The Eye

The major anatomical features of the eye 1

The eye is only about an inch (25 mm) in length, but it is one of the most sophisticated organs in the human body. The colored part of the eye is called the iris, and the black center is called the pupil. The pupil is actually a hole that regulates the amount of light that is let into the eye, the size of which is controlled by the constriction or relaxation of the iris. The white part of the eye is called the sclera, and the surface of the eye that covers the iris and pupil is called the cornea.

Light passes through the pupil to the lens, which sits directly behind the iris. The lens helps to focus light like a magnifying glass. For near vision, the muscles that control the lens allow it to become rounder so that its refracting power is increased, allowing the eye focus on nearby objects. When focusing on distant objects the lens is stretched, reducing its refracting power.2

The shape of the lens changes in order to focus on close or distant objects 3

The layer of tissue that covers the back of the eye, called the retina, is the component of the organ that transfers visional information to the brain. The eye is perfectly constructed to allow the detection of a wide range of light intensities and wavelengths (different wavelengths of light are seen as different colors). Light is detected by light sensing cells within the retina called rods and cones. Rod cells are more numerous and are sensitive to even very dim light, and cone cells are responsible for distinguishing color. The detection of light by a rod or cone cell will create an electrical impulse that is transmitted to a neuron that will transmit this information to the brain, where it is perceived as an image.

In the back of the eye, the macula has the highest density of rod cells, and in the center of the macula is a small pit called the fovea that is responsible for central vision. The optic nerve region is where neurons and blood vessels exit the eye.

The eyeball is filled with a gel called the vitreous that helps the eyeball keep its shape. The area that is between the cornea and the lens is filled with a more watery solution called the aqueous humor. The aqueous humor is drained out of the eye through a region located in the angle between the cornea and iris called the trabecular meshwork. This region is of particular importance in glaucoma.

Illustration of the trabecular meshwork 4

Glaucoma

Glaucoma is a leading cause of visual impairment and blindness in the world.5 It affects approximately 3 million Americans (70 million people worldwide), and about half of these people do not yet know that they have the disease.6 The disease is typically asymptomatic at its onset and progresses slowly. There is no cure for glaucoma, but very effective medical and surgical options are available to halt the progression of the disease. Early detection of glaucoma is key to slowing progression and preventing blindness. The exact cause of glaucoma is unknown. It is a disorder in which optic neurons die, which leads to irreversible vision loss. The mechanism by which the neurons are damaged is not well understood. Typically peripheral vision is lost first, then central vision. The most common form of glaucoma in the United States is primary open angle glaucoma (POAG), which is characterized by poor aqueous humor drainage due to the clogging or dysfunction of the trabecular meshwork. The second most common form of glaucoma is closed angle glaucoma (CAG). This form of the disease occurs when the drainage angle of the eye is blocked leading to an increase in eye pressure. Sometimes glaucomatous symptoms are seen even though eye pressure is in a normal range, and this is called normal tension glaucoma (NTG). The cause of NTG is unknown, but there are theories that people who develop it have an abnormally sensitive optic nerve, problems with optic blood flow, or an imbalance between the pressure in their eye and the pressure in their brain. Risk factors for glaucoma include age (60+), family history, corticosteroid use, and high eye pressure.7

Glaucoma Detection

The diagnosis of glaucoma requires the identification of structural changes in the back of the eye accompanied by evidence of visual field loss. The incorporation of ocular imaging devices as part of the clinical evaluation allows for accurate, micron scale measurements of the structures in the back of the eye. This can improve disease detection and assist in monitoring changes over time.

Functional Changes in Glaucoma

The visual field test is a subjective measure of functional vision. By shining small pinpricks of light on a screen in front of the eye it is possible to determine regions where light is detected and regions where it is not.

Plots of gradual visual field loss, with dark boxes indicating regions where light is not detected

Structural Changes in Glaucoma

Optical coherence tomography (OCT) technology gives information about the layers of tissue in the eye. Real time, noninvasive information about the thickness of eye tissues has greatly helped glaucoma detection and monitoring.

OCT macula thickness map
OCT works by using a technique called interferometry. Light is shone on the back of the eye and the machine uses the light that bounces back to create a virtual cross section using the different reflectivity of tissues at different depths. The first OCTs were called time domain (TD)-OCTs, which obtained information about different depths in the tissue by using a moving mirror. Current OCT devices are called spectral domain (SD)-OCTs, which have eliminated the need for a moving mirror by using a mathematical technique called a Fast Fourier Transform and incorporating a spectrometer and charge-coupled-device camera to separate and detect the tissue layer information.
OCT cross-section of the macula
OCT circular scan around the optic nerve region
OCT cross-section of the optic nerve head
Newer experimental OCT technologies include swept-source OCT, polarization sensitive OCT, phase-sensitive OCT, white-light OCT, and others that further enhance the ability to visualize fine structures in the eye and assess the functionality of certain tissues.
As the neurons that comprise the RNFL die this layer thins, giving ophthalmologists a way to assess the severity of glaucomatous damage. Regions of retinal nerve fiber layer (RNFL) thinning are shown here, marked with red and yellow colors, expanded over the course of 6 years of follow-up

References:
  1. Diagram of the Eye. https://nei.nih.gov/health/eyediagram/ Accessed August 12, 2015.
  2. Anatomy of the Eye. http://www.ophthobook.com/chapters/anatomy. Accessed August 12, 2015.
  3. Accommodation of The Eye. http://free-stock-illustration.com/accommodation+eye+test?image=1913319284 Accessed August 12, 2015.
  4. Trabecular Meshwork. http://www.glaucomaassociates.com/surgery.html Accessed August 12, 2015.
  5. WHO Causes of blindness and visual impairment. WHO. http://www.who.int/blindness/causes/en/. Accessed August 12, 2015.
  6. Quigley HA, Broman AT. The number of people with glaucoma worldwide in 2010 and 2020. Br J Ophthalmol. 2006;90(3):262-267. doi:10.1136/bjo.2005.081224.
  7. Glossary of All Eye & Vision Conditions. http://www.aoa.org/patients-and-public/eye-and-vision-problems/glossary-of-eye-and-vision-conditions?sso=y. Accessed August 12, 2015.

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