Barbara Jennings

lntraocular lens for cataracts

In its normal state, the human eye is said to be phakic, that is the crystalline lens is intact. When the lens is removed because of cataractous changes, the eye is called aphakic. About 400,000 per- sons in the United States-most of them elderly-undergo surgery each year for removal of cataracts. It is usually three to six weeks before such patients can be fitted comfortably with corrective lenses, either spectacles or hard or soft contact lenses.

Spectacles prescribed are extra thick. They magnify images from about 15% to 30% and seriously impair peripheral vi- sion. In patients who are phakic, mag- nification is particularly troublesome in that it makes useful binocular vision impossible.

Barbara Jenninge,Pittsburgh, Pa, is a certified orthoptist, a member of the American Orthoptic Association, and a registered ophthalmic assistant with the Joint Commission for Allied Health Personnel in Ophthalmology. Employed in the office of C S Reshmi, MD, she is a graduate of the Columbia Presbyterian Hospital School for Orthoptists. The author wishes to acknowledge

C S Reshmi, MD, and Minda Chipurnoi, illustrator.

The contact lens, on the other hand, permits adequate peripheral vision and far less distortion, but many aged pa- tients find them uncomfortable and dif- ficult to care for.

The intraocular lens introduced in the United States in the late 1960s of- fers an alternative to sight correction for the aphakic patient. The implant does not always replace glasses, how- ever, because the lens cannot be de- signed to change shape for focusing like a normal lens. The implant is designed for distance vision; thus, the patient wears appropriate glasses to compen- sate for reading and other near vision. When the cataract is replaced with an intraocular lens, the eye is then de- scribed as being pseudophakic, or ar- tiphakic.

The idea of intraocular lens was first proposed in 1766 by Jacques Tardini. In 1949, Harold Ridley of London im- planted a lens of methyl methacrylate, the same material he found imbedded in eyes of fighter pilot patients who suf- fered penetrating eye injuries. Ridley noticed through years of follow-up that these patients tolerated fragments of plexiglass (from the canopy of their Spitfires) in their eyes for indefinite pe- riods.'

The British ophthalmologist and

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Fig. 1 The spatial relationships involved in determining the power or prescription forintraocular lens are the corneal curvature, the depth of the anterior chamber, and the axial length of the eyeball.

others in Europe are credited with pioneering in intraocular lenses and techniques for implantation. It has only been since about 1968 that US physi- cians have become equally involved in the trend.

The incidence of lens implantation here has increased with improvement in lens design and materials, along with refinement in surgical techniques for insertion. Conservative estimates are that 15,000 procedures have been per- formed to date. Local and national pro- fessional societies have been formed to share knuwiedke and experience with permanent intraocular lenses. These

organizations include The American In- traocular Implant Society and Interna- tional Lens Implant Club.

For the pediatric patient, intraocular lenses have produced good and poor re- sults. On the one hand, these lenses seem potentially ideal because theoreti- cally, the lens is a lifelong appliance set as close as possible in its natural anatomical environment. On the other hand, it is generally believed that adults and geriatric patients are better candidates for lens implants because at this time, the life of lens is not fully established. Selection of patients is further extended to those with a history

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of poor contact lens wear, arthritic individuals, and the physically handi- capped.

Determining the power or prescrip- tion for the lens is based upon sophis- ticated calculations. The spatial relationships involved are the corneal curvature, the depth of the anterior chamber, and the axial length of the eyeball (Fig 1). These structures and spaces play a specific part in determin-

U Fig 2. Above, the Binkhotst- Worst medallion lens. Below, the Copeland lens.

ing how light rays form an image on the retina. Values for these measurements are considered in terms of refractive or dioptric power, the unit of measurement of the refractive power of a lens having the focal distance of one meter.

In selecting lens power in a patient with a unilateral cataract, the objective is to leave the patient slightly myopic or nearsighted. When this result is achieved, the image size in each eye will

be very close or exactly the same. In this state, the patient is able to use the oper- ated eye for reading and the unoperated eye for seeing clearly at a distance.

When bilateral lenses are implanted, the aim is to make the patient emme- tropic, that is, to make all light rays focus on the retina in a perfect image. In these cases, the eyes will refract basi- cally the same for distance vision. This requires the patient to use glasses for reading as do almost all persons middle aged or over.

Hyperopia, or farsightedness, is avoided when determining lens power, because a postoperative hyperopic eye would magnify an image and cause visual and fusional difficulty. Residual astigmatism must be corrected with eyeglasses.

There are about six intraocular lenses available today. Two are the Binkhorst-Worst medallion lens, ad- vantageous for its versatility, and the Copeland lens, noted for its simplicity (Fig 2). Both are made of polymethyl methacrylate. They differ, however, in how they are fixated in the eye.

