Thursday, 18 December 2014

What’s so special about specialty lenses?

Advancements in specialty lens manufacturing have made it possible for more patients to wear contact lenses

December 16, 2014
By David I. Geffen, OD, FAAO

Increasingly, many practitioners in today’s practices are using specialty contact lenses. These contact lenses historically have been viewed as hard to fit, difficult to find, and much higher in cost. Let’s look at what’s so special about specialty contact lenses.

Extended-range lenses

What determines that a lens is a “specialty” lens? There are many categories of these lenses and way too many to list every brand and manufacturer. The most common specialty lenses are the lenses which fall out of the parameters of normally available lenses in a category. These would be lenses that are higher in spherical power than the company normally manufactures. Biofinity (CooperVision) is a typical lens in this category. Biofinity spheres are available from +6.00 D to -10.00 D—a very large range, which is good for the vast majority of patients. The Biofinity XR or extended range lens is now available from +15.00 D to -20.00 D.

Dr. Geffen: The value of a contact lens patient

Most doctors and manufacturers consider these extended-range contact lenses to be specialty lenses. These lenses allow us to serve the need of our patients who fall out of the normal parameter ranges. These are ordered and manufactured on an as-ordered basis at this time so you may need to wait up to a few weeks to receive these lenses. There are also a number of smaller specialty lens companies that manufacture lenses on an as-needed basis. These companies have thrived in providing eyecare practitioners with lenses with most any power or base curve or diameter a patient may need. They also can get these lenses to patients in a few days.

Toric and multifocal lenses

Toric lenses are the next category of specialty lenses. Like spherical lenses, the major manufactures have a limited power range typically from +6 to -9 with 3 or for cylinder powers. The typical cylinder limit for readily available toric lenses is -2.25 D. Smaller labs are able to fill in the gaps and make a toric lens for our patients with almost any axis or power we need. These lenses are available in a few days to our patients.

Multifocal lenses are also available in many custom designs from small contact lens manufacturers. Some ingenious designs have come out of these labs and we can serve many more patients than ever before.
Lenses for irregular corneas

Now let’s cover some of the truly special designs coming out of smaller labs. Lenses for irregular corneas are changing the way we think about providing excellent vision and comfort to our patients. These lenses have provided a way for doctors to care for those patients who cannot or will not wear rigid gas permeable (RGP) lenses. Bausch + Lomb brought us the Kerasoft soft contact lens for irregular corneas. Kerasoft is available in a wide range of custom parameters and base curves to correct the irregularities in the cornea caused by keratoconus, surgical imperfections, and disease.

Dr. Geffen: R&D needed for new gas perm car systems, materials

Another lens in this category is NovaKone from Alden Optical, one of several specialty contact lens labs that have made great innovations in soft lens designs. NovaKone comes in any custom base curve you would like and has different center thickness parameters to correct for irregularities. There are several other lenses of these types from great-minded individuals creating new and improved contact lenses for every need.

What makes them special

So why are these lenses special? They seem to be treated as devices only certain doctors are allowed to use. They have a reputation as being difficult and eating up too much chair time for many doctors. However, because specialty lens companies are utilizing very advanced manufacturing processes, the reproducibility and quality has never been better. You can have the confidence that the patient will not be calling your office complaining about the lens. These lenses do take a little more time and thought. Specialty contact lens company consultants are excellent partners in your practice—they want you to succeed and make it as easy as possible. The cost of the lenses is higher than standard lenses, and this is a concern for many doctors. Remember that these patients know that they are not the usual contact lens wearer and know that they have special needs. As such, they expect to spend more for both your expertise and the cost of the device. Optometrists are notorious for not charging enough for our expertise and time. We need to be compensated for the extra knowledge and study we do to learn about these special lenses and the time we spend without patients.

So what is so special about specialty contact lenses? I can answer that question only one way. These lenses are so special because we are truly changing the life of patients who has sought our expertise to improve their quality of life!

Saturday, 15 February 2014

Tips for handling digital eye strain

Digital eye strain is caused by the overuse of digital devices such as computers and smart phones. Because these electronic devices are designed to be used and held within close range of the eyes, after a while, the eyes become strained as they continue to refocus to process the images on the digital screen.

