Timothy C. Hain, MD •Page last modified: March 5, 2021
Related pages that you may found helpful include: outline of visual vertigo•
|Image of cataract surgery from https://www.allaboutvision.com/|
Cataract surgery is very common. It involves removal of a clouded lens just behind the cornea (front of eye), and placement of a new, clear, intraocular lens within the eye. The new lens is called an "IOL". Generally speaking, IOL placement is a "one shot" deal. "Re-do's" are rare and best avoided. Thus you want to get it right the first time.
While there are a large number of reasons for patient dissatisfaction post cataract surgery (Salerno et al, 2017), here we are going to focus on dizziness. We often encounter patients who develop dizziness after cataract surgery, generally having features of visual vertigo. Here-in we discuss the potential reasons for dizziness after cataract surgery, and potential treatments. There is an explicit attempt to use ordinary words rather than jargon.
Most cataract surgery occurs in older people, who are not as good at adaptation as younger people. We will assume this as a given. Those of you that wear glasses - -remember what happens when you get a new set of glasses -- if the change is a big one, it is common to be dizzy for a few days. Eventually, most people "get used" to them. Some people don't and they are dizzy when they move about space using their eyes. They have visual vertigo.
Each of these visual changes can make people dizzy after cataract surgery.
When one is wearing glasses, the size of the world is changed compared to no glasses. Consider what happens when one looks through a magnifying glass (which is just a large plus lens). Myopic people ordinarily use spectacles that are minus lenses, or in other words, they minify the world. The magnification effect is a product both of the strength of the spectacles, i.e. degree of myopia (i.e. diopters), and distance of the spectacle lens from the eye. Spectacles make things smaller than contacts because they are further from the center of the eye.
Dimensions of the eye (from https://www.researchgate.net/publication/245681842_Thermal_lensing_in_ocular_media_exposed_to_continuous-wave_near-infrared_radiation_The_1150-1350-nm_region). a-b is the cornea, b-c is the aqueous humor. c-d is the lens thickness, and d-3 is the vitreous humor thickness. The axial length is from front to back.
Thus placing an IOL through cataract surgery, which moves the lenses into the eye itself, often makes the world appear bigger in that eye. Garcia et al (1996) computed the difference in magnification that resulted from moving correction from a spectacle to an IOL, in high myopes (i.e. about -6D). The change depends both the axial length of the eye as well as the corneal power. For an eye that is 27.0 mm in axial length, and has a corneal power of 44D, the relative magnification is 1.145 (or about 14%). Lesser amounts of myopia and thus smaller axial lengths have smaller changes in magnification. The logic is reversed with far-sighted persons wearing plus lenses.
In order to see while the head is moving, the eye must stay still in space, which means that as the head moves, the eye must counter-rotate. This is usually performed by the vestibulo-ocular reflex (VOR), where the input is from the inner ear, and the output is to the eye muscles. If the magnification of the eye is changed, this means that the world becomes bigger (or smaller), and accordingly, the amount of movement of the eye required from the VOR must change in order for things to be still.
While magnification or minification from changes in moving a spectacle to an IOL does require a change in the VOR gain, this is a relatively small change compared to normal human capabilities. Most people can adjust their the VOR gain up by roughly a factor of 2. This ability to modify the VOR gain seems to "max out" at about a factor of 2, and thus if someone has (for example), already lost half of their inner ear function, they may not be able to increase their VOR gain more. This may be the reason that studies show that "Magnification is a likely explanation of the mobility problems some older adults have with updated spectacles and after cataract surgery." (Elliott and Chapman, 2010).
Solutions to magnification problems:
The fix for this problem, at least in theory, is "practice practice practice". In other words, VOR exercises, especially those for gaze stabilization. There is a substantial ability of humans to readjust their VOR gain up or down.
The next two paragraphs are mostly theoretical -- few people go to these lengths.
If time and/or exercise fails, at least in theory, glasses that make the world bigger or smaller (i.e. telescopic lenses, magnifying or minifying lenses), might be helpful. A pure glasses solution is somewhat challenging to implement due to constraints having to do with the thickness of spectacle lenses. People don't want to wear "coke bottle" glasses.
