8.1.2 Orbital Contents, Lacrimal Apparatus and Vision Flashcards
What are the contents of the orbital cavity?
Nerves (CNII,III,IV,Va,VI)
Blood vessels (opthalmic artery and vein)
Fat +++
Lacrimal apparatus
Eyeball (globe)
Extra-ocular muscles
What is the tear film and lacrimal apparatus?
Structures involved in tear production and drainage
What are the layers of the tear film?
Meibomian-Oily
Lacrimal-Water
Goblet cells in conjuctiva-Mucus
What does blinking do?
Protects eye
Distributes tear film across surface of eye, rinsing and lubricating conjuctivae and cornea
What does the lacrimal apparatus do?
Collects and drains tear fluid, obstruction to drainage can cause epiphora (overflow of tears over lower eyelid)
Outline tear production to drainage
- Lacrimal gland produces tears
- Drains to lacrimal punctum
- Lacrimal canaliculus
- Nasolacrimal duct
- Inferior meatus of nasal cavity
How can the nasolacrimal duct become obstructed?
Infection
Injury
Stenosis
How is the position of the eyeball maintained?
Suspensory ligament (sits underneath like a sling)
Extra-ocular muscles
Orbital fat
What is the outer surface of the eyeball covered with?
Transparent mucous membrane, conjuctival membrane reflects onto inner surface of lids
Why can’t contact lenses slip behind the eye?
Conjuctiva reflects on itself, contact lenses unable to go beyond this point
What part of the outer anterior eyeball is not covered by conjuctivae?
Cornea- conjuctiva goes to the limbus (area where the sclera and cornea meet) and no further
What does the conjuctival membrane contain?
Blood vessels
What is conjuctivitis?
Inflammation of the conjuctiva,
* Uncomfortable “gritty”
* Watery, may have discharge
* Typically viral cause e.g. adenovirus
* Highly contagious
What is a sub-conjuctival haemorrhage?
Burst blood vessel in the conjuctiva
Painless
No other symptoms
Often no cause- spontaneous
What are the three layers of the eye?
Label the image
Outer: sclera (continues posteriorly as the dura mater and the cornea anteriorly)
Middle: choroid (vascular), continuous with ciliary body and Iris (=uveal tract)
Inner: retina (photosensitive layer)
What is the macula and what does it do?
Point of highest acuity vision (thinnest part of retinal layer with many cones) for central vision
Appears as a darker spot as thinnest part of the retina so can see the highly vascular choroid layer through it
Where is the fovea?
At the back of the eye, the indent
What are the different layers of the back of the eye?
Retinal ganglion cells, axons merge together forming the optic nerve
Rods and cones
Pigment epithelium to absorb excess light
What happens in a central retinal artery occlusion?
Sudden painless loss of sight in one eye developing over seconds
Due to embolus causing occlusion
How does a central retinal artery occlusion appear on a fundoscope?
Pale retina due to ischaemia
Cherry redspot as the macula is the thinnest part and you can see the vascular choroid layer beneath which is unaffected
What is the globe of the eye filled with?
Aqueous humour (watery)
Vitreous humour (firm jelly-like)
How is the lens attached to the ciliary body?
Suspensory ligaments attached to ciliary muscle
What does the ciliary body contain?
Ciliary muscle
Ciliary processes- produces the aqueous humour
How does aqueous humour flow and drain?
Flows from posterior chamber through pupil into anterior chamber
Nourishes lens and cornea
Drains through iridocorneal angle (between iris and cornea)
Via trabecula meshwork into canal of schlemm (circumferential venous channel draining into venous circulation)
What is glaucoma?
Optic nerve damage secondary to raised intraocular pressure
How does glaucoma occur?
Drainage of aqueous humour from anterior chamber blocked, causes rise in intra-occular pressure
What is chronic: open-angle glaucoma?
Most common and many asymptomatic
* Trabecular meshwork deteriorates with age
* Increased intraocular pressure, causes optic disc cupping
* Gradual loss of peripheral vision
What is acute: closed-angle glaucoma?
Less common
- Narrowing of iridocorneal angle
- Opthalmological emergency as sight threatening
How does glaucoma appear on fundoscopy?
Glaucomatous cupping- lighter region of optic disc increases in size
How does acute closed angle glaucoma present?
Older patient- 55+
Acutely painful red eye
Irregular oval shaped pupil (fixed)
Blurred vision
Halos around lights (due to corneal oedema)
Nause and vomiting
Treat with drugs to reduce IOP then surgery
For us to see what must happen?
Light must reach and be focused onto the macula
How can light reach the macula?
- Transparent lens and cornea
- Pupil- regulates light entering
- Tear film and cornea refract light to bring into focus
- Shape of the eye ball, too long (short sighted, myopic) too short (long sighted, hypermetropic)
What is the accomodation reflex?
To see closer objects, greater refraction needed to focus onto retina beyond capabilities of cornea
- Pupil constricts, limits light coming through
- Eyes converge, ensure image remains focused on same point of retina in both eyes
- Lens becomes more biconvex (fatter) by contraction of ciliary muscle
What happens to the lens as we age?
Lens becomes stiffer with age and less able to alter shape, older people hold things further away to see more easily
What is presbyopia?
Age-related inability to focus on near objects
What part of the eye refracts the most light?
Tear-lined cornea
What is phototransduction?
Photoreceptors convert light into action potentials
What are the two type of photoreceptors?
Cones
Rods
What do cones do?
Colour vision
High definition
Active at high light levels
Many at macula and fovea
What do rods do?
Non-colour vision
Low acuity
Active at low light levels
Many at peripheral retina
What happens after action potentials are produced?
APs propagated via retinal ganglion cells
RCG axons collect in area of optic disc forming optic nerve (optic disc=no photoreceptors, blind spot)
APs propagates along visual pathway to occipital lobe for interpretation