According to organizations like The Vision Council, more than 70% people don’t know or don’t believe they are at risk for digital eye strain; however, anyone who is in front of a digital screen is vulnerable. Red eyes, twitching eyes, dry eyes, blurred vision, headaches, neck pain, decreased productivity and increased work errors, fatigue from staring at a digital screen, and straining to see small fonts and images are some of the signs and symptoms that occur when experiencing digital eye strain.

“In our fast-paced society, most people use a computer throughout the day while they’re at work, and they also go online to communicate with friends, read books, and even pay bills.”

“It’s just the way we operate in the 21st century. Nevertheless, people can stay digitally connected and also maintain the health of their eyes.”

Optometrists suggests following tips for avoiding digital eye strain:

1. Follow the “20-20-20 rule.” Be mindful of the amount of time that is spent looking at a computer screen without taking a break. Every 20 minutes, take a 20-second break and look at something that is 20 feet away. Looking far away relaxes the focusing muscle inside the eye and reduces eye fatigue.

2. Reduce glare. People often see reflections from objects around their computer on their computer screen. Install an anti-glare screen on the computer monitor to reduce glare on the screen. Cover windows with drapes and blinds, and use a computer hood to block some of the overhead and peripheral light. Get anti-reflective (AR) coating on eyeglass lenses.

3. Work in proper lighting. When looking at a digital screen, the surrounding light should be half as bright as what is typically found in most offices. Try to position the computer screen so windows are on the side (instead of in front or behind) the computer screen. If the interior lighting is a concern, consider reducing the number of fluorescent tubes that are installed above the computer. Also consider turning off the overhead fluorescent lights in the office and use lamps that provide halogen or incandescent lighting, or switch to lower intensity bulbs.

4. Blink often. People tend to blink less often when they look at a computer screen—approximately one third less often as they normally blink—and a lot of the blinking that takes place when looking at a digital screen are only partial lid closures. Blinking less often can cause the eyes to become dry. To reduce the chances of experiencing dry eyes when looking at a digital screen, try this exercise: Every 20 minutes, blink 10 times by closing the eyes very slowly, as if falling asleep. This will moisten the eyes, and it will also help the eyes refocus.

5. Revise the workspace. When working on a computer, people often look back and forth between the computer screen and a printed page, which can cause eye strain. To alleviate the stress and strain on the eyes, put the printed pages on a copy stand that is next to the computer monitor. Make sure the paper on the copy stand is well-lit by using a desk lamp. Poor posture can also lead to problems with clearly seeing a digital screen. Consider purchasing ergonomic furniture where the computer screen is positioned 20 to 24 inches from the eyes. The center of the digital screen should be 10 to 15 degrees below the eyes.

6. Get a regular comprehensive eye exam. Computer users should have eye exams once a year. Before the exam, be sure to measure the distance between the eyes and the digital screen. Share that measurement with the eye care provider, and remember to let the doctor know how often computers and smart phones are used. 

Friday, 14 February 2014

Bielschowsky's head tilt (3 step) test

When a healthy individual tilts their head, the superior oblique and superior rectus muscles of the eye closest to the shoulder keep the eye level. The inferior oblique and inferior rectus muscles keep the other eye level. In patients with superior oblique palsy, the superior rectus muscle’s action is not counteracted by the superior oblique muscles. This leads to vertical deviation of the affected eye when the head is tilted towards the effected eye. However, there is no deviation when the head is tilted towards the unaffected eye because the superior oblique muscle is not stimulated in the effected eye, but rather it is stimulated in the unaffected eye. When there is a discrepancy in ocular deviation based on which way the head is tilted, the patient is diagnosed with unilateral palsy of the superior oblique muscle due to damage in the Trochlear Nerve.

People with superior oblique palsy on one side experience double vision, which is improved or even abolished by tilting the head towards the shoulder on the unaffected side. Tilting the head towards the shoulder on the affected side will make the double vision worse by causing increased separation of the two images seen by the patient.

