Potential Concerns and Contraindications for IOL Monovision

Pseudophakic or intraocular implant (IOL) monovision has been widely used in surgical cataract practice for more than 3 decades. More and more premium IOLs compete presbyopia management, but IOL monovision still remains the most commonly used modality with good spectacle independence and high patient satisfaction. Little attention, however, has been paid to its contraindications and concerns in the ophthalmology literature considering the widely used scope of this modality. Due to the length limitation, those well-known contraindications for IOL monovision will be just briefly mentioned, but those not easily recognizable ones are fully discussed. To author's knowledge, this is the first attempted review to address this important issue. Fuxiang Zhang* Department of Ophthalmology, Henry Ford Health System, USA Fuxiang Zhang Clinics in Surgery Ophthalmic Surgery Remedy Publications LLC., | http://clinicsinsurgery.com/ 2016 | Volume 1 | Article 1084 2 Even within the 120-degree binocular field of view shared with both eyes [12] the difference in image size of the same object with the left and right eyes are potential sourcefor confusion with monovision if the disparity in image size is too large. One of three outcomes will happen: Fusion with increased depth focus without any major issue beside a minor compromise in fine stereopsis as occurs in the majority of patients with IOL monovision; one image can be suppressed, so that only the other is seen, as in amblyopia; If fusion and/or suppression do not occur, two images of a single object are seen with symptomatic diplopia. Theoretically, any external ocular muscle abnormality can compromise binocular function in Panum’s fusional area. Anisometropia and aniseikonia are an additional demand on the fusional capacity for patients with monovision. We probably do not need to spend much time with detailed discussion about some well accepted facts: IOL monovision should not be offered to those patients who have history of external ocular muscle (EOM) surgery, diplopia, prism usage; those who have signs of tropia, significant phoria of > 8 prism diopters, or EOM restriction. (Some studies have shown a unique function for extreme monovision to correct symptomatic diplopia, [2,13-15] that will not be discussed in this article). It is important to know that those patients are not good candidates for IOL monovision in most clinical situations. Some ocular conditions, however, are not that easy to recognize as potential contraindications for IOL monovision. This will be the main topic to discuss for this paper. To author's knowledge, this is the first attempted review to address this important issue. Long-standing unilateral traumatic cataract Patients with long-standing unilateral dense cataract, especially traumatic cataract, may already have compromised fusion function. If preoperative strabismus is noted at examination, even if the trauma happened during adulthood, there is a good chance that that patient will have diplopia after cataract surgery [16-18] due to disruption of fusion. Pratt-Johnson [16] reported 24 cases of unilateral longstanding traumatic cataract from 1984 to 1988. All 24 cases had unilateral traumatic cataract and developed intractable diplopia after their vision was restored with IOL or contact lenses to 20/40 or better. None of the 24 cases had a known history of interrupted binocular function prior to their trauma and the average age when trauma occurred was 18 years. There was no central nervous system trauma associated with the ocular trauma and the study noted that risk of diplopia increased if the interval of cataract formation prior to vision restoration reached 2.5 years or longer. The authors also noted that these patients typically had secondary strabismus in the injured eye one year or longer after the injury. It can be difficult or sometimes impossible to accurately evaluate ocular alignment if the vision is very poor and if the strabismus is very small or if it is in the transitional process of becoming deviated. For those patients, warning of the possibility of postoperative diplopia is warranted even if the eyes appear straight. It may worsen the risk if crossed monovision is planned. In this circumstance, therefore, it is reasonable to correct the affected eye aiming for slightly more myopia than the fellow normal eye. After the first eye surgery is done, if postoperative Worth-4-Dot test at that time shows intact fusion with 4 dots at 6 meters, or if 4-diopter base out test does not suggest small central scotoma, or if the patient has good stereo acuity, then it is probably safe to consider IOL monovision option with the fellow eye aiming at plano, if the patient requests spectacle independence. The Worth-4-Dot test, 4DBO prism testand Titmus stereopsis test prior to the first eye surgery in the presence of long-standing dense cataractis typically not possible in the presence of poor vision. Fixation switch diplopia Fixation switch diplopia has been described as an acquired diplopia in adults who have a history of strabismus or amblyopia since childhood [19,20]. With a history of childhood strabismus or amblyopia, the patient may not have diplopia if the affected eye is not the fixation eye. Diplopia can happen if the amblyopic eye starts to be the fixation eye when refractive status changes, such changes can happen as the original fixation eye becomes more myopic (as happens as cataract forms), intentional or unintentional monovision modality introduced, or if an inaccurate refraction prescription is provided. Kushner [19] reported 16 adult patients with fixation switch diplopia. All 16 had a history of strabismus since childhood. Six of the 16 developed diplopia owing to their monovision correction. In all 16 patients, symptoms were completely eliminated when proper optical correction was instituted to encourage fixation with the dominant eye at all viewing distance. Boyd et al. [20] reported a group of 24 patients as “Fixation Switch Diplopia” who had spontaneous intermittent unilateral diplopia. All 24 patients had the following features: When they were asked to demonstrate the production of diplopia, each patient fixed with the non-preferred eye and no suppression was present in the preferred eye; when fixing with the preferred eye, suppression could be demonstrated in the non-preferred eye and the diplopia disappeared. They were all able to alternate fixation, but not able to alternate suppress. There is no alternate suppression present in this entity of patients. Each of the 24 patients also had strabismus onset before age 7 years old and the preferred eye had better vision than the non-preferred eye. The vision of the preferred eye in all cases was 20/20 or better. The severity in the non-preferred eye can be quite variable. In some of the mild cases, the symptom was trivial and 5/24 was even not able to tell the duration of their intermittent diplopia. Vision was only mildly affected at 20/40 to 20/20 level in 17 out of 24 cases. 5 out of 24 had good stereovision with 80 to 40 arcs of seconds. The EOM deviation could be minimal to less than 10 prism diopters. This finding had some similarity with what Parks had noted in his 100 cases of monofixation syndrome21Parks. This study also noted that intermittent fixation switch diplopia happened more if the non-preferred eye vision had good visual acuity. For that reason, the authors intentionally treated some cases by decreasing the vision with glasses in the non-preferred eye. These studies raised an important concern when we do IOL monovision. If we happen to choose the non-preferred eye as distant fixating eye, it may cause fixation switch diplopia. From this perspective, the pretty common practice pattern of routinely choosing the worse eye or denser cataract eye aiming plano and the follow eye for near regardless of dominant eye test may be a concern if we also missed the history. It is important to ask every single prospective patient of IOL monovision: “Do you have a history of an eye turned in or out?” “Can you recall any double vision in your whole life?”, “Do you always have one eye weaker than the other eye?”. For this entity group, crossed IOL monovision is contraindicated, and conventional IOL monovision should be avoided too so we do not add anisometropia as an extra burden to an already compromised binocular function.