The pediatric ophthalmologist has to perform this skill routinely, but what about the rest of us?  This skill comes in handy.  How about that little old lady with a spine problem who just can't straighten up behind the phoropter?  How about that gentleman with a permanent head tilt?  You can get accurate retinoscopy readings on anyone, without a phoropter, and the procedure is relatively easy once you understand it.

When retinoscopy is performed with trial lenses, using a cylinder is not the most efficient (fastest) method to determine the astigmatic correction.  The optical (power) cross works nicely for this task.  With this procedure, only spherical lenses are needed. The steps, assuming there is astigmatism, are as follows:
 
1. Neutralize the most minus meridian.  You will be holding up spherical lenses only in front of the eye.  Just as with a phoropter, add minus power if necessary until all meridians have "with" motion.  Then add more plus power until one of the meridians is neutralized.  Write down the power of the spherical lens that was used to reach neutrality.  Also write down the meridian that is being streaked.  By "meridian", I mean the axis that you are sweeping, not the axis of the streak.
2. Neutralize the meridian 90 degrees away from the one that was just neutralized.  This meridian will still have with motion, so you will be using a more plus (or less minus) lens to arrive at neutrality.  Write down the power of the spherical lens that was used to reach neutrality. Write down the number (axis) of the meridian that was scoped; this will be 90 degrees from the first meridian.
3. Use the optical cross to compute the refractive error.
 
Example:  Suppose the first meridian to neutralize is at 90 degrees.  This means that your streak is aligned with 180 and you are moving the streak along the 90 degree meridian.  Suppose that it neutralizes with a -4.50 sphere held up in front of the eye.  Now you will streak the 180 degree meridian and suppose it neutralizes with a -3.00 sphere.  Your notes are as follows:
 
-4.50 at 90      -3.00 at 180
 
You will draw an optical cross as follows:
 
oc7.gif
 
 
Now we will convert the optical cross to a prescription:
 
We will start with the vertical meridian and write down -4.50 as our sphere power.  On the number line we travel from -4.50 to -3.00, which is a distance of +1.50, which we write down as our cylinder power.  The axis is the same as the meridian that corresponds with our starting sphere power, which is 90 in this case.  This results in:
 
-4.50 +1.50 x 90   or   -3.00 -1.50 x 180
 
You will also need to subtract the working distance from the sphere power.  The two most common working distances are 26 inches (subtract 1.50) and 20 inches (subtract 2.00).  If using 26 inches, your retinoscopy result will be:
 
-600 +150 x 90  or  -4.50 - 1.50 x 180
 
 

Wide Field Imaging: The Future of Retinal Photography?

I recently had the opportunity for an extended trial use of the Optos 200Tx ultra-widefield high resolution imaging device. As the name implies, the main advantage over "conventional" retinal photography is that the field of view is expanded from 30 degrees or 50 degrees to a 200 degree field of view, which covers approximately 80% of the retina in a single view.

When I first heard of the device, my first thought was that this device would have a very limited application, because most "diagnostically significant" retinal diseases have manifestations in the posterior pole, which is covered in a 30 degree field of view. I had also heard that this device was primarily marketed to optometrists, as an "add on" service for a more "state of the art" eye exam. In other words, for an additional fee, the patient can have an instant examination of the entire retina without the discomfort of an indirect ophthalmoscopic examination. Because of these pre-conceptions, I had thought of this instrument as a gimmick rather than a serious diagnostic device. After using this instrument extensively for several weeks, it has changed my thinking.

Going into our trial period with the instrument, I had three major questions:

1. Does this instrument provide you with information that is not attainable with a conventional fundus camera? If so, how often does this occur?

2. Is the resolution of the posterior pole sufficient for diagnostic purposes? In other words, if you have this instrument, are you also going to need a 30-50 degree fundus camera for the posterior pole?

3. How "operator friendly" is the instrument? In other words, will you need a degree in medical photography in order to obtain good quality images?

For most imaging needs, the instrument is very "operator friendly". The software is intuitive and the basics of patient management/ image capture can be taught/learned in a few minutes time. The patient "peeks" through a hole and the eye fixes on a bright target. The operator aligns the eye by guiding the patent's head. Because the imaging device is a scanning laser ophthalmoscope, no focusing is required, and no exposure adjustments are needed. Some operator skill is required if you desire an image that is not in the central 200 degrees. If you want an image farther out into the periphery, which is possible, then some "unconventional" techniques are required.

