THE SCLERA

Question 1

Which of the following areas of the sclera is the thickest?

Answer 1

Near the optic nerve

Question 2

The episcleral blood supply is derived from which 2 of the following ophthalmic artery branches?

Answer 2

Muscular, Posterior ciliary

Question 3

Which 2 of the following characteristics are most consistent describing the scleral stroma?

Answer 3

Disorganized lamellae, Dehydrated

Question 4

Which 2 of the following structures passes through the posterior scleral foramen?

Answer 4

Ganglion cell axons, Central retinal artery

Question 5

Which of the following openings provides passage for the vortex veins?

Answer 5

Middle emissaria

Question 6

Which of the following structures serves as the anterior boundary for the internal scleral sulcus?

Answer 6

Schwalbe’s line

Question 7

Where is the sclera thickest? Thinnest?

Answer 7

Sclera is thickest near the optic nerve and thinnest immediately posterior to the insertions of the rectus muscles, then there is gradual thickening of the sclera: it increases at the rectus muscle’ tendinous insertions and in the area of the limbus

Question 8

For the episclera:
Where is it thickest?
Which blood vessels supply this part of the sclera?

Answer 8

The episclera is thickest just anterior to the insertions of the rectus muscles and starting behind the rectus muscle insertions, the episclera becomes progressively thinner
towards the back of the eye.

The episclera has a rich blood supply anteriorly.
– This richer blood supply is due to the episcleral arteries that are derived from the anterior ciliary arteries
– At the equator and posteriorly the episcleral blood supply is poorer and is derived from the long
and short posterior ciliary arteries

Question 9

Which blood vessels are dilated if a patient has “ciliary/perilimbal injection”?

This would be due to the inflammation of these ocular structures:

Why does that inflammation lead to ciliary/perilimbal injection?

Answer 9

Anterior 2-3mm of conjunctiva adjacent to limbus = perilimbal conjunctva, linked to anterior ciliary artery circulation while rest of bulbar conj. served by posterior conjunctival arteries.

Inflammation of the iris/ciliary body can cause dilation/congestion of anterior anterior ciliary artery vascular network, causing perilimbal conjunctiva to be red but the rest of bulbar conj. is uninvolved.

This pattern is referred to as “perilimbal/circumlimbal injection”, “ciliary injection” or “ciliary flush” and is an indication that the focus of the inflammation is more likely intraocular rather than conjunctival in origin.

Question 10

Compare sclera and corneal stroma in terms of:

• collagen fibril diameter
• collagen fibril organization (lamellae present? branching?)
• ground substance amount and content

Answer 10

COLLAGEN FIBRIL DIAMETER
Sclera: larger variation and larger fibrils
Corneal stroma: small, uniform diameter

FIBRIL ORGANIZATION
Sclera: disorganized; lamellae extensively intertwined and branch/extend into deeper or more superficial layers of stroma (but this makes it more resistant to separation)
Corneal stroma: highly organized and arranged into stacked layers of lamellae

GROUND SUBSTANCE AMOUNT AND CONTENT
Ground substance of corneal stroma contains more GAGs + water than the scleral stroma

Question 11

What is an Axenfeld’s loop and how is its position marked on the sclera?

Answer 11

The point of entry of the anterior ciliary arteries’ major perforating branch into the sclera is often marked by a pigment spot, called an “Axenfeld’s loop”.

An Axenfeld’s loop represents a LPCN looping through the scleral channel used by the major perforating branch of the anterior ciliary artery, coming to the surface of the sclera and then looping back into the eyeball to continue its normal path.

Question 12

What is the lamina fusca?

What does the supraciliary space lie between?

The suprachoroidal space lies between: _____ & ____

What lies in these 2 spaces?

Answer 12

It is the innermost layer of the scleral stroma, which has a brownish tint due to melanocytes forming thin irregular layer from the choroid.

It is separated from the choroid by the suprachoroidal space (between sclera and choroid) and from the ciliary body by supraciliary (between sclera and CB) space.

These are the potential spaces through which long posterior ciliary nerves & arteries (LPCN, LPCA) and short posterior ciliary nerves & arteries (SPCN, SPCA) pass to reach the tissues they will innervate and supply with blood.

Question 13

What structures pass through the:

• posterior emissaria
• middle emissaria
• anterior emissaria

Answer 13

Additionally, the sclera is pierced by a series of smaller channels called “emissaria, through which blood vessels and nerves pass as they travel to or from the eyeball • The emissaria are lined by a layer of loose connective tissue.

