The Eye and Orbit

Question 1

Describe the shape and boundaries of the orbit. (5)

Answer 1

Pyramid shape
Base of pyramid: orbit opening
Lateral wall: sphenoid and zygomatic 
Medial wall: ethmoid and lacrimal - nasal cavity. 
Roof: frontal - anterior cranial fossa
Floor: maxilla - maxillary air sinuses.

Question 2

What are the three openings at the back of the orbit? (3)

Answer 2

Optic canal
Superior orbital fissure
Inferior orbital fissure

Question 3

Describe the neurovascular supply to the orbit. (6)

Answer 3

Artery: opthalmic
Vein: superior and inferior opthalmic veins into cavernous sinus, pterygoid plexus and facial vein.
General sensory (inc conjunctiva and cornea): ophthalmic branch of trigeminal Vc
Special sensory: optic
Motor: occulomotor, trochlear, abducens.

Question 4

Describe orbital ‘blow out’ fractures. (4)

Answer 4

Caused by increased pressure inside the orbit (punch) or inside the maxillary sinus (sinusitis) just below the floor, which can burst the weak inferioromedial wall and cause orbital contents to leak into the sinus. Presents with trapping of the extraoccular muscles so can’t look up.

Question 5

Describe the structure of eyelids. (6)

Answer 5

Skin, subcutaneous sptissue, tarsal plate containing Meibomian glands, muscles like aponeurosis of levator palpebrae superioris, and sebaceous glands assocaited with lash follicles. The tarsal plate gives shape and the glands produce the tear film.

Question 6

Explain the differences between styles and Meibomian cysts. (2)

Answer 6

Styes - blocked eyelash follicles causing infection.

Meibomian cysts - blocked Meibomian glands - not infectious.

Question 7

Describe the septum of the eye. (2)

Answer 7

Thin sheet of fibrous tissue that runs from orbital rim to tarsal plates to stop superficial infections from spreading into the orbit.

Question 8

Describe periorbital or preseptal cellulitis. (3)

Answer 8

Secondary to wounds of infections, confined superficial so doesn’t affect eye movements. Usually no treatment needed.

Question 9

Describe orbital or postseptal cellulitis. (6)

Answer 9

Infection within the orbit which can cause exophthalmos, reduced eye movements and reduced visual acuity. Potential route for infection to spread intracranial via venous drainage causing meningitis or cavernous sinus thrombosis. Needs abx and surgical drainage.

Question 10

Describe the lacrimal apparatus. (4)

Answer 10

Structures involved in tear film production (lacrimal gland) and drainage (lacrimal sac, canaliculi and nasolacrimal ducts). Obstruction can lead to epiphora (overflow of tears).

Question 11

Describe the anatomy of the eyeball (6)

Answer 11

Eyeball has 3 layers:
Anteriorly the conjunctiva.
Outer sclera (white) continuous anteriorly as the transparent cornea.
Middle choroid, ciliary body and iris - the vascular layer.
Inner retina (photosensitive layer on outer pigmented layer).

Question 12

Describe how the eyeball position is maintained. (3)

Answer 12

Suspensory ligament, extra-ocular muscles and lots of orbital fat.

Question 13

Describe the conjunctiva and two pathologies of it. (4)

Answer 13

Anterior surface (except cornea) covered in conjunctiva which covers whites of eyes and lines the eyelids, creating a conjunctival sac.
Conjunctivitis - inflammation and redness
Subconjunctival haemorrhage - wholly red by asymptomatic patch.

Question 14

What is the macula? (1)

Answer 14

The area at the back of the eye where light is focussed.

Question 15

Describe the differences between rods and cones. (6)

Answer 15

Rods: active at low levels of light, not colour vision, abundant at peripheries.
Cones: high definition colour vision at high levels of light only, concentrated within the macula. Fovoa = only cones.

Question 16

Explain why older people tend to hold books further away to read them. (5)

Answer 16

Things that are closer have more divergent light rays, so more effort is undertaken to focus them on the macula.
This is beyond the cornea, so the eye accommodates - pupil constricts (less light) lens becomes more convex (ciliary muscle contraction)
As we age, lens gets stiffer in presbyopia so you can’t focus on near things. Through occipital higher thinking, they hold stuff away so the rays are less divergent to start with.