Does this instrument provide you with information that is not attainable with a conventional fundus camera? Yes. Border to border images of nevi/tumors in the periphery are attainable with this camera, which are not attainable with a conventional fundus camera. Peripheral diabetic eye disease that is easily missed with a conventional fundus camera (even with a survey), is easily seen with this camera. Name any peripheral eye disease, and what may be impossible to document with a conventional fundus camera now becomes relatively easy to image.

Is the resolution of the posterior pole sufficient for diagnostic purposes? Yes, see images below. Photographs can be captured in two "zoom" modes, 200 degrees and 100 degrees. The 100 degree mode provides a zoomed in view of the posterior pole that includes the Superior and inferior arcades.

Is this the future of retinal photography/fluorescein angiography? Why would you settle for 50 degrees when you can have 200 degrees? Why would you drive a Ford instead of a Ferrari? Yup, it always boils down to the price tag, especially if that Ford is already paid off.

Optos images: Keep in mind that the resolution of these images has been reduced by me in order to fit the article and to keep the download time reasonable. The images on the instrument would easily fill a 32 inch screen and would zoom in to nice detail.

Below is a typical 200 degree color image from the Optos. This is an image of a diabetic retina with laser scars. Notice the eyelashes in the image superiorly and inferiorly. This is typical. Because of the ultra-wide angle, it is difficult to totally eliminate lashes from the image. The lashes can be cropped out to produce an image for presentation.

Below is an image of a horseshoe retinal tear. This was taken on the 200 degree setting, but I have cropped it for presentation.

The image below of a treated malignant melanoma is in the 200 degree mode and it is not cropped. This is not the typical straight ahead view. The patient is looking up and temporally in order to get the entire lesion in view. There are peripheral fixation aids for the patient to fix on, but this technique does require some operator skill.

The 200 degree image below is a fluorescein angiogram of diabetic retinopathy. Again, this is not the typical straight ahead view. The patient is looking temporally in order to get a better view of the far periphery, where diabetic disease is evident that would not typically be documented with a 30 or 50 degree fundus camera. The image has been cropped at the top and bottom of the image. We have found that fluorescein angiography on diabetics with this instrument often reveals pathology that would not otherwise be detected.

The image below is a 100 degree field of view that has been cropped to correspond to roughly the 50 degree angle of view. This is to demonstrate that the detail of the macula is very nice and is comparable to the detail of a 30 or 50 degree fundus camera.

The image below is the full 100 degree view, without any cropping. This is an auto-fluorescence image of optic nerve head drusen.

 
 
 

logo_copyright2.jpg

Frame Foundation

 

The changes to the JCAHPO exam content areas has done away with some of the optics subject matter, specifically some of the "opticianry" subject matter, such as lens design and frame specifications. This may be good news for exam takers, but it is unfortunate in that assistants and technicians will be less educated regarding skills that directly affect the visual performance of the patient. This knowledge will make you a better technician and a better caregiver, so eyetec.net is going to cover it.

As a technician who performs refractometry, you are likely to be asked to make recommendations regarding glasses design, and you are likely to be asked to solve glasses complaint problems.  Having a basic knowledge of lens design will help you head off potential spectacle problems, and help you resolve many glasses complaints.

Although it is an asset to be able to consult with a knowledgeable optician, don't assume that every optician that you deal with is more knowledgeable than you are concerning lens/glasses design.  Some of the discount optical shops employ salespersons with little actual opticianry kowledge.  The more you know about opticianry, the better the technician you will be.

We will explore how lens design can have a profound affect upon the comfort and performance of your patient's glasses.

Lens Thickness

One of the main goals of opticianry is (should be) to produce a pair of glasses with lenses that are as thin as they can practically be.  Thick lenses are not only cosmetically unappealing, the thicker the lens is, the more aberrations will affect the patient's vision.  Of course, a main factor that affects the lens thickness is the prescription.  However, there are several other factors that can be controlled in order to minimize thickness.

