma and nasal step on perimetry.

ISNT rule: normal rim thickness is greatest inferiorly, then superiorly, then nasally, then temporally; violation suggests glaucoma.

Acute angle closure presents with a mid-dilated fixed pupil, corneal oedema, haloes, headache and nausea; treatment is pilocarpine, timolol, apraclonidine, acetazolamide, osmotic agents and laser peripheral iridotomy.

Prostaglandin analogues lower pressure mainly by increasing uveoscleral outflow; side effects include iris darkening, eyelash lengthening and periorbital fat atrophy.

Topical beta-blockers are contraindicated in asthma and heart block because of systemic beta-adrenergic effects.

Pseudoexfoliation syndrome shows dandruff-like material on the lens capsule, weak zonules and marked pressure spikes.

Pigment dispersion syndrome occurs in young myopic patients with a Krukenberg spindle and iris transillumination defects.

Disc haemorrhage, thin central corneal thickness and family history are risk factors for glaucoma progression.

The fovea is avascular and nourished by the choroid; the inner retina is supplied by the central retinal artery.

Diabetic retinopathy: microaneurysms, dot-blot haemorrhages, hard exudates and cotton wool spots; proliferative disease is treated with panretinal photocoagulation.

Central retinal artery occlusion causes sudden painless vision loss with a pale retina and cherry-red spot at the fovea.

Central retinal vein occlusion gives a 'blood-and-thunder' fundus with disc oedema, venous tortuosity and macular oedema.

Wet age-related macular degeneration is driven by choroidal neovascularisation and treated with intravitreal anti-VEGF agents.

Rhegmatogenous retinal detachment requires a full-thickness break plus vitreous liquefaction; symptoms are flashes, floaters and a curtain.

Cotton wool spots are infarcts of the retinal nerve fibre layer.

Central serous chorioretinopathy affects young to middle-aged men and is associated with corticosteroid use.

Retinitis pigmentosa shows bone-spicule pigmentation, vessel attenuation, ring scotoma and reduced electroretinogram.

Anterior uveitis produces cells and flare in the anterior chamber, ciliary injection and photophobia; HLA-B27 is a common association.

Hypopyon is characteristic of Behçet disease and HLA-B27-associated uveitis.

Posterior synechiae are prevented by cycloplegic mydriatic agents given with topical steroids.

Granulomatous uveitis shows mutton-fat keratic precipitates and iris nodules, seen in sarcoidosis, tuberculosis and sympathetic ophthalmia.

Ocular toxoplasmosis produces a focal retinochoroiditis adjacent to a scar, described as 'headlight in the fog'.

Sympathetic ophthalmia is bilateral granulomatous uveitis after penetrating trauma; risk is reduced by enucleation of a blind injured eye within about two weeks.

Masquerade syndromes include intraocular lymphoma and retinoblastoma, which mimic chronic uveitis.

Treatment uses topical steroids and cycloplegics for anterior disease, with systemic steroids or immunosuppression for posterior or severe inflammation.

A relative afferent pupillary defect indicates optic nerve or extensive retinal disease; it is absent in media opacity and cortical blindness.

Papilloedema is bilateral disc swelling from raised intracranial pressure, with enlarged blind spots and preserved acuity early.

Optic neuritis causes painful unilateral vision loss, reduced colour vision and the Uhthoff phenomenon; it is associated with multiple sclerosis.

Third nerve palsy gives ptosis and an eye that is down and out; pupil involvement suggests a compressive lesion such as a posterior communicating artery aneurysm.

Horner syndrome is ptosis, miosis and anhidrosis; a postganglionic lesion may indicate internal carotid artery dissection.

Anterior ischaemic optic neuropathy produces altitudinal field loss with sector disc oedema and a pale swollen disc.

Pituitary adenoma compresses the chiasm and causes bitemporal hemianopia.

Argyll Robertson pupils are small and irregular with light-near dissociation, classically from neurosyphilis.

Cortical blindness gives severe visual loss with normal fundi and no relative afferent pupillary defect.

Infantile esotropia presents before 6 months with a large constant angle; accommodative esotropia is linked to uncorrected hypermetropia.

