Cataracts
Cataracts are the opacification of the lens, a natural component that helps focus light in your eyes. While normally transparent, there are many ways for a lens to become opaque, affecting vision and quality of life.
The Lens: Structure and Function
The lens is a transparent, biconvex, avascular structure in the eye that helps focus light onto the retina. It is flexible and can change shape through accommodation, allowing the eye to focus on objects at varying distances. The lens provides one-third of the eye's total refractive power in combination with the cornea to focus light on the retina. In addition to being avascular (depending on aqueous humor for nutrition and waste removal), it is also non-innervated and therefore cannot feel pain.
The lens is located behind the iris and pupil, in front of the vitreous body, suspended by the zonular fibers (suspensory ligaments of the lens), which attach it to the ciliary body.
Anatomy of the Lens
Lens Capsule
This is a thin, elastic outer membrane composed of type IV collagen and glycoproteins. It provides the basic shape and acts as a protective barrier.
Anterior Lens Epithelium
This is a single layer of cuboidal epithelial cells beneath the anterior capsule that is responsible for producing new lens fibers throughout life.
Lens Fibers
These are long, thin, transparent cells with no nuclei in mature regions. They are arranged in concentric layers like an onion and filled with crystallin proteins that maintain transparency and refractive power.
Nucleus and Cortex
- Cortex: The outer region, composed of newer fibers.
- Nucleus: The central region, made of older, compacted fibers that harden with age.
Supporting Structures
- Zonular fibers (suspensory ligaments): Attach the lens to the ciliary body
- Ciliary muscle: Changes the lens shape during accommodation by altering tension on the zonules
Causes of Cataracts
Aging is the most common cause for the development of cataracts. As a person ages, the lens becomes less flexible, less transparent, and thicker. This is due to the denaturing of crystallin proteins. These misfolded proteins aggregate and scatter light, causing lens opacity.
Diabetes is another major cause due to excess glucose leading to non-enzymatic glycation of lens proteins, forming advanced glycation end products (AGEs). AGEs alter protein structure and increase light scattering.
Oxidative stress caused by UV light, smoking, diabetes, and aging leads to oxidative damage from reactive oxygen species (ROS). ROS damage DNA, lipids, and proteins within lens fibers. Antioxidant systems (glutathione, superoxide dismutase) become overwhelmed or decline with age and cannot properly compensate for the buildup of ROS.
Furthermore, loss of lens homeostasis due to disruption of ion pumps (like Na⁺/K⁺-ATPase) leads to increased intracellular Na⁺ and Ca²⁺, resulting in lens swelling and opacification.
Other notable causes of cataracts include medications (particularly corticosteroids), injuries, and surgeries.
Types of Cataracts
Cataracts can be classified into four different types: Nuclear Sclerotic (NS), Cortical, Posterior Subcapsular (PSC), and Congenital.
- Nuclear Sclerotic (NS) cataracts involve central lens hardening and yellowing due to protein oxidation.
- Cortical cataracts appear as spoke-like opacities in the outer lens from disrupted hydration and protein balance.
- Posterior Subcapsular (PSC) cataracts are opacities under the lens capsule that are typically associated with steroid use or diabetes.
- Congenital cataracts are caused by genetic factors or intrauterine problems that disrupt normal lens development.