Telescopes on the First Visit
By: Stanley Woo, OD, MS, FAAO
Including a telescopic evaluation for all low vision patients is an important first step in any low vision rehabilitation regimen. Too often patients with visual impairments limit themselves in their potential, and may have conceded many visual objectives as being out of reach. By evaluating patients with telescopes, it may present to them a way of extending their visual reach that they hadn’t considered.
In general, to estimate the amount of magnification we aim for a goal of 20/40. Thus, we can take the patient’s measured visual acuity, for instance 20/200, and compare the ratio of both denominators. In this case, to go from 20/200 to 20/40 we recognize that 200/40 = 5x. Thus, a 5x telescope may be necessary to obtain 20/40 acuity in this example.
Having established our target magnification of 5x, it may still be prudent for the clinician to use a low power telescope like a 2.5x or 2.8x for the initial evaluation. Alternatively, an afocal 2.2x full diameter telescope in a trial ring may also be helpful. The reasons are that with greater magnification we have a smaller field of view and the image swim through the higher power telescope is exaggerated. It is important for the patient to get started off on the right foot and be able to see something quickly and easily at the outset.
With the low power telescope, like a 2.8x hand-held, it’s important to pre-focus the scope. That way, when the patient holds up the telescope over their better seeing eye the image should already be as clear as we can make it. Directing their attention to a large super-threshold target, we evaluate whether the patient is able to line up the target, the scope, and their eye along the same visual axis. In our earlier example, the patient was 20/200 and thus we expect them to only be around 20/70 – 20/80 with the 2.8x telescope. However, what we give up in acuity initially we gain in ease of use in introducing the telescope.
Depending on how quickly they get the hang of it as they read off letters from the chart, the clinician may elect to evaluate a stronger power hand-held to see if the patient responds predictably to the increase in magnification. For example, with a 4x12 hand-held telescope, we would expect the patient with 20/200 to now be able to see 20/50. Does the decreased field of view pose a problem with alignment or are they doing just fine?
Once the response to magnification is established and the patient has a sense of the potential for improved visual function the initial telescope evaluation is complete. At the follow-up visit, distance visual objectives may be further elucidated and more rigorous training in spotting, focusing and tracking may be introduced. Bioptic telescopic spectacles may also be an option to be considered. However, without subsequent training, the probability of success is relatively low. Remember that the aims of the initial low vision evaluation are to identify visual objectives, to determine if the response to magnification is predictable and consistent, and to motivate the patient to work on their rehab plan in order to maximize their functional vision. Subsequent visits may be used to refine the choice of prescriptive device and map out the necessary training regimen to make progress towards the patient’s visual objective.
- Measure distance visual acuity
- Choose a goal e.g. 20/40 unless more demanding tasks
- Calculate how much magnification is required for the better seeing eye by looking at the ratio of denominators
- VA = 20/160; goal 20/40; 160/40 = 4x
- 4x telescope system may ultimately be required
- Introduce a low power telescope with wider field of view to maximize probability of a good first impression during the initial evaluation
- 2.5x or 2.8x hand-held telescope OR
- 2.2x afocal full diameter telescope in a trial ring
- Measure VA with telescope over better seeing eye
- assess whether improvement in acuity was predictable and consistent
- increase telescope power if good initial response
- Counsel and educate the patient about the role of a telescope in extending visual reach
- Map out plan for further evaluation and training if visual objective established