Functional Anatomy of the Orbit and Eye

Question 1

What are the three openings into the orbit?

Answer 1

Optic canal
Superior orbital fissure
Inferior orbital fissure (goes into the infra temporal space behind the maxilla)

Question 2

Why does the orbital ridge not fracture?

Answer 2

Because it is tough. The skin splits instead.

Question 3

What is the main arterial supply to the orbit?

Answer 3

Ophthalmic artery and its branches e.g. central retinal artery and lacrimal artery.

Question 4

What is the venous drainage of the orbit?

Answer 4

Superior and inferior ophthalmic veins. These drain venous blood into the cavernous sinus, pterygoid plexus and facial vein.

Question 5

What is the nerve supply to the orbit?

Answer 5

General sensory from the eye (inc conjunctiva and cornea) - Ophthalmic vein (CN Va)

Special sensory vision from retina - Optic nerve (CN II)

Motor nerve to muscles - CN III, IV, VI

Question 6

What are some anatomical relations to the orbit and why are they important??

Answer 6

Paranasal air sinuses (maxillary and ethmoid) -Spread of infection through the medial wall of the orbit (as thin) into the ethmoid air cells.
Nasal cavity
Anterior cranial fossa

Also important for orbital trauma as the medial wall and the floor are the weakest parts so are easily fractured.

Question 7

What are orbital blow out fractures?

Answer 7

This is when a sudden increase in intra-orbital pressure (e.g. from retropulsion of eye ball by fist or ball) fractures the floor of the orbit.

Thus could result in orbital contents prolapsing and bleeding into the maxillary sinus.

A fracture site can trap structures e.g. soft tissue, extra ocular muscle located near orbital floor.

This will prevent upward gaze on the affected side.

Question 8

How will a patient with an orbital blow out fracture present?

Answer 8

History of trauma to the eye / orbit
Peri-orbital swelling, painful
Double vision (worse on vertical gaze)
Anaesthesia over affected cheek (upper teeth and gums) on affected side (because the infraorbital nerve that supplies the skin of the eyelid cheek and gum goes through the floor).

Question 9

What makes up the eyelid?

Answer 9

Skin,
Subcutaneous tissue,
Tarsal plate,
Muscles - obicularis oculi -palpebral part and levator palpebrae superioris,
Glands -Meibomian glands, sebaceous glands associated with lash follicles

Question 10

What are Meibomian glands?

Answer 10

These are glands in the tarsal plate that secrete an oily (lips rich) substance onto the edges of the lips to help prevent evaporation of tear film and tear spillage.

If these glands block they can cause a Meibomian cyst (non-infective hard swelling on eyelid).

Question 11

What is the orbital septum?

Answer 11

Thin sheet of fibrous tissue originating from orbital run periosteum blends with tarsal plates.

Question 12

What is the role of the orbital septum and tarsal plates?

Answer 12

They separate subcutaneous tissue of the eyelid and obicularis oculi muscle from intra-orbital contents.
This acts as a barrier against superficial infection spreading from the pre-septal to post-septal space (oral cavity proper).

Question 13

What is periorbital (pre-septal) cellulitis?

Answer 13

This is an infection occurring within the eyelid tissue, superficial to the orbit and the septum.

It often occurs secondary to superficial infections (bites, wounds)

It is confined to tissues superficial to orbital septum and tarsal plates

The ocular function (eye movements / vision) remains unaffected

Can be difficult to differentiate between peri-orbital and more severe cellulitis - if in doubt, refer urgently (IV antibiotics and surgical drainage).

Question 14

What is orbital (post-septal) cellulitis?

Answer 14

This is an infection within the orbit posterior or deep to the orbital septum.

Most commonly caused by sinusitis of the ethmoid air cells.

The orbital veins drain to the cavernous sinus, pterygoid plexus and facial veins so, there is a potential route for it to spread intercranially causing meningitis or cavernous sinus thrombosis.

Question 15

What are the symptoms of orbital cellulitis?

Answer 15

Proptosis / exophthalmos

Reduced and maybe painful eye movements

Reduced visual acuity.

Question 16

What is in the orbital cavity?

Answer 16

Eyeball
Fat
Associated extra-ocular muscles
Nerves and blood vessels;s
Lacrimal apparatus

Question 17

What structures make up the lacrimal apparatus?

Answer 17

Lacrimal gland (tear production)

Lacrimal sac

Lacrimal duct -canaliculi and nasolacrimal duct (tear drainage)

Question 18

What is the lacrimal apparatus?

Answer 18

These are structures involved in tear film production and drainage.

Blinking washes tear film across front of eye, rinsing and lubricating conjunctiva and cornea.

