Visual System

Question 1

Eye anatomy

Answer 1

Palpebral fissure
Lateral canthus
Medial canthus
Iris
Pupil
Caruncle
Limbus (border between cornea and sclera)

Question 2

Lacrimal System

Answer 2

Tears – basal (constant), reflex and emotional (crying)

Afferent – cornea, cranial nerve V1 – ophthalmic trigeminal
Efferent – parasympathetic
Neurotransmitter - acetylcholine

Tears produced by lacrimal gland
The lacrimal gland is located within the orbit, Latero-superior to the globe.
Drain through the two puncta, openings on medial lid margin
Flow through superior and inferior canaliculi
Gather in tear sac
Exit tear sac through tear duct into nasal cavity

Question 3

Tear Film

Answer 3

Maintains smooth cornea-air surface
Oxygen supply to Cornea – normal cornea has no blood vessels
Removal of debris (tear film and blinking)
Bactericide

Composed of three layers
Superficial lipid layer to reduce tear film evaporation - produced by a row of Meibomian Glands along the lid margins
Aqueous (water) tear film (tear gland)
Mucinous Layer corneal surface - maintains surface wetting

Question 4

Conjunctiva

Answer 4

Thin, transparent tissue that covers the outer surface of the eye
It begins at the outer edge of the cornea, covers the visible part of the eye, and lines the inside of the eyelids
It is nourished by tiny blood vessels that are nearly invisible to the naked eye

Question 5

Coat of the Eye

Answer 5

3 layers
Sclera - hard, fibrous and opaque - maintain shape - High water content
Choroid - pigmented and vascular - provide circulation
Retina - neurosensory tissue - convert light into electrical impulses

Question 6

Cornea

Answer 6

5 layers
1 – Epithelium
2 – Bowman’s membrane
3 – Stroma – its regularity contributes towards transparency
4- Descemet’s membrane
5- Endothelium – pumps fluid out of corneal and prevents corneal oedema

The transparent, dome-shaped window covering the front of the eye.
Low water content
Powerful refracting surface, providing 2/3 of the eye’s focusing power. Like the crystal on a watch, it gives us a clear window to look through

Question 7

Uvea

Answer 7

Vascular coat of eyeball and lies between the sclera and retina.
Composed of three parts – iris, ciliary body and choroid.
Intimately connected and a disease of one part also affects the other portions though not necessarily to the same degree.

Question 8

Choroid

Answer 8

Lies between the retina and sclera. It is composed of layers of blood vessels that nourish the back of the eye.

Question 9

Iris

Answer 9

Controls light levels inside the eye similar to the aperture on a camera.
Round opening in the centre is the pupil.
Embedded with tiny muscles that dilate (widen) and constrict (narrow) the pupil size.

Question 10

Lens

Answer 10

Outer acellular capsule
Regular inner elongated cell fibres – transparency
May lose transparency with age – cataract

Transparency
Regular structure
Refractive Power
1/3 of the eye focusing power - higher refractive index than aqueous fluid and vitreous
Accommodation
Elasticity

Question 11

Retina

Answer 11

Very thin layer of tissue that lines the inner part of the eye.
Responsible for capturing the light rays that enter the eye. Much like the film’s role in photography.
These light impulses are then sent to the brain for processing, via the optic nerve.

Question 12

Optic Nerve

Answer 12

transmits electrical impulses from the retina to the brain
connects to the back of the eye near the macula
visible portion is called the optic disc

Question 13

Optic Nerve: Blind Spot

Answer 13

Where the optic nerve meets the retina there are no light sensitive cells. It is a blind spot

Question 14

Macula

Answer 14

Located roughly in the centre of the retina, temporal to the optic nerve
A small and highly sensitive part of the retina responsible for detailed central vision
The fovea is the very centre of the macula. The macula allows us to appreciate detail and perform tasks that require central vision such reading.

Question 15

Fovea

Answer 15

Fovea is the most sensitive part of the retina
It has the highest concentration of cones, but a low concentration of rods
This is why stars out of the corner of your eye are brighter than when you look at them directly.
But only your fovea has the concentration of cones to perceive in detail.

