Lens Anatomy and Types of Cataracts
By Leos Kral
The previous article on Lens Development described the process by which an eye lens forms during embryogenesis. This article builds on that information to describe how the detailed cellular structures of the lens can be abstracted into a higher level of anatomical description. Different types of cataracts are characterized, in part, by their anatomical location within the lens.
As shown in the drawing below, a lens is made up of a central core of primary lens fibers. This core is surrounded by layers of secondary lens fibers. Each layer of secondary lens fibers originated from opposite sides of the periphery of the lens and individual lens fibers meet to form the sutures. A layer of cells covers the anterior surface of the lens. The entire lens is enclosed within a lens capsule. The anterior side of the lens is the side exposed to the outside of the eye and the posterior side of the lens is the side inside the eyeball.
When describing the anatomy of the lens, the terms "primary lens fibers" and "secondary lens fibers" are not normally used. As shown in the drawing below, the area composed of the primary lens fibers is called the lens nucleus. The area made up of the secondary lens fibers is called the lens cortex. The layer of cells that covers the anterior surface of the lens would then be said to be located along the anterior surface of the lens cortex.
Cataracts are classified by CERF according to their location in the lens and by their size, shape or appearance. As shown in the drawing below, cataracts fall into 4 major categories by location. A cataracts is classified as "nuclear cataract" if it is located in the lens nucleus (N). If the cataract is located in the anterior portion of the cortex (AC), it is called an "anterior cortical cataract" or just an "anterior cataract". If the cataract is located in the posterior portion of the cortex (PC), it is called a "posterior cortical cataract" or just a "posterior cataract". Those cataracts which are at the periphery of the lens (EC) are called "equatorial cortical cataracts" or "equatorial cataracts". Note that while the drawing shows an equatorial cataract located at the top of the lens, it can also be located at the bottom or side of the lens.
In addition to location, a cataract can also be described by size or appearance. For example, a very small cataract confined to a specific location would be referred to as a "punctate" cataract. A larger cataract would be referred to as an "intermediate" cataract, etc. While a cataract will not change it's location in the lens over time, a cataract may change in size over time.
CERF records over the last 7 years show that in the Australian Shepherd, the most common form of cataracts is the posterior cortical cataract. Specifically, 52% of diagnosed cataracts are posterior cataracts, 26% are anterior cataracts, 14% are nuclear cataracts, and 9% are equatorial cataracts.
Copyright 1998, 1999 Leos Kral. Australian Shepherd Health and Genetics Information Resource and Health Registry. All rights reserved.