S8) Functional Anatomy of the Eye Flashcards
1
Q
Describe the structure of the orbital cavity
A
The orbital cavity is pyramidal shaped with its apex pointing posteriorly and base anteriorly
2
Q
Identify the different boundaries of the orbit
A
- Roof (superior wall)
- Floor (inferior wall)
- Medial wall
- Lateral wall
- Apex
- Base
3
Q
Describe the structures forming the roof of the orbital cavity
A
- The roof is formed by the frontal bone and the lesser wing of the sphenoid
- The frontal bone separates the orbit from the anterior cranial fossa
4
Q
Describe the structures forming the floor of the orbital cavity
A
- The floor is formed by the maxilla, palatine and zygomatic bones
- The maxilla separates the orbit from the underlying maxillary sinus
5
Q
Describe the structures forming the medial wall of the orbital cavity
A
- The medial wall is formed by the ethmoid, maxilla, lacrimal and sphenoid bones
- The ethmoid bone separates the orbit from the ethmoid sinus
6
Q
Describe the structures forming the lateral wall of the orbital cavity
A
The lateral wall is formed by the zygomatic bone and greater wing of the sphenoid
7
Q
Describe the structures forming the apex and base of the orbital cavity
A
- The apex is located at the opening to the optic canal (optic foramen)
- The base (aka orbital rim) opens out into the face, and is bounded by the eyelids
8
Q
The orbital cavity has 4 bony walls. Identify them.
A
- Base = Tough orbital rim
- Medial wall = Ethmoid bone
- Floor = Maxillary bone
- Roof = Orbital plates of frontal bone
9
Q
Identify 3 important anatomical relations of the orbital cavity
A
- Paranasal air sinuses (maxillary and ethmoid)
- Nasal cavity
- Anterior cranial fossa
10
Q
Identify two important implications of the anatomical relations of the orbital cavity
A
- Orbital surgery e.g. lobotomy
- Spread of infection (into and out from orbit)
- Orbital trauma
11
Q
Identify the weakest parts of the orbital cavity and explain why they are most easily fractured
A
Medial wall and floor of the orbit as they are thinner and contain air cavities ( they aren’t chunky bones) .: most vulnerable to fracture when there is direct impact to the front of the eye by a ball/ fist.
12
Q
What is an orbital blowout fracture?
A
An orbital blowout fracture is a fracture leading to the partial herniation of the orbital contents through the the orbital wall due to blunt force trauma to the eye
13
Q
How does an orbital blowout fracture occur?
A
- Direct impact to front of eye e.g. ball/fist
- Leads to sudden increase in intra-orbital pressure from traume to the eye/ orbit
- Results in retropulsion of contents in the orbit e.g. eyeball
- This fractures floor of orbit (maxilla) → orbital blow-out fracture
14
Q
How does an orbital blow out fracture present?
A
- Periorbital swelling (painful)
- Double vision (especially on vertical gaze)
- Impaired vision
- Anaesthesia over affected cheek (infraorbital nerve damage)
15
Q
How do orbital blow out fractures occur?
A
Sudden increase in intra-orbital pressure from retropulsion of eye ball e.g. by fist/ball fractures floor of orbit
16
Q
What is the result of an orbital blow out fracture?
A
- Orbital contents and blood can prolapse into maxillary and ethmoid sinuses respectively
- The fracture site can ‘trap’ structures e.g. muscles and soft tissue such as the extra orbital muscle located near floor or orbit
17
Q
Look at the images and describe what is seen in the orbital blow out fracture?
A
18
Q
In orbital blow out fractures, the mastoid is often fractured. Although the medial wall of the orbit made by the ethmoid bone is thinner than the floor of the orbit, what makes it stronger?
A
- presence of walled air cells (anterior, middle, posterior and ethmoidal air cells) act as buttresses + convey an added strength to the medial wall.
19
Q
What risk is there with the presence of air ethmoidal air cells in relation to the orbit?
A
- air cells can be come infected (acute sinusitis) and due to their proximity to the orbit → infection can sometimes break through the thin lamina papyracea (part of ethmoid bone forming medial wall of the orbit) → .: track into the orbit → causing orbital cellulitis
20
Q
Which nerve is at risk during an orbital blow out fracture?