Question 17

Describe 3 causes of vision blurring. (6)

Answer 17

Transparency of structures anterior to the retina is reduces (cataracts)
Ability to refract light is reduced (presbyopia, shape of eyeball)
Retina or optic nerve pathology (retinal detachment, optic neuritis).

Question 18

Explain the pinhole thing. (3)

Answer 18

When decreased visual acuity is due to a refracted error, viewing things through a pin hole can help because all the light travels perpendicular to the retina so doesn’t need to be refracted.

Question 19

Describe the chambers of the eyeball (3)

Answer 19

Vitreous chamber is behind the lens and is filled with vitreous humour.
Anterior chamber is between cornea and iris
Posterior chamber is between iris and ciliary muscle.

Question 20

Describe the production of aqueous humour. (3)

Describe its purpose, and it’s draininage. (4)

Answer 20

Secreted by the ciliary processes within the ciliary body. Flows from posterior chamber through pupil to anterior chamber.
Nourishes the avascular lens and cornea.
Drains through the iridocorneal angle (iris to cornea) via The trabecular meshwork of Canal of Schlemm.

Question 21

Describe glaucoma. (10)

Answer 21

Optic nerve damage secondary to raised intraocular pressure due to a blockage in drainage of the aqueous humour from the anterior chamber.
Chronic = open angle glaucoma: trabecular meshwork deteriorates with age, asymptomatic but with optic cupping. Gradual but irreversible loss of peripheral vision.
Acute = closed angle glaucoma: narrowing of iridocorneal angle leading to an emergency presenting with irregular, oval and fixed pupil, a halo around lights (corneal oedema), nausea and vomiting.

Question 22

Describe the actions and innervation of Superior rectus (4)

Answer 22

Elevates
Intorts and adducts
Occulomotor

Question 23

Describe the actions and innervation of Inferior rectus (4)

Answer 23

Depresses
Extorts and abducts
Occulomotor

Question 24

Describe the actions and innervation of Medial rectus. (2)

Answer 24

Adducts

Occulomotor

Question 25

Describe the actions and innervation of Lateral rectus. (2)

Answer 25

Abducts

Abducens.

Question 26

Describe the actions and innervation of Superior oblique. (4)

Answer 26

Intorts
Depress and abduct
Trochlear

Question 27

Describe the actions and innervation of Inferior oblique. (4)

Answer 27

Extorts
Elevate and adduct
Occulomotor

Question 28

Describe the testing of the extra-ocular muscles. (2)

Answer 28

Done in a H shape to avoid secondary muscle actions covering for weakness.
IO         SR
 I             I
MR —— LR
 I             I
SO         IR

Question 29

To what are all the cranial nerves of the muscles of the eye vulnerable to? (2)

Answer 29

Raised intracranial pressure

Vascular disease - hypertension and diabetes.

Question 30

Describe the signs on eye examination of an occulomotor nerve lesion. (7)

Answer 30

Most muscles lost - only LR and SO remain - down and out position.
Loss of LPS in eyelid - ptosis
Parasympathetics to pupil lost in raised ICP - pupil dilated, but fine in microvascular - pupil spared.

Question 31

Describe the signs on eye examination of A trochlear nerve lesion. (4)

Answer 31

Superior oblique lost - eye is extorted, elevated and abducted - pt often tilts head to compensate. Diplopia often worse when looking down and medially - SO lost to depress and adduct.

Question 32

Describe the signs on eye examination of an Abducens nerve lesion. (3)

Answer 32

Lateral rectus lost - cannot abduct the eye - can’t look laterally.

Question 33

Explain the results of a Snellen chart. (5)

Answer 33

Top number - distance of test - always 6m (or 20ft)
Bottom number - row number - the number of meters a healthy average individual can see that from.
Ideally it’s 6m or longer because shorter than 6m indicates you need to be closer to it than the average.