Lecture 30: Regions of the Head: The Orbit & Eye Flashcards
What is a blowout fracture of the orbit?
Common with blunt trauma to they eye, bones break to relieve pressure blood is putting on eye and fall into maxillary sinus
What bones form the supraorbital margin and what is significant about this margin?
Frontal Bone
Supraorbital notch - passage or vessels and nerves to the forehead
What bones form the infraorbital margin?
Zygomatic laterally and maxilla medially
What are the bones of the orbit (roof, floor, lateral, medial)?
- Roof: frontal bone and lesser wing of sphenoid
- Floor: maxilla (medial), zygomatic (lateral), palatine
- Lateral: zygomatic, greater wing of sphenoid
- Medial: maxilla, lacrimal bones, ethmoid, body of sphenoid (lacrimal and ethmoid very thin)
How does double vision happen?
Break orbital bones, fall into maxilla and extra ocular muscles get trapped and cannot move eye in normal way
If broken orbital bones are replaced with a titanium plate and trauma happens again what happens?
If trauma is repeated after replacement with titanium plate - no bones to break to reduce pressure on eye from blood (reduce damage) - irreversible damage
What are the weakest bones in the orbit?
Lacrimal and ethmoid
What is the superior orbital fissure?
Gap between lesser and greater wings of sphenoid where most vessels and nerves to the eyes move through
What are the 3 layers of the eye and their functions?
Outer coat: cornea and sclera, function for strength
Middle coat: uvea, formed by a number of different structure (carotid, ciliary body and iris), function is nutrition
Inner coat: retina, nerves for vision
What are the features of the sclera?
- 5/6 of the eyeball
- Maintains shape and offers resistance against forces
- Provides attachment for extraocular muscles
- Point of extreme strength and resistance
- Made up of collagen laid down in a muddled way – series of whirls for greatest strength
What are the features of the cornea?
- Anterior 1/6 of eyeball
- Refracting component of eye
- Avascular and transparent
- 5 layers
What is special about the layers of the cornea?
Epithelium: constantly dividing cells
Stroma: fatty layer, transparent
Endothelium: finite number of cells, damage will require transplant, maintains water balance and thickness of cornea
Why is the cornea transparent?
Collagen fibrils are uniform in diameter, evenly spaced and run in parallel to each other (each lamellae)
How many lamellae in each stroma?
200-300
What happens when there is a scratch on different layers of the cornea?
Epithelium - repair in 7 days
Stroma - scarring due to disruption of collagen fibres
Endothelium - cornea cannot repair, transplant needed
What is the anterior chamber?
Area between iris and cornea
Where does the aqueous humour drain?
Anterior chamber angle - trabecular meshwork - canal of schlemm - venous system
What happens is anterior chamber angle is blocked?
Glaucoma
What are the functions of the ciliary body in the uvea?
- Forms aqueous humour
- Tethers lens via ligaments and ciliary processes
- Ciliary muscle for accommodaition
What are the components of aqueous humour?
Watery - lots of ions, nutrients and minerals but little protein
What is accommodation?
Under normal circumstances the ciliary muscle is relaxed with the lens sitting in the middle - ligaments pulling, lens is thin. When you contract ciliary muscle ligaments are not under tension and so lens gets fat when not under restriction - focus on something close up (accommodation)
What are the features of the iris?
- Colour part of eye
- Changes pupil size via muscles
- Sphincter pupillae – constricts pupil, PNS
- Dilator pupillae – dilates pupil, SNS (muscular epithelium)
What are the features of the choroid?
- Middle layer of the 3 layers (sclera, choroid, retina)
- 3 layers of blood vessels (big, medium, small) – most important is the choriocapillaris (sits just below the retina)
- Small vessels sit just below the retina
- Blood vessels are crucial for suppling nutrients to the outer retina – photoreceptors
What of the optic nerve can you see in the eye?
Optic nerve head
What is the macula?
Where no blood vessels are located, important for central vision, most important part is the fovea
What is the posterior pole?
Region outside of the macula
What is the ora serrata?
Edge of the retina, where the retina joins the other structures of the eyeball, looks like a serrated edge
What are the features of the fovea?
High visual acuity, avascular, gets nutrients from choroid, high density of cones (important for daytime), no rods (remove anything that may impinge on the ability to see – only have photoreceptors present here)
What forms the optic nerve?
Formed by the axons of ganglion cells as they exit the retina to pass visual info to higher cortical areas (3rd order neurons)
How does the sclera move in relation to the optic nerve?
Optic nerve has to move through the layers of the eye – sclera and choroid. 2/3 of sclera fibres do a U-turn and run down the side of the optic nerve – other 1/3 travel across the optic nerve (lamina cribosa – siv)
What is the lamina cribosa?
Meshwork of collagen fibres that go across the optic nerve to form a sieve, each hole contains the bundles of nerve fibres that project from the retina to the brain – this gives the retina structure.
What happens if the lamina cribosa is bent?
Glaucoma
Where does the blood supply for the eye originate and what are the branches?
Internal Carotid
Ophthalmic
-Central retinal
-Ciliary arteries
What are the features of the central retinal artery?
Pierces optic nerve, passes down middle and fans over the surface of the retina, supplies inside retinal neurones (not photoreceptors) - 2nd and 3rd order
What are the features of the short posterior ciliary arteries?
Supply optic nerve head and photoreceptors closest to the optic nerve
What are the features of the long posterior ciliary arteries?
Pierce globe and move to front
Where do posterior ciliary arteries travel?
Choroid
What are the features of the anterior ciliary arteries?
Anterior ciliary artery doesn’t pierce the globe and travels outside the eyeball to the front and supplies anterior structures (conjunctiva, episclera, sclera and limbus), pass along extraocular muscles
What leads to an episceral circle?
Inflammation in the eye - back flow into arteries that anastomose - poster ciliary and anterior ciliary arteries