A
- infra-orbital nerve (branch of Vb of trigeminal nerve)
21
Q
What is the management for orbital blow-out fracture?
A
22
Q
Three openings at its apex of the orbit transmitting nerves and blood vessels in and out.
Identify them
A
23
Q
What structures are carried in the optic canal?
A
24
Q
What structures are carried in the superior orbital fissure?
A
25
What structures are carried in the inferior orbital fissure?
26
27
What is the main arterial supply to the orbit and eye?
* ophthalmic artery (branch of ICA)
* branches of ophthalmic artery including _central retinal artery_ (supplies retina)
28
What is the main venous drainage of the orbit and the eye?
* _ophthalmic veins (superior and inferior)_ connections with **cavernous sinus**, pterygoid plexus and facial vein
29
There are 2 blood supplies to the retina. What are they? What is the significance of these?
1. **Central retinal artery** (branch of ophthalmic artery) → **inner retina**
2. **Ciliary arteries** (branches of ophthalmic artery) → feed extensive capillary bed within choroid layer (choriocapillaries) → these then supply retina too → **outer retina**
Retina **requires both** circulations to **function** properly.
30
Describe the structure and function of the eyelids (palpabrae)
- **Structure**: consists of **skin**, subcutaneous tissue, **muscles, tarsal plates.**
- **Function**: protect the front of the eye
31
Which 2 key muscles are found running in the eyelid?
1. Orbicularis oculi
2. Levator palpebrae superioris
32
What does the orbicularis oculi do and what is it innervated by?
* Closes the eyelid
* Innervated by facial nerve CN VII
33
What does the levator palpebrae superioris do and what is it innervated by?
* Retracts eyelid
* Innervated by occulomotor nerve CN III
34
What do the tarsal plates do?
**Tarsal plates** are dense bands of connective tissue skeleton to the eyelid.
It strengthens and gives the shape of the eyelid.
35
There are several glands found within the eyelids. Identify 2.
* **Meibomian/ Tarsal glands within tarsal plate**
* **Glands associated with lash follicle**
36
What is the function of the Meibomian/ tarsal glands found within the tarsal plates?
* Modified sebaceous
* Provides a lipid layer of tear film
* This to prevent tear evaporation too quickly + spillage over eyelid.
37
If the tarsal/ Meibomian glands gets blocked, what can you present with?
* Meibomian cyst - presents as a lump within the eyelid
38
How does a Meibomian cyst present?
39
How can you treat Meibomian cyst?
* ⅓ resolve spontaneously
* Surgical incision if persists
40
What is the function of the glands associated with lash follicle?
* sebaceous - secretes oily substance → nourishes the hair follicle
41
What can arise from infected eyelash hair follicle or sebaceous gland?
**Stye**
42
How does a stye present?
43
How can you treat a stye?
* Warm compresses +/- oral antibiotics
44
What is blepharitis?
* inflammation of eyelid margin
45
What are the causes of Blepharitis?
* Staphyloccocus
* Meibomian gland dysfunction
46
How does someone present with Blepharitis?
crusting, dry eye lids +/- swollen + red
47
How can we treat blepharitis?
* Treat with warm compress and lid hygiene (clean up debris, helps gland drain normally)
48
What is the orbital septum?
**Orbital septum** is a thin sheet of fibrous tissue originating from orbital rim and blends with the tendon of LPS and tarsal plates
49
What does the orbital septum do?
The **orbital septum** separates intra-orbital contents from muscle and subcutaneous tissue of eyelid.
50
What is the clinical significance of the orbital septum?
The **orbital septum** acts as a barrier against infection spreading from the pre-septal space to post-septal (orbital cavity proper)
51
What is the difference between pre-septal/ periorbital cellulitis and post-septal cellulitis?
* **Pre-septal cellulitis** → infection inv. superifical tissues (anterior to orbital septum)
* **Post-septal cellulitis** → infection inv. tissues within the orbit (posterior to orbital septum)
52
What is periorbital cellulitis?
