The Anterior Chamber and the Aqueous Humor

The aqueous serves three primary functions:

1. It provides a clear refractive media through which light passes to be focused on the retina.

2. It carries nutrients which serve the metabolism of the cornea and the lens.

3. It creates an intraocular pressure which serves to maintain the shape of the globe.

The aqueous plays an important roll in the disease called glaucoma.  Glaucoma results when the optic nerve is damaged by a sustained increase in intra-ocular pressure above a certain level.  Increased intra-ocular pressure is thought to be caused by either too much aqueous being produced, or aqueous not being drained from the eye fast enough, or a combination of the two.

Examination of the Aqueous

Inflammation inside the eye (e.g. iritis) can cause a release of proteins and inflammatory cells inside the aqueous, causing the so-called "flare and cells".  These are observed by using the slit lamp in a darkened room to project a narrow beam of light through the anterior chamber.  Ordinarily, the aqueous is so clear that you will not be able to see the beam.  If flare is present, the beam will be visible, much like a flashlight beam that is visible on a foggy night. 

Cells are observed by keeping the narrow beam stationary for a few moments.  Cells can be observed as small specks that float with the current through the beam.  In the normal aqueous, no specks are visible.

Position your mouse over the image to play the animation. 

In the first frames, a slit beam is turned on and the height of the beam is reduced and then widened.  We view the space in the anterior chamber between the corneal slit beam the slit beam that falls on the lens, with the dark pupil as a background (arrow).

If there is flare in the anterior chamber, we will be able to see the beam as it passes through the chamber.  If there is no flare, we will not be able to see the beam.

In the first frames, a slit beam is turned on and the height of the beam is reduced and then widened.  We view the space in the anterior chamber between the corneal slit beam the slit beam that falls on the lens, with the dark pupil as a background (arrow).

In there are cells in the anterior chamber, we will be able to see them as they float through the beam.

Photo courtesy of Ted Montgomery (www.tedmontgomery.com)

The Angle

The angle is formed by the intersection of the cornea and the root end of the iris.  In the angle is a structure called the trabecular meshwork, through which the aqueous passes as it exits the eye.  From the trabecular meshwork, the aqueous passes into the canal of Schlemm and then into the episcleral veins to the bloodstream.  The shape of the cornea and depth of the anterior chamber can have an effect on the drainage of the aqueous.  If the angle is too narrow, the angle can be blocked by dilation of the pupil, which causes the iris tissue to bunch up into the angle.  If the angle is blocked, pressure can build up rapidly inside the eye, causing an angle closure glaucoma attack.  The eye becomes red and painful, the cornea may become edematous, and permanent damage may occur to the optic nerve if the pressure is not reduced.

Anatomy of an angle closure:  (1) Notice that the anterior chamber is shallow.  (2) The pupil dilates, the angle is blocked by the peripheral iris, and the iris bows forward.

Examination of the Angle

Anterior chamber depth estimation and assessment of the angle opening are important techniques to utilize before a patient is dilated.  As discussed, if the angle is not sufficiently "open", dilation can cause blockage of the angle and an increase in intra-ocular pressure.

To be confident of chamber depth estimation you should observe the anterior chamber of every patient that you do a pressure check on.  This will help you learn the range of chamber depths in the general population, and you will then be able to recognize a dangerously shallow chamber.  If in doubt, have your eye doctor exam the angle before the patient is dilated.

The eye doctor can exam the angle in more detail with a gonioscopy lens (pictured below).  This lens has a curved surface that fits on the cornea.  Inside the housing of the lens are one or more mirrors that reflect the slit lamp beam into the angle of the eye, permitting a more detailed view (right image, below).  A grading system is used to access how open the angle is, with grade 4 being the widest opening and grade 1 being the narrowest opening.