In theory, a telescope type solution for someone who was far-sighted before cataract surgery, or a minifying solution for people who were near-sighted prior to cataract surgery might be implemented with a combination of a spectacle and contact lens, which allows for a larger distance. Contacts are not always practical in older people. One might also consider laser surgery and spectacles. It is rare for people to go to these lengths however.
Differences in magnifications between the eyes is another issue. This is called "anisometropia". It occurs, for example, in situations where one eye has had cataract surgery but the other is still waiting to become ready.
Going back to consideration of the situation where there is a substantial difference in size between the eyes, the eyes are mainly driven using the rule called "Herring's Law", meaning that the same command is sent to both eyes. When one eye sees a world that is larger than the other, this strategy no longer works, and after a change in eye position driven by a need to shift gaze, the two eyes end up seeing different maps of the world. This then provokes a need to realign the eyes using convergence or divergence. These have limits.
Another strategy is to just ignore one eye, throwing binocular vision "off the bus", but eliminating visual confusion. There does not appear to be much capability for the brain to change the rules for each eye, although it can certainly change rules for both eyes together. (Viirre et al, 1988)
The ignoring of one eye, or suppression, is likely the strategy used when one eye has markedly better acuity - -such as when one eye has been operated but the other is not. This strategy eliminates the chance of using both eyes together to judge depth, and forces the person to rely on other cues such as the size of things, or parallax.
The "fix" for anisometropia, at least in theory, would be an optical arrangement that corrects for size - -similar to a telescope, or a reversed telescope. As was commented above, these might be implemented with a combination of a spectacle and contact lens or laser eye surgery, which allows for a larger distance, but the process would be more complicated as each eye might need a different combination of contact/laser surgery and spectacle lenses.
Considering the "monovision" group, these are usually people who wear one contact lens for far, and use the other eye for near. There are also some people who are just this way naturally. These people don't have the anisometropia problem to any great extent, because contacts are close to the center of the eye. Similarly, people who choose to have one IOL for near and the other for far, should not notice much difference from the contact situation. People with "monovision" are throwing most of their ability to use both eyes together "off the bus", because one eye is blurry while the other is clear. So if you are dizzy anyway, throwing out your depth perception with "monovision" is probably not a good idea, but it should not make you dizzy if you have already adapted to it with contact lenses.
Normal Eye (from https://www.aao.org/eye-health/diseases/what-is-astigmatism) Eye with astigmatism (https://www.aao.org/eye-health/diseases/what-is-astigmatism) Illustration of one type of astigmatism due to a deformed cornea (From "https://antelopemalloptometry.com/learn-about-astigmatism-and-astigmatic-refractive-error.php">Antelope mall vision), with "meridians" shown.
Astigmatism means that the cornea or lens (or both) is irregular, and instead it having spherical symmetry, it might be an ellipse -- somewhat like a football (viewed from the front), or even worse, a football that is partially deflated. This causes blur because light going through one part of the lens, ends up converging at a focal point that is different from other regions of the lens. There are a lot of possibilities for shapes, and thus a lot of possible types of astigmatism. Astigmatism is "corrected" by lenses that have cylindrical symmetry, meaning that they bend light about an "axis". A simple system is used where a "meridians" are constructed -- similar to protractor angles. This is really just appropriate for simple distortions as shown on the right above. The two angles where the refracting power changes the most and the least are called the principal meridians. From these measurements, a spectacle lens can be constructed that has a correction for the "sphere" on one surface, and for the "cylinder" on the other surface. If we think about this a little, it can easily be appreciated that while this might account for simple focusing issues, there are a lot of other possibilities not correctable. If the cornea or lenses is distorted in an irregular way, with a dent on one side, it is obvious that no "cylinder" could ever fix this. Think of what could be done to the cornea during Lasik eye surgery -- perhaps many local irregularities ! On the other hand, if astigmatism is from a "dented" lens of the eye, putting in a new lens might fix the astigmatism. Well lets return to our discussion of cataract surgery...
According to Elliott and Chapman (2010), who were discussing changes in magnification from IOLs, "Changes in astigmatism can cause even more problems initially because different amounts of magnification occur along two meridians and along different meridians in the two eyes, so that objects look distorted. Symptoms can include walls, doors, and floors sloping. Contact lenses can correct for corneal astigmatism, and of course spectacles can be made with astigmatic correction. Laser refractive surgery can also be used to correct for corneal astigmatism."