Tears are ultimately drained into the nasal cavity.

Obstruction to the drainage system leads to epiphora (overflow of tears over lower eyelid).

Question 19

Describe the anatomy of the eyeball

Answer 19

It has three layers:
-Outer: sclera (white of eye) continuous anteriorly as transparent cornea (also continuous posteriorly with dura mater, covering the optic nerve).

Middle: Choroid, ciliary body and iris (vascular)

Inner: retina (made photosensitive layer lying on an outer pigmented layer.

Question 20

How is the eyeball maintained in position?

Answer 20

Suspensory ligament (sits underneath like a sling)
Extra-ocular muscles
Orbital fat

Question 21

What is the anterior surface of the eyeball covered with?

Answer 21

Conjunctival membrane.

Question 22

What is the conjunctiva?

Answer 22

It is a transparent mucous membrane that produces mucous and tears.

It covers the white of the eyes (sclera) and lines inside of eyelids (forming a conjunctival sac). It does not cover over the cornea.

It is highly vascular with small blood vessels within the membrane.

Inflamed and injected in infections e.g. conjunctivitis

Haemorrhage from blood vessels readily visible as a subconjunctival haemorrhage (looks bad but not to worry about).

Question 23

How does light reach the back of the eye and get focused onto a point?

Answer 23

Transparent structures and medium

Refract light to bring to a focal point

Shape of eyeball affects ability to focus.

Question 24

What structures in the eye refract light?

Answer 24

Cornea and associated tear film

Lens

Aqueous humour and vitreous humour

Question 25

What is the accommodation reflex?

Answer 25

This is when the eye accommodates because focusing on near objects requires greater refraction (beyond capabilities of the cornea) of light as light rays from near objects as more divergent.

Question 26

What does the eye do to accommodate?

Answer 26

Pupil constricts -limit amount of light coming through

Eyes converge - ensure image remains focused on same point in both eyes

Lens becomes more biconvex (fatter) by contraction of the ciliary muscle.

Question 27

Why do old people hold their phones far away from their face?

Answer 27

Because as we age our lease becomes stiffer and less able to change shape so people have to hold stuff further away to focus them.

It is called Presbyopia

Question 28

What is the difference between rods and cones?

Answer 28

Rods: active at low light levels, do not mediate colour vision and abundant in peripheral parts of retina.

Cones: high definition, colour-vision, active at high light levels, concentrated within the macula of the retina and int he fovea there are only cones.

Question 29

How do light wave get converted into what we see?

Answer 29

Action potentials generated in response to lgiht pass via retinal ganglion cells.

RGC axons collect in area of optic disc forming the optic nerve. -Optic disc = blind spot as no photoreceptors present here.

Action potentials propagated along visual pathways to occipital lobe for interpretation.

Question 30

What causes blurry vision?

Answer 30

Pathology affecting:

• Transparency of structures anterior to retina e.g. opacity in lease such as cataract.
• Ability of structures to refract light e.g. irregularity of cornel surface (astigmatism), ability of sense to change shape (presbyopia) or shape of eyeball.
• Retina or optic nerve e.g. retinal detachment age-related macular degeneration, optic neuritis.

Will all cause blurring of vision.

Question 31

Describe the production and drainage of aqueous humour

Answer 31

Aqueous humour is secreted by ciliary processes within the ciliary body.

Flows from posterior chamber through pupil into anterior chamber.

Nourishes lease and cornea.

Drains through iridocorneal angle (between iris and cornea)

Via trabecular meshwork into canal of Schemm (circumferential venous channel draining into venous circulation)

Question 32

What is glaucoma?

Answer 32

Optic nerve damage secondary to raised intraocular pressure.

It is when the drainage of the aqueous humour from anterior chamber is blocked causing rise in intra-ocular pressure. It can develop chronically or acutely.

Question 33

What is chronic (open-angle) glaucoma?

Answer 33

More common

Trabecular mesh work deteriorates as we age.

Many asymptomatic (picked up on routine eye tests)

Increased IOP - increase optic disc cupping

Gradual loss of peripheral vision

Question 34

What is acute (closed-angle) glaucoma?

Answer 34

Narrowing of the iridocorneal angle

Occurs quickly

Ophthalmological emergency as it is site threatening!

Question 35

What are the symptoms of acute-angle closure glaucoma?

Answer 35

Older patient (55+)
Acutely painful red eye
Irregular oval-shaped pupil (fixed)
Blurring of vision
Halo’s around lights (due to corneal oedema)
Nausea and vomiting
Medical (drugs to reduce IOP) then surgical treatment