ANTERIOR EMISSARIA lie anterior to the insertions of the rectus muscles

• provide passage for anterior ciliary arteries & veins & aqueous veins
• aqueous veins are veins carrying aqueous humor from the canal of Schlemm and use the anterior emissaria to pass through the sclera and terminate in episcleral veins

Middle emissaria lie near the equator
-lie 4mm behind equator and provide passage for vortex veins (four main vortex veins with possible accessory veins)

POSTERIOR EMISSARIA lie near the optic nerve

• transmit the long & short posterior ciliary arteries (LPCA & SPCA) and nerves (LPCN & SPCN)
• LPCA supply choroid anterior to equator, LPCN carry postganglionic sympathetic fibers to dilator muscle of iris and sensory fibers from cornea/iris/CB/bulbar conj./anterior sclera
• SPCA supply posterior sclera/choroid
• SPCN carry postganglionic parasympathetic fibers to CB/sphincter pupillae and sensory fibers from cornea/iris/CB/bulbar conj. + POSTERIOR SCLERA

Question 14

Compared to the episclera, the scleral stroma has a ______ blood supply. Why?

Answer 14

poor blood supply

Considered avascular because although several vessels pass through it to reach other ocular structures, it contains no capillary beds.

It is supplied by the episcleral vascular network and to lesser extent by the choroidal vascular network as well as by some branches of the LPCA & SPCA (where they pierce the sclera and as they travel through the suprachoroidal space) & anterior ciliary arteries (via their perforating branches)

Question 15

Why is the scleral stroma considered avascular?

Answer 15

It has no capillary beds

Question 16

Why is scleritis painful? Why does the pain increase with eye movement?

Answer 16

Scleritis = more serious inflammation of deep layers of the sclera (scleral stroma) that results in a dull aching pain and since the extraocular muscles are inserted into the sclera, ocular movement makes the pain worse.

Recall that the sclera is well innervated. Deep episcleral vessels adjacent to the scleral stroma are the vessels involved in a scleritis and show maximum congestion.

Symptoms: tenderness, tearing, photophobia, severe pain that radiates to surrounding facial structures

Question 17

Why is the sclera blue (not white) in young children?

Answer 17

In young children, the sclera is thin & transparent, so the pigment in the uveal layer will show through, giving the sclera a bluish tint

Question 18

Why is the sclera in elderly people yellowish in color?

Answer 18

The sclera of elderly adults may have a yellowish color due to fatty deposits within the sclera

Question 19

Why does the sclera appear yellow if a person has jaundice?

Is the “material” creating the yellow sclera appearance located in the sclera? If not where is it located?

Answer 19

The “yellow sclera”, seen in patients with jaundice, is due to the accumulation of billirubin in the conjunctival vessels initially, NOT in the sclera. But when profound hyperbilirubinemia occurs, then bilirubin is deposited in the sclera.

Question 20

(CO) 1. Describe the sclera in terms of variations in thickness from anterior to posterior (especially where it is thinnest and thickest)

Answer 20

see previous question

Question 21

(CO) 2. Describe what covers the external surface of the sclera (i.e. bulbar conjunctiva, Tenon’s capsule or both. Include a comparison of anterior vs. posterior sclera

Answer 21

Anteriorly the sclera is covered by both the bulbar conjunctiva and Tenon’s capsule. The bulbar conjunctiva is the outermost layer and begins around 1mm anterior to the corneoscleral limbus.

Posteriorly, the sclera is just covered by Tenon’s capsule since bulbar conjunctiva ends at the fornix so it does not cover all of the sclera.

The attachment of Tenon’s capsule to the sclera are strongest anteriorly (1.5mm posterior to corneoscleral limbus) and posteriorly at the point of exit of the optic nerve where it fuses with both the sclera and dura mater surrounding the optic nerve.