Fourth Nerve Palsy

A fourth nerve palsy typically causes diplopia that is worse in downgaze; hence, patients almost always report diplopia (or the tendency to close 1 eye) while reading. In some cases, examination of the affected eye reveals limited downgaze in the adducted position, but, in most cases, ocular motility appears grossly normal. Accordingly, it is essential to perform cover-uncover or Maddox rod testing to demonstrate a hypertropia that worsens on contralateral downgaze. Ipsilateral head tilting usually increases the vertical strabismus, and, therefore, patients typically (subconsciously) tilt their head to the opposite side to avoid diplopia.

The Parks-Bielschowsky 3-step test is a time-honored algorithmic approach to identifying patterns of ocular motility that conform to dysfunction of specific vertically acting extraocular muscles. The 3 steps are

1.     Find the side of the hypertropia.
2.     Determine if the hypertropia is greater on left or right gaze.
3.     Determine if the hypertropia is greater on left or right head tilt.

Beyond these 3 steps, it is also useful to determine if the vertical separation is greater in upgaze or downgaze (a fourth step) and check for relative cyclotropia.
The 3-step test is most helpful in determining whether a vertical strabismus conforms to the pattern of a fourth nerve palsy; for example, a right fourth nerve palsy shows right hyperdeviation that worsens on left gaze, right head tilt, and downgaze, with relative excyclotropia of the right eye.

Occasionally, a skew deviation mimics a fourth nerve palsy on the 3-step test but can distinguish itself by nonconformity to these rules. Practically speaking, the specific muscle(s) involved and the etiology of a vertical strabismus not due to a fourth nerve palsy is often not resolved by the 3-step plus fourth step test, because acquired vertical strabismus is often the result of the dysfunction of more than one muscle. In particular, thyroid eye disease, myasthenia gravis, or dysfunction of multiple ocular motor cranial nerves produces a wide variety of nonspecific patterns of ocular motility. The reliability of the 3-step test in identifying patterns of vertical strabismus lessens somewhat over time because of the phenomenon known as “spread of comitance”.
Bilateral fourth nerve palsy should always be considered whenever a unilateral palsy is diagnosed, especially after head trauma. Bilateral fourth nerve palsy presents with:-
  • ·      crossed hypertropia (ie, the right eye is higher on left gaze, and the left eye is    higher on right gaze)
  • ·        Excyclotorsion of 10° or greater (each eye rotates outwardly; best measured with  double Maddox rod testing)
  • ·         a large (≥25 D) V pattern of strabismus
Brazis PW. Palsies of the trochlear nerve: diagnosis and localization—recent concepts. Mayo Clin Proc. 1993;68(5):501–509.

Fourth nerve palsies are often congenital. An anomalous superior oblique tendon, an anomalous site of its insertion, or a defect in the trochlea are now recognized as causes of some congenital fourth nerve palsies; similarly, some cases of presumed congenital fourth nerve palsy are secondary to a benign tumor (eg, schwannoma) of the fourth nerve. Patients are often asymptomatic until the fourth to sixth decades of life, when their vertical fusional amplitudes diminish and diplopia develops. Most patients maintain a chronic head tilt. The long-standing nature of the head tilt can often be confirmed by reviewing old photographs . Patients with a long-standing fourth nerve palsy have a relatively large vertical fusional range (>3 prism diopters).

In patients older than 50 years, isolated fourth nerve palsy is typically caused by micro-vascular ischemic disease, and function always improves and typically resolves within 3 months. The fourth nerve is particularly vulnerable to closed-head cranial trauma due to the unique dorsal midbrain crossing anatomy. In addition, the fourth nerve can be damaged by disease within the subarachnoid space or cavernous sinus.

Diagnostic evaluation for isolated, non traumatic fourth nerve palsy usually yields little information because most cases are congenital, ischemic, or idiopathic. In patients in the vasculopathic age group, a full medical evaluation looking for vascular risk factors, including diabetes, hyperlipidemia, and hypertension is appropriate. Older patients should be followed to ensure recovery. Lack of recovery after 3 months should prompt neuroimaging directed toward the base of the skull to search for a mass lesion. Other possible causes of an acquired vertical strabismus include orbital restrictive syndromes (eg, thyroid eye disease or previous trauma). Skew deviation, partial oculomotor nerve palsy, or myasthenia gravis should be considered in atypical cases.

