Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84; site ski.UUCP Path: utzoo!linus!philabs!cmcl2!seismo!lll-crg!dual!ptsfa!ski!dr From: dr@ski.UUCP (David Robins) Newsgroups: net.med Subject: Re: Contact lenses and astigmatism... Message-ID: <191@ski.UUCP> Date: Tue, 6-Aug-85 14:05:27 EDT Article-I.D.: ski.191 Posted: Tue Aug 6 14:05:27 1985 Date-Received: Sat, 10-Aug-85 22:57:45 EDT References: <3420@fritz.UUCP> Organization: Smith-Kettlewell Institute, S.F., CA. Lines: 59 > Hello. I hope to glean some information from informed persons on the > topic of contact lenses. First, I have a mild astigmatism --------------> > from the incorrect curvature axis. This space is filled with tear > solution such that the combination of hard lens, tear, and natural > cornea together form a `new' cornea with the correct optical > properties and curvatures in both axis. Thus any `floating' of the > hard lenses won't greatly affect vision quality, unlike soft lenses > (that correct for astigmatism). > > I would appreciate if any informed netters out there could tell me if > the above information is pretty much correct, or if the optometrist > was just passing gas. Thanks in advance. The optometrist gave you a very thorough explanation, and your understanding of it is correct. Soft contact lenses for astigmatism do need to stay oriented to the corneal axes in order to compensate for the astigmatism. This is accomplished by either ballast weighting (thickening) the bottom of the lens, so gravity pulls this edge down, or by truncating the edge, so contact with the lower lid keeps it in position. Trouble is, at each blink, the lids move sideways slightly, thereby torting the lens. After the blink, the lens realigns itself, but this takes a short time (less than 1 second, usually). However, if there is a significant astigmatism, or if one is acutely aware of one's vision, the time when the lens is not aligned may cause blurred vision, and can thus be annoying. These problems are largely eliminated by rigid contact lenses. Hard contact lenses (non-permeable) and gas-permeable contact lenses are spherical on the front and back surfaces. As long as there is not too much astigmatism (less than about 2.50 or 3.00 diopters of keratometer- reading curvature), the spherical surface sits on the toric surface of the astigmatic cornea reasonably well, thereby forming a new, spherical front optical surface. This eliminates the optical aberrations. In high astigmatism cases, the posterior curve of the contact lens can be made toric, instead of spherical, to get a better fit. However, then the anterior surface must be toric also. Manufacturing such lenses is an art, and fitting them is not easy. Gas-permeable CL's are not as rigid as hard CL's, and they can bend a bit if there is much corneal toricity. However, on the whole, gas-permable CL's are healthier for the cornea. Hard CL's, with no oxygen transmission, have sometimes caused permanent corneal warping. Any rigid CL will put pressure on the toric cornea, causing a temporary curvature change. This may be noticeable after taking the lenses off and putting on one's glasses. The usual CL should not take more than about 1/2 hour to bounce back. Corneal moulding takes longer to undo, even up to several weeks in severe cases. -- ==================================================================== David Robins, M.D. Smith-Kettlewell Institute of Visual Sciences 2232 Webster St; San Francisco CA 94115 415/561-1705 {ihnp4,qantel,dual}!ptsfa!ski!dr