Low vision or visual impairment affects mostly the elderly. With the rapid aging of the population distribution in the USA and other industrialized nations, both the numbers and proportions of people with visual impairment are expected to increase rapidly in the next two decades. The most common cause of vision disability is age related macular degeneration (ARMD), which affects the fovea, the central section of the retina used for high-resolution vision (Klaver et al., 1998). Prevent Blindness America (1994) reported about 3.5 million Americans over the age of 40 years have moderate to severe vision loss including blindness (about 300,000). About half of these impairments are due to ARMD. Like ARMD, diabetic retinopathy, optic neuropathy, central retinal vein occlusions and other conditions also cause central field loss (CFL). CFL reduces the patient’s resolution and affects their ability to conduct activities of daily living such as reading, recognizing faces, watching television and driving.

Normally when reading, people move their eyes rapidly across the text scanning about 5 words per second, leading to a reading rate of as much as 300 words per minute, However, with CFL reading rates drop below 100 words per minute. A variety of magnifying devices have been use to aid patients with CFL read. Most magnifying devices have a restricted field of view and have to be manually scanned across the text, magnifying a small portion at a time. In some cases the text is scanned in front of the magnifying device and held at a very short, fixed viewing distance. In all cases the manual scanning is slow and limits the reading rate.

The bioptic telescope is mounted at the top of the spectacle lens, above the pupil, with a slight inclination upwards. Most of the time, the patient views the environment through the regular spectacle lens (the carrier lens) enjoying the benefits of intact peripheral vision. When a distant object is detected and cannot be recognized due to reduced resolution (CFL), the patient tips his head slightly down, bringing the telescope in front of the eye and the object of interest into the field of view of the telescope. A short examination (1 to 2 sec) of the target through the telescope provides the patient with the required high detail information needed to recognize the target. This intermittent use makes the bioptic telescope an effective, comfortable, and safe device. Despite some adverse opinions (Fonda, 1983), low vision telescopes are permitted as visual aids for driving in 27 states in the USA (Peli and Pely, 1999).

Despite all these advantages, spectacle and head mounted telescopes have gained limited use by patients with CFL as reading or distance vision aids. The reasons for the limited acceptance of the devices are presumed to be: the obvious and unattractive appearance of the devices (Spitzberg et al., 1989), the need to use slow head scanning movements in place of the eye movements used in natural vision, and the vestibular conflict resulting from the increased motion which accompanies head-mounted magnification (Demer et al., 1989). The latter effect may cause a conflict between the visual and vestibular system leading to discomfort or motion sickness. While the vestibular system can adapt easily to low levels of magnification (i.e. a few percent) as created by spectacle lenses, the ability to adapt to the large magnification needed in low vision telescope have not been demonstrated.

In this article we describe the development and preliminary clinical results of an effort to develop an implantable telescopic lens. With the telescopic lens inside the eye, we get an eye which performs as a telephoto lens with a focal length three times the focal length of the normal eye. In this case, most of the limitations of the head-mounted telescopes are avoided. We believe this should lead to a successful use of the telescope as both a reading and a distance vision aid.

As shown in Fig. 1, the IMT is a miniature Galilean telescope mounted on a PMMA carrier intraocular lens implant (Lipshitz et al., 1997). The IMT is implanted in the posterior chamber of the eye, in place of the crystalline lens. It is held in position in the lens capsule by haptic loops and the IMT bulges forward into the anterior chamber through the pupil (Fig. 2). The IMT is implanted using a modified cataract extraction procedure. It is designed to be implanted in one eye of a patient with symmetrical CFL. With no additional lens, the IMT should provide 3.05 magnification at a distance of 50 cm, providing better resolution for many activities of daily living. With the use of additional spectacle lenses, we can focus at any distance from 25 cm to infinity. With the use of an additional lens for reading at shorter distances, the relative effective magnification can be increased, up to 85 . Also, the IMT can be used for distance vision with the required refractive correction in the spectacles (-2.00D).

Fig. 1. The Miniaturized Telescope mounted in a carrier intraocular lens with haptic loops.

Patients with refractive error prior to this surgery will need similar correction to their pre-surgical correction to achieve the same effect. The useful field of view through the telescope is limited to 6.6º (with a wider extent of field available at low resolution and contrast). For this reason the telescope is implanted in one eye only to provide high-resolution central vision, while the fellow eye continues to be used for peripheral vision and safe mobility. With the telescope, vision is expected to improve sufficiently to provide comfortable reading at a conventional distance. While the instantaneous field of view is small, the ability to scan using eye movements should provide much more comfortable reading than would be possible with a head mounted telescope of the same field.

