4- Ophthalmology (Anatomy and physiology) Flashcards
Skeletal anatomy
- X2 orbital cavities
- Pyramidal shaped with apex pointing posteriorly and medially (i.e. towards thwe centre)
walls and anatomical relationships of the orbit
Roof (superior wall)
- Formed by the frontal bone and the lesser wing of the sphenoid.
- The frontal bone separates the orbit from the anterior cranial fossa.
Floor (inferior wall)
- Formed by the maxilla, palatine and zygomatic bones
- The maxilla separates the orbit from the underlying maxillary sinus.
Medial wall
- Formed by the ethmoid, maxilla, lacrimal and sphenoid bones
- The ethmoid bone separates the orbit from the ethmoid sinus.
Lateral wall
- Formed by the zygomatic bone and greater wing of the sphenoid.
Apex
- Located at the opening to the optic canal, the optic foramen.
Base
- Opens out into the face, and is bounded by the eyelids. It is also known as the orbital rim.
nasal cavity relation with orbit
Nasal cavity also important anatomical relationship – connection between he orbital and nasal cavity by nasolacrimal duct -> spread of infection after orbital trauma
fissures and foramens of the orbit
artrial blood supply to the ye
Ophthalmic artery (first branch off the internal carotid sinus after cavernous sinus) and subsequent branches
- Enter eye via optic canal
- Including central retinal artery (most important branch – supplying the internal surface of the retina)
Clinical correlate: occlusion of this artery will quickly result in blindness
venous supply to the eye
- **Ophthalmic veins **(superior and inferior ophthalmic veins)
- Via the superior and inferior orbital fissures
- Drains venous blood into cavernous sinus, pterygoid plexus and facial vein
Central retinal artery occlusion
- Ciliary arteries undisturbed -> still good perfusion of choroid layer
- Distal branches of the central retinal artery that arise and supply the retina are starved of blood
- Therefore in central retinal artery occlusion the retina looks very pale
- Cherry red spot
o Obvious in area of macula
o Arises because macula and fovea are thinnest part of the retina- can see underlying choroid much more clearly than usual contrast is much more obvious when rest of the retina is very pale due to loss of blood supply, but still very well perfused choric layer
Giant cell arteritis
- Risk of visual loss
- Vasculitis can affect posterior ciliary arteries – which feed the choroid and retina- also supply the anterior proportion of the optic nerve
o Optic nerve ischaemia
nervous supply to the eye: sensory
- Vision- Optic nerve – CN II
- Touch, pain – Opthalmic division of trigeminal nerve- CN Va
nervous supply to the eye: motor
- Oculomotor - superior, inferior, medial rectus and LPS
- Trochlear- superior oblique
- Abducens- lateral rectus
Contents of orbit
- Eye balls
- Extraocular eye muscles
- Lacrimal apparatus
- Fat
- Nerves
- Blood vessels
Orbital blow out fracture
MOA
- Sudden increase in infraorbital pressure (e.g. from retropulsion of eye ball (globe) by fist or ball) fractures floor of orbit (maxilla)
- Ethmoid and maxillary bone have paranasal air sinuses -> therefore more vulnerable to fracture
- Floor of orbit is weakest
Presentation
- History of trauma with large object e.g. fist or elbow
- Periorbital swelling
- Painful
- Diplopia worse on vertical gaze
- Numbeness over cheek, lower eyelid and upper lip e.g. problem with infraorbital nerve
KEY Always check for vertical double vision and numbness over cheek
what are the eyelids made out of
o Skin – most superficially
o Loose subcut tissue
o Muscles
muscles of the eyelid
1) Orbicularis oculi (palpebral part) – Facial nerve CN VII)
- Closes eye
- Protects front of eye
2) Levator palpebrae superioris
- Arises from within orbital cavity and swings forwards anterior becomes aponeurotic and blends into tissue of upper eyelid
- Retracts and elevates eyelid (opens eye)
- Small smooth muscle components- mullers muscle- small contribution to elevation of the lid via sympathetic action
- Bulk of muscle- CN III – oculomotor
Orbital septum and Tarsal plate
The orbital septum is a thin, fibrous membrane that serves as a barrier between the superficial lids and the orbit. The septum arises from the orbital periosteum at the orbital rim and extends to the tarsal plates of the eyelids
- Orbital septum and tarsal plates separate subcutaneous tissue of eyelid and orbicularis oculi muscle from infraorbital contents
- Acts as a barrier against superficial infection spreading from the pre-septal and post-septal space (orbital cavity proper)
o Pre-septal- preorbital cellulitis
o Post-septal – Orbital cellulitis
Preorbital sepsis
- Infection occurring within eyelid tissue, superficial to orbital septum
- Secondary to superficial infection e.g. bites, wounds
- Confined to tissues superficial to orbital septum and tarsal plates
- Therefore:
o Ocular function- eye movement and vision remains unaffected - Can be difficult to differentiate between peri-orbital and more severe orbital cellulitis
- If in doubt refer urgently- high dose IV antibiotics and surgical drainage
orbital cellulitis
Background
- Infection WITHIN the orbit posterior or deep to the orbital septum
- Can arise from pre-septal cellulitis
- Usually arises from infection from within the orbit e.g. bacterial sinusitis
Signs
- Proptosis - eye pushed forward
- Reduced +/- painful eye movement
- Reduced visual acuity (optic nerve involved)
Most dangerous
- Inferior and superior ophthalmic veins can spread infection to the cavernous sinus causing intracranial infections
- Venous sinus thrombosis
- Meningitis
Management
- Surgical decompression
- IV antibiotics
Lacrimal apparatus: lacrimal gland
Lacrimal gland
- Is located anteriorly in the superolateral aspect of the orbit, within the lacrimal fossa – a depression in the orbital plate of the frontal bone.
