EBOD flashcards
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32 cards. Read the term and the definition here. The deck at the top of this page is optional.
- Snell's law
- The law governing refraction at an interface: n1 sin θ1 = n2 sin θ2, where n is refractive index and θ is the angle to the surface normal. Light bends toward the normal when entering a denser medium.
- Prentice rule
- Prism power in prism dioptres equals the decentration in centimetres multiplied by the lens power in dioptres (P = c × F). Used to calculate induced prism from spectacle decentration.
- Purkinje images
- Four reflections from ocular surfaces: images I and II arise from the anterior and posterior cornea, while images III and IV arise from the anterior and posterior lens.
- Hyperopic LASIK profile
- Hyperopic excimer laser treatment ablates peripherally, steepening the central cornea to increase plus power. Myopic treatment does the opposite, flattening the centre to reduce myopia.
- Keratometric power conversion
- Corneal dioptric power is estimated as 337.5 divided by the corneal radius in millimetres. This assumes a standard keratometric refractive index of 1.3375.
- Spherical equivalent
- The spherical equivalent equals the spherical power plus half the cylindrical power. It represents the single spherical lens that places the circle of least confusion on the retina.
- Dendritic corneal ulcer
- A branching epithelial lesion with terminal bulbs seen with fluorescein, typical of herpes simplex keratitis. Topical steroids alone are contraindicated because they promote stromal necrosis.
- Keratoconus slit lamp signs
- Inferior corneal steepening, Fleischer iron ring, Vogt striae that disappear on pressure, apical thinning and Munson sign of lid distortion on downgaze. Corneal topography confirms the diagnosis.
- Acanthamoeba keratitis
- A painful contact-lens-associated keratitis with ring infiltrate and radial keratoneuritis. Caused by Acanthamoeba cysts; diagnosis by confocal microscopy or culture; treated with biguanide and diamidine drops.
- Posterior capsular opacification
- Opacification of residual lens epithelial cells on the posterior capsule months to years after cataract surgery, causing reduced vision and glare. Treated with neodymium:YAG laser capsulotomy.
- Marfan syndrome ectopia lentis
- Zonular weakness causes superotemporal lens subluxation in Marfan syndrome, usually bilateral. Homocystinuria instead causes inferonasal subluxation and carries a high risk of thromboembolism.
- Post-cataract endophthalmitis organism
- Coagulase-negative Staphylococcus epidermidis is the commonest cause of acute endophthalmitis after cataract surgery, presenting with pain, hypopyon and vitritis within days of the operation.
- Acute angle-closure glaucoma
- Sudden rise in intraocular pressure with a mid-dilated fixed pupil, corneal oedema, halos, headache and nausea. Treated with topical and systemic pressure-lowering drugs and laser peripheral iridotomy.
- Prostaglandin analogue action
- Topical prostaglandins such as latanoprost increase aqueous outflow mainly through the uveoscleral pathway. Side effects include iris darkening, eyelash lengthening, periorbital fat atrophy and conjunctival hyperaemia.
- ISNT rule
- In a normal optic disc the neuroretinal rim is thickest inferiorly, then superiorly, then nasally, then temporally. Violation of this order suggests glaucomatous rim loss.
- Cherry-red spot
- A pale oedematous retina with a red fovea, seen in central retinal artery occlusion because the thin fovea shows the underlying choroidal colour. Also occurs in storage diseases such as Tay-Sachs.
- Rhegmatogenous retinal detachment
- Separation of neurosensory retina from retinal pigment epithelium caused by a full-thickness retinal break allowing vitreous fluid to enter the subretinal space. Risk factors include myopia, lattice degeneration and pseudophakia.
- Central serous chorioretinopathy
- Serous detachment of the macula with a focal leakage point on fluorescein angiography, typically in young to middle-aged men under stress or using corticosteroids. Often resolves spontaneously.
- Mutton-fat keratic precipitates
- Large greasy keratic precipitates on the corneal endothelium associated with granulomatous uveitis such as sarcoidosis, tuberculosis or sympathetic ophthalmia. They indicate a chronic granulomatous inflammatory process.
- Sympathetic ophthalmia
- Bilateral diffuse granulomatous uveitis following penetrating trauma or surgery to one eye, thought to be immune-mediated against uveal antigens. Prevented by early enucleation of a blind injured eye.
- Relative afferent pupillary defect
- A reduced direct pupillary response when light swings from the normal eye to the affected eye, indicating optic nerve or extensive retinal dysfunction. It is absent in media opacities and cortical blindness.
- Papilloedema
- Bilateral optic disc swelling from raised intracranial pressure, with preserved visual acuity and enlarged blind spots initially. It is a sign rather than a diagnosis and requires neuroimaging.
- Horner syndrome
- A triad of mild ptosis, miosis and anhidrosis from sympathetic denervation. Anhidrosis suggests a preganglionic lesion; otherwise a postganglionic cause such as internal carotid dissection is likely.
- Amblyopia treatment
- Occlusion of the better eye by patching, or penalisation with atropine, forces use of the amblyopic eye. Best results occur before about seven to eight years of age.
- Leukocoria differential
- A white pupillary reflex suggests retinoblastoma, congenital cataract, toxocariasis, retinopathy of prematurity or persistent fetal vasculature. Retinoblastoma must be excluded urgently with fundoscopy and imaging.
- Orbital blowout fracture
- Fracture of the orbital floor causing restricted upgaze, diplopia, enophthalmos and infraorbital nerve numbness. Coronal CT confirms the bony defect and any muscle entrapment.
- Dacryocystitis
- Infection of the lacrimal sac presenting with a painful red swelling below the medial canthus, tearing and discharge. Caused by nasolacrimal duct obstruction; treated with antibiotics and later dacryocystorhinostomy.
- Uveal melanoma
- The commonest primary intraocular malignancy in adults, arising from choroidal melanocytes. Features include a dome or mushroom-shaped pigmented mass with subretinal fluid; treated with plaque brachytherapy or enucleation.
- Sebaceous carcinoma
- A malignant eyelid tumour arising from meibomian glands, commoner in the upper lid of older patients, with pagetoid spread. It can masquerade as a chalazion or chronic blepharoconjunctivitis.
- Trachoma
- Chronic conjunctival infection by Chlamydia trachomatis serovars A to C causing follicles, scarring, trichiasis and corneal opacification. It is the leading infectious cause of blindness and is treated with azithromycin.
- Bitot spots
- Foamy grey conjunctival patches over the temporal sclera caused by keratinised epithelium in vitamin A deficiency. They are associated with night blindness and other features of xerophthalmia.
- WHO blindness threshold
- The World Health Organization defines blindness as best-corrected visual acuity worse than 3/60, or a visual field less than ten degrees, in the better eye.