Jim Owen, OD, MBA, FAAO

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Although the cause is unknown, central toxic keratitis may occur after PRK or femtosecond laser-created flaps, within 3-9 days. DLK usually precedes CTK. Since CTK is noninflammatory, it’s important that ODs recognize it as soon as possible, to avoid unnecessary topical steroids.

Laser vision correction has progressed in many ways since its U.S. Food and Drug Administration (FDA) approval. The outcomes are improved, and the complications are reduced.

 Technology has made improvements in the ability to identify good candidates, and in the hands of a skilled surgeon patients are as happy as ever.

Therefore, it is interesting when ODs see complications that they do not completely understand, such as central toxic keratitis (CTK). The good news is that incidence is rare with studies estimating its occurrence from 0.0076 percent to 0.016 percent.

Refractive surgery in 2020: From minimally invasive LASIK to light adjustable lenses


	Central toxic keratitis has occurred in laser-assisted in situ keratomileusis (LASIK), photorefractive keratectomy (PRK), corneal collagen cross-linking (CXL), and even contact lenses patients, but LASIK is the most common surgery leading to the condition.

Interestingly, CTK has not been observed in enhancement procedures

-the thinking is the condition may be intrinsic to each individual. It is an acute, self-limited, non-inflammatory reaction of the central cornea. It usually appears on Day 3 to Day 9 after surgery but is often preceded by diffuse lamellar keratitis (DLK) on Day 1.


	Differential diagnosis includes Grade 3 to Grade 4 DLK, pressure-induced stromal keratitis (PISK), and infectious keratitis. CTK often results in opacification of the central corneal along with a significant hyperopic shift in the patient’s refraction.5 Confounding early diagnosis of CTK is the fact that it is often preceded by DLK on postop Day 1 and Day 2, and the two have overlapping features.

DLK is defined by infiltrative white cells at the interface of the LASIK flap. These infiltrative cells are diffuse in the central or peripheral flap interface and respond to increased topical steroids. DLK will typically resolve in 5 to 8 days and only rarely has any impact on the final refractive outcome.

Unlock the potential of refractive surgery

A clinical pearl of treating DLK in 2020 is identifying the specific steroid being used (anecdotally my colleagues and I have found generic prednisolone acetate has been less effective), make certain the patient is shaking the bottle vigorously prior to instilling the drop, and if I am still concerned, switch the patient to difluprednate ophthalmic emulsion (Durezol, Novartis). I have found that most Grade 1 and Grade 2 DLK patients are improved the following day by increasing the steroid from qid to q2h. If the patient is not improved the next day, contact the operating surgeon.

Several features delineate CTK from infectious keratitis. While central opacification and edema associated with the conditions can be similar, CTK eyes are white and quiet. There is no injection nor an anterior chamber reaction that often accompanies infectious keratitis. Inflammatory cells often surround the central opacification in infectious keratitis in a “fluffy” pattern that are not seen in CTK.

 Theories include patients having a hypersensitivity to materials within the tear film that, when exposed to the excimer laser, results in apoptosis of keratocytes. There is a toxic reaction when these “substances” go though photoactivation by the laser. The keratocyte apoptosis has been documented by confocal microscopy and can affect the stroma on the posterior of the flap as well as in the corneal bed.

The rate of apoptosis appears to be greatest early in the the condition based on optical coherence tomography (OCT) images and pachymetry. Corneal thickness gradually returns because a proliferation of keratocytes and fibroblasts aid in reconstructing the stromal matrix. Therefore, treatment of CTK is controversial because steroids slow the proliferation of keratocytes and inflammatory mediators.

Laser vision correction: Look backward to move forward


	CTK is preceded by or assumed to be DLK upon initial diagnosis. Conventional treatment for DLK is increasing the topical steroid, usually from qid to q2h.