- Exocrine gland
- Fluid spread over eyes by blinking
Tear film
- Important fluid to ensure anterior surface of the eye (conjunctiva and cornea) are kept lubricated ad hydrated
Role:
- Cleans
- Nourishes
- Lubricates
Drainage
Through small ducts made up of the lacrimal punctum (which you can see if you pull inferior tarsal plate down) and lacrimal canaliculus which drains into the nasolacrimal duct
tear film production and drainage process
Process
1) After secretion by the lacrimal gland, lacrimal fluid circulates across the eye, and accumulates in the lacrimal lake – located in the medial canthus of the eye.
2) From here, it drains into the lacrimal sac via a series of canals.
3) The lacrimal sac is the dilated end of the nasolacrimal duct, and is located in a groove formed by the lacrimal bone and frontal process of the maxilla.
4) Lacrimal fluid drains down the nasolacrimal duct and empties into the inferior meatus of the nasal cavity.
clinical correlates with problems with the lacrimal apparatus
- Injury to medial aspect of eye
- Scarring and fibrosis -> obstruct drainage
- Lead to over flow of tears over lower eyelid - epiphora
Location of the globe
- Globe does not sit on the floor of the orbit
- Eyeball position maintained by
o Suspensory ligament of Lockwood- sits under like a sling
o Extraocular muscles
o Orbital fat ++
3 layers of the eye
1) Sclera and cornea (fibrous) - outer
2) Choroid (vascular) - middle
3) Retina- inner
the sclera and cornea
Fibrous layer
- Outermost layers
- Sclera becomes cornea most anteriorly
- Most anteriorly becomes transparent -> cornea ->so we can see
Function: provide shape to eye and supports deeper structures
- Sclera provides attachment to extraocular muscles (white in real life- opaque)
- Cornea
- Refracts light entering eye (2/3s of total refractive power, lens 1/3)
choroid layer
vas
vascular
- Middle red layer in the image
- Lies beneath the sclera
- Vascular : Huge plexus of blood vessels within this layer
- Supply the retina
- Choroid is continuous with the ciliary body and iris
Ciliary body
(includes 2 parts: ciliary muscle and ciliary processes)
- Important role e.g. muscle important in altering the thickness of the lens and formation of aqueous humor
Iris (coloured part of the eye)
- Iris is a muscular diaphragm with a central aperture (opening in a lens through which light passes) – the pupil
- Sphincter papillae and dilator papillae (ANS) constrict and relaxed aperture of pupil
the retina
- Photosensitive layer
- Cells within retina which convey action potentials out of the back of the eye via the optic nerve (optic canal) to the occipital lobe
Area of the retina lying lateral to the optic nerve called the macula
the macula
- Area of the back of the eye which is responsible for our highest acuity vision –> colour vision
- High proportion of cones- detect colour- type of photoreceptor
- At the very tip of the macula- central depression called the fovea
- Thinnest part of the retina and central part the macula-> light doesn’t need to travel as far to photoreceptors (esp cones)
Cells found in the retina:
Pigmented layer acts as the site of absorption of light- to modulate amount of light being received by the photoreceptors
o People with albinism (lack of pigment- melanin) have photophobia
o Anchors photoreceptor
Photoreceptors
- Rods- black and white vision
- Cones – colour vision- high acuity vision (ound in the fovea (macula densa))
Bipolar cells
- Act to connect photoreceptor cells to the ganglion cells
- Ganglion cells axons come together to form the optic nerve
- Optic nerve exits via the optic disc
- Blind spot- not photoreceptor cells
Horizontal cells
o Lateral inhibition- detects area where image is coming from and detects the photoreceptor that is most in line to pick up that signal and inhibits photoreceptors next to it prevents too many neural impulses
Vitreous Body
The vitreous body is a transparent gel which fills the posterior segment of the eyeball (the area posterior to the lens).
