Contact-Spectacle Telescope Improves Low Vision
BY ROBERT CAMPBELL, M.D. & PATRICK CAROLINE, C.O.T.,
About three million people in the United States have best corrected acuities of 20/70 or less which cannot be improved medically or surgically or by standard eyeglasses or contact lenses. For many of these low vision patients, optical or electronic imaging devices can augment or restore visual function. One such system is the contact-spectacle Galilean telescope, which consists of a high minus contact lens (the eyepiece) and a high plus spectacle lens (the objective). The two lenses are vertexed to create a single telescope that produces an erect, magnified retinal image. The resulting magnification is the ratio of the power of the high plus objective lens and the high minus eyepiece (Table 1).
|Power of Contact Lens||Vertex Distance (mm)||Power of Spectacle||Magnification|
|Monocular temporal half field: 13mm VD = 53.13°; 15mm VD = 50.58°; 17mm VD = 46.40°|
High minus rigid contact lenses may produce physical and optical problems, including edge thickness and prismatic effects secondary to lens movement and position. A custom, high minus, soft contact lens by Flexlens has dramatically lessened these problems.
Our case history involves a 32-year-old man with juvenile proliferative diabetic retinopathy. The patient underwent photocoagulation of both eyes in 1990, and his visual acuities have remained stable at 20/400 OD and 20/100 OS. The systemic condition has been well-managed with diet and insulin.
Our patient had some success with a spectacle-fixed external 2x telescope
and wanted to try a contact-spectacle system OS. Manifest refraction OS
-2.50DS. We prescribed a soft contact lens, base curve 8.6mm, diameter 14.5mm, power -40.00D. This made the effective power of the eyepiece lens
-37.50D. We determined the power of the spectacle (objective) lens by placing a
-20.00D lens into a trial frame at a vertex of 15mm. Loose lenses of
± 3.00D were alternately placed in front and bracketed to smaller dioptric steps until a maximum distance acuity of 20/70 was obtained. The final objective lens power was +24.00D (Fig. 1), providing an effective magnification of 1.56x.
FIG. 1: CONTACT-SPECTACLE TELESCOPE CONSISTING OF A -40.00D SOFT CONTACT LENS AND A +24.00D SPECTACLE LENS.
We explained to the patient how to bring near objects into focus by pushing the spectacles down his nose. Eventually, we prescribed a pair of +32.00D spectacles for near.
LIMITATIONS OF GALILEAN TELESCOPES
Potential problems associated with contact-spectacle telescope systems include:
- ocular intolerance of high minus soft contact lenses;
- unacceptable cosmetic appearance of high plus spectacle lenses;
- limited functional field;
- rapid and opposite movement of the visual field with head movement; and
- difficulties acclimating to extended periods of magnification.
When using high power contact-spectacle systems, it's often best to correct only one eye, even when both eyes have approximately the same acuity. Central vision of the uncorrected eye will usually be suppressed when the patient uses the telescope. However, binocular correction is sometimes possible with low power telescopes. CLS
Dr. Campbell is medical director of the Park Nicollet Contact Lens Clinic & Research Center, Minnetonka, Minn. Patrick Caroline is an assistant professor of optometry at Pacific University, Forest Grove, Ore., and director of contact lens research at Oregon Health Sciences University in Portland