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Show ]. Clin. Neuro-ophth.l/nwl. Z: 1°-22, \082 Hemianopic Fresnel Prisms J. LAWTON SMITH, MD. IRA G. WEINER, MD. ALBERT I. LUCERO Abstract Patients with homonymous hemianopic visual field defects have been treated by using a 30-diopter Fresnel plastic paste-on prism on their glasses. A patient with a left homonymous hemianopia, for instance, can have a 30.diopter base left (i.e., base out) Fresnel prism placed on the temporal half of the left spectacle lens. The prism need be only on one lens. A small 1- to 1.5mm portion can be cut out of the center to prevent diplopia. This device is widely available and easier to obtain than a hemianopic mirror on the frames. Use of this device is illustrated in the report. The visual disability imposed by a homonymous hemianopia is particularly bothersome to some patients. Generally speaking, those individuals with good central visual acuity, and who are otherwise neurologically intact but for the hemianopia, are the prime candidates to consider for some form of optical relief of their field limitation. In 1966,1 we published a short note advocating the use of hemianopic spectacles and have continued to use them in some patients since then. This entails having an experienced optician placing a small mirror mounted on the outside of a pair of glasses frames so that the patient can learn to look into the mirror to avoid bumping into objects in the hemianopic field. For example, in a patient with a left homonymous hemianopia, a mirror theoretically could be placed on either the temporal side of the right glass or on the nasal side of the left glass. In practice, it has been found preferable to put the hemianopic mirror on the nasal side of the left lens in a case where other factors, such as acuity, are equal. The reason for this is to avoid having the externally placed mirror compromise the temporal field of the right eye, which is generally the best available portion of ambient field in such a patient. However, hemianopic mirrors are not available to many ophthalmologists, the majority of opticians are not familiar with constructing From the Department of Ophlhalmolo~y. Ba,com Palm"r [Yl' Institute. University of Miami School of M"dicin". Miami. Florida. March 1982 or fitting them, and therefore, this modality has been somewhat limited in general use. In August ]980, we first used a Fresnel press-on prism to help a patient with a homonymous hemianopia. A young woman suffered a bleed from a right occipital arteriovenous malformation which left her with a permanent, total left homonymous hemianopia. She was seen by one of us (lew) and a hemianopic Fresnel prism was advised. The patient was fitted with a 3D-diopter Fresnel prism placed on the temporal half of her left lens. It was necessary to trim a very small central area with a radius of about 1-1.5 mm from the center of the press-on prism to avoid difficulty with diplopia bumping into the prism (Fig. 1). However, by means of this device, the patient was able to g~t about IS degrees of useful vision to her left, and was very happy with this. Figure 2 shows that the patient has a slight residual left facial paresis, and that when the examiner's finger came in from her left, she could not see it until it reached the midline. Figure 3 shows the patient wearing a pair of tinted lenses with the hemianopic Fresnel prism in place, and by this means she was able to catch the finger when it came in from her left at about ]5 degrees from the midline. She considered this very helpful in avoiding objects to her left, not bumping into doors, etc. The optical principle is seen nicely in Figure 4. This shows her glasses .1S one would (001... into them from the front, or from an observer's point of view. Note that the patient's right eye lens show the temple of the glass going essenti.111y straight back on that side, but the left lens (\Vher~ the hemianopic Fresnel prism is in pl.lee) sh,)\\'s the temple on that side notably deviated t,)'V.Hds her right, and it is seen actually 'Clmling right to the center of the gl.1ss on that side. The optical principle is simply this-since.1 prism displ.1ces objects towards its .1pPX, if "Ill' pllh .1 prism base Idt on the left lens in .1 Idt hemi.llwpi.1. the prism simply "grabs" the tpmpor.11 (hpmi.lI11)pic) field in the left eye and "pulls" it over l'r displaces it towards the apex or into the seeing nas.11 field of that eye. It is not necessary to put prisms iJefore both eyes in a patient with a hemi.lllopi.1 involving both eyes. Indeed, it is preferable to put the hemianopic Fresnel prism on only one eye in such cases. This is because a press-on prism usually reduces the acu- 19 Hl'mi.lI1l'pil r rl'~I1l'I~ Figure I. !'