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As an eye care professional, problem solving and resolution of adaptation issues is an integral part of the job. Many patients have to go through an adaptation process when presented with a new prescription or lens design. Unfortunately, adaptation and its process are usually not fully understood by the ECP and therefore difficult to explain to the patient. This can result in frustration by all parties involved.
So, what is adaptation?
Adapting to the World
Adaptation is a survival skill that allows us to live comfortably within our surroundings. It enables individuals to find harmony with their environment through a series of adjustments that our mind subconsciously makes. When one of the five senses is involved, the adaptation is called sensory adaptation. A prime example of a sensory adaptation is a sight sensory adaptation called light adaptation. When an individual walks from a bright room to a dark room, he or she is completely blind in the darkness.
However, after a couple of minutes, the brain and eye goes through a series of steps and adapts. The pupil dilates, allowing more light into the eye and stimulate the retina. The rod photoreceptors, the receptors that are sensitive to low levels of light, are activated and objects that were once shadowed in darkness becomes more visible to the individual. Although this response occurs in all individuals, regardless of their visual acuity, there is another sensory adaptation that typically only occurs in an individual who is wearing some type of visual correction. This is the hand-eye sensory adaptation.
Hand-eye sensory adaptation is an adaptation to a person’s ability to coordinate eye movement with hand movement. How hand-eye coordination works is a multistep process in which the eye fixates on a target. Information about the object, such as size, shape and distance, is then passed along the optic nerve to the brain. The brain processes the visual information and then passes it along the central nervous system to the hand. However, this visual process of hand-eye coordination is altered from its natural state when a prism is placed in front of the eye.
The Prism
A prism in its most basic sense is a transparent, triangular shaped object with a base (thick portion) and an apex (thin portion). When light enters a prism from air, it slows down and the light is bent or deviated towards the base. If an object is being viewed through the prism, the image is displaced toward the apex. Because the objective of a prism in regards to optics is to deviate, or refract a beam of light as well as to displace an image, it is the building block upon which visual correction is based.
All lenses are prisms. A plus lens is two prisms placed base to base and a minus lens is two prisms placed apex to apex. As a result, the two lenses have very different ways of passing information to the brain. A plus lens converges light and magnifies the images received by the brain and a minus lens diverges light and minifies the images received by the brain. The image changes are compounded by a spherocylinder lens that has a correction for astigmatism as well as a lens that has a multifocal addition. It is easy to see how these image changes can cause an initial feeling of imbalance and discomfort to the patient. It is when these uneasy feelings are experienced by the patient he or she undergoes the hand-eye sensory adaptation process.
Hand-Eye Sensory Adaptation
The best example of hand-eye sensory adaptation occurs when a progressive wearer gets a new, stronger prescription or is fitted for a new lens design. When the patient looks down, objects seem closer to him or her than they actually are. The result is the patient may feel clumsy and disoriented when walking down stairs or stepping off curbs or they may reach for an item such as a book and it is not where he or she thinks it is. Day by day the feelings of uneasiness decrease until it eventually disappears within a few days.
Easing the Adaptation Process
One of the primary responsibilities of the eye care professional is to aid the patient in the adaptation process. In order to achieve this goal, the ECP must help the patient through all steps of the fitting process.
Share knowledge with the patient
When patients receive a prescription, it is usually just a bunch of numbers. It is up to the ECP to decipher the numbers and explain it to the patient so that he or she knows what to expect when picking up their new glasses. Since lenses are governed by the laws of physics, each lens type will react in a specific way. For example:
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Plus lenses
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used to correct hyperopia
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cause images to appear larger and therefore closer
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the farther away from the eye the lens sits, the stronger the perceived power to the patient
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causes an image to be displaced when the patient looks through the bottom of a lens if there is a significant difference in the prescription of both eyes
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Minus lenses
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used to correct myopia
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cause images to appear smaller and therefore farther away
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the closer to the eye the lens sits, the stronger the perceived power
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causes an image to be displaced when the patient looks through the bottom of a lens if there is a significant difference in the prescription of both eyes
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Spherocylinder lenses
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a compound lens that is used to correct an astigmatic error
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has a power ground in two principal meridians placed 90 degrees apart on the same surface
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power of the lens changes as the eye looks around the lens
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may cause a “swimmy” feeling, images may appear slanted and slightly misshapen especially if the astigmatism has an oblique axis
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Multifocal lenses
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An abrupt image jump, especially with lined bifocals and trifocals
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Vertical imbalance in patient that has good visual acuity OU as well as anisometropia, or a power difference of an imbalance of at least 1 diopter between each eye
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Distortion along the sides of the corridor in a progressive lens
Since the ECP knows how each lens reacts before it is dispensed, he or she can better prepare the patient on what to expect if there is a change in the prescription or lens style.
Measure Properly
As discussed earlier, lenses are prisms placed together. The point at which the two prisms meet is the optical center. This is the point where the light entering the lens is not refracted or deviated. In order for the patient to see properly, the optical center must be placed in the proper position. Horizontally, this placement is placed at the pupillary distance. It is usually best to take this measurement with a pupilometer and use the monocular measurements.
Vertically, this measurement is called the OC (optical center) height and should be placed along the optical axis. The optical axis is not in front of the pupil. The rule of thumb for OC height is to place the measurement 1mm below the pupil for every 2 degrees of pantoscopic tilt had by the frame. Since most frames are worn by the patient with 8 to 12 degrees of pantoscopic tilt, an average OC height would be 4 to 6mm below the pupil. When these measurements are off, the patient experiences unwanted prism.
Unwanted prism is a major reason why a patient has trouble adapting to his or her new lenses. When the pupillary distance is off the effect is excessive Base In or Base Out prism. The patient will experience a slanting feeling when wearing their glasses. An image will appear high toward the prism base and low toward the prism apex. When the OC height is wrong the result is excessive Base Down or Base Up prism. Unwanted Base Up prism will give the wearer the feeling of walking down hill, of standing at the top of a hill, and vertical objects will seem shorter. Excessive Base Down prism wearers will feel as if he or she is at the bottom of a bowl, will experience a feeling of walking uphill and vertical objects will appear taller.
Don’t forget to Adjust
It does not matter how much time was taken in measuring for the lens if the frame does not fit properly. If the frame is crooked, too tight or too loose, the optical center will not follow the optical axis and the patient’s vision will be off. Proper adjustment is crucial both at the time of fitting and at the time of dispensing. Make sure that the weight is distributed properly along the fitting triangle of the bridge of the nose and the top of the ears. Ensure that the frame front is level and is placed at good vertex distance from the eye. Adjustment to the temple and along the temple bend will keep the lenses stable in relation to the optical axis. Also, make certain that the pantoscopic tilt is proper for the OC height.
When to Refer
There are certain times when the problem is not an adaptation situation and the patient needs to be referred back to the doctor. If the problem is visual, it should be done only after a thorough analysis of the glasses and troubleshooting with the patient is completed. Generally speaking, if the patient’s vision is clear and sharp and the spectacles are made correctly, then any complaints should be normal adaptive symptoms. Other situations however require an immediate referral. These situations include pain, decreased vision, light sensitivity or redness.
Conclusion
Adapting to glasses is normal. Although every ECP loves the patient that puts on a new pair of glasses and can see everything perfect, this cannot happen in every instance. In fact, it is these patients that provide the best opportunity for developing a life-long patient. By providing a thorough understanding to all parties, proper measurements, and patience, the adaptation process can run much smoother for both the patient and ECP.
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