2 Optics and Neurophysiology of Pseudophakic Monovision Abstract Keywords: IOL monovision, pseudophakic monovision, mini monovision, modest monovision, full or traditional monovision, depth perception, not-sharply-focus-zone, hole-in-card, plus-lens test, dominant eye test The first symptoms of presbyopia—eyestrain, difficulty seeing in dim light, and problems focusing on fine print—are usually noticed between the ages of 40 and 45. The ability to focus on near objects declines throughout life, from an accommodative amplitude of about 20 D (ability to focus at 5 cm) in a child, to about 10 D at age 25 (10 cm), and levels off at 0.50 to 1.00 D at age 60 (ability to focus down to 1–2 m only). The main purpose of designing IOL monovision is to increase depth of focus to eliminate or decrease spectacle dependence by choosing different IOL powers in the two eyes. A successful IOL monovision patient should be able to see at all distances, spanning from the far focal point of the far eye when the eye is not accommodating to the near focal point of the near eye when the eye is accommodating, although in optical theory there may be a zone where there is no sharp focus. If a patient’s eyes are both phakic and emmetropic at the age of 60, with an average accommodative amplitude of 1.00 D at that age, that patient will be able to see well from 20 feet away to infinity when the eyes are not accommodating (lights coming from 20 feet and farther away are considered as zero vergence power, or parallel to the optic axis ray),1 but for near, he or she will be able to focus only up to 1 m in front of him or her, not any closer without an optical addition (
By choosing appropriate intraocular lens (IOL) power differences between the two eyes, we can create IOL monovision in such a way that the depth of focus is increased within the physiological range with minimal compromise. Mini and modest monovision are likely more physiological situations where binocular summation rather than monocular blur suppression plays the main role. Optically, it is also demonstrable that the not-sharply-focused-zone is shorter in –1.50 D monovision than –2.50 D monovision. The hole-in-card sighting dominance test likely reflects more the monocular situation, while the plus lens sensory test reflects more binocular perception and balance.
2.1 Optics of Increased Depth of Focus
Fig. 2.1).
2.1.1 Equation 1
Near focal point of each eye (N) | = 100/1 D |
If the patient’s status is now post cataract surgery with plano in each eye with 20/20 distance vision in each eye, assuming natural nonaccommodating pseudophakic accommodation power of 1.00 D for each eye, he or she will still see well 20 feet away, but at near he or she will only be able to focus up to about 1 m, not any closer with the same formula and calculation as above (
Fig. 2.2).
Fig. 2.1 Phakic age 60.
Fig. 2.3 OS –1.50 pseudophakic.
Fig. 2.4 OD plano.
2.1.2 Anisometropia – 1.50 D Monovision
If we keep the patient’s OD the same, at plano, but give the OS more myopic defocus, say adding extra power to the IOL to give the patient –1.50 D monovision (for the sake of discussion simplicity, we do give two different powers at IOL plane and at glasses plane, although we all know that when we add + 1.00 D to the IOL typically we will get roughly about 2/3 D at the glasses level), then the patient’s OS will have a focusing power of 2.50 D (extra 1.50 D + 1.00 D from assumed pseudophakic accommodation = 2.50 D) with a new near focal point of 40 cm in front of him/her.
2.1.3 Equation 2
Near focal point of OS (N) | = 100/2.5 D |
2.1.4 Equation 3
Far focal point of OS (F) | = 100/1.5 D |
Clear zone of OS is between 40 and 66.7 cm (
Fig. 2.3).
The patient’s OD should be able to see clearly from infinity to up to 100 cm with this eye due to assumed pseudophakic accommodation of 1.00 D.
2.1.5 Equation 4
Near focal point of OD (N) | = 100/1 D |
2.1.6 Equation 5
Far focal point of OD (F) = infinity
Clear zone of OD is 100 cm to infinity (
Fig. 2.4).
Thus, the patient should be able to see well in the range from infinity to 40 cm in front of him or her when both eyes are working together. That is what we mean when we say that IOL monovision increases the depth of focus (
Fig. 2.5).
Optically, the left eye’s far focal point is not at infinity any more. It is at 66.7 cm in front of the patient. Any farther away, the OS alone is not able to see sharply without myopic correction for the left eye. That is the compromise of monovision. This is actually the most common “negative” comment from IOL monovision patients if it is not clearly explained to them prior to the surgery. However, the reality of IOL monovision in this scenario is that they see well for both far and near and in between without much noticeable downside in daily life since we live our lives with both eyes open, although optically in theory there is still a zone that is not in sharp focus. This concept is helpful when we discuss the pros and cons of modest versus high anisometropia.
Not-Sharply-Focused-Zone
What happens if the object is located at 50 cm in front of the patient? His or her OD alone is not going to focus well since the refraction is plano, although the assumed pseudophakic accommodation of 1.00 D will enable the right eye to see everything clearly from infinity to up to 100 cm away to the eye. The object located at 50 cm is within the clear zone of OS (40–66.7 cm) (
Fig. 2.6).
What happens if the object is located at 75 cm in front of the patient? It is located within the not-sharply-focused-zone (NSFZ).
The NSFZ is defined as the distance between the near focal point of the distance eye and the far focal point of the near eye in monovision.
The length of the NSFZ is 33.3 cm between 100 and 66.7 cm when we create –1.50 D IOL monovision (
Fig. 2.7).
Optically, within the “NSFZ,” neither the OD nor the OS will have perfectly sharp focus. This is the compromise of monovision when the anisometropia level gets high. It may have some negative impact in full monovision, but it is close to negligible at mini and modest monovision.
2.1.7 Anisometropia – 2.50 D Monovision
Now, what if the anisometropic level is –2.50 D, instead of –1.50 D for the OS? Let us assume the right eye is still plano with pseudophakic accommodation amplitude of 1.00 D. The total focusing power for OS will be 2.50 + 1.00 = 3.50 D.
2.1.8 Equation 6
Near focal point of OS (N) | = 100/3.5 D |
