Functional Anatomy of the Eye Flashcards
Describe the structures forming the walls of the orbital cavity
- Roof - frontal bone, anterior cranial fossa, sphenoid bone
- Lateral wall - zygomatic bone, sphenoid bone
- Floor - maxilla, part of zygomatic bone
- Below the floor of the orbit is the maxillary paranasal sinus - air filled cavity
- Medial wall - ethmoid, maxilla, lacrimal, part of sphenoid
- Ethmoid bone contains ethmoid air cells
What are the openings at the apex of the orbital cavity
- Superior orbital fissure
- Inferior orbital fissure
- Optic canal
Outline which walls of the orbital cavity are most susceptible to fracture
- Medial wall and floor of the orbit are the weakest parts of the orbital cavity as they contain paranasal air sinuses behind the bone
- Fracture can break into sinus and cause infection to spread to and from orbit
What are signs and symptoms of orbital blow out fracture
- History of trauma to the eye/orbit
- Periorbital swelling, painful
- Double vision - especially on vertical gaze
- Impaired vision - restricted eye movement
- Anesthesia over affected cheek (upper teeth and gums) on affected side
- Infra-orbital nerve from maxillary nerve runs on orbital floor
What are the implications of a blow out fracture on the orbital contents
- Sudden increase in intra-orbital pressure fractures floor of orbit
- Retropulsion (forceful backward movement) of eyeball by fist or ball
- Orbital contents and blood can prolapse into maxillary sinus
- Fracture site can trap structures
- Extra orbital muscle located near floor or orbit
- Eye cannot look up due to trapped contents - prevents movement
State the connective tissue that form the eyelid
- Eyelid (palpebrae) formed from superior and inferior tarsus
- Provide a connective tissue skeleton to the eyelid - firmness and shape
- Fibrous ring surrounding it - orbital septum
Explain the role of fibrous ring in the eyelid
- Orbital septum
- Thin sheet of fibrous tissue originating from orbital rim
- Blends with tendon of LPS and tarsal plates
- Separates intra-orbital contents from eyelid fat and orbicularis oculi muscle
- Act as a barrier against infection spreading from the pre-septal space to post-septal space
Explain the presentation and complications of periorbital cellulitis
- Secondary to infection from bites, periorbital trauma, sinuses (fronto-ethmoidal sinuses)
- Pre-septal orbital cellulitis - in front of orbital septum
- Localized infection and swelling
- Complication include abscess formation and spread of infection intracranially
- Cavernous sinus thrombosis - veins of orbit drain to cavernous sinus, pterygoid venous plexus and facial veins
- Post-septal orbital cellulitis - infection can spread into orbital cavity
Describe the glands of the eyes and the pathology associated with each
- Meibomian glands and glands of Zeis located on the edge of eyelids
- Meibomian glands secrete oily substance onto edge of eye
- Help prevent evaporation of tear film and tear spillage
- Can block causing Meibomian cyst
- Glands of Zeis are eyelash follicles
- Can block causing styes
Outline the secretion and drainage of tears
- Lacrimal apparatus involved in secretion of tears into conjunctival sac
- Lacrimal gland - arranged around edge of levator palpebrae superioris
- Lacrimal punctum drains conjunctivae into the lacrimal sac and down the nasolacrimal duct
- Lacrimal sac located within the lacrimal fossa
Describe what conjunctivae is and how its tears affect it
- Conjunctivae is a secretory mucosa lubricating the conjunctival and corneal surfaces
- Blinking washes tear film across front of eye, rinsing and lubricating the conjuctivae and cornea
Explain the conjunctiva
- Transparent mucous membrane that produces mucous and tears
- Covers white of eye (sclera) and lines inside of eyelids (forming a conjunctival sac)
- Does not cover over cornea
- Highly vascular with small blood vessels within the membrane
Outline pathology of conjunctiva
- Conjunctivitis - inflammation and infection
- Pink eye
- Haemorrhage from blood vessel readily visible as a subconjunctival haemorrhage
- No treatment needed - just a bruise without skin
Describe the blood supply to and from the orbit
- Main arterial supply through ophthalmic artery branching off internal carotid artery
- Ophthalmic veins drain venous blood into cavernous sinus, pterygoid plexus and facial vein
State the nerves which innervate the orbit
- General sensory from the eye through ophthalmic division of trigeminal nerve
- Special sensory vision from retina through optic nerve
- Motor nerves to muscle through occulomotor, trochlear and abducens
Describe the layers of the eyeball
- Outer - fibrous, tough sclera (white of eye) continuous anteriorly as transparent cornea
- Middle - vascular consisting of choroid, ciliary body and iris
- Inner - retina (inner photosensitive layer lying on an outer pigmented layer)
Describe the iris and lens
- Iris is a muscle that can dilate and constrict under autonomic control to help focus
- Lens are suspended by suspensory ligament to ciliary muscle
- Under parasympathetic control where contraction of muscle reduces tension of ligament and causes lens to become fatter
Outline how light is perceived in the retina
- Light must reach the photosensitive retina to be detected by photoreceptors (rods and cones)
- Action potentials generated in response to light
- Pass via ganglion cells whose axons collect in area of optic disk to optic nerve
- Rods are for vision in low intensity light
- Cones are for vision in high intensity light and colour vision - concentrated in macula
Describe refraction and where it occurs
- Refraction is the change in direction of light on passing through boundary of two different mediums
- Light will be refracted from air into liquid tear film
- Cornea is where most light refraction occurs (air-cornea interface)
- Refraction through lens and vitreous humour before reaches retina
Explain the accommodation reflex
- Focusing near objects requires greater refraction of light