Question 16

Central vision

Answer 16

Detail day vision, colour vision – fovea has the highest concentration of cone photoreceptors
Reading, facial recognition
Assessed by visual acuity assessment
Loss of foveal vision – Poor visual acuity

Question 17

Peripheral Vision

Answer 17

Shape, movement, night Vision
Navigation vision
Assessed by visual field assessment
Extensive loss of visual field – unable to navigate in environment, patient may need white stick even with perfect visual acuity

Question 18

Retinal Structure

Answer 18

Outer layer
Photoreceptors (1st order neuron)
Detection of Light

Middle layer
Bipolar Cells (2nd order neuron)
Local signal processing to improve contrast sensitivity, regulate sensitivity

Inner layer
Retinal ganglion cells (3rd order neuron)
Transmission of signal from the eye to the brain

Question 19

Visual processing - photoreceptors

Answer 19

Rods
Longer outer segment with photo-sensitive pigment
100 times more sensitive to light than cones
Slow response to light
Responsible for night vision (Scotopic Vision)
120 million rods

Cones
Less sensitive to light, but faster response
Responsible for day light fine vision and colour vision (Photopic Vision)
6 million cones

Question 20

Photoreceptor distribution

Answer 20

Rod (scotopic) vision
Peripheral and night vision More photoreceptors, more pigment, higher spatial and temporal (time) summation
Recognizes motion

Cone (photopic) vision
Central and day vision
Recognizes colour and detail

Question 21

Frequency Spectrum

Answer 21

Like a CCD camera the eye captures different colours through different photoreceptors:

S-Cones: Blue
M-Cones: Green
L- Cones: Red

Rods are used for night vision and spatial recognition and are not really sensitive to any particular colour

Question 22

Colour Vision Deficiencies

Answer 22

Deuteranomaly also known as Daltonism is the most frequent form of colour blindness.
People with deuteranomaly are not completely colour blind but they don’t perceive the colour red.
Full colour blindness which occur only in a very small percentage of the population is called achromatopsia

Question 23

Index of refraction

Answer 23

Speed of light in vacuum/ speed of light in medium

Question 24

Adequate correlation between axial length and refractive power
Parallel light rays fall on the retina (no accommodation)

Answer 24

Emmetropia

Question 25

Mismatch between axial length and refractive power | Parallel light rays don’t fall on the retina (no accommodation)

Answer 25

Ametropia (refractive error) Near-sightedness (Myopia) Farsightedness (Hyperopia) Astigmatism Presbyopia

Question 26

Myopia

Answer 26

Parallel rays converge at a focal point anterior to the retina Etiology : not clear , genetic factor Causes excessive long globe (axial myopia) : more common excessive refractive power (refractive myopia Blurred distance vision Squint in an attempt to improve uncorrected visual acuity when gazing into the distance Headache

Question 27

Hyperopia

Answer 27

Parallel rays converge at a focal point posterior to the retina Etiology : not clear, inherited Causes excessive short globe (axial hyperopia) : more common insufficient refractive power (refractive hyperopia) visual acuity at near tends to blur relatively early nature of blur is vary from inability to read fine print to near vision is clear but suddenly and intermittently blur asthenopic symptoms : eyepain, headache in frontal region, burning sensation in the eyes, blepharoconjunctivitis Amblyopia – uncorrected hyperopia > 5D

Question 28

Astigmatism

Answer 28

Parallel rays come to focus in 2 focal lines rather than a single focal point Etiology : heredity Cause : refractive media is not spherical-->refract differently along one meridian than along meridian perpendicular to it-->2 focal points (punctiform object is represent as 2 sharply defined lines) ``` Symptoms Asthenopic symptoms (headache , eyepain) blurred vision distortion of vision head tilting and turning Treatment Regular astigmatism : cylinder lenses with or without spherical lenses (convex or concave), Sx Irregular astigmatism : rigid cylinder lenses, surgery ```

Question 29

Near Response Triad

Answer 29

Adaptation for Near Vision Pupillary Miosis (Sphincter Pupillae) to increase depth of field Convergence (medial recti from both eyes) to align both eyes towards a near object Accommodation (Circular Ciliary Muscle) to increase the refractive power of lens for near vision

Question 30

Presbyopia

Answer 30

Naturally occurring loss of accommodation (focus for near objects) Onset from age 40 years Distant vision intact Corrected by reading glasses (convex lenses) to increase refractive power of the eye ``` Treatment convex lenses in near vision Reading glasses Bifocal glasses Trifocal glasses Progressive power glasses ```

Question 31

Types of optical correction

Answer 31

Contact lenses disadvantages : careful daily cleaning and disinfection , expense complication : infectious keratitis , giant papillary conjunctivitis , corneal vascularization , severe chronic conjunctivitis Intraocular lenses replacement of cataract crystalline lens give best optical correction for aphakia , avoid significant magnification and distortion caused by spectacle lenses ``` Surgical correction Pre operative eye Initial cutting of corneal flap Cutting of corneal flap Flipping of corneal flap Photorefractive treatment (laser) Corneal stroma reshaped post laser Corneal flap back in position ``` The Staar intra-collamer lens (ICL) is inserted into the eye for the correction of myopia and astigmatism ``` Clear lens extraction + IOL: IOL: Intra ocular lens. Same as cataract extraction. Implantation of artificial lens. Lose accommodation (patient will need reading glasses). ```