 The patient should be seen the following day. When the condition worsens, the next step for DLK is lifting the flap and rinsing the interface with steroid. While the two conditions can appear similar under the slit lamp, often the Grade 3 to Grade 4 DLK patient will experience a foreign body sensation or ocular pain.

Unfortunately, the diagnosis of CTK is made after the attempts of treating DLK have failed. Increased steroids, lifting the flap, and rinsing the stromal bed can have deleterious effects on the CTK patient. Steroids slow the proliferation of keratocytes and lifting the flap can increase the amount of tissue loss on the anterior stromal bed. This results in an increase in hyperopia and a thinning of the cornea.

Current thinking supports the use of steroids and lifting the flap because their adverse effects on CTK patients are less than the results of not treating an advanced DLK patient.

Recovery from CTK is 6 to 18 months with the central cornea gradually clearing and the anterior curvature steepening as the cornea thickens. This results in a reduction of hyperopia over time. Both oral doxycycline and topical vitamin E have been used to treat CTK, but neither has been studied.

CTK is a non-inflammatory self-limiting condition that can be a result of PRK or LASIK as well as other non-refractive procedures. Making the diagnosis that this condition is not DLK as soon as possible at minimum reduces the risk of long-term topical steroid use and may improve the long-term outcome of the patient. Of course, collaborating with operating surgeon is best for everyone.

Comanaging intraocular lens power calculations

1. Chua D, Htoon HM, Lim L, et al. Eighteen-year prospective audit of LASIK outcomes for myopia in 53 731 eyes. Br J Ophthalmol. 2019 Sep;103(9):1228-1234
	2. Hainline BC, Price MO, Choi DM, Price FW. Central flap necrosis after LASIK with microkeratome and femtosecond laser created flaps. J Refract Surg. 2007 Mar;23(3):233-42.
	3. Sonmez B, Maloney RK. Central toxic keratopathy: description of a syndrome in laser refractive surgery. Am J Ophthalmol. 2007 Mar;143(3):420-7.
	4. Mari Cotino JF, Suriano MM, et al. Central toxic keratopathy: a clinical case series. Br J Ophthalmol. 2013 Jun;97(6):701-3.
	5. Liu A, Maloney RK, Manche EE. Toxic peripheral kera-topathy: a syndrome in laser refractive surgery. J Cataract Refract Surg. 2012 Sep;38(9):1684-9.
	6. Galal A, Artola A, et al. Interface corneal edema secondary to steroid-induced elevation of intraocular pressure simulating diffuse lamellar keratitis. J Refract Surg. 2006 May;22(5):441-7.
	7. Linebarger EJ, et al. Diffuse lamellar keratitis: diagnosis and management. J Cataract Refract Surg. 2000 Jul;26(7):1072-7.
	8. De Paula FH, Khairallah CG, et al. Diffuse lamellar keratitis after laser in situ keratomileusis with femtosecond laser flap creation. J Cataract Refract Surg. 2012 Jun;38(6):1014-1019.
	9. Hau SC, Allan BD. In vivo confocal microscopy findings in central toxic keratopathy. J Cataract Refract Surg. 2012 Apr;38(4):710-2.

Articles

Quick diagnosis leads to better outcomes and reduces long-steroid usage.

How to differentiate CTK from DLK in post-surgical patients

As laser vision correction improves and lasers become more precise, ODs are able to better identify great candidates for LASIK. And if, during that process, ODs remember old beliefs and conventional wisdoms regarding refractive surgery, it can allow for happy patients with even better vision outcomes.

Listening to Joe Rogan and Robert Downey, Jr. discuss laser vision correction in a recent episode of the “Joe Rogan Experience” podcast was both surreal and amusing.

 To paraphrase Rogan, “LASIK corrects vision, but it does not fix vision for things like we have, like macular degeneration.”

What they have is presbyopia, so he is correct to some degree. Unfortunately, I have had conversations with optometrists who are also confused about 

, mostly due to the evolution of knowledge surrounding laser in situ keratomileusis (LASIK). This article will cover a few common misconceptions.