The medallion lens is equipped with two thin loops of platinum or specially coated suture. The lens is placed in front of the iris while its loops, (located at the twelve o’clock and six o’clock positions), are inserted through the pupil and bent into place behind it. The loops may be fixated by sutures placed in the intact posterior capsule or sutured back to front of the opening made by the peripheral iridectomy. Another method of fixation preferred by some surgeons is to leave a ring of cortical material to serve as a snow pile into which the lens loops may be anchored. The pupil is 2 x 2 mm square as a result of the medallion implant,

Another type of intraocular lens is manufactured by Copeland. Designed Like a Maltese cross, it has four append-

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Fig 3. The intraocular lens provides patients and surgeons a choice of methods for restoring sight to the aphakic patient.

ages that fit to the iris like a hub cap fits the rim of a tire. Its horizontal hap- tics position anterior to the iris and its vertical haptics fit posterior to the iris. (A haptic is the portion of a scleral con- tact lens that rests on the scleral con- junctiva.) The Copeland intraocular lens requires no sutures for fixation. With this lens the pupil is square (as the iris molds to the square base of the lens) and has an aperture of 3 to 4 mm.

Any lens that has no suture for fixa- tion requires a miotic agent, such as pilocarpine, to keep the iris from dilat- ing too widely permitting the loops to slip back through the pupil. Most pa- tients with sutured lenses are given pilocarpine only in the immediate post- operative course.

In addition to complications that can occur with any form of cataract extrac- tion, there are some specifically as- sociated with lens implantation. These include prolonged inflammation, rosy

vision, malposition of the lens, and cys- toid macular edema, or swelling of the macula by means of fluid seepage from tiny capillaries that surround it. The swelling causes decreased vision.

Prolonged inflammation may appear as iritis or vitritis (inflammation of the vitreous humor secondary to iritis). These conditions are controlled with steroid therapy.

Rosy vision may occur because the pupil is kept from maximally constrict- ing. This allows more light than usual to enter the eye and causes the macula to be dazzled.

Malpositioning of the lens rarely oc- curs but should it happen, the patient’s pupil is first dilated and then his head maneuvered into a position to allow gravity to reposition the lens into the desired place. When the lens is repositioned, the pupil is again con- stricted. If this procedure is unsuccess- ful, the lens may have to be surgically

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removed for it can harm the cornea by touching the sensitive endothelium. An ophthalmologist usually decides to re- move an implant when it frequently needs repositioning, it causes long-term corneal edema or dystrophy, or there i s recurrent or uncontrolled inflamma- tion.

Advantages of the intraocular lens include the following: (F ig 3)

0 eliminates the diff iculty o f inser- tion, removal, adaptation, and lens care routine associated w i t h con- ventional contact lenses

0 provides a solution for patients who cannot wear cataract glasses and w i l l not wear contact lenses

0 cannot be lost or misplaced l i ke spectacles and never have to be re- placed

0 provides a permanent form of vis- ion for patients. Since the image size is so close to normal, the pa- t ient can benefit from normal vis- ua l f ield ranges and fusion w i t h depth perception.

In conclusion, it must be emphasized that the intraocular lens w i l l probably never replace tradit ional methods of correcting aphakia, ie, spectacles or hard or soft contact lens. However, w i t h the increasing numbers of lenses im- planted and the statistical success tha t i s growing in favor of such lenses, it i s evident that lens implants provide pa- tients and surgeons involved in cataract surgery a choice of methods of restoring

sight to the aphakic patient. 0 Notes

1. Henry Hirschman, “Five hundred intraocular lenses in the treatment of cataracts,” paper deliv- ered at the 123rd annual convention of the Ameri- can Medical Association, Chicago, June 25, 1974.

Big renin’s role in blood pressure Scientists have reported the first successful attempt to purify a recently discovered chemical that may play a major role in regulating blood pressure.

The kidney-based substance called “big renin” has brought into question basic ideas about how the body controls blood pressure levels. The availability of purified big renin should help scientists clarify the issue and may well lead to better diagnosis and treatment of hypertension.

The report on the successful purification of big renin was presented at the 48th annual scientific sessions of the American Heart Association by Tadashi lnagami and Kazuo Murakami, biochemists at Vanderbilt University School of Medicine, Nashville, Tenn.

Current theories atput blood pressure regulation are based on the belief that renin exists only in active form. Released by the kidney into the bloodstream, renin leads to the production of angiotensin II, the most potent constrictor of blood vessels known. This squeezing action on the vessels causes blood pressure to rise.

Experts believe that abnormalities in this renin-angiotensin system lie at the root of a great number of cases of high blood pressure, affecting 23 million Americans and a major factor in heart attacks, stroke, and kidney disease.

large-molecule, less-active or latent form of renin, has now complicated the picture. For one thing, the Nashville researchers discovered in their successful efforts toextract and purify the agent from hog kidneys that the renin in the kidneys exists almost entirely in the latent or big form. This observation, said the scientists, raises puzzling questions about how big renin is converted into the smaller, active form and what role this transformation may have in the blood pressure regulating process.

According to investigators, any one of several aspects of the big renin to renin conversion-such as the speed and site of this conversion as well as the biochemical mechanism of change-may have some role in the development of high blood pressure.

However, the recent discovery of big renin, a

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