Study Guide: Limbus and Anterior Chamber
I.
II.
III.
Anterior Chamber Structure
A. Bounded on the anterior side by the posterior of the limbus and the cornea
B. Bounded on the posterior side by the lens and iris
C. Depth of anterior chamber- 3.5 mm ± 0.35 mm
D. Width stretches from the juncture of the iris and limbus on the medial side to the
same juncture on the temporal side. (12.5 mm)
E. Volume of anterior chamber is between 240-280 μL.= ¼ ml = ¼ cc
F. Anterior chamber is 4% of the total volume of the eye
G. The anterior chamber angle is comprised of the iris, limbus, and cornea
i. open angle: approx. 40 degrees and is associated with a wide anterior
chamber
ii. closed angle: approx. 15 degrees and is associated with a shallow anterior
chamber ( i.e. an iris with a papillary margin that is set farther forward in
the anterior chamber.)
Limits and Structures of the Limbus
A. dividing lines
i. cornea to limbus: terminations of Bowman’s layer and Descement’s
membrane
ii. limbus to sclera: imaginary line running from the apex of the anterior
chamber angle to a region on the surface that transitions from limbal to
bulbar conjuctiva
iii. the outermost tissue is the limbal conjuctiva, which is composed of 8 to 10
layers of epithelial on top of a thin layer of stroma
iv. the episclera lies underneath the limbal conjuctival and is extension of the
sclera’s episclera into the limbus; consists of loose connective tissue,
interspersed with fibroblasts and blood vessels
v. interior to the episclera is the limbal stroma, which contains blood vessels
that important in corneal nutrition, and drainage of aqueous
vi. the most interior structures are the canal of Schlemm, trabecular
meshwork, and scleral spur.
Aqueous flow
A. derived from blood flowing through the ciliary processes
i. transparent in normal eye
ii. lacks proteins
B. enters anterior chamber through the pupil
i. higher temperature upon entering causes the aqueous to rise as it flows
outward toward the cornea
ii. once at the cornea, aqueous will cool and drop to flow along the
endothelium.
C. exits the anterior chamber at the angle of the anterior chamber angle
i. 85-90% of the fluid exits using the limbal route; aqueous flows through
the trabecular meshwork to the canal of Schlemm and into the episcleral
veins
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UAB School of Optometry
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ii. 10-15% of the time aqueous flows into spaces between ciliary muscle
fibers to the supracilliary space between the ciliary body and the sclera
(Uveoscleral flow)
iii. possible third route of drainage is through the blood vessels of the iris
IV.
Anatomy of Aqueous drainage (Limbus) The aqueous drainage follows a path of
circulation from the ciliary processes, through the posterior chamber-over the lensaround the iris-through the pupil into the anterior chamber the draining through
trabecular meshwork into the canal of schlemm
A. the limbus is a 1.5 mm wide circular ring of tissue and is the junction of the
sclera and cornea.
B. Trabecular meshwork (a.k.a. the Gatekeeper)
i. the uveal portion lies closest to the anterior angle and is composed of
slender cords with lots of open spaces
ii. the corneascleral portion has broader and flatter cords with fewer and
smaller spaces
iii. fluid is less restricted through the uveal than it is through the
crorneascleral portion
iv. surrounding the collagen cords are endothelial cells, fibroblast, and elastin
v. trabecular endothelial cells are able the change shape and move around,
believed to be amitotic, and phagocytic; along with fibroblasts they
synthesize proteoglycans
B. Canal of Schlemm 85-90% of outflow path
i. large vessel that forms a circle beneath the angle of the anterior chamber
ii. limbal and aqueous veins are found around the circumference of the canal
iii. the inner side of the canal contains collecting channels that are closed to
the trabecular meshwork; aqueous passes directly through the tissue of the
meshwork. The internal collecting channels of Sondermann increase the
surface area for collecting aqueous. Aqueous enters the canals (Schelmm
& Sondermann) through large vacuoles in the endothelium lining the
canal. (see figure 9.16)
iv. Aqueous drains either through the aqueous vein (of Asher) where it is not
mixed with blood, or through the deep scleral plexus and then into veins
in the limbal stroma.
v. From the veins of the limbal stroma (deep scleral plexus) aqueous flows
into the episcleral vein, the aqueous vein dumps directly into the episcleral
vein.