Tuesday, 11 February 2014

Magnification and magnifiers

Many people with low vision find magnifiers useful to help them do short everyday tasks such as reading their post or instructions on a packet. Magnification increases the retinal image size. For people with a scotoma this may make an object easier to see, because although the retinal image size increases the area of visual loss remains the same size (Figure 1).

Figure 1: A schematic and simplified representation of how magnification can help a person to read short text.

1. Relative size magnification

This is a linear relationship: doubling the size of the object makes the image on the retina twice as large, creating x2 magnification. This form of magnification is usually limited to about 2.5x because of the physical limitations of enlarging an object. Examples of this type of magnification are large print books, watches or timers (Fig. 2).

2. Relative distance magnification

This is also a linear relationship: halve the distance of the object and the retinal image becomes twice as large, creating x2 magnification. For example, viewing the television from 2m rather than 4m gives x2 magnification (Figure 3).

This type of magnification can also be used for near tasks, e.g. bringing print closer to the eye from 40cm to 10cm gives x4 magnification.

Children and young adults can use accommodation to provide this form of magnification, mainly for short duration near tasks. Myopes who take off their glasses can achieve some magnification without the need for accommodation.

Plus lens magnification

A plus lens creates magnification by allowing the person to adopt a closer viewing distance. When the plus lens is placed so that the object viewed is at the anterior focal point of the lens, the object is focused clearly on the retina and accommodation can be relaxed. Most hand and stand magnifiers work on this very simple principle. The plus lens can be close to the eye, in a spectacle lens, or remote from it, in a hand or stand magnifier.

Limitations of plus lens magnifiers

Field of view: Patients often ask for larger magnifiers, hoping that this will

Figure 2: Making things bigger creates relative size magnification.

Figure 3: Moving things closer creates relative distance magnification.

Figure 4: A wide range of hand magnifiers is available, including folding and illuminated versions

increase their field of view. However, as the power of a magnifier increases, the diameter of the lens decreases, due to the weight of the lens and physical constraints in manufacturing. Instead, they should be encouraged to hold the magnifier as close as possible to the eye, thereby increasing the field of view.

Short working distance: Although the distance from the eye to the magnifier can be varied, the distance from the magnifier to the object is often very short, especially with stronger magnification. This makes it difficult to place implements such as a pen or screwdriver under stronger magnifiers, and directing adequate light on to the object can be problematic.

Hand magnifiers

Hand magnifiers are useful for short ‘survival’ tasks such as looking at packets or the dials on a cooker. Most people find them socially acceptable and they are easy to carry in a pocket or handbag. There are countless designs available at low cost in a wide range of powers, and many are internally illuminated (Figure 4). People with hand tremors or grip problems may, however, find them impossible to use.

Stand magnifiers

Stand magnifiers allow the maintenance of a precise magnifier-to-object distance, which is advantageous because of the small depth of focus of plus lens magnifiers. This means they are particularly useful for sustained tasks or where there are physical difficulties, such as tremor. The most commonly prescribed stand magnifiers are internally illuminated because the stand can obstruct light from getting to the object (Figure 5). Some lower-powered stand magnifiers allow tools, such as a pen, to be used (Figure 6). The disadvantage is that they are very bulky.

Spectacle-mounted plus lens magnifiers

The best optical solution to the difficulties of plus lens magnifiers is to mount them in spectacles: this gives the best magnification and greatest field of view. However, the majority of patients do not like any magnifier that focuses less than 25cm from the spectacle plane. For people who are able to accept shorter working distances, spectacle-mounted plus lenses are sometimes tolerated because they give the best magnification and field of view, and allow their hands to be free (Figure 7). Spectacle-mounted low vision aids can be prescribed monocularly or binocularly

Figure 5: Illuminated stand magnifiers are the most commonly prescribed stand magnifiers. 

Figure 6: A pen may be used under some low powered stand magnifiers.

Figure 7: Spectacle-mounted low vision aids allow the person to do tasks that need both
hands free, but only at a short working distance.

if prisms are incorporated to help convergence. Over +10.00DS, the person is unlikely to maintain binocularity. As well as providing magnification, some allow for the correction of refractive errors; high powered bifocal near additions are also available.