About 15 to 20 years ago, every laser center was equipped with a Colvard pupilometer and measuring pupil size in scotopic conditions was necessary. Prior to wavefront-and topography-guided methods of treatment, night vision problems were a significant side effect and concern for those seeking surgery for myopia.

It was believed that scotopic pupil size was a significant contributor to night vision problems. A 2013 article published in the 

 reviewed 19 papers that evaluated pupil size and night vision symptoms.

In the conclusion, Dr. Schalhorn stated, “Modern LASIK has negated the role of the low light pupil in predicting adverse visual outcomes after LASIK outside of the early postoperative period.”

Related: Unlock the potential of refractive surgery

The majority of studies found no correlation between the low light pupil’s role in predicting visual outcomes after LASIK. Those that found a slight correlation were either evaluating night vision in the early post-operative period or were using optic zones of less than 6 mm.

Related: Measurements matter in cataract surgery, the new refractive procedure

So, what should patients be told about night vision symptoms after laser vision correction?

I suggest ODs first ask their patients about night vision problems with their current method of correction. I have found that patients who experience night vision symptoms with their glasses and/ or contact lenses will likely experience the same symptoms after laser vision correction.

Additionally, as the tear layer is getting back to normal, patients will experience night vision symptoms more often than not.

The easiest way to glean that diagnosis from a patient is to ask about vision right after the instillation of an artificial tear. If it clears up, even just for a few seconds or so, the culprit is the tear layer. If it does not clear, then uncorrected refractive error is the more likely culprit.

While a refraction of +0.25-0.50 x 175 does not impact daytime vision for some patients, it can have a significant effect on the quality of night vision, despite uncorrected visual acuity (UCVA) of 20/20.

Related: Surgical, medical approaches making waves in presbyopia

Furthermore, this prescription would deter most surgeons from performing an enhancement. In this case, night-time glasses would be the best solution.

In my clinic, pupil size does not have an impact on determining whether or not a patient is a good candidate. Patients who make poor candidates are those who want

 to eliminate glare they have with glasses or patients who indicate they never need to wear glasses, especially if they have current night vision challenges.

Related: How to determine an ideal LASIK candidate

Another confusing topic within the optometry world is enhancement surgery. In the days of the microkeratome, surgeons would brag about how far out they could successfully lift a flap.

I remember being curious when a surgeon talked about lifting flaps that were seven years old and the procedure itself had been approved for only five years.

Over time what we learned was that not all flaps behaved the same when lifted and not all flaps were made equally. Surgeons would lift flaps that they, themselves, created but not those of other surgeons, and that practice became the norm. The greatest concern with this was epithelial ingrowth.

ODs would sometimes encounter that one patient, usually an older hyperope, who had recurrent ingrowth so problematic that amputation of the flap was the only feasible treatment option. This led to photorefractive keratectomy (PRK) becoming the preferred and most common method of enhancement because PRK eliminated the risk of ingrowth.

The pendulum has swung so far away from where it started, in fact, that some surgeons will not risk lifting a flap, even if it is less than three months old and they, themselves, created it.

Related: Managing dry eye key to patient satisfaction after cataract, refractive surgeries

 has become very accurate and the enhancement rate is around 2 percent or less for quality surgeons.

A study from the Journal of Refractive Surgery has shown us that outcomes from PRK enhancement is significantly poorer than outcomes from flap lift enhancements. Only 72.5 percent of patients achieved 20/20, compared to 91.5 percent who achieved 20/20 with the flap lift three months after the enhancement. 

Related: Surgical update 2019: What every OD needs to know

Even more alarming, these patients started with a SE of 1.20 D. One would expect better results from the PRK group, considering the procedure is the current best practice. 

The study postulated that epithelial hyperplasia was the likely cause of poor outcomes. The rubric for our clinic has shifted to where we lift all flaps from the original surgeon of myopes under the age of 50. 