1. The amount of light reaching the retina is controlled.  If there is too much light (e.g bright sunlight), then the pupil constricts to block excess light.  If there is too little light (e.g. evening or at night), then the pupil dilates to let more light in.  Too much light can bleach out the visual pigments in the retina, causing vision to "blank out".  This happens when we look directly into a camera flash at a short distance.  Vision is lost momentarily as the pigments recover from over-saturation.  Visual function is also lost if there is not enough light reaching the retina.  The first to go is cone function and color vision.  Notice that colors become less saturated in the evening light and eventually fade into gray as the light dims and rod function takes over.

2. A smaller pupil serves to sharpen our vision.  The visual system of the human eye is not optically perfect.  Most eyes have some "lower order" aberrations such as myopia, hyperopia, and astigmatism.  All eyes have higher order aberrations, the most significant of which are spherical aberration, coma, and trefoil.  If the pupil is widely dilated and light is entering the eye from the far corners of the cornea, then these aberrations are maximized and visual acuity is reduced.  However, if the size of the pupil is reduced, then peripheral corneal light rays are blocked and only the central rays pass through to the retina.  These rays are the least distorted and thus visual acuity is improved.  This is demonstrated by the pinhole effect, which is a valuable visual acuity estimating tool.  Even a person with a high refractive error can see relatively small letters on the visual acuity chart while looking through a pinhole, assuming the eye is otherwise normal.

3. A smaller pupil improves the visual depth-of-field.  Depth-of-field is a photography term which refers to the range of focus in the field of view.  If objects in the foreground are in focus and objects in the background are also in focus, the the depth-of-field is said to be good or great.  If objects in the foreground or in the distance are in focus, but objects elsewhere are out of focus, then depth-of-field is said to be poor or shallow.  A smaller lens opening in the camera, and a smaller pupil size in the eye, improves the depth-of-field.  The photo below demonstrates excellent depth of field.  Notice that the tulips in the foreground are in focus, and the building in the background is in focus.

We can see that a small pupil size can be visually beneficial.  The catch is that there must be enough light available.  Some years ago, glaucoma was commonly treated with an eye drop called pilocarpine, which is a medication that makes the pupil very small (1-2mm).  Elderly patients on pilocarpine did very well on bright sunny days.  Many with significant refractive errors could see well outside without their glasses, and many could read well without reading glasses if there was enough light.  But when the sun went down, these patients had a hard time seeing, especially when trying to drive at night.

The flip side of the coin is the person who has relatively large pupils most of the time.  Young people, blue eyed people, and myopes tend to have larger pupils.  They, of course, get plenty of available light, but the large pupil increases the effect of aberrations, particularly at dusk and at night.

Some people do OK when driving home after dilation at the eye doctor's office and other people have to bring a driver with them.  Each of them has some trouble with the increased brightness (sunglasses can be worn).  Each of them has trouble focusing up close (accommodation is paralyzed, but we use our distance vision for driving).  The dilation brings out the worst in the optical aberrations of the eye.  The difference is that some people have more optical aberrations in their visual system than others do, particularly if lens opacities or cataracts are present.

By the way, it can be very difficult to refract a person who has small pupils.  First of all, it is difficult to perform retinoscopy, auto or manual.  Secondly, the person with small pupils is mostly seeing with central rays of light  that are refracted very little by the cornea and the rest of the visual system.  Thus, when you present lens power changes to the patient, he/see may see very little difference in the choices.  A saving grace is that the patient may not want or need glasses anyway.

Pinhole Glasses

Wow, pinhole glasses, I can hardly wait to get mine in the mail.  They are so stylish.  We certainly know how they work, but do I really want to wear something like that?  I bet they work great at night.  There's even an anti-pinhole conspiracy.  Let me know if the hyperlinks dry up and I will find some new ones.  There's always someone out there trying to make a buck.

The iris is basically two kinds of muscle held together by some colorful (pigmented) tissue, the iris stroma.  The two muscles are the dilator muscle and the sphincter muscle.