Question 32

Visual Pathway Anatomy

Answer 32

Visual Pathway transmits signal from eye to the visual cortex Visual Pathway Landmarks Eye Optic Nerve – Ganglion Nerve Fibres Optic Chiasm – Half of the nerve fibres cross here Optic Tract – Ganglion nerve fibres exit as optic tract Lateral Geniculate Nucleus – Ganglion nerve fibres synapse at Lateral Geniculate Nucleus Optic Radiation – 4th order neuron Primary Visual Cortex or Striate Cortes – within the Occipital Lobe

Question 33

Visual Pathway Retina

Answer 33

First Order Neurons – Rod and Cone Retinal Photoreceptors Second order Neurons – Retinal Bipolar Cells Third Order Neurons –Retinal Ganglion Cells Optic Nerve (CN II) Partial Decussation at Optic Chiasma – 53% of ganglion fibres cross the midline Optic Tract Destinations Lateral Geniculate Nucleus (LGN) in Thalamus – to relay visual information to Visual Cortex

Question 34

Optic Chiasma

Answer 34

Optic Chiasma – Important Landmark in Visual Pathway Lesions anterior to Optic Chiasma affect visual field in one eye only Lesions posterior to Optic Chiasma affect visual field in both eyes 53% Ganglion Fibres cross at Optic Chiasma Crossed Fibres – originating from nasal retina, responsible for temporal visual field Uncrossed Fibres – originating from temporal retina, responsible for nasal visual field

Question 35

Visual Field Defects

Answer 35

Lesion at Optic Chiasma Damages crossed ganglion fibres from nasal retina in both eyes Temporal Field Deficit in Both Eyes – Bitemporal Hemianopia Lesion Posterior to Optic Chiasma Right sided lesion – Left Homonymous Hemianopia in Both Eyes Left sided lesion – Right Homonymous Hemianopia in Both Eyes

Question 36

Bitemporal Hemianopia

Answer 36

Bitemporal Hemianopia Typically caused by enlargement of Pituitary Gland Tumour Pituitary Gland sits under Optic Chiasma Homonymous Hemianopia Stroke (Cerebrovascular Accident)

Question 37

Homonymous Hemianopia with Macular Sparing

Answer 37

Damage to Primary Visual Cortex Often due to stroke Leads to Contralateral Homonymous Hemianopia with Macula Sparing Area representing the Macula receives dual blood supply from Posterior Cerebral Arteries from both sides

Question 38

Pupillary Function

Answer 38

Regulates light input to the eye like a Camera Aperture In light: pupil constriction decreases spherical aberrations and glare increases depth of field – see Near Response Triad from Previous Lecture reduces bleaching of photo-pigments Pupillary constriction mediated by parasymapthetic nerve (within CN III) In dark: pupil dilatation increases light sensitivity in the dark by allowing more light into the eye pupillary dilatation mediated by sympathetic nerve

Question 39

Pupillary pathways

Answer 39

Afferent pathway (Red & Green) Rod and Cone Photoreceptors synapsing on Bipolar Cells synapsing on Retinal Ganglion Cells Pupil-specific ganglion cells exits at posterior third of optic tract before entering the Lateral Geniculate Nucleus Afferent (incoming) pathway from each eye synapses on Edinger-Westphal Nuclei on both sides in the brainstem Efferent pathway (Blue) Edinger-Westphal Nucleus -> Oculomotor Nerve Efferent -> Synapses at Ciliary ganglion -> Short Posterior Ciliary Nerve -> Pupillary Sphincter

Question 40

Direct vs. Consensual Reflex

Answer 40

Direct Light Reflex –Constriction of Pupil of the light-stimulated eye Consensual Light Reflex – Constriction of Pupil of the other (fellow) eye Neurological Basis Afferent pathway on either side alone will stimulate efferent (outgoing) pathway on both sides

Question 41

Afferent vs. Efferent Defect

Answer 41

Right Afferent Defect E.g. damage to optic nerve No pupil constriction in both eyes when right eye is stimulated with light Normal pupil constriction in both eyes when left eye is stimulated with light Right Efferent Defect (Pupil Constriction) E.g. Damage to Right 3rd Nerve No right pupil constriction whether right or left eye is stimulated with light Left pupil constricts whether right or left eye is stimulated with light Unilateral Afferent Defect Difference response pending on which eye is stimulated Unilateral Efferent Defect Same unequal response between left and right eye irrespective which eye is stimulated

