Thursday, March 29, 2018

amblyopia (lazy eye)

That darn lazy eye...

What is a "lazy eye?"
The term "lazy eye" is most often used to refer to a condition called amblyopia. Amblyopia is reduced vision in one eye, or less frequently both eyes, due to abnormal vision development in childhood (1)

What causes amblyopia?
In simple terms, functional amblyopia occurs when something prevents the eye from focusing clearly. Any of the below factors can disrupt the normal development of the visual pathway and visual cortex: 
  • Deprivation: This type of amblyopia is a result of an obstruction in the line of sight, like a cataract or a lid droop. The first step in treating this type of amblyopia is to remove the obstruction as early as possible. 
  • Strabismus: This is the fancy name for an eye turn. When one eye is constantly misaligned, the brain is receiving two dissimilar images. So in order to prevent double vision, the brain ignores the visual input from the misaligned eye, which causes amblyopia. Some cases of strabismic amblyopia require surgery as a part of the treatment plan. Amblyopia can also occur when the eye turn is intermittent or alternating, but that is less frequent and less severe. 
  • Refractive error: Refractive amblyopia occurs when there is unequal (anisometropic) refractive error or, less often, high equal (isoametropic) refractive error between the two eyes that goes uncorrected. If one eye is seeing significantly more clearly than the other, the brain will disregard the blurrier eye, causing amblyopia. This is especially difficult to detect because the eyes look normal and the child often won't complain about blur because he/she can see out of one eye. That's why eye exams are crucial!! Optometrists and ophthalmologists are trained to know what kind of prescriptions can cause amblyopia. 

The visual pathway is developing from birth to age 6-8, so amblyopia occurs during this time.

Many of the studies we'll discuss refer to amblyopia in terms of severity. This classification is based on the best correct distance visual acuity, which is how far down the eye chart a patient can read THROUGH their best prescription.
  • Mild amblyopia: best corrected distance vision better than 20/40
  • Moderate amblyopia: best corrected distance vision 20/40-20/80
  • Severe amblyopia: best corrected distance vision worse than 20/80

How is amblyopia treated?
There are many valid answers to this question, and many theories that are currently being studied. We will focus on the evidence-based methods for treating amblyopia. Much of what we know about treating amblyopia is thanks to a group of optometrists and ophthalmologists known as the Pediatric Eye Disease Investigator Group, or PEDIG. This group has conducted numerous Amblyopia Treatment Studies, or ATS

Amblyopia treatment is dependent on many factors, such as the severity and type of amblyopia as well as the age and compliance of the patient, so treatment needs to be patient-specific. That being said, here's a general guideline for treating most cases of amblyopia. 

1. OPTICAL CORRECTION: The first line of treatment in most cases of amblyopia is correction of refractive error. Translation: glasses or contact lenses that correct vision to provide equally clear images to the retinas. Your eye doctor will likely use a dilating drop called cyclopentolate to assess the refractive error of the eye. Cyclopentolate relaxes the focusing system and gives the eye doctor a better idea of a patient's true prescription. Because of this, vision up close remains blurry for around 24 hours after the appointment.
Once the eye doctor determines the appropriate prescription, the prescription is written for full-time wear (that can be tricky with young children, but it is imperative). The eye doctor will follow-up every 4 to 6 weeks to check how vision is improving. 

Just having the optical correction in place makes a big difference, so don't underestimate the power of glasses! Glasses can improve vision even if the child still has a crossed eye with the glasses on.

In some cases, optical correction is all your need. But often, more therapy is involved.

2. DEPRIVATION: If further vision improvement is needed after full-time wear of glasses/contacts, deprivation of the better-seeing eye is added to the treatment regimen to encourage the use of the amblyopic eye. This deprivation is often achieved via patching, atropine, or Bangerter filters.      
  • Patching
    • Patching therapy involves using a patch (over the prescribed glasses/contacts) to cover the non-amblyopic or "good" eye, forcing use of the amblyopic eye. This can be done using an adhesive patch that goes on around the eye, or a soft patch that slides over the glasses.
Slide-on patches by OKeye on Etsy

