What are 'lazy' eyes?

Many of you may have come across the term 'lazy eye'. What exactly does it mean?

In truth, there is the term that is understood medically, and then there are the other interpretations or myths-which sometimes makes it difficult to know what people mean when they use this term.

Lazy eye medically speaking, refers to the condition called amblyopia. In this situation, the amblyopic eye is unable to see well, even though the eyeball itself is structurally normal. An amblyopic eye develops because connections between the nerves of the eye and the brain are not fully developed at birth. This process requires well focused images on the retina of the eye together with good signals reaching the brain, and continues for several years after birth. The system generally matures around the age of about 8-9 years of age, after which a lazy eye will not develop even if that particular eye develops a disease which prevents it from seeing well.

So, a lazy eye develops because something prevents a well focused image from reaching the retina of the eye and the brain. These possible causes include high longsightedness or astigmatism, a constant (present all the time) squint (eg crossed eyes, which cause double image, resulting in the brain blocking the image), or even more rarely things like corneal scars or cataract.

Do you think the above is a lazy eye?

Answer: Maybe...

The above left eye has drifted outwards, and some would call this a squint. If a squint happens only some of the time, it is very unlikely to have amblyopia, or be 'lazy'. In other words, the eye would be able to see well at whatever it was looking at.

However, if the squint is present all the time, the child would have double vision-so the brain automatically blocks the image from the squinting eye. This blockage, or 'suppression', can cause the eye to become amblyopic, so that this particular 'lazy' eye would be unable to see clearly even if any long/shortsightedness/astigmatism was corrected.

Many children with 'lazy' eyes do not display any outward sign of the poor vision in that eye. This is the case especially when the 'lazy' eye is caused by different and high degrees of spectacle power/astigmatism. That is why screening the vision of young children of 5-6 years of age is so important. In Singapore, schools and kindergartens routinely screen student's eyes once a year.

What 'lazy' eyes are not...

So just to recap, lazy eyes are not squints (although constant squints can cause lazy eyes), and lazy eyes do not mean an eye with very high spectacle power (although an eye with high spectacle power/astigmatism can be lazy). As long as an eye can see well (with or without glasses), then it is not considered 'lazy'. It follows from the explanation above that lazy eyes can only develop in childhood, and that if it is detected much later-eg in adulthood when the visual system is mature, there are no effective cures.

Lazy eye treatments

When 'lazy' eye or amblyopia is detected at a young age less than 7 years of age, treatment is generally straightforward and involves patching the good eye to force the child to see with the poorer, amblyopic eye. The younger the child, the shorter the time needed for patching.

There are several different methods, but in my practice the child patches the good eye one hour /day/year of life (age). So a 3 year old may patch for 3 hours a day, while a 6 year old patches for 6 hours a day. Once the child gets older than 7 years of age, the eye is less responsive to patching, and in some cases even whole day patching may be required for a period of time. For somebody above 12-13 years of age, patching essentially does not work anymore. There exists an FDA approved treatment for amblyopia for those older children and adults, consisting of eye exercises (called Neurovision), however, my personal experience with it is disappointing.