Monovision LASIK is a specialized technique in LASIK surgery to reduce or eliminate the need for reading glasses. Monovision LASIK works by correcting one eye for emmetropia (distance) and the other eye for myopia (near vision).
Presbyopia and the Loss of Accommodation
Patients who are seeking refractive surgery around or over the age of forty will experience a universal phenomenon known as presbyopia. Presbyopia is a condition caused by the loss of accommodation, or the inability to focus our lens as we age. The loss of accommodation occurs as our natural crystalline lens begins to harden and lose it’s flexibity as it continues to enlarge throughout our life. Once the loss of accommodation begins, the patient will have increasing difficulty focusing at near, heralding the need for reading glasses. Presbyopia tends to occur in the early forties to fifties depending on the refractive state of the patient.
Preoperative Counseling for Monovision LASIK
When a presbyopic patient is considering refractive surgery it is paramount that a thorough discussion is given on presbyopia and options for the patient. All too often, patients will seek refractive surgery and not realize they will still need the use of reading glasses. Counseling for presbyopic patients depends on the refractive state. The first catagory of patients are the hyperopic or farsighted patients, and many astigmatics. This type of patient is usually wearing bifocal glasses full time. They tend to understand the concept of presbyopia as they are wearing glasses full time. This type of patient has two choices, see clearly at a distance and continue to wear glasses for reading, or employ monovision to see both near and far. The second type of patient is the myopic or nearsighted patient. This patient can see clearly to read without glasses, but wears them for distance. These patients often do not realize that once refractive surgery is performed, they will lose the ability to see up close, since they do not have a problem in their current state. This is often a hard concept to grasp. For some patients, they will decide not to undergo refractive surgery if they will lose their near vision. Others will decide distance vision is more important, and will wear the reading glasses, and others will choose monovision. The last type of patient is the patient who only needs reading glasses. This patient may elect to have monovision inorder to relieve their need for reading glasses. This patient will usually only need one eye corrected.
Side Effects of Monovision
Side effects of monovision LASIK is an important concept for both the surgeon, as well as the patient to understand, and never to be “brushed under the rug.” Side effects are a direct result of the imbalance or anisometropia caused by monovsion LASIK. Side effects include; blur or fog in distance or reading vision, glare and halos, especially at night, reduced night time vision, especially driving, reduced depth perception, an uncomfortable feeling, or even transient diplopia caused by temporary strabismus. Side effects for most patients will tend to decrease overtime as a patient adapts to their new vision. Patients who have previous experience with monovision through contact lenses wear, tend to adapt much quicker. The discussion of side effects begins immediately for the presbyopic patient during their initial consultation to determine if monovision is the right choice for them.
Ocular Dominance Testing
Ocular dominance testing is important in monovision since yypically, the dominant eye is the eye best suited for distance vision. There are numerous ways to test for dominance. The most common test is the Miles test. The patient extends both arms, and brings them together to form a small opening. The patient then with both eyes open, looks at a distant object through the opening. The patient than alternates closing each eye. Whichever eye the object is in the circle, is the dominant eye. Other tests include; Porta test, Fogging Test, Pinhole test, Dolman test, or simply the camera test whereby a patient is simply asked, “ Which eye do you use to take a picture.” Two thirds of the population is usually right eye dominant and 1/3 left eye dominant, with a very small percentage nondominant.
Monovision Contact Lens Trial
Once a patient decides on monovision, the next important step which must be done on all patients unless they already employ monovision with contact lenses, is a monovision contact lens trial. Once dominance has been ascertained, the patient is fitted with soft contact lenses. The patient dominant eye is set for emmotorpia and the nondominant eye is set for myopia. If a patient has never worn contact lenses, than they are placed in their eyes and sent out for the day to experience monovision. At the end of the day, they return to have their contact lenses removed. A patient who already wears contact lenses, has the ability to try monovision for longer periods of time if needed. This is a crucial step in the process of evaluating a patient for monovision, since this will mimic the effects of monovision, to see if the patient has any unwanted side effects. Patients need to go out and spend the whole day using the monovision to get a better understanding of the type of vision this produces. If they are golfers or play tennis for example, than they should do these activities with the monovision, since these will be the times they are demanding their best vision. If patients have significant difficulties with the vision, than they should be discouraged from pursuing monovision, or trying the contact lens trial longer. Patients who do well with the contact lens trial overwhelmingly do well with monovision LASIK.
Once a patient has decided that monovion is right for them, than they will undergo the normal rigorous LASIK workup. Special attention should be paid to the patient’s occupation or lifestyle. For example, if a patient is a pilot, or truck driver who spends many nights on the road, than having monovision may interfere with their distance vision, or have unwanted side effects that could make their job harder. Other patients such as active outdoors or sports driven patients may also decide they do not want any compromise in their distance vision. Again, counseling along with the contact lens trial is key, so that expectations are met for these patients. Secondly, the amount or degree of monovision must be determined. The more minus “pushed” on a patient, the more reading ability the patient will have, but at the expense of more anisometropia and more side effects. A good rule of thumb to start with is; an early presbyope (early-mid forties) around -1.75, moderate presbyope (late forties) -2.00 and older presbyopes (fifties) -2.25 to -2.50. This type of monovision is full monovision, whereby the patient will acquire excellent reading vision even for small print, and will maintain this monovision as they age. However, with full monovision, the maximum amount of anisometropia is experienced which will have a higher risk of side effects. If a patients has a hard time adapting to this high a degree of monovision with the contact lens trial, than reducing the amount of myopia may reduce the side effects the patient experiences. This is known as partial monovision. The drawback to partial monovision is less reading ability which may mean the need for reading glasses some of the time. However, the gain in distance vision may outweigh the drawback to wearing reading glasses some of the time.
The operative procedure and cross checking numbers is no different than any other patient undergoing LASIK surgery. Special attention should be paid to which eye is dominant and will be set for distance, and which eye is nondominant and set for myopia. This should always be confirmed by the patient. The amount of monovision must also be re-confirmed.
Postoperative care is managed the same as any other LASIK patient. However, careful attention must be paid to their complaints, as side effects are common early after monovision LASIK. Patients must be re-counseled that the side effects are normal, and will improve with time. These patients may need more frequent visits, often to help alleviate stress and uncertainty, while encouraging them that things will improve. The vast majority of patients who undergo rigorous testing and counseling and who underwent a contact lens trial, will adapt to monovision without problem. For the few who may continue to have side effects, options are available. After all the normal LASIK post-operative issues are addressed (dry eyes, flap striae, ect.), than attention is turned to helping alleviate monovision side effects. These side effects can be reversed by simply wearing glasses to correct the anisometropia. For instance, a patient who continues to have problems driving at night, may want a pair of distant glasses in the car to use when needed. Other patients who are adapting normally, but may struggle at certain times, are encouraged to wear glasses when needed, but not to over wear, which would result in a longer time to adapt. If for the rare patient who cannot tolerate the monovision, than an enhancement can be done to reverse the monovsion. Lastly, it important to know that enhancement rates may be higher in these patients. Since each eye is doing one job, even a small amount of over or undercorrection can cause the patient problems. For example, if the distance eye in a patient is undercorrected by even -0.5 to -0.75, the patients distance vision may not be strong enough since the contralateral eye is blurred for distance. If patients are bothered by the small residual corrections, enhancements are needed.
- American Academy of Ophthalmology. Refractive Management/Intervention: Monovision for correction of presbyopia Practicing Ophthalmologists Learning System, 2017 - 2019 San Francisco: American Academy of Ophthalmology, 2017.