**Periorbital cellulitis** is the cellulitis of orbital structures confined to skin and tissues of eyelid, superficial to orbital septum.
53
What can cause peri-orbital cellulitis?
occurs secondary to infection from bites, periorbital trauma, sinuses (fronto-ethmoidal sinuses)
54
How would peri-orbital cellulitis present?
* Painful
* Ocular function - all eye movements + function NORMAL
55
Identify 2 complications of periorbital cellulitis
- Abscess formation
- Intracranial spread of infection → cavernous sinus thrombosis
56
It can be difficult to differentiate between peri-orbital and the more severe orbital cellulitis. What should you do?
* if any doubt, urgently refer
57
What is orbital/ post-septal cellulitis?
* infection within the orbit (posterior or deep to orbital septum)
58
How does one present with orbital cellulitis?
* Proptosis/ exopthalmous
* **Reduced +/- painful eye movements**
* **Reduced visual activity - ‘blurred vision’**
59
What could cause orbital cellulitis?
* spread of infection from paranasal air sinus
60
What is the danger with orbital cellulitis?
* orbital veins drain to cavernous sinus
* potential route for infection to **spread intracranially**
resulting in :
→ cavernous sinus thrombosis
→ meningitis
61
How can we treat orbital cellulitis?
* IV antibiotics
* close monitoring to get infection under control
62
Describe the arterial supply of the orbit cavity
Arterial supply via ophthalmic artery and its branches
63
Describe the venous drainage of the orbital cavity
The **veins of orbit** drain to cavernous sinus, pterygoid venous plexus and facial veins
64
Describe the general sensory, special sensory and motor innervation of the orbital cavity
- **General sensory**: opthalmic nerve (CN Va)
- **Special sensory**: optic nerve (CN II)
- **Motor**: oculomotor (CN III), trochlear (CN IV), abducens (CN VI)
65
Identify the glands of the eye
66
What is the Meibomian gland and what does it do?
**Meibomian glands** secrete a lipid-rich substance to lubricate the edge of the eyelids and mix the tear film over the surface of the eye to prevent tears from evaporating to quickly
67
What is a Meibomian cyst?
A **Meibomian cyst** (aka chalazion) is a lump in the eyelid caused by the blockage and resultant inflammation of the Meibomian gland
68
What is a stye?
A **stye** is a small painful lump in the inside of the eyelid due to the blockage of eyelash follicles
69
Identify 6 contents of the orbital cavity
- Lacrimal apparatus
- Nerves
- Blood vessels
- Orbital fat (loads - protective)
- Globe of the eye (eyeball) and its internal structures
- Extra-ocular muscles
70
Which structures are involved in tear film production and draingage?
* tear film and lacrimal apparatus
71
The tear film consists of 3 layers. What are they?
1. Meibomian glands - OILY
2. Lacrimal glands - WATER
3. Goblet cells in conjunctiva - MUCUS
72
**Blinking** distributes tear film across the surface of the eye. What is the physiological effect of this?
* **Blinking** washes tear film across front of eye, rinsing and lubricating the conjunctivae and cornea
* (sweeping any dust + other foreign material across to the medial angle of the eye to be removed)
73
What is the purpose of the lacrimal apparatus?
* series of structures that collect and drain tear fluid
74
If there is obstruction to drainage along the lacrimal apparatus, in particular the **nasolacrimal duct**, what can this result in?
* **Epiphora** - overflow of tears over lower eyelid ('excessive tearing of eye')
75
How can there be obstruction in the lacrimal apparatus?
* due to infection
* injury
* stenosis
76
The lacrimal apparatus is also involved in secretion of tears into conjunctival sac.
Identify the specific structures involved
- Lacrimal gland
- Lacrimal caniculi
- Lacrimal sac
- Nasolacrimal duct
77
What does the lacrimal gland do, where is it found and how is it controlled?
- The **lacrimal gland** secretes lacrimal fluid (tears)
- It lies in the **lacrimal fossa** on the superolateral part of the orbit
- It is under parasympathetic control via the **facial nerve**
78
Describe the process of secretion from the lacrimal gland
- **Lacrimal fluid** enters the conjunctival sac through the **lacrimal canaliculi** and passes into the lacrimal lake at the medial angle of the eye
- The fluid then drains into the **lacrimal sac** before passing into the nasal cavity via the **nasolacrimal duct**
79
What are the small holes on the medial side of the eyelid and what purpose do they serve?