The risk factors for epithelial ingrowth have always been microkeratome (who don’t do any), hyperopes, and older patients.

 Following this guideline we have not had any of the disaster epithelial ingrowth patients from earlier years. 

More and more, I hear patients tell me they have been told they are “too old” for LASIK. Often, the patient is 45 years old. Chronological age is not what limits success with laser vision correction as much as the clinical findings that often accompany increased age. 

The first ocular age-related change that occurs is presbyopia. Laser vision correction does not correct this. For low hyperopes and high myopes, presbyopia doesn’t even confound the laser correction discussion. 

Related: Combining MIGS approaches paves way for future glaucoma surgeries

For single-vision myopic contact lens wearers, the discussion about reading glasses should have already occurred, leaving the successful multifocal 

While telling these patients they are “too old” for LASIK shortens the conversation, it won’t stop them from seeking it, particularly if they are fed up with glasses.

It is better to spend the extra time explaining the pros and cons rather than letting the patient decide which method of correction is best. 

There are many types of age-related, ocular pathologies that should affect decisions to forego or undergo LASIK. One common challenge are cataracts, and more specifically, determining when a 

Certainly, a decrease in best corrected visual acuity is a valid reason to move from a cornea-based surgery to a lens-based procedure. Also, patients who glare test to a level where insurance will consider covering cataract surgery may be best suited for a lens-based procedure. A grayer area is when a patient experiences a myopic shift,but visual acuity remains 20/20. That is not a refractive error I would want to chase with laser vision correction. 

As laser vision correction improves and lasers become more precise, ODs are able to better identify great candidates. And if, during that process, ODs remain open minded about old beliefs and conventional wisdom regarding refractive surgery, it will allow for happier patients.

More by Dr. Owen: Recognize signs and treatment for patients with persistent PSP

1. Joe Rogan Experience podcast #1411. Available at: http://podcasts.joerogan.net/podcasts/robertdowney-jr. Accessed 1/31/20.

2. Myung D, Schallhorn S, Manche EE. Pupil size and LASIK: a review. J Refract Surg. 2013 Nov;29(11):734-41.

3. Pop M, Payette Y. Risk factors for night vision complaints after LASIK for myopia. Ophthalmology. 2004 Jan;111(1):3-10.

4. C Chan, M Lawless, G Sutton, C Hodge. Re-treatment in LASIK: To Flap Lift or Perform Surface Ablation. J Refract Surg. 2020;36(1):6-11.

5. Friehmann A, Mimouni M, Nemet AY, Sela T, Munzer G, Kaiserman I. Risk Factors or Epithelial Ingrowth Following Microkeratome-Assisted LASIK. J Refract Surg. 2018 Feb 1;34(2):100-105.

There is a lot to learn from the history of LASIK if ODs are willing to unearth it.

Prior to laser vision correction, ODs carefully examine the cornea. They evaluate the tear film, meibomian glands, curvature of both the front and back of the cornea; and measure overall corneal thickness.

, the same evaluation is necessary for cataract patients.
	
	

How to determine an ideal LASIK candidate

 was not a refractive procedure, it was just becoming a safe medical procedure.

y benefits from precise measurements of the length of the eye and curvature of the cornea, and precise placement of the 

 to achieve a desired refractive outcome. The removal of visually disruptive cloudy lens seems almost secondary.
	
	

Choose your words carefully with surgery patients


	
	Once a visually significant cataract has been identified in a motivated patient, the task of setting expectations begins. All clinical facts need to be gathered before the discussion of premium lens can start.

Findings as benign as posterior vitreous floaters can create visual disturbances that may impact the success of multifocal 

 and should be pointed out to patients prior to surgery. Additionally, pupil motility and iris defects can signal pseudoexfoliation (PXF), which is important to know prior to surgery.
	