Although spectacles are supposed to correct for astigmatism, spectacles are fixed to one's head, while the cornea moves around in the eye socket (orbit). What this means is that the correction for astigmatism only works when you are looking straight ahead, and may be wrong when you are, lets say, looking at something with your eyes turned far to one side. Thus you end up with a systematic change in focus and magnification, depending on where you are looking. This provides a somewhat challenging task for the brain -- which has to deal with variably blurry things that are moving around at different speeds, as the eye moves in the head.
Using a contact lens to correct for astigmatism makes a little more sense, and is less challenging. The idea here is that the contact provides a better approximation to a sphere than does the cornea, and thus smoothes out local irregularities. It will not do much for a substantial thinning of the cornea one one side. Hard contacts or toric contact lenses may help for some situations. These solutions have the advantage that they are temporary and reversible. On the other hand, contacts are not as practical in older people due to a higher frequency of dry eyes.
Laser corneal refractive surgery, including PRK and LASIK, offers another approach. According to Braga-Miele et al (2014), "However, aside from the disadvantage of an additional surgery, these procedures are associated with limitations such as limited predictability, dry eye, and wound-healing problems." We suppose what they are trying to say, is that you are combining two "no return" type of surgeries -- the cataract surgery first, and the Lasik/PRK surgery second. It might be safer to try less permanent procedures first, and consider laser surgery if the contact solution helps but is not well tolerated.
If one has both astigmatism and magnification issues, the problem and solution starts to become rather daunting. One would think a combination of contacts or Lasik surgery (that eliminate corneal astigmatism), and spectacles to recalibrate magnification might be something to consider.
Single vision IOL's are the most common type. They are similar to single vision spectacles, but of course, they are located inside the eye rather than perched on one's nose. These are the safest choice. It is expected that with a single vision IOL, you will need reading glasses as well. Persons with astigmatism due to a non-spherical cornea, who stop using glasses that correct for astigmatism, will probably have astigmatism again after insertion of a single vision IOL.
The size of the world may change with a single vision IOL, as discussed above. This is going to be mainly a problem in persons who have strong corrections (e.g. 6 diopter), and wear glasses.
People who have an IOL placed in one eye, but leave the other one without surgery, may have to deal with a difference in magnification, and perhaps also astigmatism between the two eyes.
These are more complex devices. Generally speaking, these devices trade off contrast sensitivity for greater spectacle independence (Braga-Mele et al, 2014). It will always be safer to avoid a multifocal IOL and just go with the simple single-vision IOL.
Different lenses for each eye --one for near and one for far -- this is called "monovision".
These are similar to contact lenses, one IOL for near and one for far. This is not a great vision solution for anybody with dizziness, as this method makes it hard for the eyes to work together to judge depth. For people who were wearing glasses, and go to monovision IOL, this situation both changes magnification as well as breaks fusion -- posing a more complex task for the brain to sort out.
A toric IOL is an attempt to correct for astigmatism built into the IOL. Because once your IOL is in place, you can't really go back, if your surgeon chose the wrong device, you are just stuck. Toric IOLs can rotate when people blink, adding to visual distortion .Accordingly, we don't think this is a very wise approach for dizzy people.
Rarely, IOL's move within the eye, and become "off center", causing visual distortion and dizziness. This problem usually needs surgical remediation.
When dense cataracts are replaced by clear IOL's, there may be light more getting to the retina. Occasionally people with photophobia (such as due to Migraine) find that this increases their dizziness. Fortunately, the age range affected by migraine is generally not much overlapping of the cataract age range. This problem is usually best addressed with sunglasses.
There are many ways that cataract surgery can make people dizzy. It is usually safest to chose simple types of cataract surgery (i.e. single vision IOL, same in both eyes), rather than choosing a method that allows you to avoid glasses, but sacrifices clarity and balance.
If it is too late and you have already had your surgery done, it is best to have an evaluation done by an eye doctor, who can diagnose and direct you towards a possible solution. At Chicago Dizziness and Hearing, we offer evaluations for visual vertigo, for situations like this.