Lens Size

For a given prescription, as the lens size increases, lens thickness increases proportionally.  For a plus lens, the center thickness will increase as the lens size increases.  For a minus lens, the edge thickness will increase as the lens size increases. 

   
 

 

Lesson learned: smaller lenses are generally better optically than larger lenses.

   
 

 

Lens Decentration

For a given prescription, as the amount of lens decentration increases, lens thickness also increases.  Lens decentration occurs when the optical center of the lens is not the same as the geometric center of the lens cutout for a particular frame.  In the picture below, the patient's PD lines up perfectly with the center of the lens cutouts in the frame.  The lens must be ground from a blank that will accommodate the size of the lens shape.

 

 

In this picture, the frame is wide and the patient's PD is narrow.  The optical centers of the lenses must be decentered inward relative to the geometric center of the lens cutout in the frame.  In order to align the optical center and accommodate the size of the frame cutout, a larger blank must be used.  A larger bank means a thicker lens.

 

   
 

Lesson learned:  It is better to choose frames that line up the patient's pupils close to the geometric center of the lenses.

So how would you know how much decentration would be required for a given frame and given patient?

First of all, we should be familiar with frame measurements:

   
 

   
 

The "A box" is the distance between the temporal edge of the lens and the nasal edge of the lens.  The "DBL" is the "distance between lenses" as mounted in the frame.

The "frame PD" is the A box measurement plus the DBL measurement.  This is equal to the distance from the center of the left lens opening to the center of the right lens opening, on a horizontal line.  This can easily be measured using a millimeter ruler from the outside edge of the right lens opening to the inside edge of the left lens opening.

 

 

The total decentration can be calculated by subtracting the patient's PD from the frame PD.  This measurement assumes that the patient's face is perfectly symmetrical.  Monocular decentrations can be calculated by taking monocular PD measurements and subtracting from half the frame PD.

With this simple information a comparison can be made between the decentration values for two different frames that the patient may be considering, or between the new glasses/frames that the patient is complaining about and the patient's old frames.  Remember that less decentration means thinner lenses.  A decentration value of zero would be optimum.

   
 

Lens Shape

For a given prescription, the more irregular the lens shape is, the larger the lens blanks must be, and consequently the greater the lens thickness will be. 

The key is the effective diameter (E.D.) of the lens shape.  The effective diameter of a lens shape is defined as twice the longest radius of the shape.  The longer the effective diameter is, the thicker the finished lens will be.

 

Let's look at an exaggerated "aviator" sample lens shape:

 

The first thing we do is draw a box around the shape and draw diagonals in the box.  This will give us the center point of our shape.

 

Next we will draw a radius line (red line) from the center point to the lens edge that is farthest from the center point.  This will give us the longest radius of the shape.

 

The lens blank for this shape would have to have this same radius.

 

Compare the above to the size of a lens blank that would accommodate a circular lens that would fit into the same box.

   
 

Lesson learned: Irregular lens shapes increase the lens thickness.  More regular (circular) lens shapes keep the lens thickness to a minimum.

 

Lens Style

Lens style refers to mounting and edge treatments of the lens that are either dictated by the style of the frame or that are added to alter the cosmetic appearance of the lens.  Examples would be a grooved rimless mounting or a metal rim.  Other examples would be rolled and polished edges or faceted edges.

The thickness of a plus lens can be minimized by grinding the edge of the lens to a very thin edge.  The thickness of a minus lens can be minimized by keeping the center thickness of the lens to a minimum.  When we do this, we minimize the saggital value (sag) of the lens.  The saggital value is the millimeter distance of a perpendicular line from the front vertex of a lens to the back plane.

 

 

A grooved rimless mounting of minus lens would work well because there is usually plenty of edge thickness to insert the groove without affecting the sag value.  On the contrary, a grooved rimless mounting of a plus lens is not desireable because thickness would have to be added to the lens edge in order the accommodate the groove.  This would increase the sag value and add to the overall thickness of the lens.  The same can be said for edge facets, edge scallops, and rolled edges.  They work well with minus lenses but add sag value to plus lenses.

   
 

Lens Weight

It is obvious to all of us that excessive lens weight is a major irritant for glasses wearers.  Lens weight is affected by the same factors that affect lens thickness.  The more volume there is to a lens, the heavier it will be.