There are two **lacrimal puncta** in the medial portion of each eyelid which function to collect tears produced by the lacrimal glands
80
What can happen if there are dirt or particles damaging the cornea?
* Corneal abrasions
* corneal ulcerations
81
Corneal abrasions and ulcerations can arise if there is dirt or particles damaging the cornea. How is the cornea recovered?
* **Outer epithelial layer of the cornea is constantly undergoing mitosis**.: easily regenerates if damaged
* However, if there are injuries deep into the cornea → can lead to permanent scarring + potential sig visual impairment
82
How is the eyeball maintained in position?
* Sensory ligament (sits underneath like a sling)
* Extra-ocular muscles
* Orbital fat ++
83
Covering over the sclera is a **thin transparent layer of cells** called the ..
conjunctivae
84
What is the conjunctiva and what does it do?
**Conjunctiva** is a transparent, secretory mucous membrane which produces mucous and tears to lubricate the conjunctival and corneal surfaces
85
Describe the location and structure of the conjuctiva
- **Location**: covers sclera and lines inside of eyelids, forming a conjunctival sac (**does no**t cover over cornea)
- **Structure**: **highly vascular** with small blood vessels within the membrane
86
What is conjunctivitis?
**Conjunctivitis** (aka pink eye) is an eye condition that causes redness and inflammation of the conjuctiva due to infection or allergy
Redness due to blood vessels dilating.
87
What is the most common cause of conjunctivitis?
viral causes
88
How would one present with conjunctivitis? Is it contagious?
Contagious - can spread to the pt's other eye or other people!
89
What is the treatment for conjunctivitis?
* Reassurance
* hygiene advice - washing hands/ not sharing towel etc
* short course of topical choramphenicol eye drops - reduces risk of secondary bacterial infection
90
Name an infective organism that can cause conjunctivitis esp from a mother giving birth.
* Chlamydia - picked up from mother's vaginal mucosa during birth → maybe a cause of **conjunctivitis in the neonatal period**
91
What is the treatment for chlamydial conjunctivitis?
systemic antibiotics - erythromycin
92
What is a subconjunctival haemorrhage?
A **subconjunctival haemorrhage** (aka hyposphagma) is bleeding underneath the conjunctiva due to the rupture of one of the blood vessels often spontaneously in this mucosal layer
93
How would one present with subconjunctival haemorrhage? Is it painful?
94
How do we treat subconjunctival haemorrhage?
* Looks more concerning that it acc is
* Such haemorrhages are common
* Just reassure pt that it will slowly resolve → much like a bruise would elsewhere on the body
95
Name an example of a serious condition presenting as an acutely painful red eye?
* **Uveitis**
96
What is uveitis?
* **inflammation of the choroid layer**
(uvea - collective term for choroid, ciliary process and iris)
97
How do patients with uveitis present?
* red painful eye
* pain often worse when focussing / looking at bright lights
98
What is uveitis typically associated with?
autoimmune conditions such as:
* **ankylosing spondylitis**
* **inflammatory bowel disease**
99
How would we manage and treat uveitis?
* If uveitis is suspected, pt must be **referred urgently to ophthalmology** for treatment (corticosteroids)
100
The eyeball has three layers.
Identify and describe them
- **Outer**: tough, fibrous sclera which continues anteriorly as transparent cornea
- **Middle**: choroid which continues anteriorly as ciliary body and iris (vascular) = uveal tract
- **Inner**: retina (inner photosensitive layer)
101
What does the sclera do?
- Provides **attachment** for the extra-ocular muscles
- Gives **shape** to the eyeball
- Continuous with the **dural sheath** covering the optic nerve at the back of the eye
102
What is sclera continuous of?