	The cataract patient should undergo a similar work-up of the ocular surface and its supporting structures as a 

patient or a laser vision correction patient. The significant difference is that positive findings do not prevent the procedure-they simply guide the 

Blog: Dive into the widening landscape of refractive surgery 


	Begin by evaluating structure before moving on to function.
	
	I first image the meibomian glands using meibography. Showing patients meibography images and the absence of glands gets their attention to the importance of maintaining a healthy ocular surface.
	
	Next, I evaluate the lid margin documenting erosion, telangiectasic vessels, and capped meibomian glands. By pressing on the meibomian glands, evaluate the quality of the meibum from absent, inspissated, or cloudy, to an olive oil consistency.
	
	

Offer options to your cataract patients 


	
	Noting how many glands produce meibum determines the productivity of those glands. There are treatments such as iLux (Alcon), LipiFlow (Johnson & Johnson Vision), and intense pulsed light (IPL), along with supportive therapy of hot compresses and omega-3 supplements.
	
	Fluorescein and lissamine green dyes provide information on both the structure and function of the tear film. Evaluate both cornea and conjunctival staining along with measuring tear break-up time (TBUT). An unstable tear film not only affects the quality of vision after the surgery but also the accuracy of the measurements that determine IOL power.


	In the Prospective Health Assessment of Cataract Patients’ Ocular Surface (PHACO) study, more than 50 percent of patients who were evaluated for cataract surgery had a TBUT of less than 5 seconds, and 75 percent had corneal staining.

Hyperosmolar patients also had a greater variability in their keratometric measurements and a subsequent IOL power calculation of greater than 0.50 D difference.

 Therefore, the refractive “surprise” may be the simple result of a poor ocular surface.
	
	

 Blog: How to manage refractive surgery surprises 


	
	As indicated, the corneal curvature plays a crucial role in the refractive power of the eye and determining

 power. In fact, corneal astigmatism determines the power for a toric IOL. The corneal astigmatism is often similar to the refractive astigmatism-but not always.

In my experience, refractive spheres can have a significant amount of corneal astigmatism and- if not identified and addressed-can lead to postoperative refractive astigmatism.
	
	The optical quality of the cornea impacts postoperative results. Therefore, corneal scars, epithelial basement dystrophy, and other corneal dystrophies need to be identified and have their potential impact on a patient’s vision discussed prior to surgery.
	
	Additionally, keratoectasia can have a significant impact on the quality of vision after surgery. Pre-operative tomography can reveal forme fruste keratoconus (FFKC) and eyes that are not suited for corneal surgery to “fine-tune” a final refractive outcome. Those patients with even subtle corneal irregularities can experience poor postoperative quality of vision and less satisfaction, especially if they have a multifocal implant.

Technology helps to diagnose cornea ectasia


	
	In a study of 19 patients with mild keratoconus, refractive astigmatism was significantly reduced with the implantation of a toric

and the spherical equivalent was within ±0.50 in 68 percent of the eyes.

Of note, the corneal astigmatism and total higher-order aberrations (HOA) were not significantly different.

 fitting postoperatively may still be complex.

. The precise measurements and surgical techniques allow a skilled surgeon to provide successful refractive outcomes.
	
	Nevertheless, there are limitations that need to be identified and discussed prior to surgery. When properly evaluated, IOLs can provide patients with a quality of vision they have not experienced before, often without the aid of glasses or contact lenses. Those successful patients require evaluation and consultation from their referring doctors.
	
	

Read more refractive surgery content here

1. Trattler WB, Majmudar PA, Donnenfeld ED, Stonecipher KG, Goldberg DF. The prospective health assessment of cataract patients’ ocular surface (PHACO) study: the effect of dry eye. 

. 2017 Aug 7;11:1423-1430.
	2. Epitropoulos AT, Matossian C, Berdy GJ, Malhotra RP, Potvin R. Effect of tear osmolarity on repeatability of keratometry for cataract surgery planning. 

