There is important information to learn about a patient who has undergone previous ocular muscle surgery before treating him with refractive surgery. Constant diplopia is a rare but reported complication that occurs after LASIK surgery. Performing a few simple tests preoperatively can help you identify those patients more likely to develop double vision and thereby enable you to avoid surgical surprises.
VISUAL PERFORMANCE
Before treating a strabismus patient with refractive surgery, it is most important to know the type of strabismus the patient had and what procedure was performed. I recommend that you obtain the operative notes of all previous surgery, as well as the patient's preoperative deviation and initial postoperative alignment. Sensory status is equally important. Patients with sensory fusion and a residual phoria may decompensate to a true tropia if their fusion is disrupted for even a short time. However, if the patient had no sensory fusion and suppresses either eye, postoperative diplopia is unlikely, even after refractive surgery for monovision.
An accommodative esotrope who is severely farsighted and requires glasses in order for his eyes to work together may also benefit significantly from refractive surgery. Be aware that esotropic refractive surgery patients require 100% correction to fuse; otherwise, they may experience double vision postoperatively or may be unhappy with the appearance of a residual esotropia.
PREOPERATIVE MUSCLE ALIGNMENT AND STEREO TESTING
It is particularly important for refractive surgeons to conduct stereo testing on patients to learn their sensory status, which is critical to avoiding postoperative diplopia. This is especially true for patients who are comanaged or referred to you. You need to know how extensively the previous physician checked the patient's alignment and the types of stereo tests he performed. For instance, optometrists tend to check muscle alignment using a phoropter, which can produce erroneous results. All patients should undergo alternate cover testing at both distance and near, as well as a Titmus stereo test at near. Additionally, patching a patient's eye for 30 minutes prior to motility testing can reveal a latent deviation that may break down in the immediate post-LASIK period.
TESTING THE CORRECTION
To test whether a patient will be able to tolerate a refractive correction, I strongly recommend placing him in a trial frame or contact lenses to mimic the intended final correction. This approach is especially helpful for severely nearsighted or farsighted patients in whom a 100% correction would be difficult to achieve. Test these patients' ocular alignment in what you believe to be a reasonably attainable correction. Ideally, the patient should wear this undercorrection for several weeks, after which you must retest his alignment. If he shows any evidence of worsening phoria, explain that he may have to continue to wear a light prescription after LASIK surgery to maintain fusion.
THE PERILS OF CREATING PERMANENT MONOVISION
Of the reported cases of patients who experienced diplopia after refractive surgery, almost all had been corrected for monovision and had an underlying, unrecognized strabismus. Moreover, even after undergoing additional correction to remove the monovision, the majority of these patients still exhibited strabismus and had to wear prism glasses or undergo strabismus surgery.1,2
Be wary of inducing monovision in any presbyopic patients, especially those who have had strabismus. Testing monovision on a patient for just a few minutes in the office is not sufficient; it is imperative to test presbyopic patients preoperatively in contact lenses for an extended period of time (at least 2 weeks to 1 month) in the prescription you intend following the refractive surgery. Insist that these patients wear the contact lenses constantly and then check their eyes for a disruption of fusion, worsening phorias, or the development of an intermittent tropia. Obviously, these patients should not be corrected to monovision, but rather treated for the full distance refraction and informed of the continued need for reading spectacles postoperatively. I recommend against attempting monovision on a strabismic patient with fusion, unless he has significant suppression. If you have reservations, you may want to consult formally with a strabismus pediatric ophthalmologist, a strabismologist, or an orthoptist.
If you decide to induce monovision in a patient with strabismus, do not produce more than 1.25 D of difference between the two eyes. The highest amount of blur a patient can tolerate is 1.25 D; the brain is unable to fuse images at a difference greater than that. Studies have shown, however, that a difference of less than 1.25 D between the eyes offers patients comfortable stereo vision at both distance and near.3 If the patient is in his 40s, explain to him that, by inducing only this degree of monovision, the effect will weaken by the time he reaches his 60s and he will have to resume using reading glasses.
There are some patients who, despite successful monovision testing with contact lenses, still cannot tolerate the effect after refractive surgery and must be placed in their full refraction in order to restore binocularity. Any significant delay in treating these patients could change a well-controlled phoria into a frank tropia that requires surgery.
ACCOMMODATIVE ESOTROPIA
Often, farsighted, accommodative esotropia patients who are well controlled and fuse with glasses benefit the most from refractive surgery (Figure 1).1,2 Interestingly, some purely refractive esotropes do not straighten completely with LASIK; although they may correct to 20/20 in both eyes, their eyes still cross postoperatively. In contrast, some partially accommodative esotropes (those who do not entirely straighten in spectacles), who tend not to align after refractive surgery, perform very well.
