N

naevus Any localized area of pigmentation or vascularization of the skin or eye tissues, usually benign and congenital. Note: also spelt nevus. Plural: naevi.

choroidal n. A benign accumulation of melanocytes in the choroid. It affects some 10% of the population. Ophthalmoscopically it appears as a slate-grey lesion, flat or minimally elevated, oval or circular. It is asymptomatic. With time drusen may also appear.

See melanocyte; melanoma, choroidal.

conjunctival n. A naevus located on the conjunctiva, most often near the limbus. It appears as a yellowish-red area or deeply pigmented mass usually before the age of 20. A pigmented conjunctival naevus must be distinguished from an acquired melanoma of the conjunctiva which occurs later in life (after the third decade, is typically unilateral and may become malignant). A conjunctival naevus rarely becomes malignant. It can be excised if cosmetically undesirable or has enlarged to such a degree as to irritate the eye.

flammeus n. See syndrome, Sturge–Weber.

iris n. A pigmented spot of variable size on the surface of the iris. It is composed of an accumulation of melanocytes in the iris stroma. It is usually benign but occasionally it may transform itself into a malignant melanoma.

See melanocyte; melanoma, iris; syndrome, ICE.

n. of Ota A benign, congenital, usually unilateral, accumulation of melanocytes on the cheek, eyelids, forehead, nose or sclera. Some naevi may become malignant melanoma. Syn. oculocutaneous melanosis; oculodermal melanocytosis.

See melanocytosis, ocular; melanosis.

Nagel anomaloscope See anomaloscope.

nanometre (nm) SI unit of length equal to one-millionth of a millimetre (or 10 ångströms or 10^–9 m). Syn. millimicron (obsolete).

See ångström; micrometre.

nanophthalmos See microphthalmia.

naphazoline hydrochloride A sympathomimetic vasoconstrictor, which may be used as a topical decongestant in 0.1% eyedrops. It causes slight mydriasis. It also comes as naphazoline nitrate.

See adrenaline (epinephrine); decongestant, ocular.

narrow-angle glaucoma See glaucoma, angle-closure.

nasal step See Roenne nasal step.

nasolacrimal duct See lacrimal apparatus.

nasotemporal overlap A vertical strip passing through the fovea and above and below it, within which retinal ganglion cells may send their axons either ipsilaterally or contralaterally. The width of this strip is less than 1° in the central retina and increases to several degrees in the upper and lower areas. Therefore information from the fovea and a small area around it is projected to both sides of the visual cortex and this may have some involvement with stereopsis. Syn. bilateral strip.

natamycin See antifungal agent.

nativism The belief that knowledge or behaviour is inborn.

See empiricism; theory, nativist.

nativist theory See theory, nativist.

Nd-Yag laser See laser, neodymium-yag.

near addition See addition, near.

near point of accommodation See accommodation, near point of.

near point of convergence See convergence, near point of.

near point retinoscopy See retinoscopy, dynamic.

near point rule; point sphere See under the nouns.

near point stress See asthenopia.

near reflex See reflex, accommodative.

near sight See myopia.

near triad See reflex, accommodative.

near vision; visual acuity See under the nouns.

nearsightedness See myopia.

nebula See leukoma.

Necker cube Perspective drawing of the outline of a cube that can induce two perceptions, either a three-dimensional cube orientated upward or a three-dimensional cube orientated downward (Fig. N1).

See figure, Blivet; Rubin’s vase; Schroeder’s staircase.

necrosis Death of some or all cells in an organ or tissue. The process involves swelling of the nucleus (pyknosis), fragmentation of the nucleus (karyorrhexis) and complete dissolution of the nuclear chromatin (karyolysis). Necrosis is caused by disease, trauma or interference with blood supply. There are many sequelae to ocular necrosis (e.g. inflammation, reduction in aqueous humour production following ciliary epithelium necrosis, corneal opacity following necrosis of corneal epithelial cells, and visual loss and floaters following retinal necrosis).