. 2015 Aug;41(8):1672-7.
	3. Montano M, Lopez-Dorantes KP, Ramirez-Miranda A, Graue-Hernandez EO, Navas A. Multifocal toric intraocular lens implantation for forme fruste and stable keratoconus. 

. 2014 Apr;30(4):282-285.
	4. Kamiya K, Shimizu K, Miyake T. Changes in astigmatism and corneal higher-order aberrations after phacoemulsification with toric intraocular implantation for mild keratoconus with cataract. 

 Measurements matter in cataract surgery, the new refractive procedure

I have enjoyed working in a laser referral center for the better part of my career. I have seen procedure volume grow, plummet, then grow and decline again, and now back to growing. The procedure currently uses a femtosecond laser for the cornea flap and an excimer laser guided by topography or wavefront aberrometry.

What has not changed is the fact that the first patient a new OD sends to our clinic is likely a non-candidate for 

. I know ODs want to find a solution to help patients, and we all have that patient who has struggled with glasses and contact lenses. So laser eye surgery (LASIK) must be the solution.
	
	

In 2019, laser vision correction patient come in all ages, sizes, and prescriptions. Based on years of experience and learning from earlier errors, an OD today can determine if a LASIK patient is clinically a good candidate before he walks into the exam room. That leaves fear and expectations to talk about.

Let’s discuss high-level guidelines to help determine which patients may have the best outcomes from LASIK.


	I first evaluate a patient’s age. The Food and Drug Administration (FDA)-approved age for LASIK is 18 years old. Linked to age is refractive stability.

While some 18-year-old myopes are refractively stable, many are not. Waiting until a patient has refractions within 0.50 D spherical equivalent a year apart is necessary to determine stability.

I have found that most patients are stable by age 25. They also have finished college and have a job. If LASIK were a new ophthalmic device, there would be a red mark next to those patients under 18, a yellow mark for those ages 18 to 25, and a green mark for those above 25.

Consider two other ages: the age of presbyopia and the age of cataracts. While presbyopia does not eliminate LASIK, it certainly changes the discussion. Treating both eyes for distance correction and needing glasses for closer vision is the most predictable result. 
	
	


	Monovision refractive surgery is successful in patients who have been successful with 

 monovision prior to surgery. This is even more so if they are age 55 as opposed to age 45. The least successful are those patients who are successful with multifocal contact lenses.  That visual world cannot be “recreated” with LASIK. The discussion with a -1.25 D 50-year-old patient can be difficult. 

When these patients understand what they are gaining and what they are losing, they can be the happiest patients. However, only they can properly weigh out the difference.

Working backward, a visually significant cataract patient requires a lens-based surgery and can achieve the desired refractive outcome. Lens changes that are not yet affecting daily activities but are reducing vision fall into the most challenging situation. Medical insurance will likely not pay for cataract surgery. But I am not eager to perform LASIK. This patient may need cataract surgery in the next year or two.

6 steps to prepare former LASIK patients for cataract surgery


	A few absolute contraindications should be gathered on a patient’s history. They can be medical, ocular, or related to the medicine a patient is taking.

The absolute contraindications are part of the FDA label for lasers approved in the U.S. and include pregnancy or nursing and presence of a collagen vascular, autoimmune, or immune-deficient disease.

Pregnancy may be an all-or-nothing condition, but collagen vascular disease and autoimmune diseases present in a broad continuum of severity. Some surgeons will not treat anyone with these conditions, while others will treat based on severity.

A 2016 retrospective study analyzed 1,224 eyes of patients with a diagnosis of collagen vascular and other immune-mediated inflammatory diseases and who had excimer laser surgery between 2008 and 2015.

The disease was well controlled in all cases with no flare or symptoms for a minimum of six months preoperatively. Researchers concluded these patients could be safely treated.