- Pupil constricts - increase focus so lens don’t have to refract as much light
- Eyes converge - image is brought to focus on same point of retina in both eyes
- Lens become more biconcave (fatter) - ciliary muscles contract
Describe problems of lens
- As we age, the lens become stiffer and less able to change shape
- Presbyopia - age related inability to focus near objects
- Progressive opacities can also occur within lens
- Cataracts - non-transparent lens
Explain what the macula and optic disk are
- Macula - area of the retina upon which an object in your visual field falls if you gaze direct at it (center of vision)
- Optic disk - accumulation of retinal axons that leave the eye as the optic nerve
- No photoreceptors present (blind spot)
Explain what is found on a retinal scan
- Macula is a dark spot and optic disk is a light spot
- Veins are thicker than arteries due to larger lumen
- Optic disk always medial to macula (to tell which eye)
Levator palpebrae superioris action, innervation, damage presentation
- Action - elevates upper eyelid
- Innervation - CN III
- Damage - ptosis
Palpebral part of orbicularis oculi action, innervation, damage presentation
- Action - blink/close eyes
- Nerve - temporal branch of CN VII
- Damage - can’t close eyes properly
Superior tarsal muscle action, innervation, damage presentation
- Action - keeps upper eyelid elevated
- Innervation - sympathetic
- Damage - partial ptosis
State the extra-ocular muscle actions and innervation
- Lateral rectus
- Action - abduction of eye
- Innervation - CN VI
- Medial rectus
- Action - adduction of eye
- Innervation - CN III
- Superior rectus
- Action - elevation, intorsion, adduction of eye
- Innervation - CN III
- Inferior rectus
- Action - depression, extorsion, adduction of eye
- Innervation - CN III
- Inferior oblique
- Action - elevation, extorsion, abduction of eye
- Innervation - CN III
- Superior oblique
- Action - depression, intorsion, abduction of eye
- Innervation - CN IV
- LR6SO4
State what extra-ocular muscles are involved in looking left/right, up and down
- Looking left and right due to medial and lateral rectus muscles
- Looking upwards due to superior rectus and inferior oblique muscles
- Looking downwards due to inferior rectus and superior oblique
How are lateral and medial rectus tested
Abduction and adduction of eye
How are inferior rectus and superior oblique tested
- Superior oblique - move eye into medial position (looking towards nose) then move eyeball down
- Inferior rectus - move eye into lateral position then move eyeball down
How are inferior oblique and superior rectus tested
- Inferior oblique - move eye into medial position then move eyeball up
- Superior rectus - move eye into lateral position then move eyeball up
Describe the presentation of CN III damage
- Eyeball in down and out position
- CN III innervates all muscles except lateral rectus and superior oblique
- Severe ptosis
- CN III innervates levator palpebrae superioris
- Pupil dilation
- CN III carries parasympathetic function to sphincter pupillae
Describe the presentation of CN IV
- Eyeball is held extorted, and up and in
- CN IV innervates superior oblique muscle
- Compensate for the slight extortion of eyeball by tilting the head slightly
- Difficulties when looking down and medially
Describe the presentation of CN VI
- Unopposed pull of medial rectus muscle
- Innervates lateral rectus - Unable to abduct the eye on affected side
- Diplopia
Describe the route of aqueous humour in the eye
- Aqueous humour secreted by ciliary processes within ciliary body
- Flows from posterior chamber through pupil into anterior chamber
- Anterior chamber - between cornea and iris
- Posterior chamber - between iris and lens
- Drains through iridocorneal angle - angle between iris and cornea
- Trabecular meshwork helps Schlemm’s canal drain the aqueous humour into the venous circulation
Describe the role of aqueous humour
- Nourishes lens and cornea
- Lens and cornea are avascular to allow light to pass through
- Need aqueous humour to supply oxygen and nutrients
Describe the type of glaucoma
- Drainage of aqueous humour from anterior chamber can be blocked
- Trabecular meshwork deteriorates - old age
- Open angle glaucoma
- Blockage within the trabecular meshwork
- Narrowing of iridocorneal angle - acute
- Close angle glaucoma
- Access to travecular meshwork is blocked off
- Trabecular meshwork deteriorates - old age
- Rise in intra-ocular pressure and damage to optic nerve
- Optic disk cupping
- Visual field loss
- Sight threatening
- Treatment - topical medications (eye droplets) that reduces production of aqueous humour or increase its drainage
Explain the principal of colour blindness
- Colour blindness - 3 types of cones (red, green, blue sensitive)
- Absence or dysfunction of one of these three cones leads to colour blindness
Describe papiloedema
- Optic disk swelling due to increased intracranial pressure
- Usually occurs in both eyes
- Increased size of the blind spot means patients may lose vision briefly
Describe blepharitis
- Inflammation of the eyelids
- Inflamed, irritated, itchy, and reddened eyelids
- Can be due to bacterial infection, skin disease, blockage of Meibomian glands
Outline the steps for a clinical examination of the eye
- IVA FROM
- Inspect - general, eyelids, conjunctiva, sclera
- Visual Acuity
- Snellen chart with one eye
- 6/18 means that standing 6 meters away, the patient can only see the letter size that should be seen from 18 meters away in a person with normal vision
- Visual Fields
- Move hand slowly towards the centre and see where the patient first sees your finger
- Reflexes - pupil light reflex, accommodation reflex
- Ophthalmoscopy
- Dim lights and sit opposite the patient and shine light
- Pupil should appear pink/red (retina)
- Slowly move the light towards the eye at an angle to observe the optic disk on the medial side
- Follow blood vessels out from the optic disk and check macula
- Dim lights and sit opposite the patient and shine light
- Movements of the eye
- Keeping head still, follow fingers with your eye