See apoptosis; retinal necrosis.

necrotizing scleritis See scleritis, necrotizing.

nedocromil sodium See mast cell stabilizers.

negative convergence See divergence.

negative eyepiece See eyepiece, negative.

negative relative accommodation See accommodation, relative amplitude of.

negative relative convergence See convergence, relative.

negative scotoma See scotoma, negative.

negative spherical aberration See aberration, spherical.

neglect, visual See visual neglect.

neodymium-yag laser See laser neodymium-yag.

neomycin sulfate A broad-spectrum antibiotic agent effective against gram-negative and gram-positive organisms, although it is not effective against Pseudomonas aeruginosa. It may be applied topically as eyedrops or eye ointment, but it is most commonly combined with bacitracin and polymyxin B. Syn. framycetin (a mixture of neomycin A, neomycin B and neomycin C).

neonatal conjunctivitis See ophthalmia neonatorum.

neostigmine A reversible anticholinesterase drug, which neutralizes the effect of acetylcholinesterase and thereby allows the prolonged action of acetylcholine on the iris and ciliary muscle. Its action is similar to physostigmine but it is not so irritating a miotic. Both are occasionally used in the treatment of glaucoma.

See acetylcholine; parasympathomimetic; physostigmine; miosis; miotics.

neovascular glaucoma See glaucoma, neovascular.

neovascularization Development of new blood vessels, especially in tissues where circulation has been impaired by disease or trauma.

choroidal n. (CNV) Abnormal growth of blood vessels, originating in the choriocapillaris, which pass through Bruch’s membrane and then proliferate under the retinal pigment epithelium (type 1) and/or under the retina (type 2). It may occur as a result of a rupture of Bruch’s membrane, release of cytokines (e.g. VEGF), inflammation, oxidative stress to the retinal pigment epithelium, or vascular insufficiency. The condition is the main cause of exudative (wet) age-related macular degeneration and it may be associated with various disorders including angioid streaks, choroidal rupture, pathological myopia, chorioretinal scars and birdshot retinochoroidopathy.

See macular degeneration, age-related.

corneal n. See pannus.

iris n. Abnormal formation of new blood vessels on the anterior surface of the iris. It is commonly associated with many conditions that have led to retinal ischaemia, such as diabetic retinopathy, occlusion of the central retinal vein, carotid arterial disease, uveal melanoma, long-standing retinal detachment, etc. The neovascularization begins at the pupil margin and often at the same time in the angle of the anterior chamber and spreads over the whole surface. New vessels are associated with fibrous tissue membranes, which may block the passage of aqueous humour through the trabecular meshwork (neovascular glaucoma) and ectropion uveae near the pupillary margin. Treatment typically includes photocoagulation to prevent the formation of new blood vessels.

nepafenac See antiinflammatory drug.

nerve A whitish cord made up of myelinated or unmyelinated nerve fibres held together by connective tissue sheath in bundles and through which stimuli are transmitted from the central nervous system to the periphery or vice versa.

abducens n. Sixth cranial nerve. It has its origin from the abducens nucleus at the lower border of the pons and at the lateral part of the pyramid of the medulla. It passes through the cavernous sinus and enters the orbit through the superior orbital fissure. It supplies motor innervation to the ipsilateral lateral rectus muscle. Additionally, interneurons leave the abducens nucleus and project to the contralateral medial rectus sub nucleus to allow conjugate gaze. A lesion in the nuclear region will cause gaze palsy, whereas an abducens nerve lesion will produce only an abduction deficit. See nucleus, abducens; paralysis of the sixth nerve.

cranial n’s. Twelve pairs of nerves, one set on each side of the brain, that emerge, or enter the cranium. They carry sensory information from the sense organs, the muscles of the head, neck, shoulders, heart, viscera and vocal tract. The motor neurons with axons in the cranial nerves control pupil diameter, accommodation, movements of the eyes and eyelids, mastication, facial expression, head movements, as well as cardiorespiratory and digestive functions.

n. fibre layer See retina.

fifth cranial n. See nerve, trigeminal.

fourth cranial n. See nerve, trochlear.

frontal n. See nerve, ophthalmic.

n. impulse See potential, action.

infratrochlear n. See nerve, ophthalmic.

lacrimal n. See nerve, ophthalmic.

long ciliary n. One of a pair of nerves that comes off the nasociliary nerve and runs with the short ciliaries, pierces the sclera, travels in the suprachoroidal space and supplies sensory fibres to the iris, cornea, and ciliary muscle and sympathetic motor fibres to the dilator pupillae muscle (Fig. N2).

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Dictionary of Optometry and Visual Science