Tip of the Month
Publishing date: March 2012
Tip Editors: Ann Hoste, John Salmon and John Thygesen
Tip reviewer: Roger Hitchings
The Science behind the Tip
Anterior chamber angle width varies dramatically with illumination (figure and video in ref. 1) (1,2). Visible wavelength based examination methods such as gonioscopy can cause pupillary constriction and thereby open an angle in which there is irido-trabecular contact at lower light levels. The narrowest, dimmest possible slit-lamp beam with which the corneal wedge is still identifiable should be used in a fully darkened room. A 1mm long beam is sufficient and should be kept well away from the pupil.
Other subtle signs of iris-trabecular meshwork apposition such as pigment smudging (3) are useful adjuncts to direct observation of iris-meshwork contact but are no substitute for adequate examination conditions. Ultrasound biomicroscopy and newer imaging techniques such as anterior chamber ocular coherence tomography allow cross-sectional imaging without inducing miosis.
Contributors: Gus Gazzard and Paul Foster, London
1. Gazzard G, Foster PJ, Friedman DS et al. Light to dark physiological variation in irido-trabecular angle width. Br J Ophthalmol. 2004; Video http://bjo.bmj.com/content/suppl/2004/10/22/88.11.DC1/88_11__report.full.
2. Gazzard G, Friedman DS, Devereux JG et al. A prospective ultrasound biomicroscopy evaluation of changes in anterior segment morphology after laser iridotomy in Asian eyes. Ophthalmology. 2003;110:630-8.
3. Foster PJ, Gazzard GA, Garway-Heath T et al. Pattern of trabecular surface pigment deposition in primary angle closure. Arch Ophthalmol. 2006;124:1062.