With modern frame materials, even stocky frames can have a minimal effect on overall glasses weight.  The most important factor affecting weight is lens size.

 

Lens Materials

Plastic lenses are much lighter than glass lenses, with polycarbonate lenses being the lightest material of all.  High index materials provide thinner lenses for a given prescription, but they do not necessarily provide lighter lenses for a given size.  This is because high index materials are denser than lower index materials.

 

 

Back to top                      

 

 

 

  

 
  logo_copyright2.jpg

Banish "Count Fingers"!

     
 

We are all very familiar with the visual acuity progression. First, you attempt to get the patient to read on the Snellen acuity chart.  If nothing can be seen on the acuity chart, then you have the patient count fingers.  If fingers cannot be seen, then you have the patient look for hand motion, etc.  The problem with this method is that "count fingers" is not precise.  Anyone who works for a retina practice should be aware that there is a very wide range of functional vision between "count fingers at 4 feet" and "hand motion".  Some patients who cannot read the big E on the acuity chart are still able to function well in their daily environment.  Other patients who cannot read the big E are functionally blind.  It's not that all the patients in the second group are just complainers, its that they really cannot see as well as the folks in the first group.  The problem for us is that the "count fingers" measurment does not do a good job of differentiating the two groups.

There is a more precise way to measure visual acuity if the eye cannot see any letter on the Snellen chart at 20 feet.  As described above, the usual procedure is to check "count fingers" at this point.  Many retina specialists, who deal with low vision patients often, prefer to obtain the best Snellen visual acuity in this situation by bringing the Snellen chart closer than 20 feet.

If you have a chart hanging on the wall, you can simply take the chart off the wall and hold it closer to the patient, or ask the patient to move closer to the chart.  If you have a projected Snellen chart, then you will need a handheld chart. You can buy a cardboard Snellen chart, or you can buy a Fonda Low Vision Chart, which is very similar.  Once you have a chart, you can then run some copies on your copier, and then you will have charts for each exam room.  A standard Snellen chart is about 11 inches wide and 22 inches long, so you will need to copy the chart in 2 sections since your paper is 8.5 x 11 inches.  You will not need to copy the smallest letters, because if the patient can see those then he can see the big E on the wall chart.

Procedure:  The room light will need to be as bright as possible.  Start by holding the chart 5 feet away.  The fellow eye is occluded.  Just as with a regular Snellen acuity test, have the patient read the smallest line possible.  If no letters an be seen at 5 feet, then hold the chart at 2 feet.  If nothing can be seen at 2 feet, then hold the chart at 1 foot.  There is nothing magic about the distance. Meters can be used also, for example, a one meter distance and then a half meter distance.

Once you have the distance and the smallest line read, you will then need to calculate/record the visual acuity.  There are two options, depending upon the preference of your doctor.  The first option is to simply record the VA in regular notation.  For example, the patient can read the 20/80 line at a distance of 5 feet away.  The visual acuity can be recorded as 5/80.  The problem with this method is that there is no standard distance.  The patient may see 5/80 on one visit and 3/60 on the next visit.  It is not immediately obvious which is the better visual acuity. 

The other option is to convert to 20/xx equivalent.  This is accomplished by dividing the test distance into 20, and they multiplying the result by the line read.  For example, the smallest line our patient can read is the 20/80 line at a distance of 5 feet away.  As discussed, the regular notation would be 5/80.  Divide 5 into 20 to get a result of 4.  Now multiply 80 by 4 to get 320.  The 20/xx equivalent of 5/80 is 20/320.

To avoid having to do calculations all the time, decide on 2 standard distances to use.  A good choice is 5 feet and 2 feet.  You can then record the standard results on your charts, to be read and recorded after testing.  Here are the calculations below.  For example, next to the 20/80 line on your chart you would mark, in small figures, 20/320 and 20/800.  If the 20/80 line is the smallest line read, you would record 20/320 if you are 5 feet away, 0r 20/800 if you are 2 feet away.

   
    Line

5 feet

 2 feet
            20/400 20/1600  20/4000
            20/200 20/800  20/2000
            20/100 20/400  20/1000
            20/80 20/320  20/800
            20/70 20/280  20/700
            20/60 20/240  20/600
            20/50 20/200  20/500
            20/40 20/160  20/400