The aqueous humor

The aqueous humor

The aqueous humor. The fluid that fills the anterior chamber of the eye, that area between the cornea and the front surface of the crystalline lens, is called the aqueous humor. Aqueous is produced by the ciliary body that is just posterior to the root of the iris and extends backwards along the inner globe to the anterior aspect of the retina.. Aqueous finds its way into the anterior chamber by flowing between the crystalline lens and the iris through the pupil. Aqueous has two functions; it provides nutrients to the cornea and is part of the optical pathway of the eye. Aqueous humor is basically a fortified blood plasma that circulates in the anterior chamber, providing nutrients to the cornea and the crystalline lens. It is a transparent fluid with an index of refraction of 1.333, which is slightly less than the index of refraction of the cornea (1.376) and less than the index of refraction of the lens (gradient index of 1.406 to 1.386). As is discussed in another chapter, it is these differences in index of refraction between media coupled with the curvature of the various optical surface interfaces that result in the bending of light at each interface.
 

As nutrients are drawn from the aqueous into the cornea by the endothelium, the aqueous fluid is circulated out of the eye and replaced by newly produced aqueous produced. To move out of the anterior chamber, it flows out of the eye primarily through the trabecular meshwork, a “drainage” system that lies behind the limbus in the angle between the cornea and the anterior iris. There is some resistance to outflow of aqueous at the trabecular meshwork that serves to maintain a pressure within the eye of approximately 15 mmHg. If there were no resistance, the eye would lose its shape and therefore its optical integrity. If there is too much resistance (or too much production of aqueous), the pressure in the eye may exceed the eye’s tolerance and damage to the optic nerve may occur, a condition known as “glaucoma.” Glaucoma generally results in a loss of mid-peripheral vision with sparing of central vision until the condition has progressed significantly. It is most commonly hereditary with a higher prevalence in certain ethnic groups (Friedman et al., 2004; Leske, 2007; Rivera, Bell, and Feldman, 2008; Wadhwa and Higginbotham, 2005); however, it may occur in individuals without a family history of glaucoma or may result secondarily to blunt trauma to the eye Cavallini et al., 2003; Kenney and Fanciullo, 2005; Sihota, Sood, and Agarwal, 1995).
 

Glaucoma can be slowly progressive, as in the case of primary open angle glaucoma (POAG) or low tension
glaucoma (LTG), and the loss of vision may be initially barely noticeable. A third type of glaucoma, angle closure glaucoma (ACG), is more acute and may or may not be accompanied by pain in and around the eye when it occurs (Ang and Ang, 2008; Congdon and Friedman, 2003). During routine eye exams, measurement of intraocular pressure and assessment of visual fields are essential for early detection of glaucoma.
 

The iris
The iris is visible through the cornea and is what gives the eye its “color.” All irides have a dark pigmented posterior layer; it is the amount of pigment in the anterior or stromal layer that produces different colors. A “blue” eye results from the selective absorption of long wavelength light by the stroma of the iris and the reflection of short wavelength (blue) light by the posterior pigmented layer. In a “brown” eye almost all visible wavelengths are absorbed by the iris stroma and very little light is left to reflect out of the eye.
 

The Iris, Cornea and Lens

The main purpose of the iris, however, is to block excess light from entering the eye and to control the iris aperture or “pupil” for differing amounts of ambient light (Figure 6-2). There are two opposing muscles in the iris; the sphincter muscles that serve to constrict the pupil and the dilator muscles that serve to dilate the pupil. Parasympathetic nerves innervate the sphincter muscles and sympathetic nerves innervate the dilator muscles. It’s because the sympathetic system is heightened relative to the parasympathetic system during “fight or flight” situations that pupils dilate when danger is sensed. Most pupil responses are controlled by a complex set of signals sent through the midbrain (specifically the Edinger-Westphal nucleus) in response to the amount of light striking the retina or as part of the accommodative triad (discussed in Chapter 7, Visual Function). There are very few conditions that affect the iris directly; however, changes in the normal response of the pupil to light or accommodation can result from lesions in the neural pathways or direct trauma to the iris. If the iris does not constrict in response to light, likely the parasympathetic system has been affected by such conditions known as Adie’s tonic pupil or third nerve palsy. This lack of constriction may also occur in response to anticholinergic drugs, such as found in scopolamine patches, or adrenergic drugs, such as found in some eye drops used for “red eye.” If the iris fails to dilate under low light conditions, likely the sympathetic system has been affected by a condition known as Horner’s syndrome.