Question 22

(CO) 3. Name & describe the 3 layers of the sclera from exterior to interior

Answer 22

EPISCLERA (vascularized loose CT beneath Tenon’s capsule, connected by fine strands); more vascular than scleral stroma
-mostly collagen fibers (irregularly arranged) and abundant ground substance with elastin fibrils, fibroblasts, melanocytes
- SPCN supply episclera’s posterior portion
– LPCN supply episclera’s anterior portion
-anterior ciliary arteries (episcleral branch) provide the rich blood supply to the episclera anterior to the rectus muscle insertions
– The long & short posterior ciliary arteries (LPCA & SPCA) provide the scanty supply to the episclera behind the rectus muscle insertions (i.e. equatorially and posteriorly)

SCLERAL STROMA
Dense irregular connective tissue directly continuous with corneal stroma; made up almost entirely of irregularly arranged collagen fibrils mixed with few elastic fibers and moderate amount of ground substance/fewer fibroblasts
-supplied by the episcleral vascular network and to lesser extent by the choroidal vascular network as well as by some branches of the LPCA & SPCA (where they pierce sclera and travel through suprachoroidal space) + anterior ciliary arteries (perforating branches)

LAMINA FUSCA
Fine collagen fibers weakly connect the lamina fusca to the choroid

Question 23

(CO) 4. Describe the episclera, including:

• its location on the eyeball and relationship to Tenon’s capsule and scleral stroma
• histologic classification
• blood supply and innervation of anterior & posterior episclera

Answer 23

Vascularized, loose connective tissue lying beneath Tenon’s capsule

For the most part its superficial layers are continuous with the loose connective tissue of
Tenon’s capsule while its deeper layers are more compact as they merge with the underlying scleral stroma

– Episcleral arteries (from anterior ciliary arteries) provide the rich blood supply to the episclera anterior to the rectus muscle insertions
– The long & short posterior ciliary arteries (LPCA & SPCA) provide the scanty supply to the episclera behind the rectus muscle insertions (i.e. equatorially and posteriorly)

Question 24

(CO) 5. Discuss the episcleral vessels, including their origin & part of eyeball it supplies

Answer 24

see previous question

Question 25

(CO) 6. **Explain anatomical reason why peri-limbal injection (ciliary flush) is seen when
inflammation of cornea, iris or ciliary body occurs

Answer 25

see previous question

Question 26

(CO) 7. Describe the scleral stroma including:

• connective tissue type (include cell types and arrangement of fibers)
• **differences between corneal stroma & scleral stroma
• reasons why the sclera is opaque and not clear like the cornea

Answer 26

see previous question

Question 27

(CO) 8. For lamina fusca know:

• Location
• relationships to suprachoroidal (perichoroidal) space, choroid, sclera

Answer 27

see previous question

Question 28

(CO) 9. Describe the suprachoroidal space, including its location and what travels through it.

Answer 28

It is located between the sclera and choroid, and the branches of the LPCA/SPCA piercing the sclera travel through this space as well as LPCN/SPCN.

Question 29

(CO) 10. Be able to name and define all the scleral openings including anterior & posterior
foramina and the anterior, middle & posterior emissaria:
-describe their locations
-what passes through them (or what they are filled with) where applicable

Answer 29

emissaria- see previous question

SCLERAL OPENINGS
The sclera has two potential foramina (these are not actual openings because they are “filled in” by ocular structures)

The anterior scleral foramen is a potential foramen in the sclera, located where the sclera ends anteriorly
• it is filled in by the cornea and therefore it is not an actual opening.

Posterior scleral foramen is a potential foramen in the sclera, located where the sclera ends posteriorly
Its internal opening is smaller than its external opening
• it is filled in by the optic nerve, so it is not an actual opening

Question 30

(CO) 11. Define what an Axenfeld’s loop is. Include what it is formed by, its external appearance and its location on the eye. Be specific.

Answer 30

see previous question

Question 31

(CO) 12. Describe what the lamina cribrosa is, including location, function and what layer of the eyeball it is continuous with
- Know the lamina cribrosa is the weakest part of the sclera

Answer 31

The lamina cribosa is made of collagen & elastin fibers from the sclera that are interwoven to form holes through which the optic nerve fibers can pass (i.e. like a sieve). The lamina cribrosa provides support for the optic nerve fibers passing through it and reinforces the sclera here

Question 32

(CO) 13. Describe the blood supply and innervation to the sclera. Know that the scleral stroma is relatively avascular, especially compared to the episclera. You do not need to know specific arteries supplying the scleral stroma

Answer 32

see previous question

Question 33

(CO) 14. Discuss facts under these clinical considerations of sclera:
- variations in scleral thickness and why important
- variations in scleral color and why it happens
- why there is much pain associated with scleritis and why the pain increases with eye
movement

Answer 33

see previous question