Sclera is **continuous posteriorly** with **dura mater** covering **optic nerve.**
103
Describe the structure and function of the ciliary body
- **Structure**: vascular, muscular, consists of a ciliary process and muscle
- **Function**: connects the choroid with the iris, produces aqueous humour (ciliary processes)
104
Describe the general structure of the retina
The **retina** consists of an inner neurosensory layer and an outer pigmented epithelial cell layer
105
What does the retina do?
The **photosensitive layer of retina** converts the light energy into electrical impulses/ action.
106
The pigmented layer lies between the choroid and neurosensory layer of the retina and its cells contain melanin.
What is the role of melanin?
**Melanin** absorbs scattered light that has passed into the eye, reducing reflection and allowing us to focus images appropriately on to the retina
107
What are the photocreceptors?
rods and cones
108
The neurosensory layer is the area of the retina that senses light and is where the photoreceptors (rods and cones) are found.
What is the role of the rod cells?
The **rods** are cells found in the peripheral parts of the retina, responsible for vision in **low intensity light** (night vision) and **do not discern colours**
109
The neurosensory layer is the area of the retina that senses light and is where the photoreceptors (rods and cones) are found.
What is the role of the cone cells?
The **cones** are responsible for high visual acuity and colour vision and many are concentrated in an area of the retina called the macula
110
What is the macula?
The **macula** is an area of the retina upon which acts as the centre of your vision and is visible on fundoscopy as a slightly darker area of the retina, lateral to the optic disc
* point of highest acuity vision = central vision
111
What is the fovea?
The **fovea** is the area at the very centre of the macula where the only photoreceptors are cones
112
There are 3 types of cones: red, green and blue which respond to diff light wavelengths. Absence/ dysfunction of one of these 3 cones can lead to …
**colour blindness** - an inherited condition that affects males more frequently than females
113
Describe what happens once light is focused and reaches the photosensitive layer of the retina
* Photoreceptors convert the **light energy into electrical impulses**
* These reach the **optic disc**
* The optic disc represents the **accumulation of retinal axons** that leave the eye as the **optic nerve**
114
In central retinal artery occlusion, what occurs?
* Ciliary branches are not affected as these are spared but remember, retina needs both blood supplies .: we get
* **sudden painless loss of sight in one eye, developing over seconds.**
115
How could central artery occlusion arise?
due to embolus causing occlusion
116
What would the ophthalmoscope examination of the retina look like in someone with central artery stenosis and why?
* underlying choroidal layer unaffected .: well perfused .: macula on top it comes out very red
* whereas retina is pale - due to lack of supply coming from central retina artery
117
What is the palpebral fissure?
The **palpebral fissure** is the opening between the eye lids
118
Label the structures 1-3 below in the eye:
119
Label the structures 4-6 below in the eye:
120
Label the structures 7-11 below in the eye:
121
The eyeball has three chambers. Identify them
Anterior chamber: space between the **cornea** and **iris**
Posterior chamber: space between the **iris** and the **lens**
122
Describe the contents of the different chambers of the eyeball
- Vitreous chamber is filled with a transparent, jelly-like **vitreous** **humour**
- Anterior and posterior chambers are filled with transparent **aqueous humour**
123
How does the anterior chamber connect with the posterior chamber?
via the **pupil**
124
What is aqueous humour and what does it do?
**Aqueous humour** is a substance secreted by the ciliary processes within ciliary body found in the posterior chamber (fills both chambers via pupil) and nourishes the lens and cornea
125
Describe the flow and drainage of aqueous humour
- **Flows** from posterior chamber, through pupil into anterior chamber
- **Drains** through iridocorneal angle (between iris and cornea) via trabecular meshwork into canal of Schlemm + back into the venous circulation
126
What is the irido-corneal angle?
space **between** the anterior surface of the **iris** and the posterior extremity of the **cornea**
127
What is the canal of Schlemm?
The **canal of Schlemm** is a circumferential venous channel draining into venous circulation
128
What does the ciliary body contain?
* ciliary muscle and
* ciliary processes
129
What is glaucoma?
- **Glaucoma** is an eye condition due to the obstruction of the drainage of aqueous humour, leading to a rise in intra-ocular pressure
- If left untreated glaucoma leads to irreversible damage and death of the optic nerve, causing impairment of vision or even blindness
130
How is the optic nerve affected in glaucoma?
