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Breaking Down Strabismus : The Basics

· Happy Eyes

This post is going to be the first of a series of posts on the bread and butter of my ophthalmology world: strabismus.

What is strabismus?

'Strabismus' is the medical term used to describe when the eyes are not straight. Other words you may have heard to describe this condition include "squint", "eye turn", "cross-eyed", "wall-eyed", having "wandering eyes", or having "deviating eyes". Depending on the source of data and the region of the world, strabismus can affect anywhere between 0.8% to 5.7% of the children in a particular population. In the United States, it is estimated that about 4 percent of all children have strabismus. Strabismus can happen in adults, too.

What causes it?

This is a whole field onto itself so this is not an easy question to answer. The upcoming posts in this series will go into the different types of strabismus in more detail. For now, let's think about the big categories. Strabismus can develop when the brain hasn't learned how to use both eyes together. Certain genetic syndromes or early childhood conditions can be associated with this. For example, a child who is born premature is at increased risk for developing an eye misalignment. Strabismus can run in the family. Strabismus can be caused by abnormal eye muscles or when the brain nerves that send signals to the muscles to move are not working normally. Strabismus can be caused by something structurally different about the eye that makes it hard for the eye to focus (such as a cataract). Strabismus can be associated with a really high glasses prescription and not wearing glasses. If vision is poor from one or both eyes, a child may not want to put effort into keeping both eyes aligned. There is another big category of scary things that can cause strabismus. Sometimes if there are changes with the brain like an infection, tumor, inflammation, or after head trauma, strabismus can also develop.

Do I need to worry about it?

It depends on what is causing it (see all the different causes above). Rule of thumb : if your child has suddenly developed strabismus, this needs to be evaluated appropriately to understand what is causing it and to treat it. Our brains don't quite develop the ability to use both eyes together until about 4-6 months of age so a fair number of infants will have some intermittent strabismus until they reach this age. After 4-6 months of age, if a child's eyes are constantly crossing in or drifting out, or if this is a new strabismus that they did not have, this needs to be checked out. If a child has misaligned eyes, s/he can unconsciously be suppressing one of or both eyes to avoid seeing double. This suppression can lead to amblyopia and permanent visual loss.

Some basic terminology and notation

To understand the different types of strabismus, we need to go over some common words used:

Esotropia (ET): Eso = in. This is the type of strabismus where the eyes are turned inwards.
Exotropia (XT): Exo = out. This is the type of strabismus where the eyes are turned outwards.

Hypertropia (HT): Hyper = up. This is the type of strabismus where one eye is turned upward.

Hypotropia (HypoT): Hypo = down. This is the type of strabismus where one eye is turned downward.

Sometimes the misalignment doesn't happen all the time. The eyes may drift or cross in only some of the time. These types of strabismus might instead be called a "-phoria" or "intermittent -tropia". This is a nuanced difference and each eye doctor is different about how they decide whether an eye misalignment is a phoria or intermittent tropia. To make this distinction, we look at factors like how well a child is able to control her/his eyes together, how quickly s/he can recover if we break down her/his ability to use both eyes together in clinic, how big the eye misalignment is, how frequently misalignment occurs, et cetera.

The eyes may also align differently when a child is focusing at near and at a distance. You may see the following notation in a doctor's note to differentiate between distance and near measurements:

XT = this is a measurement of the eye misalignment when the child is focusing at a distance

XT' = the ' indicates that this is a measurement of the eye misalignment when the child is focusing at near

strabismus, esotropia, exotropia, Eye Dogtor Julie

First things first, let's get the orientation right here. If you are standing right in front of a patient and looking at his/her eyes, the patient's right eye would be on your left not on your right. In the picture above, that's why the eye on your left is labeled as "right". To understand these examples you have to imagine that this is what it would look like if you had a patient right in front of you.

The top most pair of eyes in the picture above is an example of a right esotropia because the patient's right eye is turned inward. The center set of eyes has no eye misalignment for reference. The bottom set of eyes in the picture above show an example of a right exotropia, where the patient's right eye is turned outward.

hypertropia, hypotropia, Eye Dogtor Julie

Here are two more examples of strabismus terminology. The top pair of eyes show an example of a right hypertropia since it is the patient's right eye that is turned upward. The bottom pair show an example of a right hypotropia, where the patient's right eye is turned downward.

What do you look for during the eye exam to figure out what's causing it?

A strabismus evaluation is pretty extensive since there are lots of things that can cause it.

