Case 3 - Vision Flashcards
What are the 3 layers of the eye?
Fibrous tunic - outer layer, sclera and cornea (avascular layer)
Vascular tunic - includes iris, choroid and ciliary body
Neuroretina tunic - neural layer, contains the retina, where images are formed
Where is visual acuity the highest?
Fovea
Compare rods and cones
Rods: low-threshold for light, sensitive to low-intensity light (as has more opsin) therefore function well in darkness. Responsible for low-acuity, non-colour vision
Cones: high-threshold for light, operate best in daylight. Higher visual acuity and colour vision
Where are rods and cones found?
Rods - round periphery of retina
Cones- round the fovea (centre of macula)
What is the proportion of rods and cones to neurones?
1 cone to 1 neurone
Multiple rods to 1 neurone
What controls the size of the pupil?
The iris - circular muscles contract to reduce the pupil in size (bright light), and radial muscles contract to increase pupil size (dim light)
What is a consensual response?
When you shine light in one eye, the pupil of the other eye will constrict (= a consensual response)
What happens when there is an efferent defect of the pupillary light reflex?
The ipsilateral eye doesn’t react to light but the other eye does
What happens when there is an afferent defect of the pupillary light reflex?
What happens in one eye also happens in the other
What is the pupillary response to light if a patient has a relative afferent pupillary defect (Marcus Gunn pupils)?
Both pupils will remain dilated (no constriction) when light is shone into the ipsilateral eye to the lesion (i.e. L optic nerve lesion, no response of either eyes if shine light onto L eye)
However, if shine light onto the other eye (i.e. R), both eyes will constrict (normal response)
What can cause a relative afferent pupillary defect?
Tumours, glaucoma, optic neuritis, nerve compression
What is the test used to diagnose Marcus Gunn pupils?
Swinging light test - shows normal constriction with light over unaffected eye but swinging to the affected side leads to dilation of eyes as there is no response
What is the pupillary response to light if someone has third nerve palsy (efferent defect)?
The affected side will have a dilated pupil that does not respond to light shone in either eye.
As afferent is normal, the contralateral pupil will contract when light is shone into the affected eye and in response to its own light
How does the shape of lens change with near vs far vision?
Far vision (distant object) - lens is flattened as ciliary muscles relax and increase tightness in suspensory ligaments
Near vision (close object)- lens is rounded as ciliary muscles contract and decrease tension in suspensory ligaments
What condition is caused when the lens is too convex? How is it corrected?
Myopia: near-sightedness, the lens is round so struggles to focus light for far objects, i.e. light convergence point falls short of the retina
Corrected with a concave lens
What condition is caused when the lens is too concave? How is it corrected?
Hyperopia: far-sightedness, light convergence point falls behind the retina so near objects are blurry
Corrected with a convex lens
How are each of the 6 extra-ocular muscles tested?
SR: look up and out
MR: look in
LR: look out
IR: look down and out
IO: look up and in
SO: look down and in
How are the extra-ocular muscles innervated?
All innervated by oculomotor nerve apart from LR = CN VI (abducens) and SO = CN IV (trochlear)
What is strabismus?
Squinting due to misalignment of visual axis - a lack of coordination between the eyes. It may be linked to amblyopia (poor vision in one eye), but stabismus does not necessarily mean the patient has have poor vision
What can cause stabismus?
Muscle weakness, genetics, brain lesions, uncorrected hypermetropia (i.e. in attempt to accomodate the eye turns in)
What are the symptoms of stabismus?
Double vision, headaches, eye strain
How is stabismus treated?
Glasses/ contact lenses, eye patches, surgery
What is version of the eyes?
When they move in the same direction as each other, i.e. supraversion = both eyes looking up
What is vergence of the eyes?
When eyes move in opposite directions, i.e. lines of sight don’t remain parallel
What is the difference between estropia and exotropia?
Estropia = convergent squint, where one eye turns in
Exotropia = divergent squint, where one eye turns out
What is the difference between hypertropia and hypotropia?
Hypertropia - one eye turns up
Hypotropia - one eye turns down
What does the snellen test for and how is it done?
Tests visual acuity
Chart of letters with a number above each line, which refers to the distance from which a person with normal vision should be able to read, i.e. largest letter numbered 60 = should be able to see at 60m
Chart is always viewed at 6m (20 ft)
How is the snellen fraction calculated?
Snellen fraction = viewing distance (m) / number on test line
What is a normal snellen fraction? What is a better than average fraction?
Normal = 6/6
Better than normal = 6/5
Describe the layers of the retina in order that light hits
1: Ganglion cell layer
2: Inner plexiform layer
3: Inner nuclear layer
4: Outer plexiform layer
5: Photoreceptor layer (outer nuclear)