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Uhthoff’s symptom Temporary blurring of vision occurring when there is an increase in body temperature (e.g. during or following exercise) in patients with multiple sclerosis, optic neuritis and other optic neuropathies. The symptom may also occur as a result of emotional stress, menstruation, increased illumination or a hot bath. Uhthoff’s symptom is sometimes considered to be a prognostic indicator of multiple sclerosis in patients with idiopathic optic neuritis. Syn. Uhthoff’s phenomenon; Uhthoff’s sign; Uhthoff’s syndrome.

ulcer A localized lesion of the skin or of a mucous layer in which the superficial epithelium is destroyed and deeper tissues are exposed.

See abscess.

corneal u. A superficial loss of corneal tissue as a result of infection that has led to necrosis. It may be caused by a bacterium (e.g. Pseudomonas aeruginosa, Streptococcus pneumoniae), by a virus (e.g. herpesvirus), or by a fungus (e.g. Candida, Aspergillus, Penicillium). It causes pain and usually reduced visual acuity, especially if the ulcer occurs in the centre of the cornea. Corneal ulcers usually look dirty grey or white and are opaque areas of various sizes and a mucopurulent discharge may be present. If induced by contact lenses, especially extended wear lenses, patients must cease wearing their lenses immediately, and the appropriate therapy instituted: antibacterial, antifungal or antiviral agent.

See corneal facet; keratitis, herpes simplex; keratitis, hypopyon; keratitis, rosacea; keratitis, ulcerative; keratocele; keratomycosis; leukoma.

dendritic u. See keratitis, herpes simplex.

von Hippel’s internal u. A depression noted in the posterior surface of the cornea. This lesion resembles posterior lenticonus, except that it is thought to be due to an infection or inflammation. The lesion can be differentiated from Peter’s anomaly by the presence of endothelium and Descemet’s membrane in the former. Due to its posterior location, the lesion does not usually disturb visual function.

See Peter’s anomaly.

marginal corneal u. Benign condition due to a hypersensitivity reaction to bacterial conjunctivitis, particularly staphylococcal blepharoconjunctivitis. It is characterized by infiltration of the peripheral cornea by white cells and by ocular irritation. The condition is usually self-limiting but painful. Treatment includes frequent cleaning of the eyelid margin with a cotton-tipped applicator or face cloth or cotton ball with baby shampoo, warm compresses, antibiotic ointment and occasionally topical corticosteroids.

Mooren’s u . A rare, superficial ulcer of the cornea of unknown origin. It starts near the limbus as an overhanging advancing edge that in severe cases spreads over the entire cornea and may even invade the sclera. The patient complains of pain and blurred vision. There are two types: a self-limiting form, usually unilateral, affecting old people, and a progressive form, bilateral, affecting young people. The condition is difficult to treat and this may include topical and systemic steroids, immunosuppressants, or conjunctival excision.

See keratitis, peripheral ulcerative.

serpiginous u. See keratitis, hypopyon.

shield u. A localized corneal ulcer noted in severe cases of vernal conjunctivitis. The lesion is usually oval or pentagonal resembling a warrior’s shield. It is located in the upper portion of the cornea as a result of irritation from the large papillae on the palpebral surface of the overlying eyelid.

ulcerative keratitis See keratitis, ulcerative.

ultrafiltration One of the mechanisms that produce aqueous humour from blood plasma into the ciliary epithelium of the ciliary processes. This mechanism takes advantage of the natural pressure gradient between the capillary vascular pressure and intraocular aqueous pressure, to drive fluid into the eye. Ultrafiltration is one of three physiological processes that create aqueous fluid, the others being active transport and diffusion.

See ciliary body; diffusion; humour, aqueous rate of outflow, transport, active.

ultrasonography A technique utilizing high frequency ultrasound waves (greater than 4 MHz) emitted by a transducer placed near the eye. The silicone probe, which rests on the eye, is separated from the transducer by a water column to segregate the noise from the transducer. The technique is used to make biometric measurements such as the axial length of the eye, the depth of the anterior chamber, the thickness of the lens, the distance between the back of the lens and the retina, the thickness of the cornea and detect ocular pathology. The ultrasound wave is reflected back when it encounters a change in density of the medium through which it is passing. The reflected vibration is called an echo. Echoes from the interfaces between the various media of the eye are converted into an electrical potential by a piezoelectrical crystal and can be displayed as deflections or spikes on a cathode-ray oscilloscope.

There are two basic techniques used for examination: a contact system (often referred to as applanation) described above in which the probe is in contact with cornea and an immersion

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