3. Real image magnification

Optical magnifying systems are limited to a magnification of about x20. Real image magnification produced electronically is available in much larger magnifications of x50 and over.

Closed circuit televisions

Closed circuit televisions (CCTVs) produce real image magnification electronically using a camera to create a magnified image on a monitor screen. They are usually used for near or intermediate tasks.

In theory, CCTVs should be the solution to all the frustrations of low vision aid users. They can produce high degrees of magnification, contrast reversal and enhancement, zoom facilities and binocularity of the image with none of the postural difficulties of many other magnifiers. In practice, however, they are expensive, quite difficult to use and often bulky. Only a small proportion of the low vision population use CCTVs, and most do so for longer, sustained reading tasks while they use optical low vision aids for short, survival tasks.

The most common type of CCTV is a TV screen mounted above an ‘X-Y’ table where the object is placed or held (Figure 8). Standard CCTVs cost about £1,500 but many models are much more expensive. TV readers are more affordable (£100 to £500). They consist of a hand-held camera which is plugged into the patient’s own television (Figure 9). The magnification is limited, often fixed at one value and dependent on the size of the television screen. Although they are cheap and quite portable, they are difficult to manipulate.

In recent years a number of head mounted CCTVs have been developed, such as the Jordy. The camera and TV screens are mounted in a virtual reality-type headset, and the control box is attached to the belt. These remain very expensive, heavy, difficult to use and cosmetically poor and, as yet, they cannot be used when walking around.

Unlike optical low vision aids, CCTVs are not provided on the NHS. Employment and education services will usually provide them if deemed necessary for the person’s

Figure 8: Various models of CCTV are available. The material to be viewed is placed on an X-Y table.

Figure 9: A TV reader.

Figure 10: A flat field magnifier.

work or schooling. Older people usually have to purchase their own. Many public libraries, some voluntary organizations for blind people and some social services departments have them available for trial use. Most manufacturing companies will let people try the CCTV in their own home for a short period before purchase. Due to the great expense and difficulty involved in using CCTVs, this approach should be strongly recommended to patients. 

Flat field magnifiers

These are single lenses of hemi-cylindrical or hemispherical form, designed to be put flat onto the object (usually text). The thicker the magnifier is in relation to its radius of curvature, the higher its magnification. This is unlikely to exceed x3 because of size and weight. Flat field magnifiers are very useful for children with a visual impairment as they look like a paperweight or ‘crystal ball’ (Figure 10).

4. Angular (or telescopic) magnification

Telescopes and binoculars are very effective in producing magnification for distance, while allowing the person to stay at their chosen distance from a task, such as viewing a street sign or blackboard. They can also be used for near tasks. Their main  disadvantage is restricted field of view. Also, distortion of space and movement perception prohibits walking around while using the telescope. Their use requires considerable manual dexterity, skill and practice, particularly to follow moving objects. Only a very small proportion of people with low vision use them (Figure 11).

Low vision therapy

Although as yet there is no conclusive evidence1, it is thought that people may benefit from training which maximizes the usefulness of low vision aids and vision in daily life. Some rehabilitation workers are trained to provide low vision therapy, which may take place outdoors with distance aids or in the person’s home environment.

Figure 11. Devices that produce angular magnification. A distance Galilean telescope used for TV viewing, terrestrial telescope and a pair of binoculars

Wednesday, 29 January 2014

Prism and Progressive Lenses


One slight drawback to progressive addition lenses in certain power ranges is thickness. Increased thickness is especially evident when the distance powers are either plus or low minus. Progressive lenses in plus or low minus power ranges will be thicker than a fl at-top multifocal lens of equal power. This increased thickness is a result of the steepening front curve in the lower half of the lens. (This same problem also occurs in “Executive” multifocals and can be solved in the same way.) As the lower progressive portion of the lens increases in plus power, the surface curvature steepens. This thins the bottom edge. To keep the lower lens edge from becoming too thin, the whole lens must be thickened.