Fifty-eight percent of accommodative esotropes (including some partially accommodative esotropes) improve with refractive surgery and either correct fully or with only a slight residual crossing.1 Most of those patients who do not improve, however, must undergo conventional strabismus surgery to correct the esotropia. Therefore, you should inform patients that refractive surgery alone may not be able to correct their strabismus 100%. Moreover, some strabismus patients' eyes may appear completely straight behind their glasses because the glasses mask a slight deviation. If refractive surgery does not completely straighten these patients' eyes, they may not like their appearance because the residual deviation is more obvious without lenses.
STRABISMIC COMPLICATIONS
As mentioned previously, a disruption in fusion causes most incidences of post-LASIK diplopia. This result can occur through the induction of monovision or a less-than-optimal visual acuity, such as in the case that one eye corrects to 20/20 but the second eye experiences delayed healing with reduced acuity. One well-defined type of strabismus, acute esotropia, is precipitated by transient occlusion of one eye or by physical or emotional stress. Other latent deviations (both horizontal and vertical) develop under similar conditions. There are reports of patients who have developed this complication following LASIK surgery, presumably because the inequality in acuity disrupted their fusion. In one reported case, a patient developed a decompensated CN IV palsy following LASIK surgery and required prism glasses.4 Another case documented the development of an intermittent exotropia with diplopia 9 months after unilateral PRK,5 and yet another reported that, after experiencing a decentered ablation, the patient developed both an exotropia and hypotropia and required both vertical and horizontal prism to fuse.6
WHICH SURGERY FIRST?
Another issue in treating strabismus patients who desire refractive surgery is which surgery to perform first. In almost all cases, the refractive surgery should be performed first, because it may change the patient's angle of deviation.
Be aware of whether a patient's vertical deviation is greater than his horizontal deviation. Patients can tolerate a horizontal misalignment but not a vertical one. You should look for latent vertical deviations such as unrecognized congenital CN IV palsies. Clues identifying underlying strabismus include the tilting or turning of the head (which may be subtle) and facial asymmetry. Do not perform refractive surgery on a strabismus patient unless you or a staff member is comfortable measuring his deviation and can accurately determine his viability for surgery. In healthy refractive surgery patients, performing a cover/uncover test at both distance and near is a good idea, but it is an absolute requirement for patients with strabismus. Check their version and ductions and also perform stereo testing in their full correction or the postoperative correction you expect to achieve.
TREATING PEDIATRIC PATIENTS
Recently, more studies have been designed to determine which pediatric patients, if any, would benefit from LASIK.7 Most obviously, children with a history of anisometropic amblyopia may be good candidates for surgery. Many of these patients were included in the studies because they had failed conventional treatment and/or refused to patch or wear anisometropic correction. Although the refractive surgery sometimes relieved these children from having to wear glasses, it did not significantly improve their vision or eliminate their need for a patch. Furthermore, some progression of myopia continued in the operated eye as the children aged, which will require future correction.7 Despite these drawbacks, LASIK may benefit a subset of pediatric strabismus sufferers that is not yet clearly defined. Researchers, particularly in Spain and Italy, continue to study this area, which may generate significant interest for pediatric ophthalmologists.
IN REVIEW
Although post-LASIK diplopia is a rare complication, the majority of cases can be avoided by exercising proper patient selection, as well as by conducting careful, complete motility evaluations and sensory testing . In individuals with a history of complex strabismus or significant underlying phorias, a consultation with a pediatric ophthalmologist or strabismologist may be beneficial. Refractive accommodative esotropia patients are most likely to benefit from LASIK, and refractive surgery may even replace strabismus surgery to straighten some of these patients' eyes. In children, refractive surgery eventually may become an integral part of the treatment of anisometropic amblyopia.
1. Stidham DB, Borissova O, Borissov V, Prager TC. Effect of hyperopic laser in situ keratomileusis on ocular alignment and stereopis in patients with accommodative esotropia. Ophthalmology. 2002;109:1148-1153.
2. Hoyos JE, Cigales M, Hoyos-Chacon J, et al. Hyperopic laser in situ keratomileusis for refractive accommodative esotropia. J Cataract Refract Surg. 2002;28:1522-1529.
3. Wright, KW, Guemes A, Kapadia MS, Wilson SE. Binocular function and patient satisfaction after monovision induced by myopic photorefractive keratectomy. J Cataract Refract Surg. 1999;25:177-182.
4. Schuler E, Silverberg M, Beade P, Moadel K. Decompensated strabismus after laser in situ keratomileusis. J Catarct Refract Surg. 1999;25:1552-1553.
5. Mandava N, Donnenfeld PL, Owens A, et al. Ocular deviation following excimer laser photorefractive keratectomy. J Cataract Refract Surg. 1996;22:504-505.
6. Yap EY, Kowal L. Diplopia as a complication of laser in situ keratomileusis surgery. Clin Experiment Ophthalmol. 2001;29:4:268-271.
7. Drack A, Alcorn D, Lingua R. Potential use of LASIK in Children. J Pedi Ophthal Strab. 2000;37:6:356-360.