Question 42

Swinging Torch Test

Answer 42

Relative Afferent Pupillary Defect Partial pupillary response still present when the damaged eye is stimulated Elicited by the swinging torch test – alternating stimulation of right and left eye with light Both Pupils constrict when light swings to left undamaged side Both Pupils paradoxically dilate when light swings to the right damaged side

Question 43

Eye Movement - Terminology

Answer 43

Duction – Eye Movement in One Eye Version – Simultaneous movement of both eyes in the same direction Vergence – Simultaneous movement of both eyes in the opposite direction Convergence – Simultaneous adduction (inward) movement in both eyes when viewing a near object

Question 44

Speed of Eye Movement

Answer 44

``` Saccade – short fast burst, up to 900°/sec Reflexive saccade to external stimuli Scanning saccade Predictive saccade to track objects Memory-guided saccade ``` Smooth Pursuit – sustain slow movement Slow movement – up to 60°/s Driven by motion of a moving target across the retina.

Question 45

The Muscles of the Eye (Extraocular muscles)

Answer 45

``` Six muscles Attach eyeball to orbit Straight and rotary movement Four straight muscles Superior rectus Inferior rectus Lateral rectus Medial rectus ``` Superior oblique Inferior oblique

Question 46

Superior and Inferior rectus

Answer 46

Superior rectus Attached to the eye at 12 o’clock Moves the eye up. Inferior rectus Attached to the eye at 6 o’clock Moves the eye down.

Question 47

Lateral Rectus

Answer 47

Also called the external rectus Attaches on the temporal side of the eye Moves the eye toward the outside of the head (toward the temple)

Question 48

Medial Rectus

Answer 48

Also called the internal rectus Attached on the nasal side of the eye Moves the eye toward the middle of the head (toward the nose)

Question 49

Superior Oblique

Answer 49

Attached high on the temporal side of the eye. Passes under the Superior Rectus. Moves the eye in a diagonal pattern down and out Travels through the trochlea

Question 50

Inferior Oblique

Answer 50

Attached low on the nasal side of the eye. Passes over the Inferior Rectus. Moves the eye in a diagonal pattern - up and out.

Question 51

Innervation of Extraocular Muscles

Answer 51

``` Third Cranial Nerve (oculomotor) Superior Branch Superior Rectus – elevates eye levator palpebrae superioris - raises eyelid (not shown) Inferior Branch Inferior Rectus – depresses eye Medial Rectus – adducts eye Inferior Oblique – elevates eye Parasympathetic Nerve – constricts pupil ``` Fourth Cranial Nerve (trochlear) Superior Oblique – depresses eye Sixth Cranial Nerve (abducens) Lateral Rectus – abducts eye

Question 52

Eye Movement Testing

Answer 52

``` Abduction – Lateral Rectus Adduction – Medial Rectus Elevated and Abducted – Superior Rectus Depressed and Abducted – Inferior Rectus Elevated and Adducted – Inferior Oblique Depressed and Adducted – Superior Oblique ```

Question 53

Directions of Eye Movement

Answer 53

Up (Elevation) Supraduction – one eye Supraversion – both eyes Down (Depression) Infraduction – one eye Infraversion – both eyes Right – Dextroversion Right Abduction Left Adduction Left – Levoversion Right Adduction Left Abduction Torsion – rotation of eye around the anterior-posterior axis of the eye

Question 54

Third Nerve Palsy

Answer 54

``` Affected eye down and out Droopy eyelid (loss of elevator palpebrae superioris) Unopposed superior oblique innervated by fourth nerve (down) Unopposed lateral rectus action innervated by sixth nerve (out) ```

Question 55

Sixth Nerve Palsy

Answer 55

Affected eye unable to abduct and deviates inwards | Double vision worsen on gazing to the side of the affected eye

Question 56

Optokinetic Nystagmus Reflex

Answer 56

Nystagmus – Oscillatory eye movement Optokinetic Nystagmus = Smooth Pursuit + Fast Phase Reset Saccade Optokinetic Nystagmus Reflex is useful in testing visual acuity in pre-verbal children by observing the presence of nystagmus movement in response to moving grating patterns of various spatial frequencies Presence of Optokinetic Nystagmus in response to moving grating signifies that the subject has sufficient visual acuity to perceive the grating pattern