ICO Advanced key terms

Reduced best-corrected vision in one or both eyes caused by abnormal visual experience during early development, without a structural cause on examination. It is the leading cause of monocular visual loss in children and is treatable if detected early.

A condition in which the refractive error differs between the two eyes, typically by at least one dioptre. Unequal image clarity can suppress one eye and cause amblyopia, especially when the difference is present in early childhood.

A perceptual condition in which the images formed by the two eyes differ in size or shape. It can cause asthenopia, impaired binocular vision, or intolerance of anisometropic spectacle correction, and is relevant after refractive surgery or cataract surgery in one eye.

The absence of the crystalline lens from the eye, either congenital or following surgical removal without intraocular lens implantation. The eye loses accommodation and requires high-plus optical correction, with refractive and amblyopia management considerations in children.

The cortical integration of slightly disparate images from the two eyes into one percept with depth perception. It requires ocular alignment, clear retinal images, and normal visual pathway development, and is assessed with tests of fusion, stereopsis, and suppression.

A time window in early development during which the visual system is highly sensitive to abnormal visual experience. Disruption during this period, such as deprivation or misalignment, can produce permanent amblyopia if not corrected promptly.

Amblyopia caused by obstruction of the visual axis or an optical problem that degrades the retinal image, such as congenital cataract, ptosis, or corneal opacity. It is the most severe form and requires urgent treatment within the critical period.

A white pupillary reflex seen on examination, indicating an abnormality within the eye. Causes include congenital cataract, retinoblastoma, persistent fetal vasculature, Coats disease, and retinal detachment; it is an urgent red flag requiring prompt paediatric ophthalmic assessment.

The reddish-orange glow observed when light is shone into the eye and reflected from the fundus. It is a key screening test in newborns and children, and an absent, white, or asymmetric reflex signals media opacity, strabismus, or retinal pathology.

Misalignment of the eyes in which the visual axes do not point at the same object. It may be constant or intermittent, horizontal or vertical, and can cause amblyopia, suppression, and loss of binocular single vision if present during development.

A cortical adaptive mechanism in which the image from one eye is actively ignored to avoid diplopia or visual confusion. In children it can become entrenched and lead to amblyopia, and it is detected with tests such as the Worth four-dot or Bagolini striated lenses.

The perception of depth derived from horizontal disparity between the two retinal images. It is a sensitive indicator of binocular single vision and is measured clinically with tests such as the Titmus, TNO, or Randot stereoacuity tests.

A clinical test in which one eye is occluded while the other fixates, allowing observation of any movement on uncovering or alternate covering. It detects manifest and latent strabismus and is fundamental to the orthoptic assessment of ocular alignment.

An alignment test using a light held in front of the patient to compare the position of the corneal light reflex in each eye. Displacement of the reflex indicates strabismus and gives a rough estimate of the deviation, useful in uncooperative patients.

The perception of two images of a single object, which may be monocular or binocular. Binocular diplopia typically arises from misalignment of the eyes, and its pattern of separation helps localise the affected extraocular muscle or nerve.

The increase in refractive power of the eye achieved by contraction of the ciliary muscle, which relaxes the zonules and allows the crystalline lens to become more convex. It permits clear vision at near and declines with age, producing presbyopia.

The age-related loss of accommodative amplitude, typically becoming symptomatic in the fifth decade, causing difficulty with near vision. It is corrected with reading glasses, bifocals, progressive lenses, or multifocal contact lenses and surgical options.

A mismatch between the optical power of the eye and its axial length, so that light from a distant object does not focus on the retina. Myopia, hyperopia, and astigmatism are the principal types and are corrected with lenses, contact lenses, or surgery.

A refractive state in which parallel rays from a distant object focus in front of the retina, causing blurred distance vision that is corrected with minus lenses. Axial elongation underlies most cases, and high myopia carries risks of retinal detachment and myopic maculopathy.

A refractive state in which parallel rays from a distant object would focus behind the retina when accommodation is relaxed. Young patients may compensate through accommodation, but sustained effort can cause asthenopia or accommodative esotropia.

A refractive condition in which the eye has different powers in different meridians, so that light is not focused at a single point. It causes blurred or distorted vision and is corrected with cylindrical lenses or toric intraocular lenses.

