O
object 1. Something that has a fixed shape or form that you can touch or see. 2. Anything from which an image is formed by an optical system.
extended o. An object consisting of many point objects separated laterally to form a certain shape (e.g. trees, people).
See light, beam of; light, pencil of; source, extended.
point o. A small component of an extended object, in relation to an optical system. If the point object is situated on the axis of an optical system it gives rise to the axial ray and it is referred to as the axial point object.
real o. An object from which emergent rays diverge.
o. of regard See fixation, point of.
virtual o. One towards which incident rays are converging after refraction or reflection. Example: a positive lens forms an image of an object placed beyond its anterior focal point. Introducing a mirror between the lens and the image makes that image become a virtual object.
objective An optical system or a lens used to provide a real image of an object. In cameras this image is situated on the film but in viewing instruments (telescopes, microscopes, etc.) this image is seen through an eyepiece. Syn. objective lens.
See aperture, numerical.
oblique astigmatism; effect See under the nouns.
oblique illumination shadow test See test, shadow.
oblique muscles See muscle, superior oblique; muscle, inferior oblique.
obliquely crossed cylinders See crossed cylinders, obliquely.
occipital cortex; lobe See under the nouns.
occluder A device placed before an eye to block vision or to partially obscure vision. Syn. eye shield.
occluder, Bangerter See Bangerter foils.
occlusion The act of blocking or the state of being blocked. Examples: vision with an occluder, a vessel with an embolus.
o. amblyopia See amblyopia; occlusion treatment.
o nystagmus See nystagmus, occlusion.
punctal o. Sealing of the lacrimal punctum, temporarily (e.g. with a plastic plug) or permanently (e.g. by heat cauterization), to preserve the natural tears or prolong the effect of artificial tears. This method is commonly used in the management of keratoconjunctivitis sicca. Occasionally a plug made of collagen is used prior to insertion of a more permanent type of punctal plug, because it dissolves within a week. This is done to determine whether permanent or semipermanent occlusion (as with a silicone plug) is likely to succeed.
See keratopathy, neurotrophic.
retinal arterial o. See retinal arterial occlusion.
retinal vein o. See retinal vein occlusion.
o. treatment A method of treating amblyopia or strabismus by covering the good eye. Such a method is most effective below the age of four years and with little effect after the age of nine years, that is beyond the critical period of development. However, this technique must be used with caution as prolonged occlusion in very young children can lead to a reversal of eye dominance in which the previously good eye becomes amblyopic (called occlusion amblyopia). Moreover, it has been shown that the effect of occlusion does not improve beyond six hours at a time. Alternate occlusion is preferred as both eyes are thus stimulated.
Syn. patching
See myopia, form-deprivation; penalization; period, critical; pleoptics.
octave The interval between two frequencies having a ratio of two to one. Example: from 4 to 8 c/deg. Two octaves is a quadrupling of frequencies, and so on. Octaves are commonly used in specifying the bandwidth of the frequencies (e.g. spatial frequencies) to which cells in the visual pathway respond.
See cycle per degree.
ocular 1. See eyepiece. 2. Appertains to eye.
ocular adnexa See appendages of the eye.
ocular albinism; appendages; apraxia; bobbing; column; cup; decongestant; dominance; dysmetria; flutter; fundus; headache; hypertension; hypotonia; impression; media; myoclonus; pathology; pemphigoid; prosthesis See under the nouns.
ocular lubricant See tears, artificial.
ocular myopathy See ophthalmoplegia.
ocular refraction See refractive error.
ocular tension See pressure, intraocular.
ocular tremors of fixation See movements, fixation.
ocularist One who designs and fits artificial eyes. See eye, artificial; prosthesis, ocular.
oculist See ophthalmologist.
oculoauriculovertebral dysplasia See syndrome, Goldenhar’s.
oculocardiac reflex See reflex, oculocardiac.
oculocentre Pertaining to the eye as a centre of reference.