* In glaucoma → raised intra-ocular pressure → compresses + damages optic nerve
131
We have two types of glaucoma. What are they? Which one is common?
* Chronic → open-angle glaucoma - **most common**
* Acute → closed-angle glaucoma
132
Drainage of aqueous humour from anterior chamber can be blocked.
Explain 2 possible ways this can occur
- **Trabecular meshwork deteriorates** → **open angle glaucoma** (chronic – most common)
- **Narrowing of irido corneal angle** → **closed angle glaucoma** (acute – less common)
133
What causes open angle galucoma?
* this is caused by **blockage within** the **trabecular network** (usually drains into aqueous humour into the **canal of Schlemm)**
134
How does open-angle glaucoma present?
* many asymptomatic (picked up on routine eye tests)
* develops painlessly and insidiously over time - hard to pick up
* Raised IOP - can be screened for in routine eye tests → see increased **optic disc cupping** and **visual field loss esp peripheral vision**
135
How can we treat open-angle glaucoma?
* **topical medications** (eye drops) - that **reduce the production** of aqueous humour e.g b-blockers e.g. Timolol **and/or increase its drainage .: help reduce IOP**
* if these fail, surgery e.g. trabeculectomy may be needed
136
Closed-angle glaucoma is less common. What causes this?
* blockage due to **narrowed iridocorneal angle ( narrowed by iris).**
* **the access to the trabecular network is blocked off** rather than within the trabecular network → results in **rapid rise** in intra-ocular pressure.
137
How would one present with closed-angle glaucoma?
* **Sudden onset of painful red eye ++**
* **blurred vision or halos** around objects (**due to corneal oedema**)
* **fixed** or **sluggish, semi-dilated** often **irregular, oval-shaped pupil**
* **nausea + vomitting**
* eye feels **hard** + **tender** to palpate through the upper eyelid
138
What is dangerous about closed-angle glaucoma?
* Ophthalmological emergency → requires **rapid recognition** and **management → irreversible sight loss can occur within a few hours**
139
How do you treat closed-angle glaucoma?
* **diuretics** e.g. acetazolamide - reduced aqueous humour production
* **muscarinic eye drops** e.g. pilocarpine → usually causes pupillary constriction .: helps open the irido-corneal angle → improves route of drainage .: helps reduce IOP
* **strong analgesia**
* **Surgical** - making hole in the iris (irdotomy) with a laser (or surgically) → allows aqueous humour to flow freely from posterior chamber to anterior chamber
140
Who is most at risk of closed-angle glaucoma?
141
What would glaucoma look like through at opthalmoscope?
142
What is the iris?
The **iris** is a thin contractile diaphragm, located anteriorly to the lens, with a central aperture (the pupil) for transmission of light
143
The iris gives the colour to the eye.
Apart from this, what else does it do?
Two muscles (**sphincter and dilator pupillae**) form the iris and control the size of the pupil, both of which are under the control of the ANS
144
What is the lens?
The **lens** is a transparent biconvex structure enclosed in a capsule, lying posterior to the iris and attached to the ciliary body by the suspensory ligaments
145
Describe the neurovascular supply to the lens
The lens is **without nerve innervation or blood supply**, receiving its nutrients entirely from the aqueous humour that surrounds and bathes it
146
What are cataracts?
**Cataracts** are progressive opacities (clouding) occurring within lens leading to decreased vision
As we age, degradation of the proteins in the lens → can cause it to become clouded and less transparent → cataracts
Treatment: surgery
147
Light must reach and be focused onto the macula.
What regulates light entry?
Where does the light get refracted to be brought into focus?
What is the shape of the eyeball?
148
What is refraction?
**Refraction** is the change in direction of light on passing through boundary of two different mediums
149
Light will be refracted as it passes through a number of structures and ‘fluids’ from outside eye towards retina.
Identify these structures
- From air into **liquid tear film** → refract
- Through **cornea** → refract
- Through **lens** and **vitreous humour** before reaches retina
150
Where does most refraction of light occur?
The most refraction of light occurs at the **tear film-cornea interface**
151
Focusing near objects requires a greater refraction of light.