The dilator muscle runs radially through the iris tissue.  This muscle pulls the iris tissue toward the root of the iris, making the pupil larger.  It is controlled by sympathetic innervation of the autonomic nervous system.  Pupil dilation is called mydriasis.  The dilator muscle is illustrated below, left.

The sphincter muscle runs in a circle around the pupil.  When this muscle contracts, the pupil gets smaller.  It is controlled by parasympathetic innervation of the autonomic nervous system.  Pupil constriction is called miosis.  The sphincter muscle is illustrated above, right.

The autonomic nervous system maintains a degree of control over bodily systems that is "automatic" in that we don't have to think about the actions that it performs.  The system has two subsystems called the sympathetic and parasympathetic.

The sympathetic system is also called the "fight or flight" system because it is most active when we are excited or threatened.  The heart and breathing rates increase and the pupils dilate.  Blood vessels in "non-essential" systems (skin, digestive tract) are constricted so that more blood is available to skeletal muscles and the heart.

When we are in a resting state, the parasympathetic system is more dominant.  As you might guess, more blood goes to the skin and to the digestive tract, and the pupils constrict.  Heart and breathing rates decrease.

Most of the time there are not wild swings of dominance of one system over the other, but rather smaller adjustments to maintain equilibrium.

PERRLA

Most of us are familiar with the acronym "PERRLA" which stands for "pupils equal, round, reactive to light and accommodation.  This acronym is sometimes used when the pupils are checked and all measurements are normal, which is the case with most eye exams.  PERRLA is a good reminder of what we should be checking: pupil size, shape, symmetry (equal size?), and reaction to light and accommodation.

Pupil Shape

The normal pupil is circular in shape.  A pupil can be said to be "regular" (round) or irregular (other than round).  Pupil irregularity can be congenital or acquired.  A congenital iris coloboma can cause an irregular pupil.  Trauma, surgery, and inflammation are common causes of acquired irregularity.  In the photo below, the pupil is irregular in shape, and it is ectopic, meaning that it is not centered.

Factors that can affect the size of the pupil include the following:

• The balance between the sympathetic and parasympathetic nervous systems

• Available light and retinal adaptation to the light

• Neurological input (e.g. third cranial nerve dysfunction)

• Drugs (eyedrops or systemic medications)

• Age (increasing miosis with age)

• Refractive error (myopes tend to have larger pupil sizes)

• Accommodation, miosis, and convergence happen together

• Iris color (irises with lighter pigmentation have larger pupils)

• Trauma or surgery

• Congenital malformation

• Acute closed-angle glaucoma (mid-dilated, fixed pupil)

• An inflammatory disease process in the anterior chamber (e.g., iritis)

Pupillary Symmetry (equality)

Normally, the pupils will be equal in size and reactivity to all light levels.  If one pupil is larger than the other, it is called "anisocoria".  It has been estimated that around 20% of the population has some degree of physiologic (non-pathologic) anisocoria that is usually of a small degree (1mm or less difference), with the incidence increasing with age.

Pathologic anisocoria can be be caused by a condition local to one eye or by a dysfunction of the nervous system.

Local causes of anisocoria include the following:

• A mydriatic or miotic drug instilled into the eye (e.g. pilocarpine or tropicamide)

• .Inflammation in the anterior chamber that may cause sphincter muscle spasm or which may result in synechiae.

• Acute closed-angle glaucoma (mid-dilated, fixed pupil)

• Trauma, surgery, or congenital malformations

• Prosthesis

Anisocoria due to a dysfunction of the nervous system is an indication of an efferent nerve pathway problem.  Anisocoria is never an indication of an afferent nerve pathway problem, meaning anisocoria is never caused by retinal pathology.  Remember, the efferent pathway is the nerve path from the central nervous system toward the periphery, in this case, toward the eye. For example, the third cranial nerve (oculomotor) controls the extraocular muscles, except for the superior oblique and the lateral rectus. This nerve also controls the upper eyelid (superior levator palpebrae), pupillary dilation, and accommodation.  Therefore, in the presence of anisocoria, the eye doctor will also be interested in a motility evaluation and the position of the eyelids.