See direction, oculocentric; egocentre; localization.
oculocentric direction; localization See under the nouns.
oculocutaneous albinism See albinism.
oculocutaneous melanosis; melanocytosis See naevus of Ota.
oculogyric Pertaining to movement of the eye about the anteroposterior axis.
oculogyric crisis (OCG) Sudden involuntary contractions of some eye muscles resulting in repetitive, conjugate ocular deviations, usually, though not always, in an upward direction. The attack or crisis may last from seconds to minutes. It occurs most frequently after the use of neuroleptic medication, but it may be precipitated or accompany, emotional stress, alcohol or general fatigue.
oculomotor Pertaining to movement of the eyes, or to the oculomotor nerve.
oculomotor nerve; nucleus See under the nouns.
oculomotor paralysis See paralysis of the third nerve.
oculomycosis Any disease of the eye caused by a fungus.
See antifungal agent; keratitis; keratomycosis.
oculorotary muscles See muscles, extraocular.
oculus Latin for eye. Plural: oculi.
oculus dexter (OD) Latin for right eye.
oculus sinister (OS) Latin for left eye.
oculus uterque (OU) Latin for both eyes.
oedema The presence of an excessive amount of fluid in or around cells, tissues or serous cavities of the body. In the eye oedema can occur in the cornea, the conjunctiva, the uvea, the retina, the choroid, and the ciliary body. Corneal oedema usually accompanies eye diseases, or contact lens wear with low oxygen transmissibility. Corneal oedema is easily seen with a slit-lamp using retroillumination or sclerotic scatter illumination. Quantitatively, it can be assessed with the addition of a pachometer that measures corneal swelling. Beyond about 4% swelling, there appear striae (wispy greyish-white lines usually vertical) in the stroma. Beyond about 8% swelling, there appear folds (dark lines) believed to represent physical buckling of the posterior corneal layers. Corneal swelling of 15% or greater, which indicates a gross separation of the collagen fibres of the stroma, results in a hazy or cloudy appearance of the cornea. There is a physiological oedema occurring during sleep in every human cornea amounting to an increase in thickness of about 4%. Corneal oedema gives rise to the appearance of haloes around lights, photophobia, spectacle blur, losses in corneal transparency and sometimes stinging. Management depends on the cause and tissue involved. If due to contact lenses, refitting with daily wear lenses of higher oxygen transmissibility and reducing wearing time usually solves the problem. Note: also spelt edema.
See blebs, endothelial; clouding, central corneal; hypoxia; lens, silicone hydrogel; oxygen permeability; oxygen requirement, critical; pachometer; syndrome, overwear.
oedema, cystoid macular (CMO) Oedema and cyst formation of the macular area of the retina. It may occur as a result of, or be associated with, systemic vascular disease, retinal vein occlusion, diabetic retinopathy, uveitis, hypertensive retinopathy, retinitis pigmentosa and following some ocular surgery such as vitreoretinal, photocoagulation, glaucoma procedures and especially cataract surgery. When cystoid macular oedema follows cataract surgery it is called the Irvine–Gass syndrome and it is sometimes accompanied by intraoperative vitreous loss or vitreous adhesion to the iris or to the corneoscleral wound. Visual acuity is affected initially but recovers in the majority of cases. In some cases antiinflammatory therapy may help in restoring visual acuity and in other cases the vitreous adhesion may be disrupted with a Nd-Yag laser.
ofloxacin See antibiotic.
Oguchi’s disease See disease, Oguchi’s.
olopatadine hydrochloride See antihistamine; mast cell stabilizers.
ommatidium One of the visual elements of the compound eye of arthropods. It is hexagonal in shape and about ten times longer than its diameter. It consists of a corneal facet below which is a crystalline cone which collects light and a sensory area called the rhabdom, all of it being enclosed in a dark pigment.
onchocerciasis A disease caused by infestation with the filarial worm (Onchocerca volvulus) spread by blackflies. It is common in tropical Africa and Central America, especially in areas near rivers. Large numbers of microfilariae are present on the skin and often enter the eye. The patient initially complains of itching, but blindness occurs as a result of chorioretinitis and optic neuritis. The disease is treated successfully with ivermectin. Syn. onchocercosis; river blindness.