In the history taking, we ask about when the strabismus started, whether it has gotten better or worse, when the family notices it the most, whether it is only one or both eyes affected. We ask if there have been any other new habits that the child has developed like head turns or head tilts, squinting of the eyes, difficulty tracking, or bringing objects close to the face to see. We ask about other eye symptoms like eye pain, light sensitivity, changes to the appearance of the eyes, or new complaints of blurry vision or double vision. We ask about any family history of eye problems. We ask about a child's birth history and whether there are any other medical problems. We ask about any history of head injuries, trauma, hospitalizations, or surgeries. We also ask about allergies and medicines.

During the eye exam, we take a close look at how the eyes move and we look at the alignment of the eyes. We measure the amount of misalignment if there is any seen. We check a child's vision and 3D vision (this is one way to see how the brain uses both eyes together). We look for any structural abnormalities in the eyes. We look to make sure a child does not need glasses. If there is any concern for something that might be affecting the brain, we might do more testing for this. Below is a flow chart showing the steps of the pediatric eye exam. Click on the image if you want to learn more about each step!

Pediatric Eye Exam, Eye Dogtor Julie,

How do you measure the amount of eye misalignment in a baby?

Good question. It can be tough and often multiple measurements are needed over time to make sure the eye alignment is stable. The age, vision, and how cooperative a child is feeling on the day of the exam all determine which method we use to measure eye alignment. Here are some of the different methods we use:

Hirschberg Test - This is a quick way to check alignment in babies by using the reflection of light on his/her eyes. We shine a penlight or use the flashlight from a cell phone onto their eyes. You can determine alignment by looking at the light reflection off the front of the eye (the cornea) and whether it is centered over the pupil of the eye. For every millimeter this light reflex is off center, this equals approximately 15 prism diopters (prism diopter = the unit for measuring eye misalignment).

Hirschberg Test, Eye Dogtor Julie,

This is how a Hirschberg test might look like in clinic. You can use a light source to create a light reflex on the front of the eyes and see whether the light reflex is centered. If the light reflex is not over the pupil of the eye, there may be some eye misalignment.

Krimsky Test - This method uses a penlight and prisms. Similar to the Hirschberg test, you start by shining light onto the child's eyes and looking at the light reflection off the front of the eyes. You then place different amounts of prism over the misaligned eye until the light reflex moves back into the center of the eye. The amount of prism that it takes to center the light reflex is a rough estimate of the eye misalignment.

Krimsky, Eye Dogtor Julie

This is an example of how the Krimsky test might look. While shining a light towards the patient's eyes, you hold different amounts of prism over the misaligned eye until the light reflex moves back to the center of the eye.

Cover-Uncover and Cross Cover Test - This is a tough one to explain. For this method, you use a paddle to cover one eye at a time to prevent the child from being able to see with both eyes at the same time. With the child focusing on an object, one eye is covered with the paddle and you watch what the uncovered eye does. If the uncovered eye is straight, it will not shift position when the cover is applied. If the uncovered eye is misaligned, its position will change because now it has to straighten out for the child to be able to continue to focus on the target object. You do this for each eye. You can then also rapidly move the paddle back and forth from one eye to the other and observe how the eyes shift in position to confirm the type of strabismus. There are two other variations to this technique called the alternate prism cover test and the simultaneous prism and cover test. Both of these variations use prisms to measure the amount of eye misalignment while you are covering and uncovering each eye with the paddle. You increase the strength of the prism as you cover and uncover the eye until there is no longer any shift in eye position. This amount of prism it takes to stop the eye from shifting is the estimate of the amount of eye misalignment. You also want to measure the amount of eye misalignment with the child looking in different directions to see if the alignment changes (for example, you can measure eye alignment while the child is looking up, down, left, and right). The mechanics of these methods are a bit more complicated than what I have described, but hopefully this is enough to understand the gist of how we measure eye misalignment in the clinic!

simultaneous prism cover test, Eye Dogtor Julie

Example of a simultaneous prism and cover test

How do you treat strabismus?

It depends on what's causing it. For many types of strabismus, we will try treatments like eye glasses, patching, or surgery to move the position of the eye muscles to help straighten the eyes.

The next series of posts will go over the different types of strabismus in more detail.

Reminder, these blog posts are for your education and not medical advice.

Please contact your pediatrician if you have concerns that your child has strabismus and needs a referral for a formal eye evaluation.