Blog: How to manage refractive surgery surprises 


	Ocular conditions affecting candidacy for LASIK patients include cataracts, keratoconus, glaucoma, macular degeneration, and amblyopia. It would be an interesting clinical discussion to delve into the subtleties of each of these conditions to find a patient for whom LASIK could be considered.

Nevertheless, there are plenty of patients who are free of any ocular disease.

The ocular surface disease patient is the most interesting and challenging patient for LASIK. Prior to surgery, a patient should be symptom free, have a normal tear break-up time, no corneal staining, and non-fluctuating vision. It is important to address ocular surface problems prior to surgery as opposed to chasing them after surgery.
	
	

Ocular surface disease limits surgical options 


	Three medicines appear on most FDA labels for excimer lasers, sumatriptan (Imitrex, GlaxoSmithKline), isotretinoin (Accutane, Roche; Claravis, Teva Pharmaceuticals), and amiodarone (Cordarone, Pfizer). Sumatriptan (Imitrex, Glaxo) is the medicine encountered most often of the three. Most surgeons are not concerned about its usage with LASIK patients.

tracked 54 eyes of 28 patients taking sumatriptan after having LASIK. This cohort of patients did not experience any increase in epithelial defects.

Because isotretinoin is known to have ocular side effects,

 any history of use should prompt a thorough ocular surface work-up.

 paper compared patients currently taking isotretinoin versus those who had taken it more than six months previously. No significant difference in outcomes between the two groups was found.

Amiodarone is an anti-arrhythmic drug used to advance cardiovascular patients. It is rarely used by patients considering LASIK, and should be considered an absolute contraindication.
	
	

How to handle vision changes over time post-LASIK


	In my experience, myopes are the best patients for LASIK. The outcomes from two recent FDA approvals (Alcon Contoura and Johnson & Johnson Vision iDesign Refractive Studio) reveal more than 50 percent of patients see 20/16 for better. This compares to several studies that find approximately 70 percent of patients see 20/20 after hyperopic LASIK.

It appears the outcomes begin to decline above -8.00 D SE. A study of high myopes found that 79 percent of patients achieved 20/20 when the range was -6.00 D to -12.00 D (with a mean of -8.65 D ± 1.51 D).8 While a -4.00 D myope might think of his vision as “blind,” he is considered an average LASIK patient achieving outstanding results.


	The goal of LASIK should be to reduce a patient’s dependency on glasses or contact lenses.

Therefore, a patient who is not dependent is not an ideal candidate. This could be the low myope who rarely wears her glasses or the younger hyperope who can accommodate and see 20/20 without correction.

Asking patients “why” and “how often” they wear their optical correction gives insight into how happy patients will be after surgery. In general, the patient who has uncorrected visual acuity better than 20/40 has less to gain.
	
	

Following the theme of less dependency, if a patient does not have refractive error after surgery but still needs prism in his glasses to fuse, the goal has not been achieved.

A stable binocular vision system is necessary for successful LASIK. The exception is alternating exotropes who perfectly suppress. Success in wearing contact lenses is another good predictor of binocular success.

LASIK removes corneal tissue, so therefore a patient needs to start with enough tissue to remain stable after surgery. This is more of a concern with myopes than hyperopes because myopic treatments remove tissue in the center of the cornea where the pachymetry is the thinnest. 

The FDA requires leaving 250 µm in the residual stromal bed. However, surgeons may have thresholds above 250 µm. The “napkin” arithmetic I calculate before I go in with a patient is 15 µm per diopter.

For example, a -4.00 D myope with a 540 µm cornea will end up with a residual stromal bed of 360 µm if the surgeon makes a 120 µm flap (540-120-[4x15]).

When a patient is near the surgeon’s threshold, I tell the patient the laser center will determine precise numbers of the residual stromal bed because the center’s diagnostic equipment will provide more details. 


	An average cornea is approximately 550 µm with a standard deviation of approximately 40 µm.