Why is this?
Light rays from near-objects are **more divergent** and require greater refraction to bring them into focus on retina
152
What is the purpose of the accommodation reflex?
* Helps to achieve **greater refraction of light** to bring image into focus when focusing _near objects_ (the tear film and cornea can't change .: lens will change - can thicken)
153
Outline the accommodation reflex
- **Pupil constricts** (limit amount of light coming through)
- **Eyes converge** & image is brought to focus on _same point of retina_ in both eyes
- **Lens becomes more biconcave (fatter) -** by **contraction of ciliary muscle**
154
contraction of the ciliary muscle is under the control of …
**parasympathetic nervous system**
155
contraction of the ciliary muscle is under the control of …
**parasympathetic nervous system**
156
What is presbyopia?
**Presbyopia** is the **age-related inability** to focus near-object as the **lens becomes stiffer** as it becomes **dense, less elastic** and less able to change shape
157
How can presbyopia be corrected?
**Glasses** - to allow near-objects to be focused more comfortably
158
What does phototransduction mean?
* conversion of photons (light particles) to action potentials
159
In five steps, describe the mechanism in which the retina responds to light
⇒ **Light** (photons) reaches the photosensitive retina
⇒ **Photoreceptors** (rods/cones) detect light
⇒ **Action potentials** are generated in response to light (photoreceptors convert light signals into action potentials)
⇒ Action potentials pass via r**etinal ganglion cells** whose axons collect in area of **optic disc** forming the **optic nerve.**
⇒ Action potentials propagated along **visual pathway** to **occipital lobe** for interpretation
160
What is visual acuity?
ability of the eye to discern shapes and details of what you see
161
How can you measure visual acuity?
* measured formally using **Snellen chart**
* Read set of letters of increasingly smaller size
* Test one eye at a time
* Pt reasons at 6m distance from the chart
162
Normal vision using Snellen Chart is said to be…
Normal vision = 6/6
(at a 6m distance, individual is reading row of letters that corresponds with what we'd expect someone to be able to read at a 6m distance)
163
What does the numerator and denominator depend on when measuring visual acuity?
164
Initial inspection of the eye appears normal. The doctor tests distance visual acuity (VA) using a Snellen Chart. VA is recorded as 6/9 in the left eye and 6/18 in the right eye.
What does 6/9 and 6/12 mean…and in which eye is VA better?
Left eye is better - 6/9
Smaller the denominator - the greater
As numerator for both is 6. That is the distance they are standing from (6m). The value at the bottom is the row they can read from the Snellen chart → the smaller letters are found on a row that is of lesser number.
165
What are 3 causes and examples of decreased visual acuity?
166
How can we check transparency of the structures in the eye? (to see if this is the cause behind decreased visual acuity)
Shining ophthalmoscope into the eye and checking for ‘**red reflex’ - normal**
**Absent** → suggests light prevented from reaching retina and reflecting back.
167
Which conditions can result in the absence of the red reflex?
* Cataracts
* vitreous haemrrhoage
* retinoblastoma
168
If there is nothing wrong with transparency of the eyes, we check to see if there are any refractive issues causing the decreased visual acuity. What refractive issues could be seen?
169
If there are no transparency issues, no refractive issues, what could be concerning behind decreased visual acuity?
170
How can we differentiate between refractive or non-refractive cause behind decreased visual acuity? Explain this
171
What is papilloedema?
**Papilloedema** is a condition in which there is optic disc swelling due to an increased intracranial pressure.
172
What could cause papilooedema?
* things that cause an increase in ICP e.g. cerebral haemorrhage, tumour, meningitis
173
Papilloedema can be associated with blurring of vision, which typically occurs in both eyes.
Why would blurring of vision involve both eyes (this does not relate to involvement of the optic chiasm…)?
Optic nerve - are **extensions of the meninges**.: any pressure in the brain i.e. raised ICP → will also affect the optic nerve
174
How is papilloedema different from optic disc ‘cupping’?
