6.1: Anatomy of the Orbit and Eye Flashcards
Describe the outer layer of the eye
The sclera and cornea
Describe the appearance and function of the sclera, which part of it is visible?
White, tough and structural. A site for muscle attachment and is relatively avascular
It;s anterior part is visible
Describe the appearance and function of the cornea
Transparent, protrudes is avascular and sensitive
Which nerves are involved in the corneal reflex?
Afferent: CN V1 (ophthalmic division) Efferent: CN VII (facial)
Which layer of the eye is vascular and what is it called? What lies within this layer?
The middle layer; known as the Uvuea
Comprises the choroid, ciliary body and iris
What are the clinical terms for a large and small pupil?
Mydriasis: large pupil Miosis: small pupil
Describe how the ANS influences the pupil
Parasympathetic: contracts the circular sphincter pupillae muscles Sympathetic: Contracts radial dilator pupillae
Describe the mechanism of action of phenylephrine on the eye
It’s an alpha-1 adrenergic agonist and has sympathetic activation which dilates the pupil
Describe the mechanism of action of tropicamide and atropine on the eye
Muscarinic receptor antagonists - blocks parasympathetic and causes dilation
Describe the mechanism of action of pilocarpine on the eye
Direct acting cholinergic parasympathomimetic and therefore has parasympathetic activation to constrict the pupil
Describe the neural layer of the eye
This is the inner layer; consisting of the retina which has optic and non-visual parts
Lecture: fundus, macula, optic disc
Which part of the eye constitutes the ‘blind spot’?
The optic disc
Where is the fundus in the eye and how can it be visualized?
Posterior part, visualized with ophthalmoscope/fundoscopy
Where is visual acuity highest?
The macula’s fovea centralis as it has the highest concentration of photoreceptors
Where is the macula and what significant structure is located within it?
The macula is near the centre of the retina (between the superior and inferior arcades; within it is a small central pit composed of closely packed cones called fovea centralis
Approximately how many rods and cones are there in the eye?
~120 mill rods ~6 mill cones
What visual symptom may arise from meningitis and why?
Photophobia (pain when looking at bright light), as the optic nerve is an extension of the brain and thus has a layer of meningeal covering
Define papilloedema
Swelling of the optic nerve
Where is the anterior and posterior chamber of the eye?
In the anterior segment:
Anterior: between the cornea and iris/pupil
Posterior: behind the iris/pupil and lens and in front of the ciliary body
Describe aqueous humour including its location, what produces it, it’s purpose and how it flows out
Aqeous humor is secreted by the ciliary body into the posterior chamber of thr anterior segment. Oxygen, nutrients and wastes are brought to and from the lens and cornea which are avascular. The watery fluid flows from the iris into the anterior chamber and out via the trabecular meshwork - and through Schlemm’s canal into the venous system
Where is vitreous humor and what does it attach to?
The vitreous chamber (posterior segment of the eye), attached to the retina at the macula, nerve and ora serrata
List the structures of the eye that light passes in anterior-posterior order. Where does the light inevitably focus?
Cornea, aqeuous humor, lens, vitreous humor and focusses on the fundus (fovea) of the retina
What is the clinical term ofr normal sightedness?
Emmetropia
Describe the refractive errors which occur in myopia and hypermetropia
lecture
Which structures form the boundaries of the orbit?
Medial: ethmoid, maxilla and lacrimal bones
Lateral: zygomatic and sphenoid
Roof: frontal and sphenoid
Floor: maxilla and zygomatic
Apex: optic canal
What happens in a blow out fracture?
Lecture
Name the eight muscles responsible for eye movement and the nerves that innervate them!
SO4LR6..ALL the rest 3!
Superior oblique: Trochlear CN IV
Lacteral rectus: Abudcens CN VI
Medial rectus, superior rectus, inferior rectus, inferior oblique and levator palebrae superioris: Oculomotor CN III
List the primary, secondary and tertiary actions of the following muscles in order
a) superior rectus
b) inferior rectus
c) superior oblique
d) inferior oblique
a) elevation, intorsion, ADduction
b) depression, extorsion, ABduction
c) intorsion, depression, ABduction
d) extorsion, elevation, ADduction
Which muscles are working in a 3rd nerve palsy of the eye? What might this condition be a first sign of?
Lateral rectus and superior oblique - “down and out”
May be first sign of posterior communicating artery aneurysm
What causes the symptoms of a 3rd nerve palsy (and what are they)?
Name five other things which may cause this condition
- Ptosis (droopy upper eyelid): due to paralysed levator palpebrae superioris
- Dilated pupil: paralysed sphincter papillae of iris allowing unopposed action of iris dilator muscle
- Down and out eyes due to functioning superior oblique and lateral rectus muscles
May be due to ischemia, tumour, demyelination, vasculitis, congenital (or trauma)
Which muscle of the eye is affected in a 4th nerve palsy?
Superior oblique
How might an individual with a 4th nerve palsy of the eye present?
Vertical and horizontal diplopia (double vision) that is worse on downgazing. They may have a compensatory head tilt away and hypertropia (eye misaligment) on the affected side
What are the primary actions of the lateral and medial rectus muscles?
Lateral: ABduction
Medial: ADduction
Which muscle of the eye is affected in a 6th nerve palsy?
Lateral rectus
How might an individual with a 6th nerve palsy present?
Can’t abduct the eye on the affected side. May have horizontal diplopia that is worse when looking to the affected side
Label the following diagram
Lecture
Which blood vessels supply the eye and which vessels do they arise from?
Inner retina: ICA -> Opthalmic artery -> central retinal artery (passes through the optic disc) -> multiple terminal branches (end arteries) to the eye, lacrimal gland and face
Outer retina:
What happens if there is a retinal artery occlusion?
