How to check the calibration of the B&L keratometer      

The B&L keratometer should be checked for accuracy periodically (perhaps once a year), but it rarely needs recalibration. All ophthalmic assistants/technicians are capable of checking the accuracy of the instrument.

Although the steps necessary for recalibration are relatively simple, they must be performed with care to avoid mistakenly adjusting the keratometer out of proper calibration.  A professional ophthalmic maintenance person should be the one who performs recalibration if the instrument is found to be out of adjustment.

In order to check the accuracy of the keratometer, you will need a set of standard spheres. These are usually highly polished steel balls that have a know curvature. Some kits have only one sphere, some have three. There is some type of mounting device included with the kit.   The kit pictured here includes a magnetized mount that attaches to the headrest of the keratometer. The steel spheres are magnetically held onto the end of the mount. The steel spheres include curvatures of 40.25, 42.25, and 44.25 diopters.                                                                  

1. Set the eyepiece.

This step is absolutely critical. If you do not adjust the eyepiece, you will be in danger of finding your keratometer to be out of adjustment when it may not be.  To set the eyepiece, dial it in the plus direction until the crosshair is out of focus. Now slowly turn the eyepiece in the minus direction until the crosshair just come into focus. Do not move the the eyepiece back and forth in the plus and minus directions, only approach the point of focus from the plus direction.

2. Mount the test sphere.

Clamp the sphere mount to the headrest of the keratometer. If the kit has removable spheres, place one (it doesn’t matter which one you start with) on the mount. If the spheres are steel they may have a dull finish if they have not been used in a while. If so, use a soft cotton cloth to polish the surface.                            

3. Measure the test sphere.

Measure the curvature of the sphere just as you would a cornea. You will find the sphere to be much more cooperative than an actual patient. If the reading on the horizontal and vertical measuring drums matches the designated diopter value of the sphere (plus or minus an eighth of a diopter), then you are finished! You have checked the calibration of the keratometer and it has been found to be accurate. If you have other spheres, you can use one or more to confirm the calibration.

As discussed, if you find your keratometer to be out of calibration, it is best to have the instrument re-calibrated by a professional  ophthalmic service person.

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You don't check near vision?!!!

Some techs that I work with don't bother to check near vision. I'm talking about at the beginning of the exam, when you check distance vision. There are various reasons given: "It takes too much time", "My doctor doesn't care if I do it or not", and "What good does it do, anyway?"  I'm glad you asked.  There are two good reasons to check near vision.

Reason #1:  Useful information regarding the patient's refractive state can be gleaned from the near vision measurement.

Reason #2:  Userful information regarding glasses difficulties can be gleaned from the near vision measurement.  For many patients, the refraction/glasses evaluation is the most important part of the examination, particularly if the patient has no ongoing disease process (e.g. dense cataracts, glaucoma, AMD).  Most patients rightly expect a good pair of glasses that function well at distance and near.  How do you know about how the patient functions at near unless you observe them trying to read?

To fog or not to fog, that is the question.

With regard to ophthalmology and optometry, fogging refers to the technique of adding plus sphere power during refraction and/or retinoscopy in an attempt to control accommodation. Accommodation refers to the ability of the natural lens in the eye to change shape, and thus to focus light on the retina for sharper vision. During retinoscopy or refraction, the technician does not want the eye to accommodate, because accommodation introduces an uncontrolled variable into the measurement process.

The goal is to move the focal point in front of the retina. The eye accommodates in order to see more clearly.  If an eye is optically fogged,  and the eye accommodates, the vision will get blurrier,  not clearer. Thus,  accommodation is discouraged.  Care must be taken to not fog the eye too much.  If the vision is blurred too much, accommodation may actually be stimulated in a effort to see better.

How fogging works

The eye in the diagram to the right has a glasses correction that has left it under-plused. When accommodation is relaxed,  light focuses behind the retina and the vision is slightly blurry.

The patient accommodates (the natural lens gets "fatter") to see better.  Accommodation moves the focal point onto the retina.

We fog the eye by adding enough plus power to move the focal point in front of the retina.

If the patient accommodates now,  his vision will get blurrier instead of clearer.  

Accommodation now causes the focal point to move furthur forward. Thus, accommodation is discouraged.

Fogging must not be used when using the cross-cylinder to check the cylinder power and axis. Cross-cylinder techniques work best when the eye is able to accommodate to see better.

In some situations, the question is not "to fog or not to fog", but rather how much to fog. The question has come up a lot recently with regard to the COT "skill" exam that the COA must take after passing the COT written examination. Some test takers are not passing the computerized retinoscopy skill exam at least partially because they failed to fog the follow eye before performing the retinoscopy exam.

I have never fogged the fellow eye during retinoscopy and my accuracy has been just fine, thank you. If you talk to an optometrist, without fail you will get the recommendation to fog the fellow eye. The problem with this practice is that it can often be "under-effective" and it can be "over-effective". Suppose you are performing retinoscopy on a +3.00 D hyperope OU for the first time, and he does not have an old Rx that would give you a clue. Using even +1.50 or the "R" lens would not get you close to fogging the fellow eye. Conversely, if the eyes where -3.00 D OU, the "R" lens over the fellow eye would be serious overkill.

Never the less, JCAHPO expects you to fog the fellow eye as prep for the retinoscopy skill evaluation. JCAHPO is not saying how much they expect you to fog (despite repeated attempts to get an answer). The consensus seems to be to fog with +1.50, or the "R" lens.