.llicnt" gIJ"c.. ,huwn J' "'('n f,,'m the pJt,,'nl" pcr-pel!,vc. W,th a left homonymllu~ hl'ml.,,,opi.l, thc 30-dil'pter Frc,nel prt,m " placed bJ'c 001 (I.c., bJ'(' left) on thc left lens. Nl'I(' thJt J ..mJII ccnlrJI JreJ mJY need Il> bc trtmmcd with J razor blJde to Jvold ~-llnfu5lon or diplopi.l .11 fix.llll,n. Figure 2, PJtient with left homl'nyml'u" hemi.,nl'piJ dl'e.. nl't detelt J finger cl,mJl1g in fr,1m her left flcld until Ihc midline Note sloght residuJI left fJClJI pJrc,i, 111 th" pJticnt. ity slightly (to 20/20, 20/25, or even 20/40 range) in the involved eye, so that if they are used before both eyes, the drop in .lcuity th.lt results often bothers the patient more than the origin.ll problem (as diplopia, or field defect) in the first pl.lce. Careful fitting and centering of the prism are necessary, however, and the size of the small central area that needs to be trimmed to .lVoid confusil)n or diplopia is e.lsily determined simply by tri.ll .md error, using a small penknife as in the example provided, Two other ppinls merit emphasis. The largest ',I{" '''1111111'<0 i.lliv .lV.1I1.1ble Fresnel prism at this time is .1 30-diopter lens. This will allow a patient to get 15 degrees of vision in the blind field. We h.lVe been in communication with Optical Sciences Group of San Rafael. California-manufacturers of the press-on prisms-and have encouraged them to Cl)nsider making Fresnel prisms of higher dioptric power. We would like to try a 50-, 100-, or even ISO-diopter Fresnel prism in some hemi. mopie patients. Now it is obvious that as the power increases, so does the thickness of the press-on prism, and hence a reduction in visual acuity entailed by its use would become a greater problem. We would expect to have to make the best com- Journal of Clinical Neuro-ophthalmology Smith, Weiner, Lucero Figure J. The same patient wearing hemial10pk prism glasses can now detect the finger coming in from her left field about IS degrees to left of midline. Figure 4. Patient's glass"s shown from th" oulsid" or from all "b"'rvl'rs p"rspeet"'l' This demonstrates nicely that th" t"mple go"s back strail(ht on thl> right sid" (wh"re h.1nd is h"lding the frames). but not" the dramatic optical shift of th" t"mpl" pi"ee ,\11 th" l<>it sid" .111 th" W.1Y to center of the glass. promise between the increase in field and the decrease in acuity that would result from such a prism. Optical Sciences Group has provided a small number of 40-diopter Fresnel prisms to try in hemianopias and point out that tooling up to make higher powers would not only be expensive, but might entail having to put the prism on the outside of the glass, which would be cosmetically less desirable than on the inside. The specific March 1982 power of Fresnel prism th.lt would be both optically ideal and commercially feasible for a hemianopia is therefore under investigation. The point of this report, however, is that any practitioner has access to 30-diopter Fresnel prisms, and they can be tried in hemianopias using the method here described; many of them will be grateful for this additional help with their field. A final word relates to selection of patients for 21 HeTllI,lI111pir Fresllels h('mi,lllopic spect,KI('s (h('mi,lIl0pic mirrors or hemi,lIl0pic prisms). In g('n('r,ll, th(' b('st candidat('s ,He p,ltiellts in whom the hemi,lIl0pi,1 is th(' primary neurologic,ll probl('m ,1110 constitut('s thdr chid lllll1pl.1int. Thus, nonoomin,lIlt h('misph('ral 1('sil) ns (e.g., right posterior cerebr,ll .ut('ry occlusil) ns producing left homonymous hemi,lIlopias with little l)ther neurologic r('siJu,l) h,lV(, b('('n th(' best (,lIldid,ltes. HowC'v('r, if ,1 p,lti('nt has ,1 domin, 1I1t hemisph('r,ll strok(' I('aving th('m with a right Iwmipl('gi,l, glob,ll aph,lsi,l, ,11C'xia, and right honlllnymOUS hemianopia, for ('xample, their inability to read is hidden under such a massive brain deficit that a slight increase in right homonymous field will be of no practical help at all. Therefore, one usually will advise a Fresnel hemianopic prism with more enthusiasm in a patient with a nondomin, lIlt hemispheral hemianopia than with a domin, lIlt hemispheral hemianopia. As for follow-up, we recently received a letter from the patient shown in this report telling that at the end of a I-year trial with the Fresnel hemianopic prisms, it meant quite a lot to her, and she was wearing it faithfully and was still happy with it. The patient pointed out that she would be glad to make another trip to this area from out of state if a larger powered prism became available in order to get even more optical help from that. An obvious advantage of the hemianopic Fresnel prism is that if the patient does not like it, it can simply be removed from their glasses after a 2- to 3-week trial. A slight tint to the glass may be cosmetically desirable, but is not totally neces~ary, since the cosmetic appearance of the press-on prism is so much less noticeable than the external appearance of the older hemianopic mirrors. In our practice, the hemianopic Fresnel prism has now displaced use of the hemianopic mirrors, for all practical purposes. Some patients might prefer two pair-one with the hemianopic press-on and another with a hemianopic mirror-since the latter might give a slightly larger external view than the press-on, but that would certainly be the exception. We do not recommend that patients attempt to drive a moving vehicle using hemianopic spectacles, however. This point must be stressed to them; i.e., the device is to make their ambient vision safer and more comfortable, but will not resolve the problem to the degree that it would be safe for them to drive a moving vehicle. However, the Fresnel prism is not too expensive, is readily available, can be placed on the patient's glasses at the time of initial presentation, does not have to be specially made for each patient, and is recommended for trial with patients with homonymous hemianopias as a clinical adjunct in making them more comfortable. Reference 1 Walsh, TJ., and Smith, jL.: Hemianoplc spectacles. Am.]. OphthalmoJ. 61: Q14-Q15, 1900. Write for reprints to: j. Lawton Smith. M.D, Bascom Palmer Eye Institute, P.O. Box 010880, Miami, Florida 33101 journal of Clinical Neuro-ophthalmology |
