L3/4 - Ophthalmology Flashcards
What path does light take through the eye?
Through the cornea > iris > lens
What is meant by power of a lens?
The power of a lens is its ability to bend light – the greater the power the greater the refraction of light.
What type of lens is the cornea? What type of lens is the lens of the eye?
What does that mean for the cornea and the lens of the eye?
- The cornea is a fixed-power lens (performs most of the refraction of light)
- The lens is a variable-power lens (power is adjustable to yield a sharply focused image on the retina (for varying object distances)).
What is the role of the zonular fibres and the suspensory ligaments of the lens?
The zonular fibres collectively form the suspensory ligament of the lens which connects the lens of the eye to the ciliary body.
What is the lens primarily composed of?
Cells
What are the two different regions within the lens?
Nucleus
Cortex
When were the two different regions within the lens laid down?
- Nucleus > laid down in utero
- Cortex > laid down since birth > will continue to get laid down and grow/thicken through lifetime
What is the biggest defect which affects the lens?
Cataract
What is a cataract?
A clouding or loss of transparency of the lens in the eye as a result of tissue breakdown and protein clumping
What are the two types of cataract?
Nuclear cataract
Cortical cataract
What type of cataract would be the most visually disturbing and why?
The nuclear cataract. The cataract is formed where you want to get the accuracy of focussing light.
What is the ciliary body? What is it composed of?
The ciliary body is a ring of tissue that encircles the lens.
The ciliary muscles and the ciliary processes. The ciliary processes attach the lens to the ciliary body via the zonular fibres which collectively form the suspensory ligament.
What is the function of the ciliary body?
The ciliary body controls the shape of the lens.
What happens when the ciliary muscles contract?
The diameter of the ciliary body reduces in size.
What happens when the ciliary muscles contract?
Describe what the effect of the change in the lens is?
- When the ciliary muscle contracts, the diameter of the ciliary body is decreased.
- This causes the zonular fibres to slacken and reduce the tension applied to the outside of the lens
- This allows the lens to become more rounded/curved (decreases diameter volume of lens, but increases front to back)
- The increased curvature of the lens surface bends the light more (increased optical power)
- The lens is adjusted for near vision (focus on things close up)
What happens when the ciliary muscles relax?
Describe what the effect of the change in the lens is?
- When the ciliary muscle relaxes, the diameter of the ciliary body is increased.
- This causes the zonular fibres to tighten and increase the tension applied to the outside of the lens
- This allows the lens to become more flat (increase diameter volume of lens, but decreases front to back)
- The decreased curvature of the lens surface bends the light less (decreased optical power)
- The lens is adjusted for long distance vision (focus on things far away)
What is accommodation of the eye?
Accommodation, the mechanism by which the eye changes focus from distant to near images, is produced by a change in lens shape resulting from the action of the ciliary muscle on the zonular fibers.
What structure produces the aqueous humour?
The ciliary epithelium of the ciliary body
What is the flow of the aqueous humour?
From the ciliary epithelium > between lens and iris > through the natural hole in pupil > to the anterior chamber > towards angle of eye (between iris and cornea) > filtered by trabecular meshwork > into the Schlemm’s canal > into the venous system
What is the function of the aqueous humour?
Nourishes the lens and cornea (provides nutrition) & maintains eye pressure
What occurs if the angle of the eye or the trabecular meshwork is blocked?
The aqueous humour is produced all the time. If the angle of the eye or trabecular meshwork is blocked, flow is stopped and the aqueous humour will accumulate.
This causes an increase in pressure of the eye which will affect the optic nerves and other structures.
What are the two principle focussing structures of the eye?
What is the converging power of each of these structures?
What is the significance of knowing the converging power of each of the structures?
- Cornea > 2/3rd > approx. 40 diopters
- Lens > 1/3rd > approx. 20 diopters
The power of the eye is about 60 diopters of convergence.
If a lens cataract is removed, the focussing power (20D) needs to be replaced. But everyone is unique:
- If myopic > long eye > require less focussing power > higher power lens
- If hyperopic > short eye > require more focussing power > higher power lens
How is cataract diagnosed?
What signs will the patient present with?