* **_Optic disc cupping_** → **optic disc same size,** **optic cup bigger** due to **increased intra-ocular pressure** ( pressure in eyeball)
* **_Papilloedema_** → swelling of optic disc (**optic disc bigger**) due to **increased intra-cranial pressure.**
175
There are six extra-ocular muscles that attach to the globe of the eye, whose contraction causes movement and therefore control our direction of gaze.
Identify them
- Lateral Rectus
- Medial Rectus
- Superior Rectus
- Inferior Rectus
- Inferior oblique
- Superior oblique
176
State the origin and attachment of the superior rectus muscle
- **Origin**: common tendinous ring
- **Attachment**: superior and anterior aspect of the sclera
177
State the function and innervation of the superior rectus muscle
- **Function**: elevation
- **Innervation**: oculomotor nerve (CN III)
178
State the origin and attachment of the inferior rectus muscle
- **Origin**: inferior part of the common tendinous ring
- **Attachment**: inferior and anterior aspect of the sclera
179
State the function and innervation of the inferior rectus muscle
- **Function**: depression
- **Innervation**: oculomotor nerve (CN III)
180
State the origin and attachment of the medial rectus muscle
- **Origin**: medial part of the common tendinous ring
- **Attachment**: antero-medial aspect of the sclera
181
State the function and innervation of the medial rectus muscle
- **Function**: adduction
- **Innervation**: oculomotor nerve (CN III)
182
State the origin and attachment of the lateral rectus muscle
- **Origin**: lateral part of the common tendinous ring
- **Attachment**: anterio-lateral aspect of the sclera
183
State the function and innervation of the lateral rectus muscle
- **Function**: abduction
- **Innervation**: abducens nerve (CN VI)
184
State the origin and attachment of the superior oblique muscle
- **Origin**: body of the sphenoid bone
- **Attachment**: sclera of the eye, posterior to the superior rectus
185
State the function and innervation of the superior oblique muscle
- **Function**: depression, abduction and medial rotation
- **Innervation**: trochlear nerve (CN IV)
186
State the origin and attachment of the inferior oblique muscle
- **Origin**: anterior aspect of the orbital floor
- **Attachment**: sclera of the eye, posterior to the lateral rectus
187
State the function and innervation of the inferior oblique muscle
- **Function**: elevation, abduction and lateral rotation
- **Innervation**: oculomotor nerve (CN III)
188
How does one test the function of the different extra-ocular muscles clinically?
189
Label the following structures on this image of the retina:
- Artery
- Vein
- Optic disc
- Macula
- Fovea
190
What are the different steps involved in examining the eye?
- **I** = Inspection
- **VA** = Visual acuity
- **F** = Visual fields
- **R** = Reflexes
- **O** = Opthalmoscope
- **M** = Movements
191
Overall ….
192
Pathology involving which structures could present with blurring of vision in one eye?
* lens → cataracts
* cornea → abrasion/ ulcer
* vitreous humour → haemorrhage / pus
* macula → macula degeneration (occurs w age)
* optic nerve → optic neuritis
* refractive problems
* astagmitism → eyeball change diff
193
Name 2 common conditions that affect the lens.
* Cataracts
* Prebyopia (inability to change lens with age)
194
As part of the eye examination, the doctor performs fundoscopy. Label the following structures on this image of a normal retina: a retinal artery (RA), retinal vein (RV), optic disc (OD) and macula (M)
195
From which artery do the retinal arterial vessels arisen? How did this artery get into the back of the eye (given that the branches emerge in the region of the optic disc…gives a clue)?
* ophthalmic artery → gives rise to the retinal arterial vessels
* Retinal artery runs in the optic nerve
196
A man attends the Emergency Department with an irritable, watery red right eye.
What is the diagnosis and why is the cornea not affected?
Diagnosis: **Conjunctivitis**
**Cornea is not affected** because it is **not covered by the conjunctiva.**
197
Give three other causes for an acute red eye, that would present with pain.
* closed angle glaucoma
* abrasions to the conjunctiva/ eye → corneal abrasions/ ulcers
* uveitis
198
What is the ‘blind spot’?
* the optic disc which contain no photoreceptors
199
What chart do you use to check visual acuity?
Snellen chart
200
Which test do you use to check colour vision?
Ishihara test