C. Scleral Spur (not very prominent histologically)
i. anchoring structure for parts of the limbus and ciliary body
ii. is a thickened ridge of the sclera that runs around the apex of the anterior
chamber angle
iii. ciliary muscle fibers insert into the posterior surface
iv. trabecular meshwork cords attach at the anterior surface
V: Development of Limbus structures:
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UAB School of Optometry
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A. Most of the structures are derived from Neural crest cells that arrive in three waves.
i. Neural crest migration into the space behind the developing corneal epithelium
and in front of the lens, this becomes the corneal endothelium and the
endothelium of the trabecular meshwork.
ii. The second wave comes in just in front of the lens and will become the iris
stroma, it also defines the posterior border of the anterior chamber.
iii. The third wave becomes the corneal fibroblast that lay done corneal stroma
B. After formation the anterior chamber basically enlarges, then final structure begin to
appear.
i. At the 5th month the Canal of Schlemm and rudimentary trabecular meshwork
appear.
ii. At 6 mos gestation : As the scleral spur develops it pushes the iris root
backward, opening the angle and defining the structure of the limbus. The
trebecular meshwork develops (4-8 mos) and attaches anteriorly, and the iris
root differentiates (atrophies somewhat) posteriorly.
Clinical Correlations
I. Increase in IOP:
- Rate of aqueous production must be balanced by the rate of outflow through the
Canal of Schlemm.
- If the rate of outflow is inhibited in some way the IOP will increase cause
damage to the eye.
A. Glaucoma:
1. Potential Causes
i. Increase in IOP due to angle closure
ii. Increase in IOP due to blockage of aqueous flow through Canal of
Schlemm:
a. This blockage is caused by debris from infections in the uveal
tract
b. This blockage is caused by loss of pigment from the iris
iii. Congenital defect that causes an increase in IOP
2. Types of Glaucoma
i. Closed Angle Glaucoma - Angle between the cornea, limbus and iris (iris
root) becomes more acute ~15 degrees
ii. Open Angle Glaucoma (chronic)- IOP is increased, but the angle is
normal and functions properly~ 40 degrees
iii. Acute Angle Glaucoma - Rapid blackage that causes extreme increase
in IOP, leading to irreversible blindness within hours
3. Testing
i. Gonioscopy
a. It is impossible to view the angle without the use of gonioscopy
because the light shone in is internally reflected
b. The use of gonioscopy changes the index of refraction route from low
to high so that the light will be reflected back to the clinician
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UAB School of Optometry
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c. Gonioscopy allows the viewing of Schwalbe's Ring - the most anterior
portion of the limbus, which seperates the cornea and the trabecular
meshwork. This is a clinical landmark of the last structure to disappear
from gonioscopy view with progressive angle closure. At the point where
Schwalbe' s Ring is no longer visible, the angle if fully closed.
d. Gonioscopy may be performed with either of two types of devices:
- Direct Gonio lens
- Indirect Gonioprism
e. Scheie's Classification (for assigning grades to the degree of angle
closure based on visible structures)
- Wide open - All structures are visible
Grade I - Ciliary band obscured
Grade II - Scleral spur obscured
Grade III - Posterior trabecular meshwork obscured
Grade IV - Only Schwalbe's ring visible
- Grades I - III are considered narrow angles, whereas
Grade IV is a closed angle
ii. Tonometer - A device that non-invasively measures IOP (normal IOP ~12
mmHg, high IOP >21 mmHg)
5. General Treatment Methods
i. Open angle glaucoma is treated so that the rate of aqueous production is
reduced, or so that the rate of outflow is increased
ii. Closed angle glaucome is treated so that the flow-inhibiting effect of the angle
is relieved
iii. Pilocarpine is an example of one type of drug that may be used to decrease
IOP by constricting the ciliary muscles and pupil, which in turn decreases the
resistance to outflow perhaps by opening the trabecular meshwork via movement
of the scleral spur.
6. Surgical Treatment Methods
i. Trabeculoplasty - a surgical treatment that burns holes in the trabecular
meshwork with a laser. These holes decreases the resistance to outflow, thus
decreasing IOP temporarily. The meshwork begins to heal and return to normal
after ~ 5 years
ii. Trabeculectomy - a surgical treatment where a small flap of the sclera is
opened so that a portion of the trabecular meshwork overlying Canal of Schlemm
may be removed. The flap is loosely replaced, allowing the outflow of aqueous
through the flap and into the connective tissue of the conjunctiva. This treatment,
if successful, not only lowers IOP significantly for ~ 5 years, but it also decreases
the
visual field loss that accompanies the progression of
glaucoma.
B. Ocular Hypertension
1. Condition in which the pressure of the eye is elevated above the acceptable range,
but glaucoma is not diagnosed due to the absence of other diagnostic sign
2. Patient is at higher risk for glaucoma due to high IOP
II. Debris in the Anterior Chamber:
1. Krukenberg's Spindle
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UAB School of Optometry
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i. Clinically seen as a vertical border of pigment on the posterior side of the
cornea
ii. Caused by either debris from infection in the uveal tract or from loss of
pigment from the iris becoming attached to the corneal endothelial cells
2. Hypopyon - a collection of pus due to infections/inflammation of the iris and/or ciliary
3. Hyphema - trauma that leads to bleeding into the aqueous
4. These conditions will potentially cause an increase in IOP which can have deleterious
effects
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UAB School of Optometry
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