Why does myopia increase the risk of retinal detachment and myopic macular degeneration?

What is axial myopia?

Axial length measures the eye from front to back. A normally focused eye is typically about 22–24.5 millimetres long, with individual variation.

Axial myopia means a longer eye. Rare extremes exceed 32 millimetres. Eye length and glasses prescription measure different things.

How is the eye like a balloon?

Imagine a balloon with a thick wall. As you blow it bigger, its wall stretches and thins. Similarly, tissues lining a longer eye cover a larger area.

The retina is the light-sensitive lining inside the eye. Stretching and thinning can make parts of the peripheral retina more vulnerable. Lattice degeneration consists of patches of thin peripheral retina and is more common in myopic eyes. Lattice degeneration is asssociated with an increased risk of retinal tears and retinal detachment. Its cause is more complex than stretching alone.

How can the vitreous cause a retinal tear?

The vitreous is the gel filling the back of the eye. With age, more of it becomes liquid and its attachment to the retina weakens. It can then collapse and separate from the retina, a posterior vitreous detachment (PVD). This often happens earlier in very short-sighted eyes.

Where the gel remains firmly attached, separation can tug on the retina. A thin or vulnerable area may tear more easily. If fluid passes through that tear and collects beneath the retina, it can lift the retina away from the eye wall, causing a retinal detachment. Most PVDs do not cause these complications.

My egg analogy for vitreous ageing explains the change from gel to a more liquid consistency in more detail.

What can happen to the macula?

The macula is the central part of the retina responsible for detailed vision. In an excessively elongated eye, the supporting layers beneath it, including the choroid and outer eye wall, can become very thin.

Some eyes develop progressive damage or loss of these tissues, called myopic macular degeneration. Abnormal new blood vessels can also develop beneath the macula. The balloon analogy helps explain stretching, but the central retina does not thin uniformly in every longer eye. 

Which changes need urgent assessment?

New flashes, a sudden increase in floaters, a dark shadow or curtain, or sudden loss of vision need urgent assessment, ideally the same day, by an ophthalmologist or an emergency department. Do not wait for a routine appointment.

The comparison below illustrates a possible detachment shadow. Real symptoms vary. Read the guide to flashes, floaters and curtain symptoms for more detail.

⇔ClearWith detachment+