‘one and one half’ syndrome See syndrome, ‘one and one half’.
opacity The condition of a tissue or structure which is not transparent, or being opaque. The location of an opacity within the eye can be determined with a slit-lamp. It can also be determined using an ophthalmoscope and asking the patient to look up or down. If the opacity moves very little, or not at all, it is situated in the lens. If the opacity moves in the same direction as the eye it is situated in front of the lens and if it moves in the opposite direction it is situated in the vitreous humour.
See cataract; muscae volitantes; ulcer, corneal.
opaque Impervious to the passage of light. See transparent.
operculum A flap of detached retina which projects forward, or is totally free in the vitreous. It can happen as a result of a retinal tear (break).
ophryosis Spasmodic twitching in the region of the eyebrow.
See eyebrow.
ophthalmagra A sudden pain in the eye.
ophthalmia Severe inflammation of the eye, especially, but not exclusively, one involving the conjunctiva.
See conjunctivitis.
o. neonatorum An acute conjunctivitis that occurs in the first month of life as a result of infection acquired in the birth canal. The most common causes are Chlamydia trachomatis, Streptococcus pneumoniae, Neisseria gonorrhoeae, Staphylococcus aureus and herpes simplex virus. The eyelids are swollen and stuck together by purulent discharge. If the cause is gonococcal, loss of the eye is a real and immediate threat. A gonococcal infection develops within 2–4 days after birth, whereas a chlamydial infection normally appears 5–14 days after birth. Differential diagnosis is facilitated by laboratory tests (e.g. Gram staining of conjunctival scrapings). Management depends on the cause: systemic erythromycin and topical tetracycline for chlamydial infection, ceftriaxone or cefotaxime for gonococcal infection, and eye irrigation with saline solution. Syn. blennorrhoea neonatorum; gonococcal ophthalmia; neonatal conjunctivitis.
See conjunctivitis, acute; conjunctivitis, adult inclusion.
sympathetic o. A rare, bilateral granulomatous inflammation of the uveal tract that usually follows perforation of one eye due to trauma, or more rarely intraocular surgery. The inflammation occurs first in the injured eye (called the exciting eye) and follows in the other eye (called the sympathetic eye). It usually occurs within 2 to 12 weeks, although some cases may appear later. The condition is believed to be a T-lymphocyte-mediated delayed hypersensitivity. Treatment usually involves enucleation of the exciting eye and high doses of systemic and topical corticosteroids in the sympathetic eye. Syn. sympathetic ophthalmitis.
See enucleation; immunosuppressants; uveitis.
ophthalmic Pertaining to the visual apparatus and its function.
ophthalmic crown See glass, crown.
ophthalmic cup See cup, optic.
ophthalmic Graves’ disease See disease, Graves’.
ophthalmic lens; nerve See under the nouns.
ophthalmic optician See optician; optometrist.
ophthalmic optics See optics, ophthalmic.
ophthalmic zoster See herpes zoster ophthalmicus.
ophthalmitis, sympathetic See ophthalmia, sympathetic.
ophthalmodynamometer (ODM) 1. Instrument for measuring the near point of convergence of the eyes. 2. Instrument for measuring the blood pressure of the central retinal artery. There are two types: the compression type (e.g. Bailliart’s ophthalmodynamometer) in which the pressure is raised by pressing on the eye, the force being produced by a spring-loaded plunger resting on the temporal bulbar conjunctiva of the anaesthetized eye, while the examiner observes the optic nerve through an ophthalmoscope. The other type is by suction in which negative pressure is applied to the eye using a scleral vacuum cup near the limbus (e.g. Doppler’s ophthalmodynamometer). The diastolic pressure is read from the gauge provided with the instrument when the central retinal artery is seen to pulsate on the optic disc and the systolic pressure is read when all arterial pulsations just cease (the instrument should be removed immediately afterwards). A low systolic pressure is indicative of an occlusive disease of the carotid artery (a comparison between the two eyes is also very informative) as such disorders are responsible for a significant percentage of ocular symptoms and strokes.