The vitreous body has three main functions:
* Contributes to the magnifying power of the eye
* Supports the lens
* Holds the layers of the retina in place
Lens
Lens
- The lens of the eye is located anteriorly, between the vitreous humor and the pupil. The shape of the lens is altered by the ciliary body, altering its refractive power (1/3, along with the cornea 2/3).
- Clinical correlate: In old age, the lens can become opaque – a condition known as a cataract.
Anterior and Posterior Chambers
There are two fluid filled areas in the eye – known as the anterior and posterior chambers.
- The anterior chamber is located between the cornea and the iris
- The posterior chamber between the iris and ciliary processes.
The chambers are filled with aqueous humor – a clear plasma-like fluid that nourishes and protects the eye. The aqueous humor is produced constantly, and drains via the trabecular meshwork, an area of tissue at the base of the cornea, near the anterior chamber.
Clinical correlate If the drainage of aqueous humor is obstructed, a condition known as glaucoma can result.
Phototransduction
When light hits the retina
1. Photoreceptors convert light signals into Ap
2. Interneurons communicated with ganglion cells
3. Action potentials propagated via retinal ganglion cells
4. RGC axons collect in area of optic disc form optic nerve
a. Blind spot= no photoreceptors
5. AP propagated along visual pathway to occipital lobe for interpretation
rods vs cones
Rods
- Active at lower light levels, do not mediate colour vision
- Abundant in peripheral parts of retina
Cones
- High definition
- Colour vision
- Active at high light levels
- Concentrated within macula of retina
- Fovea= only cones
accomodation reflex
When the ciliary muscle is relaxed
- Ligaments pulled tight
- Pulls the lens thinner good for looking at distant objects
When ciliary muscle is constricted (parasympathetic fibres of oculomotor nerve)
- Tension removed from ciliary muscles
- Rounder, fat lens
- Good for looking at nearby objects
summary of accomodation reflex
to have binocular vision the visual axis of both eyes need to be
aligned
- Misalignment of visual axes causes diplopia caused by strabismus
- Image focuses on different area of each retina and brain unable to fuse -> 2 separate images -> diplopia
diplopia can be
- Horizontally
- Vertically
- Diagonally
to have binocular vision the visual axis of both eyes need to be
aligned
- Misalignment of visual axes causes diplopia caused by strabismus
- Image focuses on different area of each retina and brain unable to fuse -> 2 separate images -> diplopia
diplopia can be
- Horizontally
- Vertically
- Diagonally
what are important in keeping the eyes aligned
- Extraocular muscles very important to keep alignment
- Can look at light reflex -> should be same place in both eyes
- Cover test
extraocular muscle
- Allow eyes to move together to maintain fixation on image
- Control:
o The eye balls movement
o The eyelid (LPS) - 6 extraocular muscles that move the eyeball
- Attached to the sclera (behind the cornea)
- Originate in the apex of the orbit (except IO which arises floor or orbital cavity anteriorly)
o 4 recti arise from a common tendonous ring
superior eyelid movement
1) Levator Palpebrae Superioris – Oculomotor (CN III)
- Full ptosis
2) Superior tarsal muscle – sympathetic nervous system
- Think Pancoast tumour -> Horners syndrome-> inhibits sympathetic chain
- Smaller than LPS, only causes partial ptosis
innervation of extraocular muscles
Innervation: LR6 SO4 R3
- Lateral rectus – abducens
- Superior oblique - trochlear
- Rectus- oculomotor
Terms of direction
- ADDUCTED- medial towards nose
- ABDUCTED- lateral
- ELEVATION- superior
- DEPRESSION- Inferior
- Extortion- external rotation
- Intorsion- internal rotation
Primary resting gaze (position at rest)
- Equal and opposite pull of all extraocular muscles
- Balanced allowing forward gaze
Changing position of gaze
- Exert greater pull through action of certain extraocular muscles, while antagonist relax
types of diplopia
Clinical correlates- diplopia
- CN palsies
o Oculomotor- sown and out
o Trochlear – up and in
o Abducens – in
- Thyroid eye disease
- Myasthenia gravis
- Blow out fractures
- Space occupying lesions
- RICP
Preorbital sepsis
- Infection occurring within eyelid tissue, superficial to orbital septum
- Secondary to superficial infection e.g. bites, wounds
- Confined to tissues superficial to orbital septum and tarsal plates
- Therefore:
o Ocular function- eye movement and vision remains unaffected - Can be difficult to differentiate between peri-orbital and more severe orbital cellulitis
- If in doubt refer urgently- high dose IV antibiotics and surgical drainage
venous supply to the eye
- Ophthalmic veins (superior and inferior ophthalmic veins)
- Via the superior and inferior orbital fissures
- Drains venous blood into cavernous sinus, pterygoid plexus and facial vein