 Therefore, a cornea that is less than 470 µm is unusually thin. Similarly, a cornea thicker than 630 µm is considered an outlier. Very thin corneas often do not have enough tissue for LASIK, making these patients non-candidates.

However, corneas that are too thick should be closely evaluated as well. Often the unusually thick cornea is a result of endothelial changes, which also makes these patients poor candidates for LASIK.

LASIK alters a patient’s prescription by changing the curvature of the cornea. 

Myopes’ corneas are flattened, whereas hyperopes’ corneas become relatively steeper. Corneas become about 1.00 D steeper for every diopter of hyperopic treatment. In fact, studies have found that hyperopic treatment tends to regress over time due to a flattening of the cornea.

Additionally, the steepest corneas regress the most.

 Surgeons are therefore less likely to treat patients where the resultant cornea is greater than 49.00 diopters.


	There are a few parameters referring ODs should know about the surgeon to whom they refer a patient for LASIK.

These include:
	• Height of correction of myope and hyperope at which a surgeon will treat
	• Amount of cornea a surgeon prefers in the residual stromal bed
	• Pachymetry at which the surgeon switches to photorefractive keratectomy (PRK)
	• Steepness the surgeon is comfortable in making the cornea

If you watch professional baseball players take batting practice, they almost all bunt the first few pitches. When it comes to referring LASIK, it is never a bad idea to bunt the first few. The 32-year-old -3.50 D myope who is “fed up” with her contact lenses will be thrilled by this procedure.
	
	

Read more refractive surgery articles here


	1. Schallhorn JM, Schallhorn SC, Hettinger KA, Venter JA, Pelouskova M, Teenan D, Hannan SJ. Outcomes and complications of excimer laser surgery in patients with collagen vascular and other immune-mediated inflammatory diseases. 

. 2016 Dec;42(12):1742-1752.
	2. Hardten DR, Hira NK, Lombardo AJ.

 Triptans and the incidence of epithelial defects during laser in situ keratomileusis 2005 Jan-Feb;21(1):72-6.
	3. Neudorfer M, Goldshtein I, Shamai-Lubovitz O, Chodick G, Dadon Y, Shalev V. Ocular adverse effects of systemic treatment with isotretinoin. 

2012;148(7):803-808.
	4. Ortega-Usobiaga J, Llovet-Osuna F, Djodeyre MR, Bilbao-Calabuig R, Gonzalez-Lopez F, Llovet-Rausell A, Druchkiv V.  Outcomes of laser in situ keratomileusis and photorefractive keratectomy in patients taking isotretinoin. 

 2018 Aug;192:98-103.
	5. Gharaibeh AM, Villanueva A, Mas D, Espinosa J, AliÃ³ JL. Corneal stability following hyperopic LASIK with advanced laser ablation profiles analyzed by a light propagation study. 

Available at: https://doi.org/10.1155/2018/3060939. Accessed 4/15/19.
	6. Plaza-Puche AB, Yebana P, Arba-Mosquera S, AliÃ³ JL. Three-year follow-up of hyperopic LASIK using a 500-Hz excimer laser system. 

 2015 Oct;31(10):674-82.
	7. Kanellopoulos AJ. Topography-guided hyperopic and hyperopic astigmatism femtosecond laser-assisted LASIK: long-term experience with the 400 Hz eye-Q excimer platform.

2012 Jun;6:895-901.
	8. Hashemi H, Ghaffari R, Miraftab M, Asgari S. Femtosecond laser-assisted LASIK versus PRK for high myopia: comparison of 18-month visual acuity and quality. 

 2017 Aug;37(4):995-1001.
	9. Feizi S, MS, Jafarinasab MR, Karimian F, Hasanpour H, Masudi A. Central and peripheral corneal thickness measurement in normal and keratoconic eyes using three corneal pachymeters.  

. 2014 Jul-Sep;9(3):296-304.
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Author | Jim Owen, OD, MBA, FAAO

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