If anisocoria is discovered and a local cause is ruled out, then the question becomes:

• Is it the smaller pupil that is too small compared to the other?

• or is it the larger pupil that is too large compared to the other?

To answer this question, the pupil size must be examined in dim light and in bright light.  Is there a difference in the measurements in dim light as compared to bright light? 

If it is the smaller pupil that is abnormal, then the difference in the pupil sizes will be greater in dim light.  For example:

Pupillary Light Responses

Obviously, the two iris muscles have to be somewhat coordinated because they have opposite actions.  The pupil size makes constant adjustments to the amount of light reaching the retina.  The action of these small adjustments is called hippus, which is a sign of normal pupillary action.

When light enters the eye and strikes the retina, a nerve impulse is sent through the optic nerve, and back through the visual pathway, to a relay station that sends an impulse back to the iris of the same eye, and this causes the pupil to contract. This is the direct light response.

This nerve impulse has a volume control. The higher the light intensity detected by the retina, the stronger the pupillary contraction is. This mechanism works to keep the photochemicals of the retina from becoming “bleached out” by too much light, such as you experience when you look directly at a photo flash.

The nerve pathway from the eye to the brain is the afferent path. The nerve pathway from the brain to the eye is the efferent path. The efferent path is a little more complicated in that the pupillary contraction nerve impulse goes back to both eyes, instead of just to the eye from whence it came. The pupillary light reflexes are cross-wired. This is the consensual light reflex.

If a person with normal eyes is sitting in a darkened room, and a penlight is flashed into the right eye, the right eye pupil will contract. This is the direct light response. The left eye pupil will also contract, even though it is still in relative darkness. This is the consensual light response.

There is also what could be called a  "darkness reflex" that is both direct and consensual. In the absence of light the reflex causes the pupils to dilate. The eye dilates in order to gather more light for better vision in low-light situations.

The direct and consensual light reflexes in each eye are normally equal, meaning the size and reaction of each pupil is the same in all lighting conditions.

Also notice that the right eye constricts when the light is directed into the left eye, and dilates again when the light is turned off.  This is the consensual light reflex.

Remember that the afferent path is the nerve path going to the brain from the eye.  A damaged retina or optic nerve will not be able to transmit the total "volume" of light signal to the brain that a normal eye would, so the brain will think that there is little or no light coming into the eye.
 

To better understand what is happening, think of two light reflexes at work. A light  reflex (pupillary constriction in the presence of light) and a dark reflex (pupillary dilation in the absence of light). Both are direct and consensual. Remember that consensual means that the right eye tells the left eye what it should be doing, and vice-versa.

The Marcus Gunn phenomenon presents us with two situations to compare:

1. An eye with an afferent pupillary defect ( the light response reflex going to the brain is diminished) is in the dark, and the normal fellow eye is flashed with a bright light. The result is that we observe a normal direct pupillary response (contraction of the pupil) in the normal eye. The direct light pupillary reflex of the normal eye has over-ridden the consensual dark pupillary reflex from the APD eye.

2. The eye with the APD is flashed with a bright light and the normal fellow eye is in the dark. The result is that we observe an initial weak contraction of the pupil of the APD eye followed by the characteristic dilation of the pupil. The consensual dark pupillary reflex of the normal eye has over-ridden the direct light pupillary response of the APD eye which has been weakened by the diseased condition.

The flashlight is moved back and forth, shining the light into one eye, then the other.  The observer looks at the response of the eye that the light is shinning into.  In the top illustration, there is a normal constriction of the pupil when the light shines into each eye.  In the bottom illustration, the left eye has a normal constriction, but when the light shines into the right eye, the eye has a momentary slight constriction, followed by quicker dilation of the pupil.