Opthalmology - Basic eye anatomy

Question 1

Gross outer structure of eyeball

Answer 1

Question 2

Define chambers of the eyeball

Answer 2

Question 3

Cornea
- Define layers
- Functions of each layer

Answer 3

The Cornea
- Clear front surface of the eye
- The cornea-air interface provides 2/3 of the eye’s refractive power
- Avascular and gets its nutrition from tears on the outside, aqueous fluid on the inside, and from blood vessels located at the periphery

5 distinct layers:
* Epithelium (does not scar, painful if injured)
* Bowman’s membrane
* Stroma (90% all corneal thickness, can scar)
* Descemet’s membrane
* Endothelium (do not regenrate, act as as pump to keep cornea dehydrated, damage causes aqueous fluid to leak into stroma and cloud cornea with edema)

Question 4

Anterior Chamber of eyeball
- Function
- Pathology

Answer 4

Anterior chamber angle

• The angle formed by the inner cornea and the root of the iris
• Contains trabecular meshwork with its underlying Schlemms Canal for aqueous fluid drainage
• Blockage of this pathway/angle is important in the pathogenesis of glaucoma

Question 5

Crystalline lens of eyeball
- Layers
- Anatomical structure
- Function

Answer 5

The crystalline lens
- Located behind iris
- No innervation or vascularization. Nutrient supply by nutrients floating in aqueous fluid
- Capsule held in place by suspensory ligaments called zonules that insert around the periphery and connect to the muscular ciliary body
- Ciliary body contraction causes the zonule ligaments to relax, allowing the lens to become rounder and increase its refracting power

3 layers:
Capsule
Cortex
Nucleus

Question 6

Retina and macula
- Structure
- Function

Answer 6

Retina
- sensory portion of the eye and contains layers of photoreceptors (rod and cone cells), nerves, and supporting cells

Macula:
- pigmented area of the retina that is responsible for central vision
- Fovea is located within central macula, used for extreme central vision. Fovea derives nutrition from underlying choroid (susceptible to injury during retinal detachments)

Question 7

Eyelid and Adnexa
- Structure and function

Answer 7

Two muscles are responsible for eyelid movement
* The orbicularis oculi closes the eyelids and is innervated by CN VII
* The levator palpebrae opens the eye and is innervated by CN III

Eyelid:
- Supported by tarsal plate, contains no subcutaneous fat
- tarsal plate is a fibrous layer that gives the lids shape, strength, and a place for muscles to attach

Question 8

Orbit

• Bony structure
• Function
• Pathology

Answer 8

7 bones of the orbit:
- Frontal, Ethmoid, Maxillary, Zygomatic, lacrimal, Sphenoid (greater wing)

Function:
- orbital apex is the entry point for all the nerves and vessels supplying the orbit
- superior orbital fissure lies between the wings of the sphenoid bones, through which many vessels and nerves pass into the orbit

Pathologies:
- The medial wall is most fragile, consisting of the lacrimal bone and ethmoid bone.
- The thinnest part is called the lamina papyracaea, representing a common site of orbital wall fracture during trauma and potential site for spread of infection from sinuses into the orbit

Question 9

Orbit
- Define extra-ocular muscles origin and insertion

Answer 9

Superior, lateral, inferior and medial rectus:
- Four rectus muscles control each eye. These muscles insert at the sclera, behind the limbus, and each pull the eye in the direction of their attachment

Superior and inferior oblique muscles
- The superior oblique also originates in the posterior orbit, but courses nasally until it reaches the trochlea (or “pulley”) before inserting onto the eye
- The inferior oblique originates from the orbital floor and inserts behind the globe near the macula.

Question 10

Outline checklist of structures examined in a dilated fundus exam

Answer 10

Optic disc
* Margins
* Color
* Cup:Disc ratio
* Other abnormalities

Retina
* Macula
* Blood vessels
* Peripheral retina

Media opacities
* Cornea, e.g., corneal scar
* Lens, e.g., cataract

Dilate pupils
* Phenylephrine 2.5% (mydriatic)
* Tropicamide 1% (cycloplegic)
* Combination drops, e.g., Mydrin-P

Question 11

Define the steps in direct opthalmoscopy exam

Answer 11

1. Sit the patient comfortably in a dimly lit room
2. Always use your right eye to examine the patient’s right eye and your left eye for the patient’s left eye. Stand on the same side of the eye you are examining
3. Stand at one arm’s length away from the patient and shine the ophthalmoscope on the patient’s pupil to look for a red-light reflex
4. Continue to fixate on the red-light reflex while you move closer to the patient’s eye. The first structure to look for is the optic disc (on the nasal side of the retina). Once you have found this, adjust the focus of your ophthalmoscope till the structures are at its sharpest
5. Then work your way along each of the four main retinal vascular branches to look for exudates and haemorrhage. Finally ask the patient to look directly at the light to see the patient’s macula.