Intermittent exotropia is typically worse for distance fixation and may follow a decompensation pattern.

Amblyopia is treated by patching or atropine penalisation of the better eye, with best outcomes before age 7–8 years.

The cover test distinguishes a manifest deviation (tropia) from a latent one (phoria); the Hirschberg test estimates angle from the corneal light reflex.

Strabismus surgery weakens an overacting muscle by recession and strengthens an underacting muscle by resection.

Retinoblastoma presents with leukocoria and intralesional calcification on imaging; the RB1 gene is implicated in heritable cases.

Retinopathy of prematurity screening applies to infants under about 32 weeks' gestation or under 1500 g birth weight; threshold disease is treated with laser.

Congenital glaucoma presents with buphthalmos, epiphora, photophobia and corneal oedema, and is treated surgically with goniotomy or trabeculotomy.

Duane retraction syndrome shows limited abduction with globe retraction and narrowing of the palpebral fissure on adduction.

Involutional ptosis is most often due to levator aponeurosis dehiscence; Horner syndrome causes a mild ptosis from Müller muscle weakness.

Dacryocystitis is lacrimal sac infection from nasolacrimal duct obstruction, producing a painful medial canthal swelling and epiphora; definitive treatment is dacryocystorhinostomy.

Orbital cellulitis causes proptosis, fever, pain on eye movement and restricted motility, with risk of cavernous sinus thrombosis; it requires imaging and intravenous antibiotics.

Thyroid eye disease features lid retraction, exophthalmos, restrictive myopathy (commonly inferior rectus) and dysthyroid optic neuropathy.

Orbital floor blowout fracture causes infraorbital numbness, restricted upgaze and enophthalmos; coronal CT confirms the defect.

Basal cell carcinoma is a pearly nodule with telangiectasia, most often on the lower lid, and rarely metastasises.

A chalazion is a lipogranuloma of a meibomian gland, whereas a hordeolum is an acute bacterial infection of lid glands.

Involutional entropion is treated by lid tightening; ectropion causes epiphora and corneal exposure.

Uveal melanoma is the commonest primary intraocular malignancy in adults; a collar-button shape and subretinal fluid suggest rupture through Bruch membrane.

Choroidal metastasis is the commonest intraocular malignancy overall, most often from breast or lung primary tumours.

Features favouring choroidal melanoma over naevus include thickness over 2 mm, subretinal fluid, symptoms and orange pigment.

Retinoblastoma presents with leukocoria and calcification; bilateral disease usually reflects a germline RB1 mutation.

Primary acquired melanosis with atypia can progress to conjunctival melanoma.

Ocular adnexal lymphoma may appear as a salmon-patch conjunctival infiltrate and requires biopsy for histological diagnosis.

Sebaceous carcinoma arises from meibomian glands, usually in the upper lid, shows pagetoid spread and can masquerade as a chalazion.

Iris melanoma is usually pigmented and may cause sector cataract, corectopia and angle seeding.

Haematoxylin and eosin is the standard stain; periodic acid–Schiff highlights basement membrane, Congo red stains amyloid and Ziehl–Neelsen identifies acid-fast bacilli.

Cataract is the leading cause of reversible blindness worldwide; glaucoma, diabetic retinopathy and age-related macular degeneration are other major contributors.

WHO defines blindness as best-corrected visual acuity worse than 3/60, or a visual field less than 10 degrees, in the better eye.

Trachoma is caused by Chlamydia trachomatis serovars A–C, causing follicular conjunctivitis, trichiasis and corneal opacification; the SAFE strategy includes azithromycin.

Vitamin A deficiency causes night blindness, Bitot spots and xerophthalmia, and is treated with vitamin A supplementation.

Onchocerciasis (river blindness) results from Onchocerca volvulus infection and is treated with ivermectin.

Systematic diabetic retinopathy screening reduces the incidence of sight-threatening retinopathy.

Snellen acuity of 6/6 is normal; LogMAR charts give more precise and reproducible measurement for research and paediatric use.

The Vision 2020 initiative targets avoidable causes of blindness through disease control and refractive error correction.