Since the central retinal artery gives off multiple TERMINAL branches, any occlusion/embolus ot the arteries supplying the retina will result in instant and total blindness
Describe the venous drainage of the eye
Central retinal vein and choroidal veins drain mostly into -> superior opthalmic vein
-> superior and inferior opthalmic veins + pterygoid plexus -> cavernous sinus
What do the eyelids protect against?
Injury, excessive light and dryness. They also keep the cornea moist by maintaining a film of tears as they blink
Which structures are responsible for allowing the eye to open and close?
Open: Oculomotor CN III, levator palpebrae superioris, superior tarsal muscle/Muller’s - sympathetic (tightens LPS’s attachment and can raise the lid a few more mm)
Close: Facial n CN II - orbicularis oculi
Where is the “blindspot” and why is it one?
The optic disc on the nasal side of the macula, it contains nerve fibres and no photoreceptor cells and is thus insensitive to light
Which part of the eye forms the largest vascular layer of the eyeball? Describe its location and what it terminate anteriorly as
The choroid; a dark layer between the retina and sclera. Terminates anteriorly as the ciliary body
Which part of the eyeball is musclular AND vascular? Which structures does it connect?
The ciliary body; connects the choroid with the iris
Describe the contents and space of the posterior segment of the eyeball
Back 2/3, contains the vitreous humor, retina, choroid and optic nerve
Where is the iris? Describe it
The anterior surface of the lens; it’s a thin contractile diaphragm with a central aperture (the pupil) for the transmission of light
Where is the lens? Describe its structure and how one other structure can influence it’s shape
Posterior to the iris; a transparent biconvex structure enclosed in a capsule attached to the ciliary body by suspensory ligaments. Contraction of the muscle fibres in the ciliary body changes the shape of the lens
Describe the structure of the retina - what process does it facilitate?
Facilitates the conversion of light into signals that can be sent to the brain
Outermost layer: darkly pigmented retinal pigment epithelium (RPE) which is firmly adherent to the underlying choroid
Resting on top (one layer in) is the neurosensory retina; consisting of an outer layer of photoreceptors cells (rods and cones)
Then there are multiple layers of nerve clel bodies and axons, with an innermost retinal nerve fibre layer which are transmitted to the brain via the optic nerve
What forms the facial sheath of the eyeball? What is it continous with?
The periosteum (periorbita) lining the bones of the orbit, it’s continous at the optic canal and supraorbital fissure with the periosteal layer of the dura mater
Over the orbital margin and through the infraorbital fissure it’s continous with the periosteum covering the external surface of the cranium
What provides mechanical strength to the eyelids?
Tarsal plates (dense bands of CT) which suspend the eyelids from the orbit and the medial and lateral canthi (corners)
What is the function of the tarsal glands?
They secrete oils which maintain the tear film by helping to prevent evaporation
What is a chalazion?
A non-infectious obstruction of a Meibomian/tarsal gland causing localised swelling
What is a hordeolum (i.e stye)?
An acute, localized swelling due to infection in a meibomian/tarsal gland or eyelash follicle
Where are the lacrimal glands?
Paired, almond-shaped exocrine glands that sit in the lacrimal fossae of the frontal lobe (superotemporal to the globe/eyeball)
What do the lacrimal glands secrete?
The aqueous component of tears onto the surface of the eye.
What happens to tears as the eye blinks?
They are swept medially and drain into the upper and lower canliculi found at the medial canthus. These canaliculi form the common canaliculus before joining the nasolacrimal duct which drains into the inferior meatus of the nasal cavity.
Where do the lacrimal glands recieve innervation from?
Parasympathetic innervation from secreto-motor fibres from the greater petrosal branch of the facial nerve CN VII
What constitutes a blowout fracture? How common is it?
It is produced by a blow to the orbit which sudden;y increases the intra-orbital pressyre and fractures a portion of the obrital bony lining (one of the walls or floor) with the orbital rim remaining intact. It’s the commonest orbital fracture
Name five complications that may occur as a result of an orbital blowout fracture
Enophthalmos: posterior displacement of the eye as the orbit is decompressed by the fracture
Diplopia: double vision due to entrapment of extraocular muscles in the fractures
Globe injury
Facial numbness due to injury to the inferior orbital nerve
Orbital emphysema or hemorrhages
What happens to the eye if the facial nerve becomes damaged?
Paralysis of the orbicularis oculi: this prevents the eyelid from clossing fully and loss of protective blinking. As a result, the cornea becomes dry and is left unprotected from dust and other particulate material
Chronic exposure to the cornea causes permanent scarring and visual loss
Where do the recti muscles of the eye arise from and where is it?
The common tendinous ring which surrounds the optic canal at the posterior apex of the orbit
Where does each of the oblique muscles arise from and insert?
Superior oblique: superior to the common tendinous ring -> along nasal side of orbit -> redirected towards globe through trochlea of superior oblique -> posterior aspect of the globe
Inferior oblique: maxilla on the orbital floor (inferonasally) -> posterior globe
Where does levator palpebrae arise from and where does it insert?
Arises superior to the common tendinous ring at the orbital apex and inserts into the superior tarsal plate of the upper eyelid as the levator aponeurosis.
Which nerves supply sensation to the structures in the orbit?
*identify which nerve provides sensation to the cornea
Optic CN II
Lacrimal, frontal, nasociliary and ciliary nerves of the ophtlamic division VI
Cornea: CN V1 (afferent branch of the corneal reflex!)
What do patients with 3rd nerve palsies require urgent investigation for?
ICA/cavernous sinus aneurysms
