Cataracts: From One Medical Student to Another
Cataract surgery is the most commonly performed procedure in the United States with an estimated 2.79 million cataract extractions in 2005 (ASCRS 2006). In 2004, 1.8 million Medicare beneficiaries underwent this procedure (AAO 2006). This is not surprising, as the likelihood of having a cataract climbs with each year lived. Up to 90% of individuals will have a cataract by the age of 65 years, and half of those individuals will have visual disability because of their cataracts (Trobe 2006). All said and done, 1 in 20 Americans over the age of 40 will have undergone cataract surgery (Congdo et al 2003). These statistics suggest that medical students such as you and me will have ample opportunity to encounter this seemingly small but largely prevalent and significant pathology whether it be during our family practice rotations, ophthalmology exams, or pediatric neonatal rounds. So what exactly is a cataract, who gets them, and how are they handled when found?
Let's start with where the cataract lives, inside the lens.
The lens is a biconcave disk found just behind the iris and in front of the vitreous body. The lens is circumferentially suspended inside the eye by cords called zonules to the ciliary body. The ciliary body is a circular muscle that lines the inside of the eye and tenses or relaxes the zonules to change the shape and thickness of the lens. This controlled transformation of the lens alters the angle of the light entering the front of the eye, which allows us to focus images nearby or in the distance onto our retinas—an action referred to as accommodation.
The lens has three major parts, all transparent and all circumferentially layered. A popular way to conceptualize the lens involves imagining it as a clear peanut M&M. We will start from the inside out.
|The Nucleus (the peanut)||In the center of the lens is the nucleus. The nucleus represents the majority of the lens that is present when we are born. As we age, additional layers are added to the outside of the nucleus creating the cortex of the lens. The nucleus will always remain the most central and oldest part of the lens.|
|The Cortex (the chocolate)||As we age, layers of transparent fibers are created and layered around the nucleus to create the cortex. Similar to the rings on a tree, the youngest layer is always the most peripheral. These layers are not refreshed or renewed. Instead, they are preserved, which means that the mass of the lens increases over our lifetime, from approximately 1.5 million fibers at birth to 3.5 million fibers at 80 years of age. This accumulation increases both the thickness and width of the lens while compressing and hardening the nucleus.|
|The Capsule (the candy shell)||The nucleus and cortex are surrounded by a transparent elastic membrane called the capsule. This is the basement membrane of the lens epithelium (which becomes those fibers mentioned above) and is referred to as "the bag" by the ophthalmologist. The previously mentioned zonules are connected to the capsule's equator. Changing the shape of the bag changes the shape of the lens, thus allowing accommodation to occur. During phacoemulsification cataract surgery, a procedure described later in this tutorial, the anterior part of the capsular bag is opened so that the nucleus and cortex of the lens can be removed. The remainder of the capsule is maintained to serve as a "bag", which holds the newly inserted intraocular lens (IOL).|
(Forrester et al 1996, Ford 2001)