A progressive non-inflammatory corneal ectasia in which the cornea thins and assumes a conical shape, producing irregular astigmatism and reduced vision. Management includes rigid contact lenses, corneal cross-linking, and keratoplasty in advanced cases.

A technique that maps the curvature and elevation of the corneal surface using reflected rings or scanning systems. It is essential for diagnosing keratoconus, planning refractive surgery, and fitting contact lenses in irregular corneas.

Measurement of corneal thickness, usually by ultrasound or optical methods. It is important in glaucoma risk assessment because thinner corneas affect intraocular pressure measurement, and in screening before refractive surgery.

The pressure exerted by the contents of the eye against the coats of the eye, determined mainly by aqueous humour production and outflow. Elevated pressure is a major risk factor for glaucomatous optic neuropathy, and measurement is affected by corneal thickness and technique.

A group of optic neuropathies characterised by progressive retinal ganglion cell loss and characteristic visual field defects, with raised intraocular pressure as the main modifiable risk factor. It is often asymptomatic until advanced and requires lifelong monitoring and treatment.

The area of space perceived by an eye when it is steadily fixating. Field testing detects and monitors defects from glaucoma, neurological lesions, and retinal disease, and the pattern of loss helps localise the pathology.

Damage to the optic nerve from any cause, producing reduced acuity, colour vision loss, a relative afferent pupillary defect, and field defects. Causes include ischaemia, inflammation, compression, and toxicity, and the pattern of loss aids diagnosis.

An abnormal pupillary response in which the swinging flashlight test shows greater constriction when light is moved from the affected eye to the normal eye. It indicates asymmetric afferent pathway disease, such as optic neuropathy or extensive retinal damage.

A bedside test in which light is alternated between the eyes to detect asymmetric afferent input. A pupil that dilates when illuminated indicates a relative afferent pupillary defect on that side and helps distinguish optic nerve from media or refractive causes of visual loss.

Examination of the eye using a variable slit of light and a microscope, allowing magnified assessment of the eyelids, conjunctiva, cornea, anterior chamber, iris, and lens. It is the core technique of anterior segment examination and is combined with lenses to view the fundus.

A technique using a mirrored or prismatic contact lens to visualise the anterior chamber angle, which cannot be seen directly because of total internal reflection. It is essential for classifying glaucoma and identifying angle closure or neovascularisation.

Examination of the retina, optic disc, and macula using an ophthalmoscope or slit lamp with condensing lenses. It detects diabetic retinopathy, hypertensive changes, optic disc swelling, and retinal detachment, and is a core skill in ophthalmic assessment.

A non-invasive imaging technique using low-coherence light to produce cross-sectional images of the retina and optic nerve head. It quantifies retinal layers and nerve fibre thickness and is central to managing macular disease and glaucoma.

Microvascular retinal disease caused by chronic hyperglycaemia, classified as non-proliferative or proliferative, with macular oedema as a major cause of vision loss. Screening, glycaemic control, laser, and anti-VEGF therapy are key management components.

Fluid accumulation within the macula causing thickening and central visual loss, seen in diabetic retinopathy, retinal vein occlusion, uveitis, and after surgery. It is detected by clinical examination and optical coherence tomography and treated with anti-VEGF agents or steroids.

Separation of the neurosensory retina from the retinal pigment epithelium, causing visual field loss and floaters or flashes when acute. Rhegmatogenous detachment from a retinal tear is the commonest type and requires urgent surgical repair to preserve vision.

Severe inflammation of the intraocular tissues, usually due to bacterial or fungal infection after surgery or trauma. It presents with pain, reduced vision, and hypopyon and is an ophthalmic emergency requiring urgent vitreous sampling and intravitreal antibiotics.

A layer of white blood cells settled in the inferior anterior chamber, indicating severe intraocular inflammation or infection. It is seen in endophthalmitis, severe uveitis, and Behçet disease and warrants urgent assessment.

Inflammation of the uveal tract, classified anatomically as anterior, intermediate, posterior, or panuveitis. It causes pain, photophobia, floaters, and visual loss and may be idiopathic, autoimmune, or infectious, requiring targeted workup and immunosuppressive or antimicrobial therapy.