To overcome the problem, the lower edge must somehow be thickened without thickening the upper edge. This can be done by adding base-down prism to the whole lens. When this is done properly, overall lens thickness will actually decrease. The technique, known as yoked base-down prism, is illustrated in Figure 1. Naturally, both right and left lenses must receive the same amount of base-down prism; otherwise the wearer will experience double vision as a result of unwanted vertical prism differences.

Figure 1 shows the use of base-down prism to thin a progressive addition lens. A, The progressive addition lens as ground without prism thinning. The dotted lines indicate how the lens would be curved if it were a single vision lens instead of a progressive lens. B, Adding base-down prism thickens the bottom of the lens only. C, The line between prism and original lens has been removed. It is now possible to see how this lens with newly added base-down prism could be further thinned because both top and bottom are thick. D, The hatched area shows how much lens thickness may be removed now that both edges are equally thick. E, Excess lens thickness has been removed and progressive lens prism thinning achieved.

The exact amount of prism needed to thin the lens effectively varies according to the strength of the addition, the size and shape of the lens after edging, and the design of the lens. As a rule of thumb, Varilux suggests adding prism power amounting to approximately two thirds of the power of the add. (The use of yoked basedown prism for Varilux lenses has been referred to by the name Equithin.)

Prism Thinning Causes Prism at the PRP

It should be mentioned that base-down prism used to thin the lens will show up at the PRP of the lens. This is particularly important to note when only one lens is to be replaced since both right and left lenses must have the same amount of vertical prism. Thus vertical prism found at the PRP of the lens is acceptable when both left and right lenses have the same amount of vertical prism.


Success in fitting progressive addition lenses depends on accurate horizontal placement of the monocular PDs. If monocular PDs are incorrect, the eyes do not track down the progressive corridor. This reduces intermediate vision. Incorrect PDs also displace the reading zone, reducing its usable size.

Success in fitting progressive addition lenses is also influenced by the accuracy of fitting cross heights. An inaccurate fitting cross height will cause one eye to track down the corridor ahead of the other. This means that the add power is not increasing equally for the two eyes. The eye farther down one corridor is looking through more plus power than the partner eye following a few steps behind. An inaccurate fitting cross height also causes the eye to track down the progressive corridor off-center, narrowing the effective width of the intermediate viewing.

When prism is placed before the eye, it causes the image of an object to be displaced in the direction of the prism apex. The eye must turn toward the apex to view the displaced image. For example, if base-down prism is placed before one eye, that eye turns upward toward the apex to fixate the displaced image.

Vertical Rx Prism Changes Fitting Cross (and Bifocal) Heights

When vertical prism is present in a prescription, it causes one of the wearer’s eyes to turn slightly up or down. But when fitting cross height measurements are taken, the prism is not present. When the wearer is able to keep the eyes working together without the prism the eyes are looking straight ahead. One eye will not likely be turned upward or downward in relationship to the other.

However, once the prescription lenses are in the frame, the eye must turn in the direction of the apex of its prescribed prism. The amount of displacement in the spectacle plane will be 0.3 mm for every 1 prism diopter of prescribed prism.

When vertical prism is present, the fitting cross should be raised 0.3 mm for every diopter of base-down prism or lowered 0.3 mm for every diopter of base-up prism.

If the entire amount of vertical prism is prescribed before one eye, the vertical displacement of the fitting cross should be carried out on one lens. But if the vertical prism is split, the displacement of the fitting crosses should also be split in the same proportion.

To be certain of vertical fitting cross positioning with perscription prism, cover the wearer’s left eye when measuring the fitting cross for the right eye. Then when measuring fitting cross height for the left eye, cover the wearer’s right eye.


A prescription reads as follows:

 R: +2.75 −1.00x180   “3 prism base up”
      L: +2.75 −1.00x180    “3 prism base down”

The frame of choice is adjusted to fit as it should when being worn. Next fitting cross heights are marked on the glazed lenses to correspond to pupil center location. Heights are measured to be as follows:

                                                 R: 27 mm
                                                 L: 27 mm

What fitting cross heights should be ordered?


Vertical prism for the right lens is noted. The amount of vertical compensation is calculated as follows:

             Vertical prism amount x 0.3 = change in fitting cross height in millimeters.

