Summary

A cataract is opacification (clouding) of the normally clear lens of the eye, which can lead to a reduction in vision and eventual blindness if left untreated. Globally, cataracts are the leading cause of preventable blindness. They may be acquired (age-related, traumatic, associated with systemic disease or secondary to primary ocular disease) or congenital. Age-related cataracts are the commonest type. Whilst there is no intervention to prevent the development of cataracts, modern cataract surgery can replace the cloudy lens with a clear intraocular lens to improve vision.

Background

The pathogenesis of age-related cataract is poorly understood, but is thought to involve:

• Hardening and compaction of the central lens material (nuclear sclerosis) as new layers of cortical (outer) fibres proliferate over time
• Changes in the ionic components of the lens
• Chemical and structural changes of lens proteins (crystallins), leading to loss of transparency
• Pigmentation of lens proteins (from yellow to brown)

Clinical features

Opacification of the lens reduces its optical quality, which may produce a variety of visual symptoms:

• Blurred (distance or near) vision
• Glare (difficulty seeing in bright light, haloes or glare around lights)
• Poor night vision and difficulty seeing in dim light
• Increasing myopia (near-sightedness) or change in refractive status (including the “second sight” phenomenon, where vision temporarily improves during early cataract development before it deteriorates)
• Loss of contrast sensitivity
• Loss of ability to differentiate colours

However, different symptoms are predominant in different types of cataracts, as will shortly be discussed.

Classification

Age-related cataracts are divided into 3 main classes based on the location of the opacification (although there can be overlap):