Can myopia affect only one eye, or is it always both?

• Myopia can affect one eye or both eyes, causing blurred distance vision.
• When it affects both eyes, it is called bilateral myopia, even if each eye has a different prescription. When one eye is affected with myopia and the other sees normally, this is known as unilateral myopia.
• Children who have myopia in one eye benefit from wearing contact lenses, by encouraging normal vision and healthy development, and reducing the risk of myopia in the other eye.

Myopia, also known as ‘near-sightedness’ or ‘short-sightedness’, is an eye condition that leads to blurred vision, especially for distant objects, such as signs, TV screens, and passing cars. Myopia commonly occurs between 8 –13 years of age, when during childhood eye development, the eyeball elongates compared to the rest of the eye.1 

A longer eyeball affects the way light travels inside the eye. Instead of focusing light onto the retina at the back of the eye, light falls short, causing objects in the distance to appear blurry. On the other hand, objects up close (such as a book or smartphone) can appear very clear. The more severe the myopia, the more noticeable the blur, and the shorter the range of clear vision becomes.

It’s possible for myopia to affect either one eye or both eyes. 

When myopia is present in both eyes, it is referred to as bilateral – bi- (meaning “two”) and lateral (meaning “side”). This does not mean that each eye must have the same prescription. It is common for one eye to have more myopia than the other, with the difference ranging from small to noticeable. Even when the degree of myopia differs between eyes, the condition is still described as bilateral myopia.

When myopia only affects one eye, it is referred to as unilateral – uni- (meaning “one”) and lateral (meaning “side”). In these cases, the eye which does not have myopia may see clearly in the distance, while the myopic eye experiences blur. Unilateral myopia usually arises from the two eyes growing at slightly different rates during childhood, leading to two eyes of different lengths.2 

The symptoms of unilateral myopia are not as obvious. When both eyes are looking at an object, the clearer eye often compensates and/or the weaker eye may be ignored. The difference may only become apparent when covering one eye at a time. In children, signs may include squinting or preferring to use one eye more than the other.

Myopia more commonly occurs in both eyes (bilateral) than in just one (unilateral), although there is limited data comparing these two groups. 

In a study of children and teenagers under 19 years old, about 14% of those with myopia had it in only one eye.3 In a study of adults aged 40–80 years, bilateral myopia was more than twice as common as unilateral myopia.4

Treating myopia starts with correcting vision, typically through wearing prescription glasses or contact lenses, though some people may elect to undertake LASIK and other types of eye surgery during adulthood. The next step is to slow the worsening of myopia, using 'myopia control' treatments. This is necessary because once myopia develops, it typically progresses and becomes more severe every few months until the late teenage or the early adult years.

Even if only one eye has myopia, the goals are similar. We want to ensure both eyes have the ability to see as clearly as possible, work well together, and avoid developing more severe levels of myopia in the future. Research has showed that it is quite common for the ‘good’ eye to develop myopia later on, so proactive treatment may be considered for both eyes.5,6

Contact lenses are the preferred way to manage unilateral myopia, as unlike glasses, they do not cause the two eyes to see images in different sizes (aniseikonia). This effect can be noticeable when the two eyes possess significantly different vision, and can lead to discomfort and depth perception issues.7  Studies suggest that ortho-k (orthokeratology) lenses and Essilor Stellest glasses lenses are effective for controlling myopia in both eyes in children with unilateral myopia.8,9