Ординатура / Офтальмология / Английские материалы / Optics Retinoscopy and Refractometry_Lens_2005

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Vertex Distance

If you have ever picked up a magnifying glass, you know that its power seems to change in relation to its distance from an object. When we use lenses to correct a moderate to high refractive error, we must take into account how far the lens is from the patient’s eye for the same rea- son—the effective power of the lens changes as the lens is moved closer to or farther from the eye.

The distance from the back surface of a lens to the front of the eye is called the vertex distance. This distance should be measured whenever using lenses greater than 4.0 D (plus or minus). There are various vertex distance measuring devices on refractors; consult your owner’s manual for instructions on their use. When measuring the vertex distance of a pair of spectacles, a distometer (caliper) should be used. The arm of the caliper with a flattened surface is placed against the closed eyelid, and the plunger is depressed until the other caliper arm is against the back surface of the lens. Distometers are calibrated to take into account the thickness of a normal eyelid. The vertex distance is read directly from the scale.

To calculate the power of a lens at any given distance, the following formula can be used: N = 1 ÷ (1 ÷ D - [VD1 - VD2]) where N is the new diopter power, D is the power of the lens, VD1 is the existing vertex distance (in meters), and VD2 is the new vertex distance (in meters). For example, suppose you have a -8.00 lens (D) with a vertex distance of 6 mm (VD1), and you would like to know what power would be required to have the same effect at a distance of 13 mm (VD2). N = 1 ÷ (1 ÷ -8.0 – [0.006 – 0.013]); therefore, N = 1 ÷ (-0.125 –[-0.007]); N = 1 ÷ -0.118; N = -8.47. In other words, a -8.47-D lens would be required at 13 mm to have the same effect as a -8.00-D lens at 6 mm.

There are charts available to convert effective powers from one vertex distance to another. The most common reason for doing this is when converting a spectacle prescription to contact lens power, or vice versa. If the power of the lens is 4 D or more (plus or minus), vertex distance must be concidered. For example, a -4.25 spectacle correction at 12 mm would only need to be - 4.00 D for a contact lens. On the plus side, a +4.25 spectacle correction at 12 mm would need to be a +4.50 contact lens. The difference in power between the contact lens and spectacle lens increases proportionately with the refractive error.

Spectacles of Refractometry (The Return of the Glasses)

Despite the best efforts to prescribe the ideal combination of lenses for a patient, some people will return to the office with a frown on their face and their new spectacles in their hand. There is a seemingly infinite number of reasons why patients are not happy with their new glasses. It could be that the lenses were made incorrectly, a different base curve was used, or a significant change in lens power is not tolerated, to name just a few. This section will briefly describe a systematic approach to help isolate the cause of the patient’s unhappiness.

The first and probably the simplest step is to measure the power of the lenses using a lensometer. Compare the measurements to what was prescribed. There should be less than 0.25 D difference in power of sphere or cylinder, and the cylinder axis should be within a couple of degrees. (Remember that those individuals with high amounts of astigmatism cannot tolerate any error in cylinder axis.)

In most cases, the power of the lenses matches the prescription provided. So, the search for the problem continues. Mark the optical centers of each lens using the lensometer. Compare the patient’s interpupillary distance with the distance between the optical center of the lenses. If the

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lenses have any significant power, the induced prism caused by incorrectly positioned optical centers can cause troubling symptoms (eg, a pulling sensation whenever the glasses are worn or the vague sensation that the glasses “aren’t right”).

In the case of bifocals, assess the position of the upper edge of the bifocal segment while the patient is wearing the glasses. The rule of thumb is that the top of the bifocal segment should be aligned with the lower eyelid margin. Bifocals that are too low will cause problems because the patient’s head is tilted back to move the segment high enough to read. When the segment is positioned too high, the patient may have visual symptoms (a blur inferiorly) or complain that the head must be tilted downward in order to see distant objects clearly.

If the mystery has not yet been solved, then it is time to go to the examination room to repeat the measurement of the refractive error. The process may take longer than usual since the original measurement may not have produced the proper result. If different values are found for the refractive error, then a decision must be made regarding whether the difference is sufficient to cause the patient’s symptoms. However, as is often the case, the result is the same as (or very close to) the previous attempt. This can leave the examiner scratching his or her head, wondering where the problem could be.

There are certain scenarios in which, despite all the proper measurements being done, the patient will have symptoms causing complaints. This situation can sometimes be avoided if the patient is warned ahead of time about what to expect with the new glasses and how long the symptoms can be expected to last. Trying to explain this to the patient after hearing his or her complaints is almost futile. The patient will perceive these explanations as a feeble attempt to avoid fixing the problem. Therefore, any time a prescription is given for a pair of glasses where the parameters do not match the existing pair, the patient should be informed that the new glasses may feel a bit strange for a couple of days or possibly even weeks. Problems that persist after a couple of weeks should be investigated.

The common situations that cause temporary problems are:

•Prescribing more plus for a hyperope than what was measured with manifest refractometry (but not necessarily the entire cycloplegic amount).

•Changing the cylindrical axis, especially when it is moved farther away from 90 or 180 degrees.

•Increasing the cylinder power.

•Changing the base curve and/or material of the lens.