Or in this case

                                                  3 x0.3 = 0.9 mm.
                                                  This is rounded off to 1 mm.

Because prescribed prism causes the pupil of the right eye to be displaced 1 mm downward, the fitting cross must be moved 1 mm downward as well. The left lens has an equal but opposite amount of vertical prism. Therefore the prism in the left lens necessitates moving the left fitting cross 1 mm upward. The end result is that the two fitting cross heights are modified and should be ordered as

                                                R: 26 mm
                                                L: 28 mm

Horizontal Rx Prism Changes PD Measurements

When horizontal prism is prescribed, failure to horizontally compensate the MRP placement will cause the eyes to track along the inside or outside edge of the progressive corridor. This greatly reduces the usefulness of the intermediate zone and narrows the field of view for near work.


Suppose a prescription reads as follows:

                             R: −2.25 −0.50 x180        “5 prism base in”
                             L: −2.25 −0.50 x180        “5 prism base in”

Using a pupillometer, the monocular PDs are measured as follows:

                                              R: 29.5 mm
                                              L: 30.0 mm

What monocular PDs should be ordered to compensate for the prescribed horizontal prism?


Noting horizontal prism, the amount of pupil displacement is calculated as follows:

                                             5 x0.3 = 1.5 mm.

Base-in prism will cause the eye to move outward by an amount equal to 0.3 mm for every diopter of horizontal prism. In this case 5 prism diopter of base-in prism will cause each pupil to be displaced outward by 1.5 mm. The resulting monocular PDs are modified to
                                             R: 31.0 mm
                                             L: 31.5 mm

When Might the Amount of Horizontal Prism Be Modified?

When prism is prescribed in conventional, nonprogressive, multifocal lenses, the PD is not modified to allow for a change in pupil location. This is quite acceptable because the widths of nonprogressive multifocals are so wide in comparison with the corridors of progressive addition lenses that there is little need for modification.

When an Optometrist tests for prism, the measuring prism on the phoropter is in front of the sphero-cylinder lens combination. As the measuring prism is increased in power, the eye responds by turning, leaving its location behind the OC of the refractive lenses. When the eye moves away from the OC of the lens combination, a second prismatic effect, caused by lens “decentration,” is induced (Figure 3). Practically speaking this second prismatic effect is of no consequence since the measuring prism is taking it into account. But what happens if the refractive MRP location is altered during fitting?

Fig 3 A measuring prism in front of the refractive lens will cause the eye to turn outward. As it turns, it leaves its previous location directly behind the optical center of the lens.

When the fitting cross is changed to correspond to the prismatically altered eye position, the decentration prism that was present during refraction disappears. Without decentration prism, the net prismatic effect that was present during refraction has changed. When prescription sphere and cylinder powers are small, this is of minimal consequence. As the refractive power increases, however, the prismatic amount becomes more evident.


Suppose a person is wearing or needs a prescription as follows:

                                      R: −3.50 sphere
                                      L: −3.50 sphere with 6Δ base-in prism
                                          +2.25 add

(Although it may not be advisable to place all prism in front of one eye, we will use this example for simplicity.)

Before refraction the monocular PDs are measured using a pupillometer. There are no refractive lenses in place. The PD measures as follows (Figure 4):

                                      R monocular PD = 31 mm
                                      L monocular PD = 31 mm

How should the monocular PDs and prescribed prism amounts be modified to allow the eyes to accurately track down the progressive corridor and still maintain the same net prismatic corrective effect?


Placing 6Δ of base-in prism before the left eye will cause the eye to deviate outward by

                                              6 × 0.3 mm = 1.8 mm,

                           which will be rounded off to 2 mm.

During phoria testing, the eye was looking 2 mm temporally through the −3.50 D refracting lens (see Figure 3).

Using Prentice’s rule, we see that prism caused by the eye being decentered in relation to the lens is

                                                             Δ = cF
                                                                = 0.2 × 3.5
                                                                = 0.7Δ

Since the lens is minus, prism caused by the eye moving in relationship to the refractive lens is base out. Therefore the net prismatic effect for the eye is

    (Prescribed Δ) + (Decentration Δ) = (Total Δ).