The IMT is constructed from two glass lenses inside a glass tube. Two flat glass windows keep the lenses separated from the aqueous humor, increasing the effective power of the lenses relative to lenses immersed in the aqueous. Also the airtight tube serves to float the telescope in the eye, so that its effective weight is only 46 mg. The tube is 3.0 mm in diameter and 4.4 mm long. The front of the IMT is positioned about 2 mm behind the posterior surface of the cornea. The iris is used to support and center the IMT inside the eye and on the optical axis, and helps block non-direct light from reaching the retina, and so increases the contrast.

Fig. 2. The IMT mounted in the lens capsule and supported by the iris

Using any head mounted magnifying device causes a disruption of image stability and perceived direction as a result of head motion (Miles, 1991). In natural viewing without optical devices, if an observer fixates a target at primary position of gaze then rotates his head, while maintaining fixation on the same target, the required change in eye position in the orbit is equal in magnitude and opposite in direction to the head rotation (Fig. 3a). Such an eye movement is generated automatically by the vestibular ocular reflex (VOR) in response to head movements and it serves to maintain a stable retinal image during head rotation. With the use of a head mounted magnifying device, the required correcting eye movement is larger, as illustrated in Fig. 3b. When using a telescopic device with a 3.05 magnification, an eye movement three times as large as the head rotation is required. While the ability of the VOR system to adapt to large changes in magnification (up to 35 %) that may occur (e.g. with diving goggles) have been demonstrated (Gauthier and Robinson, 1975), the ability to adapt to the extreme demand of this telescope (300% magnification) has not been demonstrated. Furthermore, it should be noted that when using a monocular telescope, as is commonly the case, the demand for adaptation is different between the two eyes (Fig. 3b). Some capacity to adapt the VOR of each eye differentially has been demonstrated in monkeys (Viirre et al 1987; Snow et al 1985), but again only small changes in VOR were demonstrated. If the VOR cannot adapt differentially between the eyes, every head movement with a head-mounted magnifying device will result in substantial image motion in one eye with the accompanying reduction in sensitivity (Peli et al., 1998). However, using the IMT resolves this problem completely. As illustrated in Fig. 3c, with the IMT a given head movement will require a compensatory eye movement of the same magnitude. Thus the natural VOR gain of about 1.0 will suffice. In addition, no conflict will occur between the image motions in both eyes with a monocular IMT.

While the discussion above was framed in terms of head movement it should be clear that the same arguments apply when the observer shifts fixation between two targets without any eye movements. With the IMT, a target 5º from fixation will require only 5º of eye movement to be examined with an IMT but will require 15º of eye movement with a head mounted telescope. Further, if a target is seen with the other eye to be at a certain angular distance from fixation it may be acquired through the IMT with the corresponding size of eye movement. Thus the IMT offers a distinct advantage in space and direction perception as well as clarity of vision in both eyes during eye movements, head movements and tracking of moving targets.

Clinical trials are being conducted in Europe. We report here the results of the first 24 patients implanted with the IMT and analyzed by March 31, 1999. Initial clinical trials were conducted in nine blind eyes to test the mechanical feasibility of the implant procedure. The first seven of these were operated by Open-Sky technique. The later procedures were modified to reduce the impact of the surgery on the cornea. A corneal incision of 140º was used to remove the lens using phaco-emulsification. The IMT was inserted through the incision and was placed into the bag with the help of a second tool.

Patient selection in the early trials was limited to patients with bilateral dry-type ARMD of about equal acuity in both eyes, not better than 20/100 (6/30). Follow-up of more than one month was available on 12 patients at the time of writing. Average follow-up for these patients was 2.2 months (range 1 to 6 months). All of these patients had bilateral CFL due to ARMD. The average age of the patients was 75.3 years (range 65 to 83 years). Most patients reported an improvement in visual function following a few weeks of adaptation. Distance VA through the telescopes was improved in 10 of the 12 patients. The group mean VA was better post-operatively and the difference was statistically significant. Functional improvement was reported by most patients for a number of tasks: watching television – 91%, table orientation – 75%, and reading – 66%. All these changes were statistically significant.

An implantable miniaturized telescopic device for low vision was developed and tested. Preliminary results are encouraging as the implant procedure is feasible and no serious complications were encountered. It is even more encouraging to find that the implant functions to a large extent as it was designed, providing useful magnification and functional vision. The use of two eyes with widely differing magnifications and fields of view will be a challenge for the patients. Understanding this situation and the ways patients can adapt to it effectively is the biggest challenge facing this project. If successful, the IMT will represent a new treatment option for patients with CFL and moderate loss of acuity that might be superior to all existing devices. Patients with CFL represent the majority of people affected by ARMD and low vision in general.