A couple of adorable kids showcasing their adhesive patches.
@patchwithgus on the left, @chipperspiratedays on the right
    • Patching has been used as amblyopia treatment for decades, but the way it is used has changed thanks to PEDIG's Amblyopia Treatment Studies (ATS). The ATS 2 study found the effectiveness of 6 hours of patching/day for severe amblyopia to be similar to full-time occlusion (2). Secondly, they found the effectiveness of 2 hours of patching/day for moderate amblyopia to be similar to 6 hours of patching (3)
      • That is where we get our recommendation of 2 hours of patching a day for moderate amblyopia and 6 hours of patching a day for severe amblyopia. Some children with severe amblyopia respond to as little as 2 hours of patching, so any amount is better than nothing!
    • Both of the above studies involved at least an hour of near work during the patching hours, because that was thought to stimulate the visual system. Interestingly, the ATS 6 study found similar results in those that were assigned near tasks vs those that were assigned distance tasks.  There was a greater improvement in visual acuity in the near-task group among those with severe amblyopia, but it did not reach statistical significance. Worth noting: the study looked at "common" near tasks like reading or using the computer (4). There is reason to believe that specific near tasks designed to enhance focusing, improve tracking, fixation, etc. can improve outcomes other than visual acuity, though that was not studied.
    • Another PEDIG study (ATS 15) showed that if visual acuity stops improving after 12 weeks of 2 hours/day patching therapy, increasing to 6 hours/day was more effective than continuing at 2 hours/day for another 10 weeks (5).
  • Atropine
    • Atropine therapy involves instilling a drop of atropine to the non-amblyopic or "good" eye to make vision blurry, forcing use of the amblyopic eye.  
    • The ATS 1 study found patching and atropine therapy to be similarly effective initial treatments for moderate amblyopia in children age 3 to 7 years old (6)Another study found similar findings in children age 7 to 12 years old (7).
    • Any parent of a child that is patching can attest to some difficulty with compliance, so atropine may be a less fussy option for kids (and parents). 
    • The ATS 4 study found similar effectivity between daily use of 1% atropine and use only on the weekends in children age 3 to 7 with moderate amblyopia (8). Weekend atropine may also be effective for severe amblyopia, though improvements may be greater in younger children (9)
      • That is where we get our recommendation of 1 drop of 1% atropine instilled in the AM twice per week. It has been shown to be effective for moderate and severe amblyopia. 
    • Atropine does have some systemic side effects, including dryness, flushing of skin, fever, confusion, unusual behavior, and irritability. Those rarely occurred in the studies.
  • Bangerter filter
    • Bangerter filters are translucent filters that are applied full time to the lens in front of the good eye, and they are available in different densities to degrade the image to different levels. Since this doesn't occlude vision entirely, it may help reduce suppression (10)
Bangerter filter on a pair of glasses via Fresnel Prism and Lens Co

    • The ATS 10 study compared the improvements in visual acuity between those treated with Bangerter filters and those with 2 hours of patching in moderate amblyopes age 3 to 10. The average difference between the two groups was less than half a line on the eye chart, but the study concluded that the filters were "not non-inferior" to patching. What does that mean in laymen's terms? The study did not conclude that the filter treatment effect was similar to that of patching, but it also didn't conclude that patching was definitely superior to filters. The study did show that the filters had less of a "negative impact" on patients and parents in terms of social stigma, compliance, etc (11)

As you can see, there are multiple ways to go about occluding or depriving the "good eye." There are even other forms of occlusion that we did not mention here, including flicker glasses and occluder contact lenses. What works for one child may not be the best treatment for another, so it's great to have options that have been proven effective. Also, if one of these treatments isn't producing the expected results, we have the option of switching to another.

3) COMPUTER PROGRAMS AND ACTIVE VISION THERAPY: The most recent buzz around amblyopia treatment is reducing suppression by taking a binocular approach. Research is being done on the efficacy of 
dichoptic games, which involve simultaneous and separate stimulation of both eyes (unlike occlusion of one eye). High-contrast images are presented to the amblyopic eye and low-contrast images to the "good" eye. This type of therapy was found to be effective in adults (12). However, in a study (ATS 18) comparing 1 hour of binocular iPad game to 2 hours of patching a day in children 5 to 13 year old with amblyopia, the improvement after 16 weeks was better in the patching group (13). Participants lost interest in the game, and compliance wasn't great, so that may have contributed to the poor results. More studies need to be done to determine what types of anti-suppression therapy may produce better results. 

Adding vision therapy to amblyopia treatment is helpful in improving visual skills and binocularity. Vision therapy involves activities designed to reduce suppression and improve deficiencies in accommodation, form discrimination, and fixation, all of which are skills that are often poor in patients with amblyopia (14)

Can you treat amblyopia in older kids? 
YES! In the ATS 3 study, PEDIG found that using either 2 hours of daily patching or weekend atropine as the initial treatment can be effective in improving vision for amblyopes age 7 to 12 years old, even if they've had prior treatment (15). That being said, amblyopia is MORE responsive to treatment in younger children, so this is one of the key reasons to have children see an optometrist EARLY!

Can amblyopia recur after treatment?

YES! According to ATS 2C, nearly 25% of amblyopic children under 8 years old experienced regression within a year of discontinuing treatment. The recurrence rate was similar in patients who stopped patching vs those that stopped atropine, and most cases occurred within 3 months of discontinuing treatment (16)The ATS 3 study showed that only 7% of 7 to 12 year olds studied experienced recurrence (17)The risk is much greater when those patching 6-8 hrs were stopped abruptly, so tapering off (ie: going from 6 hrs to 2 hrs) is advised, especially with younger children.  Because recurrence is possible, following up with your eye doctor is critical!

CliffsNotes: The first step in treating most cases of amblyopia is to correct vision, either with glasses or contacts. Surgical intervention may be needed in some cases. Occlusion therapy may be needed to improve vision beyond what is achieved by optical correction alone, and it's never a bad idea to enroll in vision therapy to improve eye teaming and visual skills
Amblyopia treatment may vary depending on the type and severity of the amblyopia as well as the age and compliance of the patient. Early intervention is key, so get your kids in to see their optometrist ASAP!

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