What does research have to say? I have done an extensive literature search in an effort to discover the “correct” amount to fog.  What I found was that recommendations range from .5 D to 6 D.  I did find a research study reference that concluded that the amount of fogging made very little difference in the accuracy of retinoscopy (link below).
http://www.ncbi.nlm.nih.gov/pubmed/9170797

By the way, if you don't fog during retinoscopy, accommodation can be discouraged sufficiently by having the patient view a non-accommodative target with the fellow eye. Examples are the big "E", or just a circle or square of light on the screen. Children are great accommodators, so a good practice is to perform retinoscopy on them after they are dilated with a good cycloplegic drop such as cyclopentolate.

The Maddox rod is a dissociating test that will reveal and measure a phoria or a tropia. A dissociating test is a test that presents dissimilar objects for each eye to view, so that the images cannot be fused. The MR test is most commonly used only to measure phorias.

The problem with the Moddox rod test is that it can be confusing to tester, not to mention the testee.  You might consider performing the Von Graefe technique. This test gives the same results, it is easier to remember how to perform, and it is usually faster to perform.

The Von Graefe Setup

The patient looks through the photoptor with the distance or near Rx in place.

The target is an isolated (small box) letter on the Snellen chart one line above the BVA of the worst seeing eye. 12 diopters base-in prism is dialed into the Risley prism in front of the right eye. Six diopters base-up prism is dialed into the Risley prism in front of the left eye. The setup should look like this: 

The patient should see something like this:

Confirm that the patient sees double. If not, the patient may be suppressing, or not understand. Either way, the test cannot be completed without the perception of two images.

Testing Horizontal deviations:

Always test horizontal deviations first. This is controlled by the Risley prism in front of the right eye. Move the Risley prism wheel slowly in one direction and ask the patient if the images are getting closer or farther apart horizontally. If they are going farther apart, move in the other direction. Ask the patient to tell you when the images are lined up directly one above the other. Record the prism power and base direction on the Risley prism at that point. This is the measure of the horizontal deviation. The end point should look like this to the patient:

A more precise measurement can be made by moving past the endpoint to horizontal separation again and coming back from the other direction to the endpoint once more. Record this second reading and average the two.

A base-in measurement indicates an exo deviation and a base-out measurement indicates an eso deviation.

Testing Vertical deviations:

Move the Risley prism in front of the right eye back to the 12 diopter base-in position. Confirm that the patient is seeing the two separate images. Vertical measurements are controlled by the R-prism in front of the left eye. Move the Risley prism wheel slowly in one direction and ask the patient if the images are getting closer or farther apart vertically. If they are getting farther apart, reverse the direction. Ask the patient to tell you when the images are directly across from one another on a horizontal line. This is the endpoint and it should look something like this to the patient:

Record the base direction and the amount of prism diopters on the scale.

Again, for a more precise measurement, you can dial in more prism power until there is once again vertical separation of the images. The prism power is then reversed until the the images are once again aligned on a horizontal plane. The two measurements are averaged.

Notes: There is nothing magical about the starting diopter values for this test. You can use other values, and sometimes you may have to. For instance, if the patient has a 12 D exophoria, then images will be vertically aligned at the outset. The point is that you want to start the measurement with some degree of vertical and horizontal separation of the images.

You may encounter this problem so infrequently that you can't remember the proper procedure.  So this is a reminder!  I guarantee that you exam takers will encounter this on a certifying exam.

Corneal astigmatism above 4 diopters will cause a reading error due to distortion of the mires from the normal circular shape.  It is not often that some of us encounter 4 diopters plus of corneal astigmatism, but is does happen, especially if you happen to work for a corneal surgeon.  The problem is that high astigmatism distorts the shape, and particularly the relative size of the mires.

If you were to see the image below when performing applanation tonometry, you would notice that the upper mire is larger than the lower mire.

Ordinarily this would indicate to you that the tip is too high on the cornea and that you should adjust downward in order to make the the mires the same size for an accurate measurement.  However, if this image is caused by high astigmatism, vertical adjustment would do you no good.

This error can be compensated for by aligning the red line on the applanator tip with the number on the mount that corresponds to the minus cylinder axis of the astigmatism.  Suppose the eye has a glasses prescription as follows:

 -2.00 + 4.50 x 30.  Transpose this to +2.50 -4.50 x 120.  The 120 axis number is then used to rotate the applanation tonometer tip in the mount, as follows.

Notice that the tip has been rotated so that the "120" on the scale has alignedwith the red mark on the tip holder.  This is the proper position for measuring this cornea.  You will now see an image that resembles the image below. You will have to measure using the image on the slant, but the mires will be the same size and the measurement will be more accurate.

What is the best way to troubleshoot an ophthalmic handheld device that is not working?  Common examples are a direct ophthalmoscope, a muscle light, a retinoscope, or a glare tester.  The light is not working.  What is the first thing you do to get it working again?

A. Replace the bulb with a new one. 
B. Replace the battery with a new one.
C. Put it back in the charger for another few hours.
D. None of the above.

I would argue that the correct answer is none of the above, for the following reasons:

A. The bulb is typically one of the most difficult parts to replace in these instruments, so I prefer to start with the battery.

B. A new battery may not be charged, thus providing you with no help in your troubleshooting.

C. Putting it back in the charger is generally a waste of time.

I find that the fastest way to troubleshoot a battery powered handheld device is to find an identical instrument that is working.  This way you know that you have a working bulb and you have a working battery.  Switch the batteries between the two instruments.  You want to switch both batteries because this will tell you if you have a bad bulb and a bad battery, which sometime happens.  If the non-working instrument does not light with the good battery, you know that there is a problem with the bulb.  If the working instrument does not light with the switched battery, you know that the battery is also dead.  Remember that a dead battery does not necessarily mean that the battery is bad, it may just need to be charged.  If you have many exam rooms, it is possible that you have an instrument stand with a non-working charger.  When charging a dead battery, it is best to use a stand alone charging unit that you know works.