See amaurosis fugax; plaques, Hollenhorst’s.
ophthalmologist A medical specialist who practises ophthalmology. Syn. oculist (this term is rarely used nowadays); ophthalmic surgeon.
ophthalmology Part of medical science concerned with the medical and surgical care of the eye and its appendages.
ophthalmometer See keratometer.
ophthalmopathy Any eye disease. External ophthalmopathy refers to any disease of the conjunctiva, cornea, eyelids or the appendages of the eye. Internal ophthalmopathy refers to any disease of the lens, retina or other internal structures of the eye.
ophthalmopathy, thyroid Disease of the thyroid gland which leads to ocular manifestations. There are two main types: mild and severe. The mild type occurs in Graves’ disease in which most or some of the typical signs may be present and to a different extent (e.g. retraction of the eyelids, exophthalmos, defective eye movements and optic neuropathy). The severe type is much less common and affects the sexes equally in middle age. All the signs of Graves’ disease are present but are more pronounced with the addition of oedema of the eyelids and of the conjunctiva, conjunctival injection, enlargement of the extraocular muscles and in a few cases there is also optic neuropathy due to compression of the optic nerve or its blood supply with consequent visual loss, colour vision impairment and often diplopia. Syn. dysthyroid eye disease; thyroid eye disease.
See accommodative insufficiency; keratoconjunctivitis, superior limbic; neuropathy, optic.
ophthalmophakometer Optical instrument designed by Tscherning in 1892.
See phakometer.
ophthalmoplegia Paralysis of the ocular muscles. External ophthalmoplegia refers to paralysis of one or more extraocular muscles. If the levator palpebrae muscle is also involved, the condition is usually referred to as ocular myopathy. Internal ophthalmoplegia refers to a paralysis of the muscles of the iris and the ciliary muscle. Total ophthalmoplegia refers to a paralysis of all the muscles in the eye, which results in ptosis, immobility of the eye and pupil, and loss of accommodation.
See disease, Graves’; paralysis of the third nerve.
chronic progressive external o. (CPEO) A rare disorder characterized by a progressive, bilateral ptosis and a loss of ocular motility. It is associated with mutation of mitochondrial DNA, which results in abnormalities in highly oxidative tissues such as the muscles and the brain. The initial sign is bilateral ptosis, followed later by strabismus. A related mitochondrial myopathy is the Kearns–Sayre syndrome in which there is CPEO, a type of retinitis pigmentosa characterized by coarse pigment clumping which principally affects the central retina, and cardiac conduction defects. This syndrome presents before age 20 years.
See myasthenia gravis.
internuclear o. (INO) An eye movement disorder resulting from a lesion in the medial longitudinal fasciculus, which disrupts the coordination between the oculomotor nucleus and the abducens nucleus. It is characterized by a limited adduction by the eye on the same side of the body as the lesion, and a jerky, horizontal nystagmus and overshoot by the other eye on abduction, when moving the eyes towards the side of the body opposite to that of the lesion. Convergence is usually intact, unless the lesion is widespread. Vertical gaze gives rise to nystagmus and oscillopsia. The condition is associated with multiple sclerosis, vascular disease, tumour of the brainstem or encephalitis.
See dysmetria, ocular; palsy, supranuclear gaze; syndrome, ‘one and one half’.
ophthalmorrhagia Ocular haemorrhage.
ophthalmorrhoea A discharge of mucus, pus or blood from the eye. Note: also spelt ophthalmorrhea.
ophthalmoscope