Or in this case

                     6 base in + 0.7 base out = 5.3 base in.

To position the progressive zone in front of the eye, the MRP must be moved 2 mm outward. (When the position of the MRP moves, so does the fitting cross location. The fitting cross is directly above the MRP.) When the MRP moves outward, the finished spectacle lens prescription will no longer duplicate the refractive situation. This is because the 0.7Δ of decentration prism caused by the −3.50 D lens no longer exists (Figure 5). To maintain the same total prismatic effect, the prescribed prism must be reduced from 6Δ base in to 5.3Δ base in. The PDs are ordered as follows:

                              R monocular PD = 31 mm
                              L monocular PD = 33 mm

It is helpful to note that when the MRP is moved in the direction of eye deviation, there will always be a reduction of prescribed prism for minus lenses and an increase in the amount of prescribed prism for plus lenses. In other words:

·         For minus lens: reduce the Rx prism by an amount equal to the calculated decentration prism.

Fig 4 A pupillometer normally measures the interpupillary distance with no lens correction in place
and with the eyes in a straight-ahead position.

Fig 5 If the monocular interpupillary distance were to be altered to compensate for prismatically induced eye movement and correct progressive corridor placement, the net effect would be to change the amount of  prism in the prescription. The decentration prism resulting from eye movement caused by the measuring prism will no longer be present.

·        For plus lenses: increase the Rx prism by an amount equal to the calculated decentration prism.

When filling an existing prescription, it should be noted that a modification to the Rx prism amount that is done to maintain the prescribed optical effect is no different than changing sphere and cylinder power in response to a change in lens vertex distance. Changing the amount of “Rx prism” to compensate for decentration prism does not change the prescription.

Compensating Fitting Cross Height or Monocular PDs for Prescribed Prism

A.   How to Compensate Fitting Cross Height for Prescribed Vertical Prism

             1. Measure the fitting cross heights.
             2. Multiply the amount(s) of prescribed vertical prism by 0.3.
             3. If the prism is base down, raise the fitting cross height by the calculated                   
                 amount. If the prism is base up, lower the fitting cross height by the                     
                 calculated amount.

B.    How to Compensate Monocular PDs for Prescribed Horizontal Prism

            1. Measure monocular PDs using a pupillometer.
            2. Multiply the amount(s) of prescribed horizontal prism by 0.3.
            3. Modify the monocular PD(s) by the calculated amount, increasing the PD     
                for base in prism and decreasing the PD for base out prism.

Compensation Used if Modifying the Monocular Interpupillary Distances Produces Clinically Significant Changes in Rx Prism

It is suggested that compensation be considered clinically significant if moving the MRP will cause a change in prismatic effect of 0.50 Δ or more. A change of at least 0.50 Δ  will occur if the prescribed prism totals 6.00 Δ  and refractive power in the meridian of movement is ±2.50 D or greater.

1. When horizontal prism is present, find the power of the lens in the horizontal meridian. When vertical prism is present, find the power of the lens in the vertical meridian.

2. Multiply the power in the meridian of eye movement by the change in monocular PD or change in fitting cross height. That is,

  Δ  = cF

where  Δ = change in prescribed prism power, c =change in PRP (prism reference point) location in cm, and F = power in the meridian of PRP movement.

3. For minus lenses, subtract this amount from the prescribed prism. For plus lenses, add this amount to the prescribed prism.


Prescribed vertical prism in progressive add lenses requires that the fitting cross be moved up or down by an amount equal to 0.3 times the prism amount. The direction of movement is opposite from the base direction of the prism.

Prescribed horizontal prism in progressive add lenses requires that the monocular PDs be increased or decreased by an amount equal to 0.3 times the prism amount. The direction of eye and MRP movement is opposite to the base direction of the prescribed prism. Steps to take when modifying fitting cross height are found in above box , A. Steps to take when modifying monocular PD amounts are summarized in box B.

Changing the prism amounts should only be done if there would be clinically significant changes to the prescribed prism. This does not happen unless the prescribed prism is greater than or equal to 6.00Δ and the refractive power in the prism meridian is greater than plus or minus 2.50 D. If this is the case, then prescribed prism may be altered according to the summary found in above last box.